All of the components on a motorcycle wear. One of the first items to need replacement is the drive chain and/or the rear sprocket. Both of these items are exposed to extreme conditions, and although chain and sprocket manufacturers have improved their products over the years, replacement is inevitable at some point.
Replacing the drive chain is a simple procedure, but changing the rear sprocket is more involved.
Before replacing the rear sprocket in isolation (without replacing the chain and front sprocket), it is very important to check the condition of the other components. In general, chains and sprockets wear at different rates. The rear sprocket (depending on the material it is made from) tends to wear first, followed by the chain and then the front sprocket.
However, if the chain is showing signs of wear, and needs replacing for example, it is very likely the sprockets will be worn too.
Checking the front sprocket is simply a case of a visual inspection. If the teeth are hooked or showing signs of wear, the sprocket should be replaced. A simple method of checking the chain wear is to lift the chain in the middle of the rear sprocket (see photograph). If more than half of a tooth on the sprocket is exposed, the chain has stretched beyond its service limit (generally, the sprocket is suspect at that stage too).
Classic motorcycle owners change the rear sprocket for a number of reasons:
- To replace a worn or damaged sprocket
- As part of a full final drive kit replacement (chain, front sprocket and rear sprocket)
- To alter the final drive ratio
Replacing a Worn or Damaged Rear Sprocket
In most cases, it is necessary to remove the rear wheel to facilitate sprocket replacement (the exception being motorcycles with single sided swing-arms with the sprocket on the outside). However, before removing the rear wheel, it is often easier to loosen the sprocket retaining bolts/nuts. Having an assistant to operate the rear brake lever during the bolt loosening process will help considerably.
Having placed the motorcycle on its center stand, the drive chain must be removed next. If the chain has a split-link, it is simply a case of removing this link and rolling the chain off the sprocket. If the chain is of the continuous-link type, the wheel will need to move all the way forward to give the chain sufficient slack to lift it off the sprocket.
Temporary Worktable
Once the wheel has been removed from the motorcycle, it should be placed at a reasonable working height to replace the sprocket. However, it is bad practice to lay the wheel flat, especially if it has a rotor type brake, as it is easy to bend the rotor. A hollow container such as a drum (with suitable protection for the spokes) makes a good temporary worktable.
Some rear sprockets are mounted to a separate carrier, such as when fitted with cush drives (rubber circular blocks designed to smooth out the drive). In this case, the carrier should be clamped in a vice (using soft jaws) to remove the bolts or nuts.
Hardware Replacement
Having removed the old sprocket, the entire assembly (wheel, carrier bolts etc.) must be thoroughly cleaned. The bolts and nuts should also be inspected for wear and replaced if there is any doubt about their serviceability.
Fitting the new rear sprocket is simply a case of positioning it and tightening the bolts or nuts to secure it (these must be tightened to the correct manufacturers torque specification). After the wheel assembly has been refitted to the bike and the chain reattached, the chain tension must be set. However, new chains and sprockets will need to be run-in gently (no hard acceleration) which often requires re-tensioning of the chain after a few hundred miles.