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Tyre pressure and rolling resistance (Crr)
The short answer: rolling resistance is the power your tyres turn into heat as they flatten against the road. Its coefficient, Crr, falls as tyre pressure rises, quickly at low pressures and more slowly at high ones. FondoLab estimates yours from the pressure you ride, and uses your own number if you know it.
What Crr means
The force your tyres resist with is Crr times the weight of rider and bike. Unlike air resistance it does not grow with speed, so the power it costs grows only in step with speed. That makes it a bigger share of the total on climbs and at low speed, and a smaller one on fast flat roads.
For a rider and bike of 80.5 kg at 30 km/h, every 0.001 of Crr costs about 6.6 watts.
What pressure does
In one study a rider coasted down a level indoor hallway again and again at five tyre pressures. The rolling resistance followed a smooth curve, Crr = 0.1071 × pressure^−0.477 with the pressure in kPa (100 kPa is 1 bar), and fell by 62% from 1.5 bar to 12 bar 1. Read off that curve:
| Pressure | Crr | Rolling power, 80.5 kg at 30 km/h |
|---|---|---|
| 4 bar | 0.0061 | 40 W |
| 6 bar | 0.0051 | 33 W |
| 8 bar | 0.0044 | 29 W |
Load matters too. Carrying 15 kg more raised the Crr itself by about 11% in the same study, on top of the extra weight it multiplies 1.
What this does not tell you
The measurements come from one tyre on a smooth indoor floor 1. Real roads are rougher, and modern tyres differ in width, casing and construction, so your Crr on a real road will be different, often higher. The study cannot tell you the best pressure for rough roads or for your tyres; it shows how strongly pressure acts on smooth ground, and nothing more. Comfort, grip and puncture risk matter as much on a long ride, and this study did not measure them.
How FondoLab estimates yours
The rider profile takes the tyre pressure you ride for each bike and reads Crr off the curve above. Below 1.5 bar or above 12 bar it uses the nearest end, because the study did not measure beyond them, and the profile tells you when that happens. If a tyre test gives you the Crr of your own tyres, enter it, and the profile uses that instead.
The formula and its source are on the method page.
References
- Grappe F, Candau R, Barbier B, Hoffman MD, Belli A, Rouillon JD (1999). Influence of tyre pressure and vertical load on coefficient of rolling resistance and simulated cycling performance. Ergonomics 42(10):1361–1371. doi:10.1080/001401399185009