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What is rolling resistance?

Dr Mark Jermy from the University of Canterbury explains why a tyre has some resistance to the motion of a bike as it rolls over the ground. He explains what causes some energy to be converted to heat in the tyres (hysteresis) and describes some things that can be done to reduce rolling resistance.

Acknowledgement:
Geoff Searle
©David Fiedler
©SKINS International Trading AG/Benjamin Fitzmaurice
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Transcript

DR MARK JERMY
Anything on wheels experiences rolling resistance because there is a friction, but anything with the wheel, where the wheel touches the ground, it deforms slightly. It’s got a flat section near the ground, and as you roll it over, that flat section moves around the tyre, and so you are constantly having to change the shape of the tyre. So that gives you an extra resistive force, and you notice that, if you leave your bicycle tyres to go flat, it’s much harder to pedal than if they are well pumped up and quite firm. That’s rolling resistance is what you are feeling.

The hysteresis is wherever you change the shape of the rubber, so if you think about a particular patch of rubber on the tyre, as it comes round the wheel and then comes into the flat bit where it touches the ground, that piece of rubber has to change shape, and any elastic material like rubber, you have to put some force into changing its shape, you have to do some work on it.

And so you store more energy in the tyre, you store more elastic energy. It’s a bit like a spring. And you release some of that energy as that part of the tyre comes off the ground and comes back to a circular shape, but you don’t release as much energy as you put into it. Some of it’s dissipated as heat, and so that’s a loss, because you the cyclist, as you are pedalling, you are having to generate that extra energy to make that heat, and that is making it harder for you to pedal.

The simplest thing you can do is pump your tyres up really hard. The other thing you can do is you can use different materials for the tyres. Certain types of rubber have less hysteresis, so less energy is dissipated as they are deformed and then brought back to their original shape, so by choosing the material carefully, you can reduce that rolling resistance.

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