Part 3B: FAQ and Putting it Together So Far | SILCA

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!


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