CRR at Different Loads: Examining Load Dependence of Rolling Resistance
Contents
- Introduction
- Test Set up and Conditions
- Average Test Results
- Individual Test Results (Pro Members)
- Conclusion
- Comments
After our CRR at different speeds article from earlier this year, we learned that rolling resistance doesn't increase linearly with speed. Both the speed and load are required to calculate the total rolling resistance from the measured CRR value, and we're curious how this works out when the tires are under different loads.
This test was requested, supported, and voted for by our Pro Members. We've done load tests at 32.5 kg (72 lbs), 42.5 kg (94 lbs), and 52.5 kg (116 lbs) with 6 tires: Vittoria Corsa Speed G+ 2.0 TLR, Continental Grand Prix 5000 S TR, Michelin Power Road TLR, Vittoria Corsa G+ 2.0, Schwalbe Pro One V-Guard, and the Continental Grand Prix Urban.
This test was a bit more complicated than the CRR at different speeds, as all measurements were repeated at 3 different air pressures. The result is a lot of data, and that might be a bit hard to understand if you're new to our website. The main results, and most useful for general purpose CRR calculations, are the averaged results of the 6 tires.
Pro Members can open the individual tire data and dig into the results further. This time we did find some surprising differences between the tires that still exist even when we take the margin of error of the tests into consideration.
Test Set up and Conditions
We've performed all tests on the same day to get the data as accurate as possible. The speed during all tests was our standard speed of 28.8 km/h / 18 mph.
We usually provide rolling resistance in watts, but we will stick with the CRR (Coefficient of Rolling Resistance) values for this article as it will be the only usable way to compare the rolling resistance at different loads.
Our rolling resistance data never includes the aerodynamic and bearing drag of the tire and wheel combination. We measure the total wheel drag by spinning the wheel up to speed with no load applied. We also subtract the rolling resistance of the drum and wheel bearings from the total rolling resistance, which we obtain from the SKF bearing calculator. These corrections are required to get a linear response when doing tests at different loads.
We tested at 32.5 kg (72 lbs), 42.5 kg (94 lbs), and 52.5 kg (116 lbs) as our test machine's maximum load is around 52.5 kg (116 lbs).
We tested at 3 different air pressures: 100 psi, 80 psi, and 60 psi for the 25-622 road bike tires, and 75 psi, 60 psi, and 45 psi for the 35-622 Grand Prix Urban.
We used a selection of tires for the load tests:
- Vittoria Corsa Speed G+ 2.0 25-622
- Continental Grand Prix 5000 S TR 25-622
- Michelin Power Road TLR 25-622
- Vittoria Corsa G+ 2.0 25-622
- Schwalbe Pro One V-Guard 25-622
- Continental Grand Prix Urban 35-622
Test Conditions
- Speed of 8 m/s, 28.8 km/h, 18 mph
- 700 x 17.8 mm wheel
- Temperature between 21.5-22.5 °C / 71-72 °F
- Diamond plate drum surface
- 77 cm drum diameter
- Excludes aerodynamic and bearing drag
Test Results - Average Results of 6 Tires
| Load | Average CRR Medium Pressure |
Average CRR High Pressure |
Average CRR Extra High Pressure |
|---|---|---|---|
| 32.5 kg - 72 lbs | 0.00452 | 0.00393 | 0.00362 |
| 42.5 kg - 94 lbs | 0.00473 | 0.00402 | 0.00365 |
| 52.5 kg - 116 lbs | 0.00499 | 0.00420 | 0.00378 |
| % Increase | 10.4% | 6.9% | 4.4% |
| Average CRR of 6 tires at different loads | |||
The averaged results of the 6 tested tires clearly show that CRR increases at higher loads. We also see that CRR increases more at lower air pressures, with a relatively low increase of just 4.4% at the highest air pressure but a 10.4% increase at the lowest air pressure.
The more significant increase at the lowest air pressure makes sense as the tire deformation will be higher at lower air pressures. In reality, the air pressure should be increased when running higher loads, but we decided to stick to the same air pressures as it will paint the rawest picture, resulting in the most usable data.
The individual tire test results show a difference among the tires, with some tires performing better at higher loads than others. Given these differences, it will be difficult to calculate the total rolling resistance for any tire that hasn't been tested at different loads. We do feel the averaged data of the 6 tested tires will be very useful for getting a ballpark figure on how much CRR increases at higher loads.
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 load tests.
Conclusion
Our simple conclusion is that the CRR of bicycle tires under different loads is non-linear and increases at higher loads.
Our CRR at different speeds tests also showed us that CRR increases at higher speeds. When we combine the load and speed test results, we can conclude that the simple formulas used to calculate the total rolling resistance from the CRR value of a (bicycle) tire need corrections for the non-linear response under different loads, speeds, and air pressures.
These corrections will be hard to determine without testing every single tire at different loads, speeds, and air pressures as there can be differences between the tires. On average, and with the limited variations in speeds and loads used in bicycling, using the simple formula will probably get you close enough.