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

Join the Conversation

23 Comments

  1. Did you measure and record the *actual* width of the tire once mounted with vernier calipers? Would seem logical as psi goes up width increased thus so did aerodynamic profile punitively….

  2. Unknown and John Armbruster,

    We did not measure the actual width with calipers. We only studied the aerodynamic effect.

    Take care,

    Chris

  3. Tony,

    I'd guess your body weight would have more of an effect on your rolling resistance. That said, body weight could have an effect on drag as well. The additional weight could change the profile of the tire change the drag. You'd have to do much more specific testing for me to give you a better answer than that.

    With respect to tire width, we only studied one tire width so it's hard for me to give you an accurate answer on how that effects the outcome.

    Chris

  4. Hi, great work as always Jon and Chris. Please could you post a graph and/or the data across different yaw angles? Some of us are interested in specific yaw angles and so it is very useful to see the raw data, in addition to an average figure.

    Thanks!
    Carl.

  5. Cool stuff! Always interesting to see these types of tests in the real world! Do you happen to know what kind of variation you saw between the two runs at each pressure?

    Keep it up!

  6. Jonathan Sebat,

    Each pressure was run twice and the results were averaged. Variations between runs are very small typically in the 1-2 gram range.

    Take care,

    Chris

  7. Did you have a hypothesis coming in? I am surprised to see that 100psi is the baseline yet both above (105psi) and below (90 and 95psi) are better. I would have expected that narrow would better to a point, and that the tire become less and less aero as you increased the PSI. I guess what this may be explained by is it is possible that there is a tire pressure that become high enough and changes the shape such that it now longer widens it but elongates it. Would love to see some more discussion on this. Thanks for all you do 🙂

    Cheers

    -TG
    Thomas Gerlach
    Professional Triathlete
    ThomasGerlach.com

  8. Thomas Gerlach,

    The hypothesis is that tire shape changes as pressure changes, and at some point you hit an "optimal" shape. As you know from all of the tunnels studies done on tires, the smallest changes in tread pattern, tire shape, etc, can have big changes on the aerodynamic drag. For some reason, 95psi, hit a sweet spot with this tire.

    Take care,

    Chris

  9. These results are incredibly puzzling. In fact, they are almost as puzzling as the multitude of results around the 22 tire aero tests that you presented some time ago. Those tests demonstrated a tremendous variability in the drag measurements at yaw angles > 10 degrees for a given tire size (e.g. 23 mm). Measurements of tire height and width of a tire when mounted on a given wheel reveal differences in the +or- millimeter range (bicycle rolling resistance web site). The article posted on "Blather 'bout Bikes: Wind Tunnel Playtime, Part 1" and "2" compares several wheels with several different tires and while some effect is seen at yaw angles > 10 degrees, it does not appear to be as dramatic as the effects you have measured in these two articles. Admittedly, the Blather 'bout Bikes articles only show data to 15 degrees while the FLO results go out to larger yaw angles. The Blather 'bout Bikes article gave me some comfort that the FLO wheels (they included a FLO 9) do quite well with a number of different tires which mitigates my extreme worries about using the correct tire for the FLO wheels I have ordered. The results in this article showing an aerodynamic "sweet spot" for the conti tire on a flo 60 wheel at 95 psi just does not make sense. The "airfoil" of the rim/tire cannot be changing that much when changing the air pressure by + or – 5 psi. Either the measurement process has some subtle issue or the fluid dynamics of a wheel at different "attack" or yaw angles are not well understood.

    Just a few thoughts. I appreciate your company's use of highly analytical, measurement driven design to deliver great results.

  10. Unknown,

    You stated the following:

    "The results in this article showing an aerodynamic "sweet spot" for the conti tire on a flo 60 wheel at 95 psi just does not make sense. The "airfoil" of the rim/tire cannot be changing that much when changing the air pressure by + or – 5 psi. Either the measurement process has some subtle issue or the fluid dynamics of a wheel at different "attack" or yaw angles are not well understood."

    My response is why is this not possible? Why can a change in PSI not change a tires shape enough to effects it's aerodynamics? You're stating the above as if your answer is of absolute certainty. Do you have proof?

    Take care,

    Chris

  11. Tire dimension would be the most important thing I'd like to see. -/+5psi @ 50psi would barely change the dimensions of a tube that wasn't installed. At 95psi I believe the tube is maxed out in how far it can compress the rim inward. I think it's also maxed out in width. At >95% it can only elongate the tire and the profile is now past its optimal shape relative to the wheels profile. Relative to the wheel is the important part, because the 95psi mark is only relative to this exact combination of wheel AND tire. Different tires and different wheels will likely have different "sweet spots".

    If you precisely measured the tire dimensions, I think you would find proportional expansion until 95psi. Past that point I would bet you hit a point in the curve and the height starts to grow exponentially compared to the width.

  12. Hi,So if i understand well, the best pressure for a triathlon OD (40K) will be 95psi.
    Almost all triathlete use a pressure about 120 psi or more.

    Please let me know what is right to do?

    Erik Bloklander

  13. Erik,

    Thanks for writing. That partly depends on your weight and the type of wheels you are using. What do you weigh, and what wheels are you using?

    Take care,

    Chris

  14. So 105 and 90psi tested the same? Did you try to go say to 120 and see what happened? I don't know how we rode so fast back in the day running 19mm tires pumped up to the max. 🙂

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