May 29, 2016

FLO Cycling – Does Tire Pressure Change Aerodynamic Drag?

The first time I heard someone say tire pressure would change aerodynamics, I questioned if this would be true. The more I thought about it, the more I questioned how aerodynamics would be effected by tire pressure. When you change the air pressure in a tire, you change the shape of the tires contact patch, and ultimately the profile of the tire. If tire pressure did make a difference in drag, my guess was that it would be a small change.

To answer this question, Chris and I decided to study the effect tire pressure has on aerodynamics when we visited the A2 Wind Tunnel in November of 2015.

If you are interested in our other studies from the A2 Wind Tunnel, please be sure to check them out.

Aerodynamic Difference between Sapim CX-Ray and Round Spokes
Does Covering the Valve Cutout on a DISC Wheel Matter?
How Much Drag Does a Clydesdale Build Add to Cycling Wheel?

What We Tested

We tested our front FLO 60 Carbon Clincher with a 23mm Continental GP 4000 S II at the tire pressures listed below.

– 90psi

– 95psi

– 100psi

– 105psi

How We Tested the Wheels

It’s important to define how a test is performed in a wind tunnel. There are countless variables, and if you are not clear about the test, the results are not very clear. Here is how we tested the wheels at the A2 Wind Tunnel.

1. Tare was calculated and removed from all tests.

2. Each wheel was swept from 0-20 degrees of yaw, in 2.5 degree increments.

3. Each measurement was taken twice and averaged.

4. The same Continental GP 4000 S II tire in a 23mm size was used for each test.

5. The tire pressures were calibrated with a highly accurate digital gauge.

The Results

It turns out I was wrong. By changing the tire pressure from 100psi to 95psi, you save 6 seconds over a 40km and 30 seconds over an Ironman. Considering a bike has two wheels, this savings would increase when taking both into consideration. While it won’t double, it will increase.

The table below shows the time savings for the psi measurements listed above. Since 100 psi was the worst case, I considered it as the baseline. The time values for the other psi readings show the savings when compared to the 100psi test.

While 30 seconds is not a huge number, I can tell you I’ll stop pumping when the gauge reads 95psi.

*UPDATE: If you would like to see the results on an aero graph you can click the links below.

Results in Grams of Drag vs. Yaw
Results in CdA (m^2) vs. Yaw

Take care,

Jon

4 comments

Peter,

Good question. Yes, this older work only looked at aerodynamics. As you load a wheel the contact patch changes. The larger issue is the shape of the tire. As you increase pressure the tire gets wider and affects the aerodynamics.

Our newer work combines both aerodynamics and rolling resistance and takes rider weight into consideration. To optimize wheels you have to look at all of them as a system. I hope this helps.

Ride safe,

Jon

Jon Thornham January 07, 2021

Is this only the aero component? If so what is the related sidewall flex impact of lower tire pressure for say a 200lb rider plus bike? Less pressure, longer contact footprint, more sidewall flex, more heat in tire that came from energy that shows up as rolling resistance…..

Peter Sutton January 07, 2021

James,

Good question. We’ve learned a lot since this article was published. I would recommend looking at our tire pressure recommendation charts for your wheels. We have them on each of our wheel pages.

Ride safe,

Jon

Jon Thornham November 18, 2020

Great articles. So, If I extrapolate the pressure and tire size from 23 to 28, I come up with 78psi. (It was the same ratio you used in the casing tension diagram).

James Fox November 18, 2020

4 comments

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