Part 4B: Rolling Resistance and Impedance | SILCA

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.

Impedance

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!

SILCA Tom An

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
 

 

 

 

 

 


10 Comments

Demetrios
Demetrios

November 20, 2016

Nice! Life long cyclist, have been playing with air pressure the last few months. Rider weight 170 lb. bicycle 17 lb. wheels fulcrum zero carbon clinchers 17c tire new vittoria corsa 25c. At 90 psi felt rough, at 85 psi felt faster, at 80 psi perceived effort was less more comfort. I am looking forward to rider weight pressure optimization.

Peter Liekkio
Peter Liekkio

October 19, 2016

Looking forward to further testing: General additional info here:
http://www.bikequarterly.com/images/TireDrop.pdf

Darrell Simonsen
Darrell Simonsen

September 14, 2016

There is a lot of valuable data being collected. Has anyone been working on an APP that can be used to set the pressure for a given ride? Inputs would be: rider weight, rider ability, bike weight, road condition (very general or a scale 1-10), type of ride planned (race training to leisure ride with the family), terrain to be ridden (climbs, descents, flats), others?. Does altitude or weather play a role in the tire pressure you ride? I am past my prime to race but I ride enough that if I can go farther with more comfort in the same time frame I am all over this. I also have sons and grandsons ( I should add my Daughters-In-Law and granddaughters) that could definitely use an app like this as they develop and hone their skills. I guess we could all just go out and ride create our own data points that suit us individually. It would be nice to have a starting point based on the data that is being collected here.

Thanks for a great article!

Paul E.
Paul E.

August 24, 2016

As a researcher in electrical engineering — I’ll let your adoption of ‘impedance’ pass — I would be interested in the testing protocol that you use for determining the Crr, as I would be interested in doing some tests for myself. You mention the ‘Chung Method’, but is there a reference paper you could point me to, to get started?

Another thought: I would imagine that the ‘impedance’ has frequency dependence. For the ‘rough asphalt’ you tested the tyre on a uniformly milled surface, which gives you an (approximated) sinusoidal perturbation with a given frequency, which depends on the speed you are riding. Have you tried looking at the behaviour at different speeds at a set pressure on the rough road?

Thanks for the article — interesting read!

E.J. Levy
E.J. Levy

August 01, 2016

Great research. Your questions are our questions! Thanks for keeping us informed. How might the shape change of the tire as pressure goes up affect the results?

E.J. Levy
E.J. Levy

August 01, 2016

Great research. Your questions are our questions! Thanks for keeping us informed.

Josh at SILCA
Josh at SILCA

July 12, 2016

Greg, so far there is no formula, but we are hopeful that with enough testing we can get there!
Ron, Yes, in Indiana they chip seal and then about 5 years later they repave it!! Yes, softness is very critical, we are looking at this currently and also looking at data for the Rio Olympics which was just repaved and is quite soft still!
Thomas, Thanks!
Josh

Greg
Greg

July 07, 2016

Is there a formula that will determine the correct psi range for various surface conditions given a riders weight?

Ron Ruff
Ron Ruff

July 06, 2016

Great stuff Josh! Love that you are doing this work and making the results free for all.

So in Indiana they chipseal first and then put asphalt on top? Makes too much sense. In NM they do it opposite. They lay down beautiful smooth asphalt then immediately chipseal it.

Speaking of chipseal, I’ve noticed a huge increase in resistance that seemed to be caused more by softness or stickiness than roughness. They chipsealed the local TT course a few years ago that was previously good asphalt, but just put the “seal” on the shoulder. Crr increased about ,0035 on the rough part and ,0030 on the shoulder. I was really surprised that the shoulder was that bad because it felt very nice. Age hasn’t improved either surface significantly.

Thomas Gerlach Pro Tri
Thomas Gerlach Pro Tri

July 05, 2016

Now we are cooking. Good stuff, can’t wait to read the next installment!!!

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