Bicycles

Last updated: November 11, 2009.

If you had to pick the greatest machine of all time, what would you say? If we were talking about machines that helped spread knowledge and educate people, you'd probably opt for the printing press. If we meant inventions that let people farm the land and feed their families, you might plump for the plow or the tractor. If you think transportation is really important, you could go for the car engine, the steam engine, or the airplane jet engine. But for its sheer simplicity, I think I would pick the bicycle. It's a perfect example of how pure, scientific ideas can be harnessed in a very practical piece of technology. Let's take a look at the science of cycles—and just what makes them so great!

Photo: The bicycle—a brilliantly simple form of transportation, wherever in the world you happen to be. Photo by Roger S. Duncan courtesy of US Navy.

How the frame works

A typical adult weights 60-80 kg (130-180 lb), so 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 but lightweight tubular steel (literally, hollowed-out steel tubes containing nothing but air). Racing bicycles are more likely to be made from carbon composites, which are even stronger and lighter.

The frame doesn't simply support you: its triangular shape is carefully designed to distribute your weight. Although the saddle is positioned much nearer to the back wheel, the rider leans 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!

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.

How the wheels work

If you've read our article on tools and machines, 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 distance depending on how you turn it. Bicycle wheels are typically over 50 cm (20 inches) in diameter, which is taller than most car wheels. The taller the wheels, the more they multiply your speed. That's why racing bicycles have the tallest wheels (typically about 70 cm or 27.5 inches in diameter).

Photo: Like a car wheel, a bicycle wheel is a speed multiplier. The pedals and gears turn the axle at the centre. The axle turns only a short distance, but the leverage of the wheel means the outer rim turns much further in the same time. That's how a wheel helps you go faster.

The wheels ultimately support your entire weight. So if you weigh 60 kg (130 lb), there's about 30 kg (130 lb) pushing down on each wheel (not including the bicycle's own weight). The spokes are what stops the wheels from buckling. Since each wheel has around 30-40 spokes, each spoke has to support only a fraction of the total weight—in this case, less than 1kg (2.2 lb), which it can do easily. Bicycles have spoked wheels, rather than solid metal wheels, to make them both strong and lightweight. Spokes also reduce the air resistance or drag on the front wheel when you're cornering. Read more in our article on how wheels work.

How the gears work

A typical bicycle has anything from three to thirty different gears—wheels with teeth, linked by the chain, which make the machine faster (going along the straight) or easier to pedal (going uphill). Bigger wheels also help you go faster on the straight, but they're a big drawback when it comes to hills. That's why mountain bikes and BMX bikes have smaller wheels than racing bicycles. It's not just the gears on a bicycle that help to magnify your pedalling power when you go uphill: the pedals are fastened to the main gear wheel by a pair of cranks: two short levers that also magnify the force you can exert with your legs. Read more in our main article on gears.

Photo: A gear is a pair of wheels with teeth that interlock to increase power or speed. In a bicycle, the pair of gears is not driven directly but linked by a chain. At one end, the chain is permanently looped around the main gear wheel (between the pedals). At its other end, it shifts between a series of bigger or smaller toothed wheels when you change gear.

How the brakes work

No matter how fast you go, there comes a time when you need to stop. Brakes on a bicycle work using friction (the rubbing force between two things that slide past one another while they're touching). When you press the brake levers, a pair of rubber shoes clamps onto the metal inner surface of the front and back wheels. As the brake shoes rub tightly against the wheels, they turn your kinetic energy (the energy you have because you're going along) into heat—which has the effect of slowing you down. There's more about this in our main article on brakes.

Photo: The rubber shoes of this bicycle's brakes clamp the metal rim of the wheel to slow you down.

How the tires work

Friction is also at work between the rubber tires and the road you ride on: it gives you grip that makes your bike easier to control, especially on wet days. Different kinds of bicycles have different kinds of tires. Racing bicycles have thin, smooth tires designed for maximum speed, while mountain bicycles have thicker, more robust tires with deeper treads that can withstand tougher terrain. The tires are not made of solid rubber: they have an inner tube filled with compressed air. That means they're lighter and more springy, which gives you a much more comfortable ride.

Why clothing matters

Friction is a great thing in brakes and tires—but it's less welcome in another form: as air resistance (drag) that slows you down. The faster you go, the more air resistance becomes a problem. At high speeds, racing a bicycle can feel like swimming through water: you can really feel the air pushing against you and you use most of your energy overcoming drag. Now a bicycle is pretty thin and streamlined, but a cyclist's body is much fatter and wider. In practice, a cyclist's body creates twice as much drag as their bicycle. That's why cyclists wear tight neoprene clothing and pointed hats to streamline themselves and minimize energy losses.

You might not have noticed, but the handlebars of a bicycle are levers too: longer handlebars provide leverage that makes it easier to swivel the front wheel. But the wider you space your arms, the more air resistance you create. That's why racing bicycles have two sets of handlebars to help the cyclist adopt the best, most streamlined position. There are conventional, outer handlebars for steering and inner ones for holding onto on the straight. Using these inner handlebars forces the cyclist's arms into a much tighter, more streamlined position. Most cyclists now wear helmets, both for safety reasons and improved aerodynamics.

Photo: Racing bicycles have two sets of handlebars. Inner handlebars let riders reduce air resistance by keeping their elbows closer together. Photo by Ben A. Gonzales courtesy of US Navy.

Bicycles are physics in action

All these things make bicycles incredibly efficient machines. A bicycle is so efficient that it can convert around 80 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 25 percent of the energy in the gasoline into useful power, and you'll see why I say bicycles are such brilliant examples of scientific machines.