What’s so good about bicycles?
What’s so good is that they get you places quickly without gobbling up fossil fuels like gasoline, diesel, and coal or creating pollution. They do that because they very efficiently convert the power our bodies produce into kinetic energy (energy of movement). In fact, as you can see from the chart opposite, they’re the most efficient transportation machines humans have developed so far. Harnessing the power from your muscles in an amazingly effective way, a bicycle can convert around 90 percent of the energy you supply at the pedals into kinetic energy that powers you along. Compare that to a car engine, which converts only about a quarter of the energy in the gasoline into useful power—and makes all kinds of pollution in the process.
Look at it this way: If you drive a car, you’re dragging a lump of metal that probably weighs 10–20 times as much as you do wherever you go (a typical compact car weighs well over 1000kg or 2000lb). What a waste of energy! Go by bike and the metal you have to move around with you is more like 6–9kg (14–20lb) for a lightweight racing bike or 11–20kg (25–45lb) for a mountain bike or tourer, which is a fraction of your own weight.
Better efficiency means you can go further on the same amount of fuel, which is another great advantage of bikes, although a little hard to quantify. According to the classic Bicycling Science book by David Gordon Wilson et al: “A racing bicyclist at 32km/h (20mph) could travel more than 574 kilometers per liter (1,350 miles per US gallon) if there were a liquid food with the energy content of gasoline.” Whichever way you look at it, bikes are pretty amazing!
Where does your energy go?
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).
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.
How a bicycle frame works
Assuming an adult weights 60–80kg (130–180lb), the frame of a bicycle has to be fairly tough if it’s not going to snap or buckle the moment the rider climbs on board. Ordinary bicycles have frames made from strong, inexpensive, tubular steel (literally, hollow steel tubes containing nothing but air) or lighter alloys based on steel or aluminum. Racing bicycles are more likely to be made from carbon-fiber composites, which are more expensive but stronger, lighter, and rustproof.
Photo: The bicycle’s inverted A-frame is an incredibly strong structure that helps to distribute your weight between the front and back wheels. It helps to lean forward or even stand up when you’re going uphill so you can apply maximum force to the pedals and keep your balance.
You might think that a bike frame made out of aluminum tubing would be much weaker than one made from steel—but only if the tubes are similar in dimensions. In practice, every bike needs to be strong enough to support the rider’s weight and the loads it’s likely to experience during different kinds of handling. So an aluminum bike would use tubing with a larger diameter and/or thicker walls than a bike made from steel tubing.
The frame doesn’t simply support you: its triangular shape (often two triangles joined together to make a diamond) is carefully designed to distribute your weight. Although the saddle is positioned much nearer to the back wheel, you lean forward to hold the handlebars. The angled bars in the frame are designed to share your weight more or less evenly between the front and back wheels. If you think about it, that’s really important. If all your weight acted over the back wheel, and you tried to pedal uphill, you’d tip backwards; similarly, if there were too much weight on the front wheel, you’d go head over heels every time you went downhill!
Frames aren’t designed to be 100 percent rigid: that would make for a much less comfortable ride. Virtually all bike frames flex and bend a little so they absorb some of the shocks of riding, though other factors (like the saddle and tires) have much more effect on ride comfort. It’s also worth remembering that the human body is itself a remarkably efficient suspension system; riding a mountain bike along a rough trail, you’ll very quickly become aware of how your arms can work as shock absorbers! Indeed, it can be quite instructive to view the body as an extension (or complement) of the bike’s basic frame, balanced on top of it.
How bicycle wheels work
If you’ve read our article on how wheels work, you’ll know that a wheel and the axle it turns around is an example of what scientists call a simple machine: it will multiply force or speed depending on how you turn it. Bicycle wheels are typically over 50cm (20 inches) in diameter, which is taller than most car wheels. The taller the wheels, the more they multiply your speed when you turn them at the axle. That’s why racing bicycles have the tallest wheels (typically about 70cm or 27.5 inches in diameter).
The wheels ultimately support your entire weight, but in a very interesting way. If the wheels were solid, they’d be squashed down (compressed) as you sat on the seat, and pushing back up to support you. However, the wheels of most bikes are actually formed of a strong hub, a thin rim, and about 24 highly tensioned spokes. Bicycles have spoked wheels, rather than solid metal wheels, to make them both strong and lightweight, and to reduce drag (some riders use flat “bladed” spokes or ones with an oval shape, instead of traditional rounded ones, in an attempt to cut drag even more).
It’s not just the number of spokes that’s important but the way they’re connected between the rim and its hub. Like the strands of a spider’s web, or the dangling ropes of a suspension bridge, a bike wheel is in tension—the spokes are pulled tight. Since the spokes criss-cross from the rim to the opposite side of the hub, the wheel isn’t as flat and flimsy as it appears, but actually an amazingly strong, three-dimensional structure. When you sit on a bike, your weight pushes down on the hubs, which stretch some of the spokes a bit more and others a bit less. If you weigh 60kg (130lb), there’s about 30kg (130lb) pushing down on each wheel (not including the bicycle’s own weight), and the spokes are what stops the wheels from buckling.