Choose sprocket sizes to change the gearing

The gearing of a chain bike is set by the number of teeth on the front (gearbox) sprocket and the rear (wheel) sprocket. Changing them changes how fast the engine turns at a given road speed. This is worth thinking about when you replace a worn chain and sprockets anyway: a bike that always carries a load or climbs hills may pull better with lower gearing, and a bike that revs very high on open roads may be calmer with higher gearing. Most riders are best served by the maker's standard sizes.

Safety

• Do not fit a larger front sprocket than the case, the chain guard and the chain slider allow. A chain that rubs the case can cut through it.
• A wrong chain length can leave no adjustment, or put the rear axle too far forward or back. Check before riding.
• After any change, check chain slack, the rear brake and the rear axle torque.

You need

• The tooth counts of the present sprockets (stamped on each sprocket).
• The maker's list of sprocket sizes that fit, from the manual or a parts supplier, and the chain size: Identify the chain size.

Steps

1. Write down the front and rear tooth counts. The final drive ratio is the rear count divided by the front count. A bigger number means lower gearing: more pulling power, lower top speed, more engine revs at a given speed.
2. Decide what you want. For more pull on hills or with a load: a smaller front sprocket or a bigger rear sprocket. For lower revs at cruising speed: a bigger front sprocket or a smaller rear sprocket.
3. Work out the new ratio before you buy. Example: 15 front and 45 rear gives 45 / 15 = 3.0. One tooth less at the front (14) gives 45 / 14 = about 3.2, which is about 7 percent lower. To get the same change at the rear you would need about 48 teeth. As a rough guide, one tooth at the front changes the gearing about as much as three to four teeth at the rear.
4. Change in small steps: one tooth at the front, or two or three at the rear. A big change can make first gear too short or top gear too tall to pull.
5. Prefer keeping the front sprocket large. A very small front sprocket bends the chain sharply and wears it faster. A front sprocket change is usually cheaper, though.
6. Check chain length. A bigger rear sprocket or bigger front sprocket may need one or two more links; a smaller one may need fewer. Lay the old chain on the new sprockets with the axle in the middle of its adjustment range and count.
7. Fit chain and both sprockets as a set: Replace chain and sprockets. Fit dished sprockets the same way round as the old ones.
8. Check where the speedometer is driven. If it is driven from the front sprocket or gearbox output (many fuel-injected bikes have a speed sensor there), it will read wrong after a gearing change. A drive at the front wheel is not affected.
9. Ride and compare: hill climbing, cruising revs and fuel use.

Check your work

• The chain slack is correct with the adjusters near the middle of their range.
• The chain does not touch the case, the guard or the swingarm at any point of the swing.
• The speedometer reads sensibly, or you know by how much it is off.

If it did not fix the problem

• The bike still struggles on hills with standard gearing: Bike struggles on hills and Power feels low.
• Chain at the end of the adjusters: Chain adjuster is at the end of its range.

Reference values (K-Light 125)