Rolling resistance is one of the forces that act to oppose the motion of a cyclist. It is caused mainly because of deformation of the tyre and can be understood by thinking about:
- energy losses due to heat (hysteresis)
- the forces pushing on the tyre.
Energy is lost as heat
As the tyre comes into contact with the road, it changes shape as it is compressed (pushed in a bit). This compression causes parts of the rubber tube and the casing of the tyre to heat up. It also causes the tread on the outside of the tyre to heat up as it comes into contact with the road. A tyre with knobbly tread will have a lot more rolling resistance than a smooth tyre because there will be more rubber being compressed and so there will be more heat.
The tyre is a bit like a spring or a rubber band. Some of the energy is stored as the tyre becomes compressed (squeezed in). As the tyre comes back off the road, there is not as much of this stored energy returned because some of it has gone towards making the tyre heat up. This loss of available energy due to heating is called hysteresis. This means there will be less energy available to make the bike go faster.
If you feel the temperature of your tyres when you have been biking fast, you will notice this heat. If your tyres are not pumped up hard, there will be even more heat created.
The force of rolling resistance does not change at higher speeds. However, because there is a lot more heat being produced at higher speeds, the cyclist uses more power (energy per second) to work against rolling resistance.
Forces on the front part of the tyre are greater
As the front part of the tyre touches the road, it takes a lot of force to squeeze in the tyre. This causes a backwards force on the wheel. As the tyre comes back off the road, there is a force pushing forwards on the wheel, but this forwards force is not as great. This is because of internal friction in the tyre.
If you could design a tyre that didn’t have internal friction and didn’t heat up, the force at the front section of the tyre contacting the road would be the same size as the force on the back section. This would mean zero rolling resistance, assuming there was no energy loss due to compression of the road surface.
Some things that affect rolling resistance
Rolling resistance depends on the mass of the cyclist and the bike, cornering, the design of your tyres, tyre pressure and road surface.
- Mass – more mass means more downwards force due to gravity onto the road. This results in more tyre compression so rolling resistance increases. Carrying things like extra water bottles increases your rolling resistance.
- Cornering – going through corners causes track cyclists to slow down, partly because cornering results in a greater downwards force into the track, which increases rolling resistance. (Cornering also increases rolling resistance because the rubber is twisting sideways on the road a bit, and this twisting produces more friction and heat.)
- Design of tyres – choosing materials and construction that don’t heat up as much will result in less rolling resistance. One way to do this is to make your tubes out of specialised latex rubber. Racing tyres are very smooth, narrow and pumped up hard. The casing is also designed to minimise heat loss as it deforms. With as little of the tyre being compressed in as possible, there will not be as much of this heating effect, so there will be more energy available for more speed. There are many options for cyclists to consider.
- Road or track surface – wooden cycle tracks contribute to a very low rolling resistance. Rough-sealed roads have more rolling resistance. Grass and soft dirt surfaces are much greater. If the surface can be deformed, this increases rolling resistance markedly.
Calculating rolling resistance
This equation is used to calculate the force of rolling resistance from each tyre:
FRR = CRR x weight
- FRR is the rolling resistance force.
- CRR is the coefficient of rolling resistance (a number used to rate each tyre at a given pressure),
- weight is the force due to gravity being exerted on each tyre (= mass (kilograms) x 9.8).
This equation shows that speed does not affect the force due to rolling resistance. The type of tyre and the mass of the bike plus rider are the only two variables.
The On your bikes activity requires students to analyse graphs of motion for New Zealand champion cyclist Alison Shanks. A worksheet leads students to analyse graphs of speed, force and power.
Calculate rolling resistances and determine time saved over a set distance.
Compare rolling resistance values at different pressures.