Journal | SILCA
May 04, 2017

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Is Your Spare Prepared?

The team at SILCA has supported more than 400 events collectively over the last few years, ranging from Gran Fondos to IronMan Triathlon to local criteriums and even a World Championship. One issue that seems to plague athletes at every level is the unprepared spare inner-tube. Namely, riders are getting caught out riding wheels too deep for their spare tubes! I cannot count the number of sitting/walking/crying athletes I have come upon over the years who are unable to proceed due to a few cm or even mm of valve stem length.

The Situation

There are many reasons people get caught out with this issue. Mostly, we see that riders pack a spare with standard valve length and then switch to a deeper race wheel on race day without thinking about changing anything. We also see riders who plan to ride one depth of wheel and then swap to a deeper one at the last moment. Other times, an athlete might have a flat and replace that spare with one they’ve just purchased. Lastly, and maybe most common of all, riders often match the valve length of the tube to the rim depth which leaves too little stem exposed for use with most pumps or CO2 inflators.

Valve stem too short


Many of the riders we find stuck on the side of the road had intended to just ‘swap’ the valve extender. But there are many problems with this, not the least of which is that during an event like an Ironman, neither your brain, nor your fingers will be working at their peak potential. Dropping or losing the little tool, breaking the extender, having it be corroded to the valve stem, etc. will put an end to your day. If you are already stuck, swapping an extender is completely reasonable to do (we recommend carrying an extender tool with you at all times as well as keeping the extender on your spare), but this should be a backup, not a primary strategy.

The Solution

The solution is to buy or prepare your spare tube with a long enough valve or valve extender that it is AT LEAST 15mm longer than your deepest wheel.

I look at it this way:

Valve that’s way too long for your rim = Looks funny

Valve that’s too short for your rim = You’re walking

Implementing the Solution

Take your deepest rim depth and add 15mm to it, THAT’s the minimum valve stem length you need.

Example: you ride ENVE 7.8 Clinchers. Your front rim is 71mm deep and your rear is 80mm deep. You need at least 80+15 = 95mm of valve stem to guarantee that you can use any pump OR CO2 inflator.

So if you buy a light butyl tube from Continental, it has a 47mm valve stem. You need a valve extender that it AT LEAST 50mm long for your spare.

The Vittoria Latex tube has a 51mm valve, so the SILCA 45mm extender would be a just right fit!

You find a generic butyl tube at a bike shop with 37mm valve (used to be the most common length) you will need AT LEAST a 58mm extender

 We could go on with the examples here, but you get the idea. Our dream is to never again find a rider on the side of the road at an event who is walking their bike despite having a functional tube AND functional inflation method, yet doesn’t have enough valve to connect the two. Go long, seal it up in whatever you carry your spares in, and never worry about it again.


To Sum it Up

  • Prepare your spare tube with a valve that will work in your deepest wheel
  • Carry a valve extender tool with you also (just in case)
  • Do NOT be tempted to swap your spare out for a single event, even if doing so might save you 3 grams
  • If you are using a disc wheel for your event, carry 2 tubes, one the right length for the disc and the other the right length for your deepest wheel.

Spare Unprepared? Click on the products below to change that.

                EOLO III                     


Join the discussion in the comments section below

July 12, 2016


Part 5: Tire Pressure and Aerodynamics

We will start this post with a quick refresh on the state of modern wheel aerodynamics.  

Rim Shape and Tires

In 1991 Steve Hed and Robert Haug patented a rim shape that would go on to be known as the 'Toroidal' shape.  The toroidal rim was unique in the it had no flat surfaces, a deep tire well for tubular tires and curved in such a way that the combined rim and tire formed an ellipse. 

SILCA Hed Bicycle Rim Patent Toroidal

Image from Steve Hed Toroidal Rim Patent #US5061013

In simplistic terms, this patent covers any rim that is wider than the tire and comprised completely of curved surfaces.  Now the problem with the patent was that actually making this rim turned out to be impossible with the technology of the day.  The Hed CX rim was roughly toroidal, but with an aluminum cap at the tire well and brake track could not make the beautiful curvature required for ultimate aero, and even at that, the angled aluminum brake track was problematic.

In 1997, Zipp (who owned the Haug half of the Toroidal patent, but had never made a rim of this shape) patented the 'Hybrid Toroidal' rim shape.  This patent took the concept of having the rim be wider than the tire in order to control the airflow, yet matched it with parallel brake tracks to make the concept more manufacturable. 

SILCA Leigh Sargent Hybrid Toroidal Rim Patent

Sargent/Zipp Hybrid Toroidal patent

The Rule of 105%

The rims of this era were all 19-21mm wide, and the Zipp and Hed rims were typically 23mm wide at the widest point, which was optimized for a 19-21mm tire.  During the early part of my tenure at Zipp 1999-2013 I noticed in the wind tunnel that any time the tire approached the rim width, the aerodynamics were compromised and from that formulated the rule of thumb we called the Rule of 105(%).  The Rule of 105 states that the rim must be at least 105% the width of the tire if you have any chance of re-capturing airflow from the tire and controlling it or smoothing it.  

SILCA Rule of 105 Tire Width Rim Width

Rule of 105 (%) Formulated in 2001 Based on Early Wind tunnel work with 21 and 23mm Tires

One of the most interesting aspects of the Rule of 105 is that before 2001, nobody was tunnel testing with 21 or 23mm tires.  The conventional wisdom was that you TT'd or raced Triathlon on 18-20mm tires and that was that.  However, I was at the Texas A&M Tunnel with US Postal in 2001 and Johan Bruyneel was talking about the amazing ride and grip of these new 21mm tires they received from the team sponsor.  He had made the decision to abandon narrower tires, even for the TT as the riders so preferred this new tire.  We immediately went about testing wheels with 21mm tires and found that the 21 and 22mm rims of the time just weren't wide enough.  

This would be the beginning of an amazing game of chicken and egg within the cycling industry as wheel manufacturers made rims that worked with wider tires and tire manufacturers and athletes kept pushing the limits by using tires even wider still.  In 2007 we struggled to convince cyclists at Paris Roubaix to use 27mm tires when they had always preferred the 'already very wide' 24mm tires by Paris Roubaix 2016 we had cyclists at Roubaix on 30mm front and 32mm rear tires with 25mm tires being used in TT's!

Why the Rule of 105

This CFD image from Bontrager does a great job in showing the 'Why' of the Rule of 105.  While the cycling industry has always liked to talk about aircraft wings, the reality is that no aircraft wing has ever had a bicycle tire as a leading or trailing edge.  This was the realization in the early 2000's that propelled Zipp, then Hed, then Simon Smart/ENVE, Bontrager and now many others to completely rethink the problem.  The real problem/opportunity is how to best take the dirty air off of the tire and smooth it with the rim in the front half of the wheel, and how to use the rim to impart some flow structures that will close up nicely around the tire on the rear half of the wheel. 

SILCA Trek Bontrager CFD Image

Image from Trek/Bontrager D3 Rim Shape White Paper Showing 25mm Tire

This is an image from the Trek/Bontrager white paper of 2011, you can see in the top image how the separation (in blue) is completely dominated by the tire as the rim is narrower.  The 'Zipp' image has the rim and tire at the same width as the tire, and the Bontrager at the bottom has the rim wider than the tire and able to 'recapture' the separated airflow from the tire.  These subtle differences can make for very large changed in drag, and even greater differences in handling.  Many brands have similar CFD imagery to this on their sites, the critical point is that subtle variations in rim shape can and will change aerodynamic drag as well as handling, but none of it is possible unless the rim is at least 105% of the tire width.

The Link to Tire Size and Pressure

So now that we understand how we got to now on this topic, let's revisit our Caliper Measured tires and rims from Part 1.

SILCA Continental Tire Measurements Actual

Actual Measured Widths of Tires On Various Bead Width Rims

The link between pressure and aero starts to become clearer as you look at the chart above.  Between 87 and 115psi most of these tires will grow by nearly 1mm in width.  In strict aerodynamic terms, this added width comes at a cost of roughly 1watt per 2mm of tire at low yaw angles.  However, the big penalty can come at moderate yaw angles as the tires approach the width of the rim.

First let's look at a Zipp 404 Firecrest, a rim with 26.5mm outer width and 16.5mm Bead Width.  With a 23c tire, we see less than 10 grams of drag difference between 6Bar, 7Bar, and 8Bar.  However, with a 25c Tire, we see some significant effects to the aerodynamics of the wheel with changing pressure as the tire growth over those pressures takes rim from being 102% of Tire Width to only 98% of Tire Width.SILCA Zipp 404 Firecrest 25mm Tire Data

Effect of Pressure on Zipp 404 Firecrest with 25mm Continental GP4000sII 25c Tire

All 3 pressures on the 23mm tire made a difference roughly equal to the margin of error of the wind tunnel (A2 Wind Tunnel), so 6Bar, 7Bar, or 8Bar would all be within 10 grams of the blue line .  However, the 25mm Tire is approaching the threshold of aero efficiency due to the inflated width of that tire on a 16.5c bead width rim, and at this tire width, your pressure can make a relatively large aero difference.  

At yaw angled between 10 and 20 degrees, the difference between 7 and 8 Bar tire pressure (100.5 and 115psi) in this instance would be between 1 and 9 watts.  When you consider that a full ceramic bearing upgrade for this same wheel set represents a savings of 0.8-1.0 watt it becomes clear that these aero differences related to tire pressure may be small, but are most definitely non-zero!


For setups where the rim is 105% of the measured tire width or greater, tire pressures will have very small aero effects.  Our friends at FLO Cycling recently completed a very detailed study on 23mm tires on one of their wheels in 5psi increments (rim was 105-108% of tire) and found 0.5-2.0 Watt Difference.  You can read the results HERE. For their setup, the optimal pressure turned out to be 95psi for the 23c Tire on 17.5c Rim.

Again, these are very small numbers, however, at the margins of performance, they may be critical to performance, and best of all, these gains are free of charge to those willing to experiment.

We continue to recommend measuring your tire width and carefully logging your tire pressures to help you better understand these effects.  The thinking should be that wider tires require lower pressures, and if you are violating the Rule of 105 for an Aero Critical event, then perhaps consider downsizing your tire or try and see if a slightly lower pressure may be the solution.

BONUS: Tire Wear and Aerodynamics

As an added bonus we've decided to thrown in a fun graph showing a new 23c GP4000SII and one that has seen 1000 miles of use as a rear under 175lb athlete.  The effect of tire wear was something I first noticed in the wind tunnel 10+ years ago and have been interested in ever since.  While your tire wear will vary based on surface conditions and rider weight, we can unequivocally say that tires with visible center tread wear or flat spotting on the crown of the tire are costing you time out on the course.  

This is logical if you think about it, the crown of the tire will wear flat, and flat, is a terrible aerodynamic shape!  For the sake of our limited data collection time and money, we have used a USED Rear Tire.  Front tires will wear more slowly, but remember, the aero performance of the tire will be slowly degrading every time you use it, so for 'A' races we recommend tires will low mileage!

SILCA Tire Wear Aero Study

July 05, 2016

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Part 4A: Rolling Resistance (The History and Previous Works)

Part 4A in this Series covers the history of Bicycle Rolling resistance and the how and why Pneumatic tires are so awesome.  If you just want to see the data, you can jump to Part B HERE

Back in 2007-2008 during the Paris-Roubaix wheel development, I had an interesting moment in the Arenberg Forest.  I was working with one of the most famous Roubaix winners of the last 10 years, one whom I had also worked with to win Tour Stages, Tour time trials and even a World Championship.  We had been running tires at ever lower pressures trying to find the point at which a rim/wheel failure was inevitable and right there plotted out on the screen was a trend which has been frozen in my brain for these last years:  every time we lowered pressure, he went faster.

It has long been known in CX and Mountain Bike racing that lower pressures are faster, but in road racing and triathlon we have long held onto the belief that most road and even cobble surfaces are smooth enough that higher pressures will be faster, at the expense of comfort.  Even at the beginning of my history with Paris Roubaix testing (~2005), the belief was that we needed to find pressures high enough to be fast, yet low enough that the riders could handle the bikes over cobbled sections.  And yet, right there, every which way we looked at it on the computer, repeated across multiple riders: Lower Pressure was Faster.

Fast forward to today and we have numerous good sources for Crr (Coefficient of Rolling Resistance) testing, and we have a real movement to identify and improve aspects of high performance tires.  We are, in many ways, in a golden age of tire Rolling Resistance advancement, much in the way the 2000's were the age of massive aerodynamic advancement.  However, none of Crr studies in the lab are yet to truly explain or predict the phenomenon we saw in the Arenberg Forest. 

A Theory in the Making

In the last 10 years, two sources that I know of have identified similar effects in their data, Jan Heine of Bicycle Quarterly has written about the effect he calls 'Suspension Losses' which you can read HERE.  Some of Jan's most interesting work is in looking at the power required to ride on different surfaces, including very aggressive ones such as highway rumble strips.  

The theory behind 'Suspension Losses' is rooted in pre-pneumatic tire experience and is also a topic of discussion amongst in-line skating athletes.  Solid tires make surface roughness incredibly apparent to the athlete both in terms of comfort AND speed. 

Imagine a rigid tire and wheel rolling over a 5mm bump in the road.  In this case the tire is rigid, so the entire wheel/tire and therefore bicycle will be raised and lowered by 5mm

 SILCA Rigid Tire on 5mm Bump

Model of a Rigid Tire on 5mm Bump. This scenario is quite literally how the early 'Boneshaker' bicycles earned their names, it was neither fast nor comfortable.

The rider of the bicycle becomes the suspension system to absorb the bump as the tire is incapable of handling it at the point of impact.  The forward momentum of the bike is converted into a vertical force which is partially absorbed within the rider's body as well as absorbed in friction at the contact points between the bike and rider.  

Another way of describing it is that the bump is essentially lifting the entire system by 5mm and dropping it in a sort of pavement bench-press of the bike and rider.  Think of 1000 5mm bumps in the road as the road doing 1000 mini-bench presses of a 180lb object and it becomes clear that energy is not being used wisely in this scenario.

Pneumatic tires were such a revolution as they were not only more comfortable, but proved significantly faster than the solid tires they replaced.  

Looking at the similar bump with a tire modeled at 100psi and we see that rather than lifting the system by 5mm, the system is only lifted 1mm off of the ground, with the other 4mm of displacement being absorbed by the tire.  As the pneumatic tire is very efficient, much of the energy absorbed is returned with the primary losses being small amounts of heat produced in the tire casing.SILCA Tire Absorbing 5mm Bump at 100psi

Model of 23mm Tire at 100psi Absorbing 5mm Bump.  The entire system is lifted 1mm with the rest absorbed by the tire. 

Our second data point came from Tom Anhalt who has been studying Rolling Resistance and other bicycle physics on his website HERE

Tom has picked up the baton from Al Morrison and had been measuring and posting bicycle tire rolling resistance data taken on rollers.  Tom posted a very interesting piece in 2009 related to the differences between roller testing and real world testing, where he was able to roughly match roller data at lower pressures, but saw a divergence in the data at higher pressures.  Tom's article mentioning this was published at and can be found HERE

Similar to Jan Heine's data, Tom found that rolling resistance decreased as pressure increased up to a point, and then began increasing again as shown below:

SILCA Tom Anhalt Al Morrison Bicycle Tire Rolling Resistance Data

Tom Anhalt's Real World Tire Test on 'Good' Asphalt Surface, Compared to Identical Tire Tested on Rollers by Al Morrison

Tom coined the phrase 'Break-Point Pressure' to describe the point at which the Crr changed from decreasing with pressure to increasing with pressure.  Tom was also the first to theorize that we could estimate what he called 'Transmitted Losses' which were the losses due to vibration and roughness and that we could (and should) model them into our theories about optimal tire pressure.  

A new term: Rolling Impedance or just Impedance

For the rest of this series we will be using the term Impedance to define this resistance to forward motion caused by surface roughness.  I have stolen the term impedance from electrical engineering where it is defined as the resistance of a circuit to an alternating current.  The phrase feels more natural to me than any used previously and was also approved by Tom Anhalt, so we hope it sticks.

Part 4B will take the concept of Impedance to the next level and help us begin to understand how to compensate with our tire pressures.  Click HERE to Read Part 4B



July 05, 2016


Part 4B: Rolling Resistance and Impedance

In Part 4A we covered the history of Bicycle Rolling Resistance study and discussed the concept of Impedance, a form of resistance caused directly by surface roughness.  The concept of Impedance is a relatively new and uncharted territory for cycling blogs, yet is something that each of us have a feel for.  Impedance is trying to start from a stop on cobbles, trying to ride over wash-board or a cattle grate, it is rolling full steam off of nice pavement onto a stretch of chip-n-seal and feeling your speed drop while your watts climb.  

While Crr or the coefficient of rolling resistance is inherent to the internal losses within the tire, impedance is an energy sucking force felt through your whole body.  Previously called 'Suspension Losses' or 'Transmitted Losses' this effect occurs when the tires are unable to do their job properly due to over-inflation, small size, or being ridden on unintended surfaces.

Rolling Resistance (Crr) and Casing Losses

When we typically talk about Crr or rolling resistance we are simply referring to losses within the tire.  As a tire is loaded, it will deform, and while  the air-spring in the tire is nearly 100% efficient, the casing of the tire is not.  As the casing deflects, heat is generated by the movement of the various casing materials.  This heat, is energy lost from the system.  

Historically, there were two solutions for casing losses, higher pressures to reduce casing deformation, and finer casings made from materials with greater efficiencies.  Traditional tire drum testing, the kind done by Tom Anhalt, BicycleTireRollingResistance, Al Morrison and others involve running a tire on a metal drum at various pressures.  These tests are all measuring casing losses within the tire.

Al Morrison Tire rolling resistance data

This graph is an example from Al Morrison and Tom Anhalt of a very efficient tire tested on a steel drum.  Note that the rolling resistance decreases as the air pressure increases, this is a result of the tire deflecting less at the contact patch.  This type of data has existed for many years and is partly to blame for the 'higher pressure is faster' myth which we have all believed for so long. 

This data, however, doesn't take surface roughness or the inefficiencies of the human body on top of the bicycle into account and is therefore incomplete.


Tom Anhalt was one of the first to take tires used in roller testing into the field to try and replicate data.  What he found was quite a shock!


While the data matched at lower pressures, the real world data diverged somewhat dramatically from the roller data at higher pressures! 

This divergence is the result of impedance losses overwhelming the system as the tire is over-inflated.  Most interestingly, this initial test was done on 'good' asphalt, which really brings up questions about lower quality surfaces.

The new theory on Rolling Losses is that both Surface Impedance AND Casing Losses were adding together to create total rolling loss.  This concept has been inherently known for a long time as we have often discussed tires having different Crr on different surfaces, however, the new way of looking at it allows us to break the equation into 2 parts which look like this:

SILCA Crr plus Impedance Rolling Resistance

New concept of Theoretical (steel drum) Crr Plus Impedance = Total Rolling Loss

SILCA Sum of Crr and Impedance

Sum of Theoretical (steel drum) Crr and Impedance

This theory predicts that below the Breakpoint pressure the system will be dominated by Casing Losses (though still affected by impedance) and at higher pressures the system will be dominated by Impedance Losses, though still affected by Casing Losses.

The Test

In summer 2014, the SILCA team was presented with a local repaving project which completely closed 900 meters of road.  Over the course of the project, the pavement was completely scraped away and then re-paved over a month long project.  We decided to turn this opportunity into a tire pressure and Crr test using the Chung Method to determine Crr from field testing.  For this test, a rider on a Cervelo P4 in the aero position was used.  A TT position is helpful for this type of testing as it reduces the variability of the aerodynamic drag.  A TT bike also has nearly 50/50 weight distribution, so equivalent front and rear tires pressures were used.  Rider and bike total weight was 190 lbs, we used water inside water bottles to maintain equivalent total mass over the duration of the testing.

Our initial surface was a mechanically roughened by a pavement milling machine.  The roughness of the surface was an incredibly uniform 8mm peak to valley height with 1 inch peak to peak length.

SILCA Crr Testing Roughened Concrete Surface

The Milled Pavement Surface: Our test course had 900 meters of this!

We further tested on the Chip n' Seal surface over top of this, the coarse asphalt and the final asphalt shown below. 

SILCA Crr Testing Final Asphalt

Closeup of the Final Asphalt Surface of our Test Road.  This Photo was taken 4 Days after Final Rolling of the Surface.  You can see up close that 'Perfect' Asphalt Actually Contains a lot of Imperfections.

Each test was run using 25mm Continental GP4000s II Tires on Zipp 404 Firecrest Wheels.  Tires had an installed width of 25.8mm at 100psi. 

SILCA Crr Testing Summary

Crr Vs Tire Pressure for 3 Different Surface Roughness.  The Original Tom Anhalt, Al Morrison Data is represented in Blue

From this testing, we learned that Tom Anhalt's data was repeatable, and Impedance does in fact dominate the rolling resistance beyond the breakpoint pressure as his initial testing had shown.  We are now going back for more testing with different rider weights and tire widths, but from the 5 data runs we took on in this test (only 3 are shown to keep the graph clean) all 5 showed Impedance taking over and dominating rolling losses beyond a certain pressure.  

Most interestingly perhaps is the non-linearity of these effects.  We have added Wattage values to represent the watts lost to these combined rolling forces.  Note the chart below the relative effects of being 10psi above the 'Break-Point' versus being 10psi below the 'Break-Point'.

SILCA Rolling Wattage Breakpoint Pressure Rolling Resistance

Wattage Differences at +/-10 PSI of BreakPoint Pressure for 3 Surfaces 

Lessons Learned

The SILCA team is now planning to expand testing to look at more pressures, more rider weights, more tire widths and alternate surfaces.  You can imagine the size of data set this could lead us to, but the results are fascinating and exciting!  One lesson learned, is that 4 day old pavement while 'smooth' in appearance has a higher roughness than you might think, but is also still 'soft' which appears to have both increased the total rolling losses, but appears to have also steepened the impedance line.  Testing completed recently on the identical road surface, now nearly 2 years old shows a marked decrease in the Crr as well as a decrease in the steepness of the curve after the breakpoint.  

SILCA 4 Day Old vs 2 Year Old Asphalt Crr Testing

4 Day Old vs 2 Year Old Asphalt on Same Course

Lessons Learned

While we have learned many lessons along this journey, there are clearly many more still to come!  We hope to soon be posting more information and data on this topic, but here are some key takeaways:

SILCA Tire Pressure Rolling Losses Depiction

  • Tire Pressure is NOT a Maximize or Minimize Variable, but Can be Optimized for your weight, tire size and course conditions
  • Better to set your pressure a few psi below the BreakPoint Pressure than to have it a few PSI above the BreakPoint PressureSILCA Bicycle Tire Rolling Resistance Impedance 
  • Rough and/or Soft surfaces have steeper Impedance Lines making total rolling resistance higher and Optimal Pressure Lower
  • More supple tires will have less steep Crr and Impedance Curves and are more forgiving of tire pressure errorsSILCA Bicycle Tire Rolling Resistance Stiff vs Supple Casing
    Tires with More Supple Casings Have Lower Rolling Resistance Everywhere and are More Forgiving of Over/Under Pressure






June 29, 2016

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Part 3B: FAQ and Putting it Together So Far

We've decided to push Part 4 back a week and do an FAQ about the first 3 segments of our series as we have had so many questions and comments regarding the series so far.  For a quick recap:

Part1: History of tires getting wider and the effect of rim width on actual tire width

Part2: Measuring Tire Stiffness in the Lab

Part3: How Tire Stiffness effects ride comfort for the entire bicycle


Q: I weigh 210lbs and have a history of pinch flatting.  I like what you are saying about lower pressures having more comfort but am worried about flats.

A1: What rims and tires are you riding on what frame? 

Q1: Mavic Cosmic, 23mm Continental GP4000s on Cervelo R3

A2: At your weight, you really need to consider wider tires.  Your frame will accommodate 25mm tires on 17c rims.  Looking at the chart in Part 1 your 23mm Conti's on those 15mm rims measure 22.2mm tall and 23.8mm wide.  Moving to 25mm tires on the same wheel will net you 24.2mm height and 26.2mm width.  The extra 2mm height and 2.4mm width will significantly increase the amount of energy required to bottom out the tire onto the rim and cause a pinch flat.

This graph shows 23mm tire at 8Bar, vs 25mm at 7Bar, Vs 28mm at 6Bar, Tires Are Displaced until Bottoming on the Rim. Area under each curve is Energy Required to Bottom Out.

The graph above shows the Force-Displacement curves for all 3 tire widths tested with the tires pushed to the point of beginning to bottom out.  The energy required to bottom out the tire can be approximated by calculating the area under each curve.  In this case, we have lowered the pressure by 1 Bar with each width decrease.  You can see that while the 28mm tire at 6Bar is less stiff (the steeper the slope of the line, the stiffer the tire) than the 23mm at 8Bar, but it can handle an additional 5.2mm of displacement which also results in a higher force at the point where it bottoms out on the rim.  In this case, the 28mm tire at 6Bar requires over 50% more energy to bottom out against the rim compared to the 23mm at 8Bar.  For you particularly, the 25mm tire at 7 bar will require 19% more energy to bottom out AND be 5% more comfortable.  If you optimize your tire pressure for equivalent stiffness (25mm at 8psi lower than the 23mm tire rather than 1Bar (14.5psi)) a pinch flat would require some 24% higher energy, your choice.  We like the 25mm at 7Bar for achieving the best of both worlds: more comfortable AND significantly reduced likelihood of pinching.

Q: Since wider rims make narrow tires both wider and taller, do you treat them as if they are the same as a tire of the wider width on a narrower rim?  For example, a 23mm tire on a 19c rim should be the same pressure as a 25mm tire on 17c rim as they are similar on your chart?

A:  While a wider rim may make a 'narrower' tire measure wide, it doesn't necessarily make it as tall, and it is the height of the tire that gives you the protection from pinch flatting.  Generally we suggest that lighter riders, or riders on smoother pavements can consider narrow tires on wide rims to be similar to wider tires when setting pressures.  However, on harsher pavements, gravel, cobbles, etc there is NO SUBSTITUTE for the added height of the tire with wider casing, and if possible, you should choose the wider tire AND the wider rim.  

Look at the actual size chart again:

Note that the 23c Tire on 19.5mm rim is almost exactly as WIDE as the 25c Tire on the 17C rim..However, from a pinch flat/rim damage perspective on rough pavement, cobbles, gravel, etc, the 1mm height difference between them represents a significant difference in the energy required to bottom the tire against the rim.  So we must look at both from a perspective of balancing grip, comfort and rolling resistance (to be covered in Part 4) AND from a damage pinch perspective.  If rim damage or pinch flatting are your issue, then the best solution is the wider tire followed by pressure optimization.

Q: Why did you bother measuring tire stiffness, we already knew that wider tires are stiffer and you should lower air pressure.

A: From working events around the world I would say that the statement 'we know this' is quite an overstatement.  Over 80% of the people we talk to at events (including pro team training camps, ProTour Races, major gran fondos, the Olympics, etc..) say that they run the same pressure in their wider tires or with their wider rims than they did before.  I honestly believe that much of this is just a matter of habit and the belief that it is better to have too much pressure than too little.  I see athletes and mechanics alike continuing to just throw 120psi into front and rear tires without even considering what those tires are.  

As for measuring this effect in the lab, our study was actually one of only 2 that we know of to look at actual vertical stiffnesses of inflated tires against different forms, and the other source of data is not a published source, but rather some data shared by Damon Rinard, one of the kindest cycling engineers around.  Surprisingly, the data on this topic is very thin to nonexistent, and while you can easily find dozens of tests of frame, seat post, wheel and other stiffness testing, there is almost no actual vertical spring rate data on the tires, which is ironic as the tire completely dominates the system (as was shown in Part 3). 

Much of the inspiration for this series actually comes from taking our 'Inflation Station' to races around the country and finding that in more than half the cases we end up removing air pressure from people's tires.  I would confidently say that in road and triathlon type events including Gran Fondo events, over 80% of the people are running too much air pressure.  

Q: If wider tires are stiffer and wider rims make the tires even wider, then are we doing this wrong?

A:  If you are maintaining your original air pressure as you go wide, then you certainly aren't doing it right!  The point we hope to make as we pull this data together is that a 23mm tire is rarely ever a measured 23mm.  So once we start talking about specific air pressure for a 23mm tire, we will need to be on the same page in terms of what '23mm means'.  As not everybody has a digital caliper we will try to be specific by saying things like '23mm tire on 17c rim, measures 24.9mm' or something like that.  Essentially, what we are getting as is that if you were running 120psi on your 21mm tires on 13c rims (which will measure 21mm) then you need significantly less pressure in your 23mm tires on 17c rims which measure at 24.9mm as effectively you are on a 4mm wider tire and not actually on a 2mm wider tire as it might seem.

We also hope to convey that rolling resistance, aerodynamics, comfort, and grip are a direct function of measured tire width and pressure, while impact and damage resistance are more related to tire casing circumference and tire height and pressure.  With a little bit of knowledge we can help you best make decisions for your event.

Q: What about MAX and MIN tire pressures listed on tire sidewalls?

A: We advise that you NEVER exceed a MAX or go below a MIN as stated by the manufacturer.  Those numbers on the tire sidewalls are generally driven by internal testing done by the manufacturers and are related to safety rather than speed, comfort, or efficiency.  MAX ratings are there specifically to eliminate the possibility of a tire blowing off the rim under extreme conditions such as very hot and prolonged braking in the mountains.   While MIN ratings are generally related to the minimum air pressure required to keep the tire mounted in the bead under heavy cornering.  If you feel that you need to go over or under these numbers, then you should seek out a new tire that specifically meets these criteria.

Q: What about rider weight, you aren't considering rider weight which is HUGE! 

A: This will be covered in Part 6.  As a general rule of thumb, you can scale recommendations for tire pressure by your weight compared to the weight used in the study by diving your weigh by the test weight times the pressure recommendation.  More in Part 6!

Q: This has all been covered already by Jan Heine and Frank Berto and the answer is 15% tire drop, you should go read that article HERE

A:  Thanks Mark, yes, we have read Jan Heine and have seen the Frank Berto graph shown below: Jan Heine Frank Berto Bicycle Quarterly Tire Drop Chart

Graph showing 15% tire drop for given pressure and mass.  Source: Bicycle Quarterly

For starters, we've been using the pressures from this graph for a while and on most surfaces and uses these numbers are great starting points.  It has been our opinion that the chart typically results in an under-inflated front tire as cornering feel can become a bit vague or squirmy, and also the chart does not specify road surface.  Much of the research we are doing is to try and better understand all of the interactions in play including comfort, grip, rim protection, aerodynamics and rolling resistance.  If we ultimately end up confirming the chart with all of this data, then that would be a major advance in the art as far as we are concerned.  In the mean time, we highly suggest everybody read the articles over at Bicycle Quarterly as well as Jan Heine's Blog which is full of great stuff. 

Q: I've seen Josh post some stuff to Slowtwitch comparing various component changes to tire pressure changes, where is that?

A: Josh did post some stuff a while back comparing various component and material changes to tire pressure changes.  The most important thing to note about this, however, is that generalizations are nearly impossible to make.  We have seen carbon posts that are stiffer than aluminum posts, and some that are as much as 30% less stiff.. so you cannot just say 'carbon post vs. aluminum post'.  Similarly, there are carbon aero frames that have equivalent vertical stiffness to some of the most comfortable 'endurance' frames and 'endurance' frames that are as harsh as the stiffest aero frames.  The data Josh posted to Slowtwitch is below:

  • 1 1/8 Steerer vs Tapered 1 1/8-1 1 ¼ steerer (same brand carbon fork): 1.2psi 
  • 24 vs 28 spokes Zipp 303: 1.8psi 
  • 3x vs radial spoke lacing, Zipp 303: 2psi 
  • Curved vs Straight seat stays, Carbon Frames (Model Year Change): 4psi 
  • Carbon Vs Steel Similar Geometry Custom Frames: 4psi  
  • Comfort/Cobble Frame design vs Full Aero frame design: 19psi 
  • Aluminum bar to Zipp SL: 7psi
  • Aluminum bar to Zipp SLC: 2psi
  • Zipp 27.2 Seatpost to Zipp 31.6 Seatpost: 4psi
  • Zero Offset Zipp seatpost to 25mm offset Zipp seatpost: 3psi
  • Thomson post to Canyon VCLS SeatPost: 24psi

The important takeaway here, though is that to compare two specific things, you really  need to compare those two things.  Generalizations are sure to be wrong..however, this list does a good job of putting things in perspective.  We so often talk ourselves into the importance of 4 fewer spokes, or replacing that aluminum part with a carbon one, when in reality, the difference in system stiffness that results is very, very fact, most all of these are well below the gauge error of a standard floor pump.  Perhaps most interesting of all is how the very well designed Canyon seat post can make a significantly larger difference than the two very different frame designs!

Next Tuesday we will look at our most anticipated Part of the series: Tire Rolling Resistance on various road surfaces!


June 21, 2016


Part 3: Tire Pressure and Comfort

'No single adjustment to a bicycle will affect comfort, handling and confidence as much as tire pressure.' 

 Frame styles and pressure gauge

Make that 'Harsh' Aero bike Ride like 'Plush' Comfort Bike: Change your pressure!

That's a bold statement, but one that has proven true over and over again.  One that has driven professional athletes and mechanics to using high-tech pressure measuring equipment, high-accuracy gauges on pumps and even tire pressure spreadsheets and logs covering course and conditions as well as tire choice and pressures.  For this part in the series we will look at the 'Why' behind the bold statement.

First of all, every component of a bicycle is essentially a spring.  Even things that seem very rigid will deflect at some load.  That seat post seems quite rigid, but load it and it will bend slightly.  Engineers take the amount of load applied and divide it by the amount of deflection to get the 'spring rate' of the item in question.  This is what we did with tires in Part 2, we loaded them up, divided by deflection and the result was the chart below:

 Vertical stiffness model for Surfaces: Stiffness given in N/mm

Technical Note about N/mm:  1 Newton (1N) is


The basic argument for tire pressure being the most important adjustment is that the tire is the softest spring in the entire bicycle system, and when springs are added together in system like this (what engineers call springs in series) the softest spring dominates.  Springs in Series add up like this:

Let's look at an example here using simple numbers, you can see that the lowest spring rate completely dominates the system:

The result here feels counterintuitive, the sum total of the springs is LESS than that of the weakest spring, so I like to think of it this way.  If we had our 10N/mm spring in series with a spring of infinite stiffness, we would be left with a spring rate of 10N/mm, since any other spring we put in series with the 10N/mm spring has a rate far below infinity, then the system will be less stiff.

When we talk about comfort in terms of cycling, the system from the ground up is made of springs in this order: tire, rim, nipples, spokes, hubshell, bearings, axle, frame, seat post, saddle.  For simplicity, we generally measure wheels as built and the frames as frame and seatpost.  We also have an amazing resource in Tour Magazin in Germany who measures frames for vertical stiffness from the seat post rails to the dropouts.  This gives us a single 'frame-seatspost' stiffness number to work with.  So for our purposes we will use this equation where K is the spring rate variable:

To the engineer, all of this looks something like this (and yes, I know that the 'rear' wheel has radial spokes, it's just for visualization! ;-)

Frame Wheel Spring Rate Rendering

CAD Visualization of Springs in Series Model of Bicycle/Wheel/Tire System 

This helps visualize the differences in spring rates where a wheel can be 5-10 times more stiff than the frame/seatpost which will be 2-5 times stiffer than the inflated tire.  In all non-suspension bicycle systems, the tires will be the spring dominating the system over bumps and other rough surface features.  

Putting all of it together: Effect of Different Frames

To put all of this together, we've created a small chart based on our tire data, the wheel stiffness data from the Road to Roubaix Story and some frame stiffness data based on actual data taken from Tour Magazin out of Germany.  The Tour stiffness data measure from the seat rails to the rear dropouts, so gives us an excellent model for the ride stiffness of a bicycle as it would be sold to you.

We modeled using 3 different frame stiffness, 250N/mm, 200N/mm and 150N/mm.  These are generally representative of what we see in the Tour Magazin data for frame vertical stiffness, though there are outliers worth noting.  We highly recommend looking at Tour Magazin data as there is a very aero bike with 125N/mm stiffness and another 'Comfort' looking model at over 300N/mm stiffness which goes to show that the design details of a specific frame can certainly matter more than the look of the frame!

For the sake of this study, we have broken all of the tested frames into 3 representative ranges which fall in the Stiffest Third, Middle Third, and Softest Third of the complete data, we called these ranges 'AeroRace', 'Race' and 'Comfort' to simplify things.    The actual data from the last few years spans 100N/mm-350N/mm for road frames.

The chart below uses our spring equation with actual measured data from our tire study, wheel study and frame/seatpost study, the resulting data represents the system stiffness from seat post rails to ground.

 Vertical stiffness model for 3 Frame Categories: Stiffness given in N/mm

For these models, we are looking at the system stiffness on an 8mm bump with Zipp 303 wheels, 3 different tire sizes and bicycles from the 3 general classes we defined above.  What is notable about this, is how much the system stiffness is dominated by tire pressure, in fact the difference between the 'Aero Race' frame/seatpost and the 'Comfort' frame/seatpost is LESS THAN 1bar (14.5psi) of pressure!  

Clearly the differences between the stiffest third of bikes tested and the most comfortable third of bikes tested is very real, but it just isn't very large in magnitude when you consider that you can go up a tire size and down 1Bar of pressure and be roughly equivalent or even better off.  Again, when optimizing for a specific event or race, the athlete should be maximizing every single advantage within the system for the largest possible total benefit, however, if you only can afford one very expensive race bike, you can take comfort in knowing that some clever air pressure strategy can put you right there with your comfort/endurance bike friends in terms of ride quality.    

Now, let's look at the differences within a category.  Below are the equations run on 3 different bicycles within the 'Comfort Race' category.  All of these bikes have been ridden at Flanders and all of them are top sellers from major brands.  

Comparison of Comfort Race Bikes at different pressures

Comparison of 3 Comfort/Endurance Race Bike Brands with Same tires and Pressures

These three models are all highly successful and all hotly sold against one another in the market touting the industry standard mantra 'laterally stiff yet vertically compliant.' 

While there are differences between them, it equates to about 0.2Bar or 3psi difference in tire pressure.  While this difference is non-zero, it is also less than 1/3 the gauge error and and within the range of repeatability of most bicycle pumps.  Think about that, between these 3 $10,000 models, the difference in ride quality is equal to the difference you will get if you pumped to the 'same pressure' each day and is likely a fraction of what you would get if you pumped to the 'same pressure' on somebody else's pump.

Editors Note: This was the 'smack in the face' moment for me during the Roubaix project, the realization that the quantitative difference between the bike the team was convinced was 'too stiff' for Roubaix and the bike that was completely designed and optimized for the race, was less than 1bar of air pressure.  Clearly, you take every advantage you can get, and every little bit helps, so you take the most comfortable frame, most comfortable wheel, AND you optimize the pressure.  Yet at the same time, we so often paint these decisions as being black and white and they simply are not.  The reality is the difference between the 'too stiff' bike and the 'just right' bike on a normal day is about the difference you get if you haven't pumped your tires in a few days.

Putting it all Together: The effect of wheels

The effect of wheels and comfort has been long debated, and I count myself as being completely surprised to learn that my own 'magic carpet' Ambrosio Chrono Roubaix wheels weren't actually that compliant.  These effects again point to the strong placebo effects of believing something to be true, and also point to the 'Just noticeable difference' threshold as being greater than most of the the variables we are changing here.  Look at the chart above again and you see that the difference in system stiffness between the 250N/mm and 150N/mm frames with 28mm tires at 8 Bar is right at 10%..which is right on the edge of what people can perceive when blinded to the study variables.

Of the 3 springs in our model, the wheel is by far the stiffest, and therefore has the least effect.  As to our point above, they still make a difference in the model, so optimizing wheel stiffness is worthwhile, but as you see below, the effects are significantly smaller than those of the frame and roughly 1/10th of the effects of tire pressure:

SILCA tire pressure stiffness wheel stiffness model

Vertical stiffness model for 3 Wheel Models: Stiffness given in N/mm

Here we have the best possible example of the effects of our Springs in Series model.  The 303 project detailed in the Road to Roubaix story was able to reduce the radial stiffness of the preferred cobble wheel by nearly 50%, it was a major breakthrough in both concept and execution, and yet, from the system perspective, it represents an improvement of just under 2%.  

What is 2%, well if you are a pro at Paris-Roubaix, it could be everything!  For the average cyclist reading this post, 2% is something very real that you could completely wipe out by over inflating your tires by a few PSI.  

Conclusions and homework

Bringing all of this back to the opening statement and image, we see that the difference in vertical stiffness of the 5 frames in the opening image can be offset by using the 5 different air pressures shown on the gauge.  More interestingly from a SILCA standpoint, is that the difference between 4 of the 5 frames is less than the gauge error of a standard bicycle pump (+/-5%), which has driven us to produce pumps with gauges of significantly higher accuracy.  It is our sincerest hope that you, the cyclist, can use this knowledge to begin doing your own testing and study on air pressure for your bike/wheel/tire system.  

When researching new equipment choices remember, it ALL matters.  That more comfortable frame really can be a big deal.  Those more comfortable wheels, most definitely are a big deal, and of course, top it all off with optimized tire pressure, because the wrong tire pressure can quite literally undo all of the benefits of the highly engineered equipment you are purchasing or already own.

Our recommendation is to begin keeping a log of your air pressures.  Start where you are today and reduce pressure by 5psi or 0.5Bar and ride it for a few days, then reduce it a bit more, etc.  We have worked with hundreds of athletes, both professional and amateur and find that just a few weeks of keeping a pressure log will begin to completely change the way you think about your pressures and tires.  In many cases, we find athletes deciding that they can race their aero road bike on that course they were planning to buy a more comfortable bike for.  Triathletes, some of whom are now racing at pressures 2Bar (30psi) lower than before, are telling us that they are running better off the bike as they are less fatigued from vibration, and better still, they aren't riding any slower!  

Next week in Part 4 we will be looking at tire size and pressure and how they affect Rolling Resistance!  The results will surprise you and will explain how you can lower pressures up to 2 Bar and STILL have Low Rolling Resistance, if not eve LOWER Rolling Resistance!


June 14, 2016


Part 2: Tire Stiffness (Wider is Stiffer/Harsher?)

In Part 1 we discussed actual tire width, specifically how it was affected by bead seat width of the rim, and also how it was generally NOT equal to the number printed on the sidewall.  

In part 2 we are going to look specifically at how tire width affects stiffness by measuring the vertical stiffness (more specifically the vertical force at a given displacement) of various mounted and inflated tires.

Wider is Stiffer/Harsher?

The initial design of this test was to show that larger diameter tires are actually Stiffer/Less Comfortable when inflated to the same pressure.  This is due to an effect known as ‘Casing Tension’ and is caused by the internal air pressure acting on a larger surface area in the larger tire.  Essentially, the same pressure acting on more surface area makes for higher casing tension.  The best explanation of Casing Tension we’ve seen was done by our friends at FLO Wheels and can be found HERE if you are interested!

For this part of the discussion, we used an Instron machine to look at the force required to deflect tires at various pressures and widths.  To keep things (relatively) simple, we will refer to the tires by the number printed on the sidewall while also mentioning the pressure used and the rim bead seat width.  You can refer back to the chart in Part 1 if you are interested in the actual sizing for comparison to other tire models and brands. 

An Instron machine is a large H shaped piece of lab equipment which can drive the center beam of the H up and down with extreme accuracy while measuring either tension or compression on on object in the middle.  Chances are that if you ever see anybody say that part A is X% stiffer/stronger/more elastic than part B, the numbers were generated in one of these machines.

The Test Machine with 8mm Anvil In Place (photo not taken during actual testing)

For this test, we used a solid steel wheel-holding fixture bolted to the base of the machine, and 3 different test anvils to push on the tires.  We tested tire stiffness against a Flat Surface, 8cm radius Cobble Surface, and 8mm radius Bump Surface.

Below are the actual measurements of the 3 Tires on the rim used for testing.  Please note that we will continue to use the size printed on the casing to refer to the tires as it is much less confusing than using the actual measurements, though the actual measurements will be important for determining optimal pressures.  Also, 17c is the industry standard for a bead measuring 16.5mm-17.5mm.  The Rim we are using is a 17c rim which measures 17.5mm, please consider them interchangeable for the purposes of this test.

Flat Surface Data

Our first study was just to look at the differences between 3 different tire widths on the same wheel with 17.5mm inner bead at the same pressure.

23, 25, 28mm Tires On Same 17.5mm Bead Width Wheel – Flat Surface

The first thing to notice here is that the wider tire (28mm) is actually Vertically Stiffer than the 25mm tire which is in turn stiffer than the 23mm tire.  The most common response we get to this graph is ‘that’s not possible, I went from 23’s to 28’s and it’s noticeably better.’ 

My initial thoughts about this were that the ‘Just Noticeable Difference’ which is the smallest change that can be accurately noticed by humans has been shown to be between 10 and 15% for ride stiffness (much of the work on this was done by Damon Rinard, mentioned in Part 1 and his work on this topic with Cervelo can be found HERE)..and the difference here between the 23 and 28mm tire is only about 8-9%.  

I have often seen in testing that if a rider believes something to be true, he/she will very often ‘feel’ that in the test ride, particularly if the effect in question is relatively small.  So based on this first piece of data, it would seem that perception and expectations may be driving some of the ‘wider is more comfortable’ beliefs (assuming same tire pressures)..however, there is clearly much more to learn.

So with this data in hand, we went about building a complete data sheet of all 3 tires at all 3 pressures on the 17.5mm Rim.  We used 6Bar, 7Bar, and 8Bar (87psi, 101psi and 115psi) to build this data set as it gave us a large total range of pressures commonly run in tires of these sizes.

23, 25, 28mm Tires on Same 17.5mm Bead Width Rim at 3 Pressures – Flat Surface

This graph really helps set the stage for the relative differences we are looking at.  As the tire pressures were changed in 1Bar increments (14.5psi) you can get a feel for the magnitudes of difference between the width changes, in this case increasing from a 23mm to a 28mm tire at 7Bar increased stiffness by 9%, while increasing pressure by 1Bar increased stiffness more than 21% .  The data grouping is mostly dominated by tire pressures, so clearly the effects of these 2-3mm width changes are below the 1Bar delta in pressure change used for the test.

To really take this study to the next level, we decided to not just push on the tires with the flat surface, but to also look at a simulated cobble-stone (8cm radius) and a simulated pavement lip (8mm radius) to see what the effective stiffness of the tire would be against those surfaces.

Visualization of 3 Different Test Anvils Used for Testing

'Cobble' Surface (8cm Radius) Data

Here is the data for the same 3 tires on the 17.5mm Bead Width rim at the same 3 pressures, only the ‘anvil’ in the test machine is now a machined piece of steel with 8cm radius to mimic the crown of a cobble.  

23, 25, 28mm Tires on Same 17mm Bead Width Rim at 3 Pressures – Cobble Surface

Of note is that the rounded impact anvil ‘the cobble’ resulted in considerably lower force at the 15mm displacement than the flat surface.  This is largely the result of how much of the tire is able to deform at the contact patch between the anvil and tire. 

Interestingly, with the Cobble Impact head, the radial stiffness is still mostly dominated by tire pressure, though the differences between the tire widths and pressures has condensed somewhat.  Clearly the shape of the object being pushed into the tire makes a large difference in the tire stiffness.  With this data set we are beginning to see some overlaps, for instance the 23mm tire at 8Bar is very nearly identical in stiffness to the 28mm tire at 7Bar.

'Pavement Lip' (8mm Radius) Data

Wanting to push this further, we looked at the same conditions with an 8mm radius anvil, which simulates a concrete lip, rock, or similar object that your tire may hit.  As the radius of this anvil is considerably smaller than any of the tires, we were interested in seeing how the data would change.. and wow.. did it change!

23, 25, 28mm Tires on Same 17.5mm Bead Width Rim at 3 Pressures – 8mm Radius

Look closely and you will see that the results have completely separated out by air pressure and the different width tires have become almost identical to each other when at the same pressure.  It would appear that for this small radius, the tire size is of little factor compared to air pressure.

So what has happened to drive this?  It would seem that the changes in air pressure are making similar differences to previous studies, yet the tire width is not contributing in quite the same way.  There is more to learn here for sure, but at this point it seems that for bumps smaller than the tire diameter, the shape and size of the bump is driving the stiffness more than the effective width of the tire itself.

Please see the post-script to this paper about why we believe this to be true, but keeping it simple based on the data we’ve seen here, it certainly seems that wider tires every bit as good or better at absorbing small bumps and imperfections than narrower tires at the same pressure.  This is most certainly not the expected outcome of the study as we planned it when we started, but most certainly is a fascinating one!!

Converting it to Vertical Stiffness

Up to this point, we have been using the phrase 'Stiffness' to explain these graphs which are actually Force-Displacement Graphs.  Stiffness is actually defined by the slope of the line in the graph, but adding that made the graphs even messier and harder to read!  Below are the actual calculated stiffness values from these tests.  These values will become important in part 3 where we look at how the tire stiffness affects ride quality of the entire bicycle system.

Vertical Stiffness of 3 Tires and 3 Pressures against 3 Surface Geometries

Summary and Recommendations

What we can say is that all those people who feel their larger tires are more comfortable, you may be correct for bumps smaller than 8mm radius...we could not measure that, so it is hard to know, but for larger radii, you are best to lower your air pressure a bit to truly take advantage of the larger tire widths.

The most exciting aspect of this study is that it has begun to point us in the direction of how much pressure we need to lose with tire width increases, and even better it hints that while that lower pressure will provide similar ride comfort on most surfaces, it will likely improve comfort on small bumps.  

Our recommendation is that you decrease tire pressure by 3-4% for each millimeter of tire width increase.  This will ensure similar compliance over most surfaces while providing improved compliance over small bumps and edges.  Keeping a log of your pressure experiments will help you decide what pressures are ultimately most comfortable and most efficient for your weight and road surfaces.  This will be important to know as we begin discussing Rolling Resistance and Aerodynamics.

Our next Tech Tuesday discussion will cover Ride Comfort and Compliance, and how tire pressure and stiffness affects the entire bicycle system.


I have to say that at this point, we certainly don’t have the answers for the questions brought about by this data, but we consider it a reasonable theory that there is something happening at the interface between the small anvil and the tire which is adding apparent stiffness to the smaller width tire data.  This could be the result of casing stiffness, localized distortion, surface friction or other factors.  Clearly there is more work to be done on this.

Also, for the purpose of this study, we are only showing the force at 0 and 15mm of displacement.  This is partly to keep the data clean, but also to simplify the chart reading.  We found for each test that the first few milimeters of displacement were non-linear and then the graphs would become more or less linear.  For the sake of keeping our sanity working with the data, we chose to assume that the curves are fully linear.  However, with the 8mm anvil, the data showed a larger non-linear section, so this is likely due to the relatively extreme localized deflections and deformations required within the tire at these large displacements.  

15mm was Chosen as the deflection data point to ensure that none of the tires were beginning to bottom out on the rim.  Past 15mm, the force data for the 23mm tires begins to bend upward as the casing deflections become extreme and then inner-tube materials becomes pinched within the system.  Taking stiffness data from 0-15mm deflection ensured that the force data represented the effect of the casing stiffness and the air spring only.


June 08, 2016


Tire Size, Pressure, Aero, Comfort, Rolling Resistance and More. Part 1: How we Got to Now

'23mm' Tire Measuring 24.89mm Wide at 6Bar (87psi)

Part 1: How We Got to Now

No other single component affects the comfort, handling and efficiency of a bicycle like the tires.  Tires are the sole connection to the ground, they are the sole transmitter of drive force to propel the cyclist forward, and they are the sole means of gripping the road during cornering.  They are the most dominant spring in the bike/rider system which means that more than anything else, they control comfort.  They are the sole component which will (ideally) ever have to resist the abrasive contact of asphalt, concrete or gravel with minimal damage.

In future segments, we will show data discussing the Aerodynamic, Comfort and Rolling Efficiency of tires, but for starters, we will be looking at something seemingly so simple, yet not simple at all.  Tire Width.

Years ago, tire width was quite a simple affair.  Tubular tires were sold in different casing widths, which they maintained even if not mounted to a rim.  A 21mm tire would measure with calipers at 21mm +/-0.5mm.  With clincher tires, this became more complicated as the interface between tire and rim became a factor in the tire size discussion.  Manufacturers were led by the ETRTO to recommend which tire sizes worked with which rims, and the conventional wisdom of narrower tires being faster kept everybody in check for some 30 years as racing rims measured 13mm between beads, road rims measured 15mm between beads, touring rims were 17mm, mountain rims were 19mm, etc.  

Fast forward to now and things are much more complicated.  ENVE just released their 7.8 TT/Tri wheel set with 19.5mm inner bead width.  That's 2mm wider than the Zipp 303 we developed for the cobbles of Paris-Roubaix 7 years ago and 6.5mm wider at the bead than the original 808 TT wheel I designed in 2004.  

Many of these changes have come in a stepwise motion over the last 10 years with first the availability of slightly wider racing tires (23mm as opposed to the previous 21mm standard), which led aero wheel companies to make wider wheels to try and offset the aero penalties of the wider tires.  Then athletes took advantage of the wider wheels and began trying (and liking) even wider tires.  This has gone on in 1 and 2 mm increments for some years now resulting in the world we live in today where the fastest racing tires are being launched with the smallest sizes at 24 or 25mm width!

As this was happening in Road, gravel was gaining popularity as were fat bikes.  In many ways, gravel riding has further pushed road wheel development toward wider format rims, while fat bikes have pushed Mountain to think wider as well.  Today we even have Plus sized mountain tires that are significantly wider than anything we would have imagined riding 20 years ago.

For this first part of our study, we have focused on road wheels and tires, but the learnings here are applicable everywhere.  The main lesson we want to convey is that tire size is no longer accurately linked to the number on the sidewall, but rather, needs to be measured based on the rim you are mounting it on.  These measurements will give us a foundation for future discussions of aerodynamics, Coefficient of Rolling Resistance (Crr) and other topics. 

Many road tires are still given their widths based on the old ETRTO fitment standards, so that 23-28mm tires are still often based on fitment to 13mm inner bead width rims, something you are likely only to find at swap meets anymore, and mounting these to wider rims will net you an effectively wider tire.  

This chart was inspired by my good friend Damon Rinard, former development engineer for Cervelo and now Engineering Manager at CSG (Cannondale).  I've borrowed (stolen) his format outright, and populated with data collected using Zipp, ENVE and Continental components.

This chart is important because it shows that the same tire can be a lot of things depending on what rim it is mounted on, generally none of which are equal to the number on the sidewall!  We didn't measure on a 13c rim, but I can imagine that the 23 would be more of a 23mm width on the 13c rim as would be the 25 and 28, but on a 15, 17.5 or 19.5mm rim, they are all much wider.  

Also note is how dramatically the tire pressure affects both width and height of the tire.  This will be a big deal when we start to talk about aerodynamics later on.  The width also plays a large factor in determining the optimal pressure for the tire which we will discuss in the next post in this series.

Simply saying '110psi is optimal for 23mm tires' suddenly has little meaning anymore.  Which 23?  On what rim? What does it measure?  Not easy!

In our next post we will be looking at the vertical compliance of these tires on these rims and ultimately the rolling resistance and aero performance which gets even more interesting as the tires all change size with air pressure!

In the mean time, take a look at what size tire you're riding on what size rim and see if it matches the number on the casing.  You might be surprised!




April 09, 2016


Road to Roubaix - The Complete Story

Note: Before buying SILCA I was fortunate to spend nearly 15 years as the Technical director at Zipp.  I worked with the most amazing team of engineers and production specialists to create some amazing products.  This story would not be possible without the efforts and brilliance (and blood, sweat and tears..literally) of Michael Hall, David Morse, John Fearncombe, Nic James and many others.  Please check out the video produced on this by Zipp and other info from them over HERE Thanks Everybody! 

Also Note: As we enter this Flanders-Roubaix week, SILCA pumps, gauges, tapes, and balancing products will be used by over half the pro peloton unofficially and officially by our technical partner Team Bora.  We are thrilled and honored to play a key role in both of these beautiful cycling Monuments and all of the key events of the 2017 Pro-Tour Calendar.

Part 1:

One might think that the catastrophic failure of a carbon wheel is a particularly dramatic, loud, or impressive occurrence, but you would generally be wrong. In an ironic twist, you find that impacts at 90% of the energy required to create a failure, can sound like a gunshot, can wrench your handlebars from your hands, or nearly throw you from your bike. However, an impact at 110% of the energy required to break a wheel can feel quite minor, and the failure itself often sounds like a sudden crunching of paper rather than anything very dramatic. Like that, your wheel is destroyed, in a whimper rather than a bang.

So went the early days carbon wheel development for Paris-Roubaix.   It was 2006 and I was convinced that aerodynamic, carbon wheels could change the face of racing at Roubaix, yet we first had to build product that was capable of surviving the famed cobbles, and even more difficult, we had to convince the riders to actually use it!

Looking back, I see that this was the start of my obsession with tire pressures, quality tools, and optimizing the minutia of cycling details. I had spent the previous 7 years developing products for aerodynamic benefits and it had been a long and difficult road convincing top level athletes to use it in good conditions.   A project to finish carbon wheels at Roubaix felt like an entirely new adventure involving different skills and new learnings.

We began the Roubaix project in 2007 with two teams on tap for 2008, Slipstream and CSC, between them we had access to Magnus Backstedt, Fabian Cancellara, Nicki Sorensen, Roger Hammond and a crop of young and enthusiastic pros looking to find their way in the world. Pro cycling at that point (and still today) was held in tension between old-world beliefs and new world science, often the tension resulted in catastrophic combinations of both worlds.

Our focus was to get the more aerodynamic carbon wheels under these riders. We had done some testing with Cancellara in early 2008 at the San Diego low speed wind tunnel showing that 58mm deep carbon wheels would be some 28-34 watts more efficient (at 30mph) than the 32 spoke aluminum wheels they traditionally rode at Roubaix, so there was some interest by the teams and riders, but still far too much skepticism to make it a near term reality.

Poertner with Zipp R&D Director Michael Hall, Riis and Cancellara at the wind tunnel in 2007

Like the athletes, we believed that the real hurdle to bringing carbon wheels to Roubailx was going to be comfort. The entire world believed that there would be no way to achieve the ‘comfort’ or ‘compliance’ of box section wheels in deeper wheels. Afterall, we all KNEW that deeper wheels were stiffer, and therefore harsher, it had been written a thousand times and was therefore true. So in late 2007 we set out to understand the baseline standards for both durability and comfort in these ‘classics’ wheels.

An Instron machine, is generally the cornerstone of any good mechanical testing lab. Instron is the company most widely known for making this type of machine which looks like a large H sitting on a steel table. The machine works by driving a crossbar up or down at a very controlled rate, in the center of the cross-bar is a load cell and a gripper, or a pusher (anvil) which either stretch or crush the object being tested. An Instron can be used to test the strength and stiffness of most anything provided you have clever engineers to build the fixturing.

Setting up for radial stiffness test of a wheel with tire.

The initial testing was conducted with about a dozen wheels including ptototype Roubaix wheels, vintage Mavic Roubaix aluminum box section wheels, and the 2007 race favorite 32 spoke Ambrosio Crono box section rimmed wheels. After the first full day of testing, crafting new anvil geometries, re-thinking the fixtures, re-thinking everything we could be doing wrong, we realized that we weren’t doing anything wrong at at all: The box section wheels were in fact, radially stiffer than most of the deeper carbon wheels!

Now this wasn’t universal, deep rims with V shapes were very stiff, though even then the correlation to stiffness was more stronger to spoke count than to rim depth, with only the original 16 spoke Campagnolo Shamal being the outlier, though with a perfectly V shaped rim, and bladed 14 gauge (non-butted) spokes, this made some sense.

However, we found ourselves in possession of data that pointed to the fact that the conventional wisdom of an entire generation of cyclists, mechanics and even industry engineers was just generally plain wrong. In the years since, I’ve taken part in studies and even designed studies to look at these perception vs reality situations and know that perception and conventional wisdom generally win, but at the time, my 2007 brain was blown away by this information.


2007 Radial Stiffness Testing of various wheels

This information really changed the entire vision of the project for us. It was immediately clear that what we believed to be the major problem “how to make a deep carbon wheel as comfortable as a box section wheel” was not actually a problem at all, in fact, most everything we were making was already there. The real problem was going to be convincing the riders of this.

With this information in hand, the team built some 20 pair of prototypes and headed to a team camp just outside the dreaded Arenberg Forest.

For the uninitiated, the Arenberg is considered the most brutal sector of Paris-Roubaix, massively crowned stones, few decent lines, incredibly narrow, sloppy when wet, dusty when dry, and worst of all, an ever so slightly downhill run-in to the sector which narrows dramatically at the entrance. The top teams put maximum effort into getting their riders to the front, and that means 60+kph speeds as the riders hit the stones.  Oh yeah, and occasionally people have been known to steal a stone here or there as a souvenir..making for the most unbelievably dangerous hole in the ground you'll see on any racecourse anywhere in the world.

Stones at the entry to the Arenberg Forest Sector - 2008 Testing

The goal with the prototypes was to build sets of various strengths and stiffnesses to see what the riders would prefer as well as to determine the strength requirements of the rims. The test plan was to have them ride various lines to see if they could break the wheels and also to try and determine the handling characteristic the riders were after. Afterall, we knew that we could make the carbon wheels ‘comfortable’ enough, but could we make them last?

90 Minutes

90 Minutes was all it took for 4 riders to break 20 sets of wheels on that first trip. Hard to remember the other details, but unsurprisingly comfort didn’t really come into the equation for this test. Our team was needless to say, devastated.

Yet, the testing had revealed some very useful data, and by early spring 2008, we had 404’s measuring equal in radial stiffness to the Ambrosio Crono wheels, but at more than 2x the impact strength of before. An early test with Slipstream showed no issues in more than 40 passes through the forest when paired with 28mm Dugast tires. We were ready! Or so we thought..

I don’t remember much from the day of Paris-Roubaix 2008, but I do remember getting the phone call. “Magnus broke both of his wheels and could not rejoin, we need to talk.” Devastated.

For those of you who don’t remember this event specifically, we were thrashed in the media for attempting this. A major US magazine gave us a ‘Thumbs Down’ award for ‘putting sponsor desires above rider’s safety.’ The general lament was why we would ever even attempt this as EVERYBODY knew that it wouldn’t work and that there was little benefit possible, yet so much downside risk. In those dark moments afterward I even wondered why we had even spent so much time, energy and money on this..

At 7 AM the next morning my phone rang, it was that Magnus and I felt a tremendous sense of relief when I couldn’t detect any anger in it. Turns out he was spending the day with his wife and children at Disney Land Paris, they were having a good time, and he wasn’t blaming us (too much).  He pointed to some critical factors that may have caused problems, mainly, they had made the decision to switch to 24mm tires the night before the race as conventional wisdom held that in the dry, these narrow tires were faster. He soothed my worries by pointing out that he was more than 1 stone above his weight of last Roubaix (14 pounds..I had to look it up at the time) and that he didn’t blame us entirely thinking that the tires were a mistake since the testing was all done on 28mm. He also was the first to let me know that Martijn Maaskant a young pro with the team had finished 4th on a pair of 202’s which were not anything special we had produced for the race, he suggested we look at the dynamics of those wheels as Martijn was really happy with them!

The significance of this conversation cannot be overstated, in 24 hours we had gone from the terrible people who cost Magnus his race to learning the other side of the story that we had the first carbon wheel to ever finish in the velodrome at Roubaix, and it was nearly on the podium!

The lessons learned from those 202’s with 28mm Dugast tires would pave the path to a total rethink of team wheel, tire pressure and tire management, a rethink of rim geometry, tire/rim aerodynamics, tire/rim interface and ultimately Paris-Roubaix race strategy.


2009 was spent obsessing over tires. We found that the difference between 24 and 27mm tires was the difference between making it through the forest and walking and we theorized that 28 and 30mm tires would be even better if they would fit in the frames! We learned that 10psi could increase speed over the cobbles by nearly 1kph at a fixed power. We looked back at the original stiffness data to understand how the 82mm 808 could be so much more compliant than the shallower wheels, and then we used those lessons to completely change the face of classics wheels by making a 28mm wide 303 with massively bulging sidewalls and a tire bed optimized for 27-28mm tires with an added focus on the outer rim edges where the tire bottomed out under heavy impact. These edges could be tuned to both better spread impact loading to save the rim, but also we learned that the same impact spreading could save the tire from pinch flatting.

Yes, you heard that correctly, once we had rims strong enough to survive Arenburg, we started to have pinch flatting issues..with tubulars!

Meanwhile in the test lab, we were working on solving the impact issues both for tire pinch flatting and for rim cracking. The new rim shape was very compliant under impact compared to anything else we had seen, but the effect of tires was really pretty unbelievable in terms of protecting the rim. 

Tire Height Typically runs 90-105% of tire height (Clincher shown here)

At a very basic level the key to the wider tires really that they are also taller. In general a tire will be slightly less tall than wide (this can vary with tread thickness and tire design, but highly efficient tubulars are always shorter than wide when installed.  So in an unloaded condition a 25mm tubular tire will keep your rim about 24mm off of the pavement when your bike is just sitting there, but when you are sitting on the bike the tire compresses (we call this tire drop) and for a drop of 15% (common) you will have only 20mm between your rim and the pavement.  So when we introduce bumps, road seams, potholes and (gulp) cobbles to the equation, we find that those 20mm really aren't all that much!

Clincher Rims and Tires shown with various Drop Percentages

So really the most critical aspect of tire width is that every millimeter of tire width brings a critical millimeter (or nearly) of height from the ground.  While lower pressures needed for these rough conditions increase the tire drop bringing the rim back closer to the ground.  The balance between tire height and pressure (which affects drop) is critical in balancing efficiency, ride quality and impact durability.  Solving for the most effective tire size and pressure is hard enough for standard conditions, but when you have cobbles with sharp edges and as much as 30mm of height difference between them, the problem becomes very,very hard to solve.  

All indications after the Magnus Backstedt rim failures were that the 24mm tires (which had never been tested on the pave with our carbon wheels) were the likely cause for the rim failures, but the data now could prove it.  Those 3mm of reduced height between the rim edge and cobbles meant a more than 20% reduction in the amount of impact energy the wheel and tire could handle before damaging the rim.

Evaluating the aluminum wheels used in previous Roubaix races taught us even more.  The rims were full of nicks, dings and deformations around the perimeter.  This meant that the tires were routinely bottoming our, but the aluminum was able to stay intact even when bent, dented or dinged.  Carbon fiber doesn't have these properties.  While aluminum will generally deform 10-13% before failing, carbon fiber usually will only yield 1.5-2% before doing the same.  Engineers refer to this as 'toughness' and it is critical to the amount of energy that something can absorb before failing.

One advantage of carbon is that we could design flex into the system by allowing the sidewall of the rim to act as a leaf spring.  This decouples the outer diameter of the rim from the inner, and by shaping the rim we could allow it to essentially act similarly to a tire.  While the conventional wisdom had been that the box section rim was comfortable because it could deflect radially inward, the reality is that to deflect inward in one place, it had to deflect outward somewhere else and all those spokes kept that from happening.  With carbon, we could engineer the flex into the cross section of the rim so while the inner diameter of the rim remained more or less round, the outer diameter could flex, this is almost exactly how a tire behaves.

HighSpeed Video of 303 Prototype Impact - 3.5mm Rim Compression After Tire Bottom-Out

This video of a 303 prototype shows the ability of the very bulging rim to deform similarly to the tire, the cross-section of the rim can deflect outward allowing the rim to compress under load.


Once we had the hard data on impact energy with the various tires and had developed new rim concepts that we believed could better handle the shock loads of the bottoming tire, we had an even larger problem: Perception.  The perception was that the 24mm tires were 'Faster' in dry conditions.  Now the problem with Perception and Convetional Wisdom is that it isn't always rooted in data, but is rather the result of people drawing correlations between features and benefits.  So when we talked with the riders we heard everything you can imagine about why 24's were faster in the dry and 27's in the wet, but the most logical was that the 27's allowed for lower pressure which meant more grip in the wet.  24's were also more aerodynamic according to the riders and this seemed logical as wider tires always bring a drag penalty from everything we'd ever seen.  So the challenge really became to make the wheel as fast with the 27 as the old wheel was with the 24.

 Making it Aero

In 2008 we went back to San Diego wind tunnel to look at the Ambrosio Crono with Roubaix tires along with numerous 303 concepts based around tires that wide. In a lot of ways it was a very liberating test for us. Out of desperation, we had thrown out all of the conventional wisdom and had made about 20 plastic prototype wheels (by Steriolithography) that were really, really outside the proverbial box. One of those was shape that would become the Hyper-Toroidal 303 that we went to production with and interestingly, the craziest of the lot was a wildly pear shaped thing we dubbed a ‘Pear-oidal’ rim shape that would ultimately become the impetus for the future Firecrest geometry wheels

Rendering of 2010 27.5mm wide Hyper-Toroidal Rim vs 2007 22.5mm 303 both with 25mm Tire 

You can see here the dramatic difference between the 2007 rim and what became the 2010 rim. This would be the first Pave specific, cyclocross oriented rim anybody ever made, and it was very strange looking. 

I remember the first time we showed it to our sales/marketing team at Zipp. The response was anything but positive. ‘It’ll have to win Roubaix if you expect anybody to buy such a damn ugly wheel,’ was one of our favorite comments. I think most everybody had the same response at first, and of course with our 2015 perspective, it doesn’t look at all out of the ordinary.

 However, the wind tunnel data spoke for itself. 

Wheel/Tire Tunnel Data From 2008 Testing

In many ways this testing was transformative for our entire team, and it ultimately changed much about our vision for future wheels.  Note on the graph that the 27.5mm wide rim with 24mm tire is very nearly identical in performance to the previous generation 22.5mm wide wheel with 24mm tire, and the wider rim with the 24mm tire was a very significant improvement over the previous rim with same tire.  This test was really the one that solidified the movement al all rims to very wide widths as previous to that only the 808 was 27.5 wide as it was necessary due to the depth.

The data from this test as shown above became a critical factor in getting the riders to open their minds to using them again following the issues of 2008.  The difference between the aero wheel with a 27mm tire and the old wheel with the 24mm tire was between 15 and 30 watts depending on wind angle...that is a tremendous amount.  Now we can't think for a second that this changed anybody's mind.  Cancellara made a very good point that this only mattered to him if the other guy was on the 303, otherwise he would rather pick the more proven and robust wheel that was equal to what the other riders had, than to have a faster wheel that brought risk of a rim failure.  

Changing Perceptions

For 2009, we continued to try and convince Cancellara and CSC, but we were also working with the very technically advanced Cervelo Test Team, and with the help of CTT management were able to convince the riders to have a go on the carbon wheels, but it would require extensive testing.  However, the Backstedt failures still loomed large with many riders and others just couldn't come to terms with the 27mm tires, or the idea of carbon wheels being comfortable.  These are times where data is critical, but even in the face of very good data, perceptions can drive what the riders are feeling (or think they are feeling) and perceptions can drive doubt and doubt can be a very self-fulfilling thing.

Our first test with CSC in the Arenburg that year brought highly variable results, ones which made no real sense and left our team, the riders, and the staff feeling uneasy.  The breakthrough came when we realized that amongst the 3 pumps on the team truck, we had a variability of 12psi when inflating to 70.  This all came in a fit of frustration when we plugged two of the pumps together on a valve stem ripped our of a tube, pumping the one to 70psi had the other pump showing 64psi and plumbing it to the third gave us 76psi.    Considering that we had been working to optimize pressures for riders like Thor Hushovd down in the region of 64 front / 70 rear, it was no wonder we were having intermittent failures and other issues.

We were facing as much as 12 psi pressure difference depending on which pump was being used!

We changed to only using one pump and completed testing with much better results, but the riders were still uneasy about the whole thing.  Before the CTT test we decided to improve the level of control.  We built a gauge setup we jokingly called 'The Truth' using a $500 Ashcroft 0.1% accuracy digital gauge, an old SILCA disc adapter and some precision industrial components to create a gauge with very high accuracy bleed.  The Truth was capable of bleeding pressure at a rate which could yield repeatable 0.05psi readings.  We knew that this accuracy and precision would be key to getting repeatable data and also to making sure that we got it absolutely right on race day.

Photo of 'The Truth' Gauge Hanging out at the Service Course with Team SILCA Pump (2009)

Now that we were properly prepared, the CTT testing went as smoothly as you could possibly imagine and the team was sold.  We spent a lot of time trying to find a good balance of comfort and feel for the riders by lowering pressures, but also safety margin for the wheels by not going too low.  This type of iterative testing takes a lot of time, but we found through use of power meter data that as the pressures decreased, the speeds also went up.  The better the ability of the tire to absorb the impacts of the cobbles, the more efficient the bike travelled over the cobbles.  Similarly, once the rim was routinely bottoming out, the speeds come down again.  It was as if we had opened a door into an entirely new critical variable in performance and were the first to begin imagining what benefits were there to be found.

Roubaix 2009

The Flanders-Roubaix week came and went in 2009 with little drama.  Thor Hushovd finished 3rd, the first podium for a carbon wheel at Roubaix and Roger Hammond finished 4th. 

Roger Hammond on way to 4th in 2009 Roubaix (photo from

The planning and preparation had done us well, Hushovd and Hammond 3rd and 4th.  Roger also pointed out something that would be critical to all future Roubaix attempts.  He noted that the latex tubes weep air, that is they allow air to slowly escape, which is why your tubular tires need to be inflated every day.  However, this request was that he noticed the tires were lower at the end of the race than the beginning and shouldn't we look at optimizing pressure for the Arenburg and then figure out what the race starting pressure should be since the two happen about 4 hours apart?

It was immediately clear that for 2010 we would be spending a lot of time with 'The Truth' and a stopwatch..

PART 3:  We Just Invented the Future!

The podium and 4th place for Thor and Roger at the 2009 Roubaix had been a real rush for the team, we were truly thrilled, but at the same time it felt like the learning had just begun.  

First of all..

Not much about this type of engineering is very glamorous, looking back on it I realize now that all of these stories sound rather romantic, but being there at the time reminds me of that old joke about flying where the punchline is something like 'long periods of boredom interspersed with moments of shear terror.'  Much of the time spent with the teams is spent cleaning things, fixing things, replacing things, and preparing for other things.  It's non-stop work making sure that every single detail is covered, every bolt is tight, every tire is perfect and while enjoyable it generally isn't very exciting.

Riding in the team car at Roubaix is a truly bone shaking and mind numbing experience, the roads are narrow, the riders are far ahead of you and there is no way for the cars to get past each other in the Pave sections so it makes for hours spent listening intently to race radio (in French) to hear who had a flat, needs a bottle or when crashes occur.  Every single puncture or crash announcement comes with a massive adrenaline rush, 'Oh $@#$ it is OUR guy?  Is it OUR product?! Is it a wheel failure?..'  Everybody experiences the same, the mechanics are thinking 'It is something I did (or didn't do)?!?!' the environment is unbelievably stressful, and unlike most races, you have these long periods where you can't get to them quickly..

Our biggest excitement from 2009 was tiny cracks in rims as well as some pretty serious tire cuts AFTER the race.  These evoke that certain kind of fear about things that could have or might have happened, but like a scary movie you've seen before, the ultimate outcome is known already.  So 2009 was mostly boredom during the race itself, with the fear happening before and after the event.

On the technical side, all of these emotions are really a mixed bag.  The problem for the engineers is that you NEED to see failures in order to understand how and where to improve, but at the same time, you need to see them before the event an not during.. Yet, looking at the speed and power data, there is just nothing you can do in training or race prep that comes even close to the real thing, so technically, nothing you do in testing is going to come close to the real thing..

Some fun P-R race data I've seen:

1 minute power leading onto the Arenburg Pave: 658 watts

1 second wattage on the Arenburg Pave: 1584 watts

5 minute wattage covering run into Arenburg AND 2,400 meters of Pave: 561 watts

These are truly amazing numbers, and as you can imagine not something you get in training runs.  We even tried motor pacing riders onto the Pave..but nothing matches the adrenaline and quite honestly the terror of running this stretch in th actual race!

More testing

The opposite of all that race adrenaline are the hours spend measuring the rate at which tires weep air, or impact testing tires and rims at a range of different pressures and sizes.  

For 2010, test engineer John Fearncombe developed a highly automated impact test platform at Zipp which used the basic concept of the UCI test rig, but could automatically launch the impact sled at very precise speeds and energy.  This allowed the team to very quickly test the spring rates of tires at various sizes and pressures, find the ultimate failure energies and make extremely direct comparisons between options.  For instance, a carbon wheel with 28mm tire might handle a 90 Joule impact before failure while an aluminum rim may be dented at 70 Joule and rendered unrideable at 80 Joule when used with same tire.  We could also create equivalency between systems, so if a rider liked one wheel and tire at one pressure, we could test that and them iterate the new wheel or tire using pressure to have identical spring rate.  Over a few months we impacted literally thousands of combinations:

Impact Sled Testing with Steel Cobble
The end result of this was a matrix of pressures and tire sizes to render equivalent spring rate.  We will publish something similar in an upcoming technical paper on tires, but for the team's purposes this became an invaluable tool to ride tuning.

Solving for Latex Tubes

Thanks to Roger Hammond (who is not just one of the greatest english speakers and hard-men ever to ride the Pave, but is also a mechanical engineer) we would spend the run up to 2010 Roubaix evaluating the leak-down rate of the team tires while also convincing them to go ever wider!  Turns out that a tubular tire with latex tube will lose 0.5-1.5psi per hour, which over a 7+ hour period (figure the race will be ~6:30 and the mechanics have to have the bikes ready at least 30 minutes before the start).  This turned out to be a very critical aspect of pressure optimization and planning as the comes some 4 hours after initial inflation and the nearly equally bad (but the riders are strung out and going slower..) section at the Carrefour de l'Arbre comes nearly two hours after Arenburg.

We ran testing leading up to 2010 in the Carrefour de l'Arbre looking at the minimum allowable pressures for that sector and determined that those numbers would be used, plus the leak down rate to set the starting pressure.  The wheels for the top riders were selected from the tires that had the lowest lead-down rates (near the 0.5psi per hour) and the numbers were written on the sidewalls to be sure and the race morning pressures were given to the mechanics with all of this factored in!

Doubts remain

During the week long run up to Roubaix, some of the riders, Fabian chief among them still had some doubts.  Remember that he had won the event previously on the old-world wheels and tires and wasn't sure that he wanted or needed the new technology.  This is understandable as the large tires and carbon wheels seemed to offer small benefits yet come with large risks.  One of the problems is that riders really don't 'feel' aerodynamics while riding..this is true for all of us, you go hard, you go fast, there is nothing really to compare to. 

To try and demonstrate the benefits in the real world (the wind tunnel doesn't always 'feel' real).  We did a test both on the cobbles and on a few pavement sections between some of the key sectors where it seemed an attack might be likely or where a rider might find himself isolated.  The results were eye opening.

Fabian's mechanic Roger Changing Wheels on the Recon Day (AFP)

With the larger tires, we found that the riders went faster over the Pave as pressure was reduced..until the point that the rim was bottoming out, and then the speeds reduced again.  

This is similar to the effect of why increasing air pressure can make you slower on rough roads, rather than the tires absorbing the imperfections in the road, the bike is being lifted or rather bounced over the bumps.  A tire bottoming on the rim dramatically increases the spring rate of the system causing the bicycle to bounce off of the cobble which can cause loss of traction, discomfort and loss of speed.

Coming out of the Pave portion of the testing the riders seemed to be truly sold on the larger tires once and for all.  The 28mm tire in the low-mid 60psi range proved to be nearly 1km/hr faster than the 24mm tire with pressure in the mid 70 psi range (which is what is required to prevent bottoming the tire) when the data was normalized for rider power output. 

Moving on to the pavement testing, we had Fabian do some interval efforts on the new wheels and the old wheels to try and demonstrate the aero differences.  The results even shocked our engineers!  The traditional wheels with 24mm tires required 24-26 additional watts to go the same speed as the aero wheels with the 28mm tires!

Finally, we had officially debunked the conventional wisdom that 24mm were 'faster'.

Change in Tactics

This new data was worked into all of the computer simulation models and in many ways the overall picture was even more promising than just looking at small sectors of the race.  

During a Paris-Roubiax, the top riders are expected to burn 6,500-7,500 calories.  The field data pointed to a 500+ calorie savings due to improved aerodynamics (remember, generally you aren't using aero to go any faster, it is buying you the same speed at a lower power). 

The analysis of the final 50km of the race pointed that the aero wheels would allow for an extended breakaway compared to the traditional wheels.  One estimate was that using Fabian's previous data a 20km solo effort could work and with the new tires and wheels a 30 or possibly 40km final effort might be possible!

Ultimately, races are not won on technology or computer screens.  It's real people suffering at the edge of what's possible, making thousands of decisions per hour with fatigued minds and bodies.  However, all of this data, technology, and testing started to swing the belief of these riders in the favor of technology.  It felt as if the conventional wisdom was turning in our favor.

Fabian's Bike Race Morning, with hand written notes to mechanics on tire sidewalls (James Huang)

I've seen over and over that riders who believe in something are more willing to commit to it, and it becomes a self-fulfilling prophecy.  When you believe the thing is more comfortable, you fatigue slower.  When you believe a tire has more grip, you will push it closer to the limit.  When you feel that something gives you an advantage that nobody else has, you feel empowered to use it.. and so on.  In many ways, all of this technological improvement led to the biggest improvement of all, which was that these athletes now had access to better equipment, but they were willing to optimize around that equipment, and were then able to come into the race knowing AND believing in what they had done.

Race Day

The beauty (not so much..) of being embedded with the team all week sweating all of the little details in the run up to Roubaix is that you get to fly home to the US on race day to be ready for work on Monday.  I headed for the airport while the team headed to the race depart, knowing that every possible thing that could be done was done.

Some nine hours later I landed in the US and turned on my phone as the plane taxied to the jetway.  Nothing..and then suddenly, text after text and message after message of congratulations..  Fabian had ridden an unbelievable race.  Funny enough, he attacked the lead group on the exact section of smooth road where we had done the power/aero testing during the week prior. 

You might remember that this was the attack that started the ridiculous rumor about him having a motor in the bike...the attack is simply phenomenal to watch!  

Speaking with him after the fact he had some really awesome things to say about that day, but most impressive was his mindset.  He told us, 'I remembered what you said about the advantages of technology, I felt like I was on a time trial bike while everybody else was chasing me on equipment from the Eddy Merckx era..'  which was something we had told the riders over and over.  In the end, it wasn't an equipment advantage, but rather a technology and knowledge advantage that had translated into an incredible confidence and belief.

Cancellara later commented to the media that 'Roubaix will never again be won on the old wheels,' as there was 'too much advantage' to the new technology.  Michael Hall, Director of R&D now at Zipp said at the time, 'I think we just created the future.'  


Fabian has turned out to be thus far correct about he new technology.  2011 Roubaix was won on a Mavic Roubaix wheel nearly identical in all measurements to that original 2010 wheel.  In 2012, 2013 and 2014 was won on again on the 2010 design wheel.  Best of all, the frame makers during this time joined the trend and pushed tire clearance further.  For 2014 the race was won on a 30mm rear / 28mm front tire and looking to 2015 we are working with more than half a dozen teams on gauges, pumps and other inflation related items and it is brilliant to see many of these teams running 30mm tires front and rear.  The conventional wisdom has changed, and the riders will be faster, happier and less likely to suffer equipment issues on their fatter, lower pressure tires.  

Notes: I spent 15 years developing racing wheels at Zipp with the most amazing team of engineers and technicians imaginable.  This story is about the teamwork between manufacturers, teams and athletes, but is more deeply a personal story reflecting my coming of age in understanding the importance of tire pressure optimization, the opportunity to improve pumps and gauges and ultimately the need to not just solve the technical problem, but also to educate and empower the mind so that athletes can not only make the best possible decisions, but can understand them and truly believe in them.  In many ways this Road to Roubaix was the first step in my buying and resurecting SILCA. 

Thanks for reading


SILCA today outfits nearly half of all pro teams in some way or another with products developed out of these experiences.  SILCA is also a proud Technical Sponsor of Team Bora / Hansgrohe and our current World Road Champion Peter Sagan.  You can learn more about those products, their uses and the stories behind their creation HERE


Note: Before buying SILCA I was fortunate to spend nearly 15 years as the Technical director at Zipp.  I worked with the most amazing team of engineers and production specialists to create some amazing products.  This story would not be possible without the efforts and brilliance (and blood, sweat and tears..literally) of Michael Hall, David Morse, John Fearncombe, Nic James and many others.  Please check out the video produced on this by Zipp and other info from them over HEREThanks Everybody! 

June 16, 2015


How Does it Work?: The SILCA Presta Chuck and 242 Gasket

We receive a lot of questions about our chuck and how it operates.  Over the last 30 years, the market has become flooded with pump and chuck designs of various complexity and part count, some with levers that flip up, some flip down, others like our own 'Hiro' Side Lever chuck work unlike any other, and yet the classic SILCA Presta Chuck continues to remain a classic, a favorite of both bike shop and pro mechanic alike for it's simplicity and ultimate durability.  In 

In the early days of the pneumatic tire, all pumps had hoses that threaded onto valves.  Early silca pumps were no different, with even the Impero frame pumps of the 1920's and 30's having a hose that threaded onto the valve stem.  This is a robust design, but time consuming and occasionally frustrating for both the home and professional mechanic alike!

In the 1940's SILCA developed the first variation on what we now sell as the 17-4 Stainless Presta Chuck.  This design tapped into lessons from one of the most iconic designs of all time for inspiration: The 2000+ year old Roman Arch!

Roman Arches used in the Construction of a 2000 Roman Viaduct

The idea is quite simple: Make the top of the gasket a dome or arch shape and allow the valve stem to serve as the 'keystone'.  As pressure rises on top of the gasket, it transfers the load into compression on the valve stem gradually increasing holding force as pressure rises!  

All loads on the arch are converted to compressive forces on the Keystone and the Springers

For the new SILCA we have updated this classic with modern materials and the benefit of Computer Aided Design and modeling.  The new SILCA Presta chuck replaces the classic brass with 17-4 Stainless Steel, a magnetic stainless originally designed for aircraft landing gear and revered for it's strength, corrosion resistance and  beauty, but more importantly, replaces the classic natural latex rubber gasket with an exceptionally high performance Synthetic Elastomer originally developed for use in the harsh environments experienced in oil drilling and sub-surface mining.  We have further improved upon this classic design by using a Parabolic Geometry on the top gasket surface.  This geometry allows us to carefully tune the holding power of the chuck to the pressure in the line.  Your gasket will always hold the valve stem with 25-30% more force than required for a given pressure, this ensures that it won't blow off while inflating, yet is easy to remove when you are finished!

Parabolic Elastomer converts Line Pressure into Holding Power

 These technologies are used in the SuperPista Ultimate, 17-4 Stainless Chuck and 242 Gasket