By Alan Schietzsch
The idea behind multiple gears on a bicycle — whether it’s an old “10-speed” bike or a modern mountain bike with 27 gears — is to let you change the distance that the bike moves forward with each pedal stroke. The gears at the front are called the chainwheels or “rings”. Most bikes have two or three chainwheels that look like this:
Attached to the rear wheel is the freewheel or “Cassette”, which looks like this:
The cassette has between five and ten gears on it, depending on the bike. A freewheel spins freely in one direction and locks in the other. That allows the rider to either pedal or not pedal — when not pedaling, the bike coasts (another feature that tricycles and penny-farthing bicycles lack).
To change the gears, a bicycle has front and rear derailleurs. Here’s a shot of the rear derailleur:
The rear derailleur has two small cogs on it that both spin freely. The purpose of the arm and lower cog of the derailleur is to tension the chain. The cog and arm are connected to a spring so that the bottom cog pulls backward at all times.
As you change gears, you will notice that the angle of the arm changes to take up or let out slack:
The top cog is very close to the freewheel. When you adjust the gears with the lever on the handlebar, this cog moves sideways to a different gear on the cassette and drags the chain with it. The chain naturally slips from one gear to the next as you turn the pedals.
A common mountain bicycle has wheels that are about 26 inches in diameter. The “lowest” gear on the bike might use the tiny “granny gear” front chain wheel with 22 teeth and a rear gear having 30 teeth. That means that the gear ratio is 0.73-to- 1. For each turn of the pedals, the rear wheel turns just 0.73 times. In other words, for each pedal stroke, the bike moves forward only about 60 inches (about 5 km/h at a 60-rpm pedaling rate). Like a slow walk. Just what you need to climb a steep hill.
The “highest” gear ratio on the bike might be a front chain wheel with 44 teeth and a rear gear having 11 teeth. That creates a 4-to-1 gear ratio. With 26-inch wheels, the bike moves forward 326 inches with each pedal stroke. At a 60-rpm pedaling rate, the speed of the bike is 30 km/h.
By pedaling at twice the rate, a “cadence” of 120 rpm, the bike has a maximum speed of 60 km/h. A range of 5 km/h to 60 km/h is fantastic, and it lets the rider climb the steepest hill very slowly or race almost as fast as a car! That is why a bike has gears.
Road bike’s gears typically don’t go quite as low, but also can go quite a bit higher than those on a mountain bike.
If you prefer your bike to have lower or higher gears, you can buy a cassette with a different number of teeth, which changes the ratios. Racers often have several cassettes, installing a big one with lower ratios when climbing in the mountains, and using a smaller one for flat courses where low gears aren’t required. Although it can be expensive, front chainrings can be exchanged as well. Everything about a bicycle is simple and can be customized just for you. That’s what makes it such a great machine to ride — and also a great mechanical work of art!