What are they? Well the technical definition of gear is a “toothed machine part whose purpose is to transfer energy to another part”. Sometimes they're referred to as sprockets or cogs.
Every bike has gears! Well, almost every bike – balance bikes and unicycles don’t.
In the case of every other bike, there are almost always a minimum of two gear-sprockets, and they do just what the definition says. They transfer kinetic energy from the rider (or in the case of an e-bike, from the rider and the electric motor) through the bicycle in order to turn the drive wheel, which is almost always the rear wheel. The combination of turning gear-sprockets, along with the chain, is what we call the bicycle's drive train.
The front gear is called the chain ring, and it transfers energy produced by the rider's muscles to make the chain move. The chain ring almost always the gear that has the pedals attached to it with crank arms.
The rear gear is sometimes just called the rear sprocket, and it transfers the energy carried by the chain into the wheel's hub, which turns the wheel and makes the bike go.
The teeth on gears are always the same distance apart, as the pins that hold the chain links together, because the teeth have to line up with the links of the chain in order to pull it (chain ring), or be pulled by it (rear sprocket).
But gears come in many sizes – or diameters – this affects how much energy they’re able to transfer, or how much work it takes to move the bicycle and the rider.
For the rest of this tutorial we’re going to focus on the REAR gears, also referred to as cassettes or freewheels. On just about every multi-speed bike, there is more than one sprocket – and this is where riding really becomes fun, because using a different gear size can help you set a comfortable pace, achieve a higher speed, or make it easier to climb or accelerate.
But like all moving machine parts, your chain and sprockets can get worn out with extended use, and then they’ll need replacement. You can tell your sprockets are worn out when the chain jumps while you’re pedaling, or if you see grooves between the teeth, or if the teeth have become very sharp, like sharks’ teeth. If you see this, you know it’s time to replace the sprockets.
There are two types of multiple-sprocket – freewheels and cassettes. It’s important to know the difference. Check out our tutorial on how to tell the difference between a freewheel and a cassette, and how choose the right freewheel or cassette.