Sprocket

A sprocket is the toothed ring bolted or splined to the right crank arm. It drives the chain. With the small gear on the back wheel (the driver), its tooth count sets the gear ratio of the bike.

What it does

The pedals turn the cranks. The sprocket turns with them and pulls the chain around the driver on the rear hub. A bigger sprocket moves the chain further for each pedal turn. A smaller one moves it less.

BMX bikes have one gear. That is why the sprocket tooth count matters. Changing the sprocket or the driver is how you change the feel of the bike.

Types and standards

Tooth counts and gear ratio

Gear ratio is the sprocket teeth divided by the driver teeth. Most modern freestyle bikes use a 9 tooth driver on the rear hub (see cassette hub). Common sprocket sizes run from 25 to 28 teeth.

Sprocket	Driver	Ratio	Feel
25t	9t	2.78	Light, quick to accelerate
26t	9t	2.89	Common middle choice
27t	9t	3.00	Balanced
28t	9t	3.11	Heavier, faster at top speed

Lower ratios feel easier to pedal and suit tight street lines and flatland. Higher ratios suit trails, big park lines and riders who want a fast roll.

Mounting: bolt drive vs spline drive

Type	How it fits	Notes
Bolt drive	The sprocket bolts directly to the crank arm.	Simple and common. Pairs with bolt drive cranks.
Spline drive	The sprocket slides onto a splined spindle and the crank arm clamps it in place.	Lets the arm and sprocket be set at different positions. Pairs with spline drive cranks.

These two are not interchangeable. Check what your cranks accept before you buy a sprocket.

Guard sprockets

A guard sprocket has an extended ring or plate on the outside. The guard protects the chain and the teeth from catching on a rail, ledge or peg grind. It adds some weight. Street riders who do many double peg grinds or feeble grinds often choose one. Many park and dirt riders prefer a plain sprocket.

Sizing and compatibility

Check four things:

1. Mount type: bolt drive or spline drive, matched to your cranks.
2. Chain width: most BMX sprockets take a 1/8 inch wide chain. Narrower 3/32 inch sprockets exist but need a matching chain. See chain.
3. Pitch: 1/2 inch, which is the standard for all BMX chains.
4. Tooth count against the driver: make sure the chain length and rear wheel position can handle the new ratio.

Changing tooth count changes how much chain you need. A bigger sprocket may need a longer chain or a different rear axle position. See frame geometry for how chainstay length affects this.

How to choose

1. Decide your driver size first. If you have a 9 tooth driver, start at 25 to 26 teeth for street and 27 to 28 for park and dirt.
2. If you ride mostly street and grind often, consider a guard sprocket.
3. If you are on a tight budget, an aluminium bolt drive sprocket in a standard size is fine.
4. Confirm your cranks accept it.
5. If a ratio feels too hard, drop a tooth or two on the sprocket instead of changing the driver.

Prices range from low to moderate. Light, alloy guard sprockets cost more than basic steel ones.

Materials

Aluminium (often 6061 or 7075) is the most common. It is light and wears slowly with a good chain. Steel is stronger and cheaper, but heavier. Heavily worn teeth shorten chain life, so replace the sprocket when the teeth look hooked or shark-finned.

Maintenance

• Check the sprocket bolts are tight every month.
• Look at tooth wear when you replace the chain. Replace the sprocket if the teeth are hooked.
• Check the sprocket is flat (not bent) by spinning the cranks and watching the chain.
• Clean grit and old grease from between the sprocket and the crank arm.
• See how to fix a BMX chain if the chain keeps skipping.

Related parts

Cranks, chain, cassette hub, freecoaster hub and bottom bracket.