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2020-08-04
Cycling can sometimes feel like hard work, especially if you're going uphill. In the science of cycling, "hard work" means that you sometimes have to use quite a lot of force to pedal any distance. If you're going uphill, you need to work against the force of gravity. If you're going fast, you're working against the force of air resistance (drag) pushing against your body. Sometimes there are bumps in the road you have to ride over; that takes more force and uses energy too (bumps reduce your kinetic energy by reducing your speed).
But whether you're going uphill or downhill, fast or slow, on a smooth road or a bumpy one, there's another kind of work you always have to do simply to make your wheels go around. When a wheel rests on the ground, supporting a load such as a rider on a bike, the tire wrapped around it is squashed up in some places and bulging out in others. As you cycle along, different parts of the tire squash and bulge in turn and the rubber they're made from is pulled and pushed in all directions. Repeatedly squashing a tire in this way is a bit like kneading bread: it takes energy-and that energy is what we know as rolling resistance. The more load you put on the tire (the heavier you are or the more you're carrying), the higher the rolling resistance.
For a racing bike traveling fast, about 80 percent of the work the cyclist does will go in overcoming air resistance, while the remainder will be used to battle rolling resistance; for a mountain biker going much more slowly over rough terrain, 80 percent of their energy goes in rolling resistance and only 20 percent is lost to drag.
How much energy are we actually talking about here? In the Tour de France, according to a fascinating analysis by Training Peaks, top riders average about 300–400 watts of power, which is as much as 3–4 old-fashioned 100-watt lamps or about 15 percent of the power you'd need to drive an electric kettle. For comparison, you can generate about 10 watts with a hand-cranked electricity generator, though you can't use one of those for very long without getting tired. What does this tell us? It's much easier to generate large amounts of power for long periods of time by using your big leg muscles than by using your hands and arms. That's why bikes are so clever: they make good use of the most powerful muscles in our body.
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