We've described a bicycle as a machine and, in scientific terms, that's exactly what it is: a device that can magnify force (making it easier to go uphill) or speed. It's also a machine in the sense that it converts energy from one form (whatever you had to eat) into another (the kinetic energy your body and bicycle have as they speed along). Now you've probably heard of a law of physics called the conservation of energy, which says that you can't create energy out of thin air or make it vanish without trace: all you can do is convert it from one from to another. So where does the energy you use in cycling actually go? It scientific terms, we say it goes into "doing work"- but what does that mean in practice? 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).
Bicycles work so well with the human body because they harness power from our large and very powerful leg muscles. Recumbent bicycles (ones you ride lying down) might look ultra-modern and a bit weird, but they date back at least 100 years. They're faster than conventional bicycles because their riders adopt a much more aerodynamic, tube-like posture that minimizes drag. Since the pedals are higher off the ground, the cranks can be longer, so you get more leverage, your muscles can make high power for longer, and do so more efficiently. Photo by Robin Hillyer-Miles courtesy of US Navy.
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.
