The energy lost in the tires and wheel bearings generate a force opposing the forward motion of a cyclist. This force is commonly referred to as rolling resistance, FR, and it is modeled in our aeroTest using the following equation:
FRR = CRR ⋅ FN
CRR is the coefficient of rolling resistance and FN is the resultant weight force of the bike plus rider acting normal to the road surface. Thus rolling resistance scales proportionally to rider weight and CRR.
The majority of rolling resistance is attributed to the tires, and this is heavily dependent upon model, inflation pressure and road surface. Rim width and tire wear can also have a small effect on rolling resistance. All of these variables are accounted for in the CRR, which serves as a single metric to define the rolling efficiency of the bicycle which is independent of mass and road inclination. This is similar to how CDA is used as a single aerometric which is independent of apparent wind speed and air density.
What creates rolling resistance?
For a cyclist riding on a hard surface, rolling resistance is essentially energy lost to heat. This heat is generated by motion in the tire material due to internal friction as it deforms to support the rider’s weight and accommodate small undulations in the road surface. Any action to limit this deflection (i.e. riding on a smoother surface or inflating the tire to an appropriate pressure) will result in less rolling resistance.
Specifically, each tire must deflect to form a contact area with the road of sufficient area corresponding to the applied force and tire pressure. (i.e. higher pressure results in a smaller contact patch, less deflection, and less energy lost to rolling resistance.) This contact patch has an elliptical form with a width corresponding to the effective tire width (a function of rim width and tire size) and the tire circumference (i.e. “rollout”). The tire sidewall and tread must deflect more when the tire is underinflated (due to the larger contact patch) or a narrower size (due to the necessarily longer contact patch in order to generate the same contact area). Notably, a smaller wheel (e.g. a 650c wheel as opposed to the standard 700c) will have more sidewall deflection than a larger wheel in order to generate an equivalent length contact patch with the same tire size and inflation, resulting in increased rolling resistance.
It also possible to overinflate tires with the effect of increasing rolling resistance. These “lost watts” are absorbed by generating useless vibration which is damped by the bicycle components and (at a major loss of comfort) the rider! Also, it is possible to loose energy in the tread of an overinflated rear wheel riding on a rough surface. In this case, a small fraction of the rider’s pedal torque is lost when the rear tire slips excessively at the micro scale by “rebounding” over the small-scale bumps and roughness in the road instead of transferring that energy into propulsive force.
A general rule of thumb to minimize CRR is to select your tires for minimum rolling resistance based on measured data freely available on the web. This can result in up to 20W of saved power. Also, use wider rims and tires (if possible with your brakes and frameset), but be considerate of potential aerodynamic effects, particularly for the choice of front tire. In this case, choosing a tire that is too wide or odd-shaped might cause the air flow across the wheel and tire system to separate, resulting in increased drag. This is especially true for clincher wheels with narrower internal widths (between the bead hooks) and external widths. Often, wheel manufacturers will specify a tire for which their wheel was designed together as a system. It makes sense to use this or a similar tire for the front wheel, unless test data is available that suggests otherwise or you use our aeroTEST to make your own CRR measurements.