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.

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. 

 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: 

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

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.

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. 

 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.  

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.  

 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'. 

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

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.   

 4 Day Old vs 2 Year Old Asphalt on Same Course

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: 

• 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 Pressure

• Rough and/or Soft surfaces have steeper Impedance Lines making total rolling resistance higher and Optimal Tire Pressure Lower
• More supple tires will have less steep Crr and Impedance Curves and are more forgiving of tire pressure errors

Tires with More Supple Casings Have Lower Rolling Resistance Everywhere and are More Forgiving of Over/Under Pressure.  Check out our Tire Pressure Calculator for your fastest pressure.