What exactly is rolling resistance?

Rolling resistance is the energy that is lost when the tyre is rolling. The main reason for the loss of energy is the constant deformation of the tyre.

Each bicycle tyre should, of course, roll as easily as possible. In contrast to a car a cyclist has only (very limited) physical power which he wants to apply as efficiently as possible.

In addition to the rolling resistance, there are also other resistances that must be overcome when riding a bicycle:

The air resistance rises in a squared ratio with increased speed. At a speed of approx. 20 km/h on level ground, air resistance has already become the main resistance force.

Acceleration energy is also expended. For instance, the weight of the wheels is of great importance when this mass has to be brought up to rotation.

When riding uphill, the main resistance force to be overcome is the gradient resistance (grade resistance).

In addition to these, there are other friction resistances in the chain and in other rotating parts. Yet in a well-serviced bicycle, these represent a very minor part of the total resistance.

 

Which factors affect rolling resistance?

Tyre pressure, tyre diameter, tyre width, tyre construction, tyre tread and other factors all have an effect on rolling resistance.

On a completely smooth surface the following applies: The higher the inflation pressure, the inferior the tyre deformation and thus rolling resistance.

Off road it is exactly the reverse: The lower the inflation pressure, the lower the rolling resistance. This applies equally on hard gravel roads and soft forest tracks. Explanation: A tyre with low inflation pressure can adapt better to a rugged surface. It sinks into the ground less and the whole rotational mass is held back much less by the uneven surface.

Tyres with a smaller diameter have a higher rolling resistance with the same inflation pressure, because tyre deformation is proportionally greater. The tyre is flattened more and is “less round”.

Wider tyres roll better than narrower tyres. This statement generally invokes skepticism, nevertheless, with tyres at the same pressure a narrower tyre deflects more and so deforms more.

Obviously, tyre construction also has an effect on rolling resistance. By using less material, less material can be deformed. And the more flexible the material is, such as the rubber compound, the less energy is lost through deformation.

Generally, smooth treads roll better than coarse treads. Tall lugs and wide gaps usually have a detrimental effect on rolling resistance.

Why do wide tyres roll better than narrower tyres?

The answer to this question lies in tyre deflection. Each tyre is flattened a little under load. This creates a flat contact area.

At the same inflation pressure, a wide and a narrow tyre have the same contact area. A wide tyre is flattened over its width whereas a narrow tyre has a slimmer but longer contact area.

The flattened area can be considered detrimental to tyre rotation. Because of the longer flattened area of the narrow tyre, the wheel loses more of its “roundness” and produces more deformation during the rotation. In a wide tyre, the flattened area is shorter in length and does not have so much effect on the rolling direction. The tyre stays “rounder” and therefore it rolls better.

Why do professional road racers use narrow tyres?

Wide tyres only roll easier with the same inflation pressure. Narrow tyres are used however with a higher inflation pressure, which makes them of course less comfortable.

In addition to this, narrow tyres have an advantage over wider tyres at higher speeds, as they provide less air resistance.

Above all, a bicycle with narrow tyres is much easier to accelerate, because the rotating mass of the wheels is lower and the bicycle is much more agile. The importance of this aspect becomes clear very quickly, when you ride in a fast group
of cyclists and when you have to accelerate rapidly from 20 to 40 km/h after a sharp turn in order to keep up with the others.

But at a constant speed of around 20 km/h, the ride is better with wider tyres. In practice, the energy saving is even greater than in theory, as the elasticity of the tyres absorbs road shocks, which would otherwise be transferred to the rider and thus saves energy.

And professional road racers are tending to ride wider tyres more and more. The tyre widths of 18 and 20 mm are hardly available anymore. And instead of the current standard width of 23 mm, the professional road racers choose more and more tyres with widths of 24 or 25 mm.