Have you ever seen an antique bicycle with a large wheel in the front and a small wheel in the back? It might seem strange, but bicycles used to be built like that for speed. Today's bicycles use chains and sprockets (a wheel with teeth, much like a gear) to achieve fast speeds with smaller wheels, but how does it work?
Bicycles in the 1800s had pedals attached directly to the front wheel, much like a child's tricycle. This means that for every turn of the pedals the front wheel turned once. By making the front wheel larger, the rider could go farther with each turn of the pedals. While this design made bicycles faster it was too easy for the rider to fall and get injured.
Look at the chain and sprockets on a modern bicycle. You'll notice that the sprocket in the front is larger than the one in the back. When you turn the pedals once, the rear wheel turns multiple times -- this is called mechanical advantage. Using this mechanical advantage allows us to make bicycles with smaller wheels, which lessens the chance of getting hurt in a fall.
If your bicycle has many sprockets, you can control the mechanical advantage by shifting the chain from one sprocket to the next. Shifting the chain to the largest sprocket in the front and the smallest in the back lets you turn the back wheel very fast, which helps you reach your top speed. Shifting to the smallest sprocket in the front and the largest in the back means you have to turn the pedals several times to turn the wheel once. While this won't make you go very fast, it will let you pedal up hills much easier.
The next time you ride your bike, take a close look at how it works. You might be amazed that science is all around you!
-- By Brad E. Peroney Jr.,
program development coordinator, Carnegie Science Center