Quick glossary
Collectively, a bike’s gear system is part of the drivetrain. The main parts are:
- Chainring: a toothed ring at the front, attached to the crank.
- Crankset (chainset): the crank arms plus the chainring or chainrings.
- Cog / sprocket: an individual toothed ring at the rear.
- Cassette: a stack of rear cogs that slides onto the rear hub’s freehub body.
- Freewheel: a rear cog cluster that screws onto the hub as one unit, common on older and lower-priced bikes.
- Chain: links the front and rear rings.
- Derailleurs: the mechanisms that move the chain between rings (front and rear).
- Shifters: the controls on the handlebar.
For how these parts fit together on the bike, see our bike gears diagram.
Gear, cog or sprocket: what is the difference?
In everyday speech people say “gear” for almost any toothed ring. More precisely, “chainring” is used for the front rings, and “cog” or “sprocket” for the rear ones. The two rear words are often used interchangeably. “Gear” can also mean the combination you have selected, such as “small ring, big cog”, which is a particular gear ratio.
Front: chainrings and crankset
Bikes may have one chainring (often written 1x), two (2x) or three (3x). The number of teeth is usually stamped on each chainring. A larger ring is harder to turn but moves you further per pedal turn.
Rear: cassette versus freewheel
Both give you several rear cogs, but they attach differently. A cassette slides on to a freehub body that is part of the wheel, while a freewheel threads on to the hub. They are not interchangeable, so check which you have before buying parts. The rear wheel is also a key point if you are considering adding gears to a single speed bike.
Derailleurs, shifters and chain
The rear derailleur guides the chain across the cassette and keeps it tight. The front derailleur, if there is one, moves the chain between chainrings. Both are controlled by shifters through a cable or an electronic signal. Some bikes use an internal gear hub instead, which hides the gears inside the rear hub and has no external rear derailleur.
What “21 speed” means
A “21-speed” bike typically has 3 chainrings and 7 rear cogs (3 x 7 = 21), while a “7-speed” bike usually has one chainring and 7 cogs. The 21-speed generally offers a wider range, which can help on hilly ground, though some combinations overlap and some put the chain at an awkward angle. Which is “better” depends on your terrain and how much you value simplicity. A 7-speed with a single chainring has fewer parts to adjust.
Gears for flat riding
On flat ground many riders stay in the middle of the cassette, and use the larger chainring if there is more than one. Move towards smaller cogs as speed rises and towards larger cogs when a headwind or slope makes pedaling heavy. For climbing, see which bike gears to use on hills.