Wet Grip at Different Tire Pressures: Examining Tire Pressure Dependence of Wet Grip
Contents
- Introduction
- Test Set up and Conditions
- Average Test Results
- Individual Test Results (Pro Members)
- Conclusion
- Comments
After a lot of hard work, we launched our wet grip test at the end of 2021, and to this day, you can access the wet grip test results of nearly 200 tires on our website. Testing this many tires has helped us understand the importance of the casing and compound and how each is responsible for a part of the total grip a tire can provide.
We're now ready to dive a bit deeper into our wet grip test and start exploring how changing the tire pressure affects our wet grip test results. While our standard tire pressure for testing 28-622 road bike tires is 72 psi / 5.0 bars, we've now conducted tests at 108 psi / 7.4 bars, 90 psi / 6.2 bars, 72 psi / 5.0 bars, and 54 psi / 3.7 bars. This tire pressure range covers the whole usable range for a 28 mm road bike tire, and it will provide a good indication of how tire pressures affect grip levels in our test.
Please note that our wet grip tester uses a flat, textured ceramic plate, and the test is run at a very low speed, resulting in a maximum contact area at all times. In real life, the advantage of lower tire pressures will be even higher as tires are usually the only form of suspension on a bike.
Lower tire pressures should increase the amount of suspension travel a tire provides, and that should help tremendously in situations where the road conditions are less than optimal. Unfortunately, we don't have the equipment to do real-life tests under an extensive range of conditions, but our tests can indicate how the tire pressure affects grip when the tire is in full contact with the road.
This test was requested, supported, and voted for by our Pro Members. We've done wet grip tests with 3 tires: Continental Grand Prix 5000 S TR 28-622, Vittoria Corsa Pro TLR 28-622, Tufo Comtura Prima TR 28-622. We've averaged the data from those 3 tires and made that available for free for anyone.
Pro Members can open the individual tire data and dig into the results further. We found consistent results among all 3 tires, but some tires responded more to tire pressure changes in our test.
Test Set up and Conditions
Because of the work required to test all tires, we could not complete all tests on the same day. We did all tests for a single tire on a single day to keep the results as accurate as possible.
We tested all tires at tire pressures of 108 psi / 7.4 bars, 90 psi / 6.2 bars, 72 psi / 5.0 bars, and 54 psi / 3.7 bars. If you want to compare the data to our standard tests, the normal wet grip test tire pressure is 72 psi / 5.0 bars for 28-622 road bike tires.
We selected 3 road bike tires that scored between 70 points (good grip) and 77 points (extreme grip) in our wet grip test, with 1 tire coming in on the low end, 1 tire in the middle, and 1 tire on the high end of that range:
- Continental Grand Prix 5000 S TR 28-622
- Vittoria Corsa Pro TLR 28-622
- Tufo Comtura Prima TR 28-622
Test Conditions
- Total wheel load of 42.5 kg / 94 lbs
- 700 x 19 mm wheel
- Ceramic test surface
- Ambient air temperature 21.5-22.5 °C / 71-72 °F
- Water temperature 18.0- 20.0 °C / 64-68 °F
- 3 degree center grip angle
- 35 degree edge grip angle
Test Results - Average Results of 3 Tires
| 54 psi 3.7 bars |
72 psi 5.0 bars (default) |
90 psi 6.2 bars |
108 psi 7.4 bars |
|
|---|---|---|---|---|
| Center Grip | 0.78 | 0.74 | 0.71 | 0.69 |
| Edge Grip | 0.77 | 0.74 | 0.73 | 0.72 |
| Average wet grip of 3 tires at different pressures Coefficient of friction (higher is better) |
||||
The averaged results of the 3 tires we tested clearly show that wet grip increases at lower air pressures in our test. The difference is relatively small but more significant than we initially expected, as this is on our smooth textured ceramic test surface, at low speed, with 100% tire-to-surface contact. Due to the lack of road imperfections, bumps, and vibrations, we expected the difference to be smaller or unmeasurable in our test.
When we examine the results further, we see a clear pattern where the center grip is more sensitive to the tire pressure changes than the edge grip. After thinking about this more, the tires probably become somewhat 'pointier' at higher air pressures, reducing the contact patch in the center of the tire. The change in the tire shape has a smaller effect on the edge of the tire, where the contact patch depends less on the tire pressure.
We also have to factor in the possible use of dual compounds on some tires. We're not sure which tires use a dual compound, but lower pressures on a dual compound tire means more of the high grip compound applied along the tire's edge comes in contact with the test surface in the center test. It also depends on how wide the lower grip center compound strip is, making the results hard to predict.
The individual tests show that all tires follow the same pattern of more grip at lower air pressures. All tires also show the same pattern where the center grip varies more with the tire pressure than the edge grip, although some tires respond a bit more to pressure changes than others.
In general, we feel it's safe to assume all tires will respond similarly, and the averaged results should be a good starting point when using this data for general-purpose calculations with the data found on our website.
Test Results - Individual Rolling Resistance Data (Pro Members)
The next section is only accessible by our highly valued Pro Members. This section includes all test data of the individual tires used for the tire pressure wet grip tests.
Conclusion
Lower tire pressures result in more grip in our wet grip test. When we look at the center and edge grip, we see an even stronger response in the center grip test. We attribute this stronger response to the larger decrease in contact patch of the center of a bicycle tire than the edge of the tire due to changes in the tire shape.
The changes to the contact patch and tire shape can have an amplified effect on dual compound tires that come with a grippier compound along the edge of the tire. Depending on the width of the lower grip center strip, more of the high grip compound along the edge of the tire will be in contact with the test surface.
We expect the differences to be much more significant in real life because our test doesn't consider the larger suspension travel that comes with lower air pressures. We test at a low speed with a flat surface, which always results in maximum contact area. In real life, the advantage of lower tire pressures should be even higher as tires are usually the only form of suspension on a bike.
Even though the differences between high and low air pressures are relatively limited in our tests, we feel our chosen standard wet grip test tire pressure of 72 psi / 5.0 bars for 28 mm road bike tires was the right choice.