Motorcyle chains

(, June 2001)


I’ve guddled around quite a bit with chains, so I’m writing down the stuff I’ve learned in the hope that it’ll be useful for other people.

What does the chain do?

In a motorbike, there’s limited space. The engine, being heavy, has to go in the middle of the bike. The wheels have to go at the front and the back. Since the engine and the rear wheel are far apart, we need some way to joining them together so that the engine can drive the rear wheel. One method is to have a cog (aka. sprocket) on the engine output shaft and the rear wheel and use a chain to connect the two. Another method is to have a sealed shaft connecting the engine and the rear wheel, with suitable joints at either end to turn the drive round by 90 degrees.

Structure of a chain

I need a diagram here. There’s no point in describing it in words.

What goes wrong with a chain

Chains lead a hard life. They’re exposed to the little bits of grit which get kicked up from the road. They spin round at high speed, which tends to throw off what little oil gets puts on them. Chains either break (very bad), get seized up (bad) or get worn (inevitable).

To put a chain on a bike, it has to be broken to get it round the swing arm, and then rejoined. There are two main ways of joining a chain. Firstly, you can use a special joining link which clips on to the side of a plate and holds it in place. The other way is to rivet the chain closed, which involves hammering a special pin until the end is smeared out a bit over the side plate and holds it in place.

If the chain becomes un-joined at speed, you’re in big trouble. Chains are heavy and if it becomes detached while the bike is moving, it’s going to cause serious damage and probably lock up the bike. For this reason, be very careful when joining chains. Don’t reuse the closing links, since they get weakened when you take them off. If you have a rivetted chain, check the rivet link to make sure it’s flattened enough to stop the side plate from coming off.

The most common way which my chains die is through corrosion. Winter takes a heavy toll on chains with salt and grit on the road and freezing temperatures make it less attractive to go outside and work on the bike for any length of time. Generally, by the end of winter there’s a good few seized links in the chain which hop whenever they go over the sprocket. Time to replace the chain …

There are lots of ways to prevent corrosion, though careful choice of what chain to buy, and regular maintenance. Most bike chains are o-ring chains. This means that there is a small rubber ring at each pivot point. The purpose of this o-ring is to trap grease inside the pivot and keep it lubricated. This means that o-ring chains have a much longer life than plain chains. However, one problem with the o-ring is that if the grease ever gets washed out from within the chain then the o-rings will make it very hard to get grease back in it. Additionally, the o-rings themselves can be damaged by various fluids and by rough treatment. The instructions which came with my last chain recommend cleaning the chain by wiping with a paraffin soaked rag, and then re-oiling.

All chains ‘stretch’ during their lifetime and eventually need replacing. Chains don’t stretch in the same way elastic bands do – they get longer because the metal in the links gradually wear away and makes the overall length of the chain increase. As the chain stretches, the amount of free play increases and you eventually have to move your rear wheel back a bit to take up the slack. If there’s too much slack, the chain will jump around lots whenever you change speed. If there is too little slack, the chain will get overtensioned when you slow down and the back end of the bike becomes unweighted.

Replacing the chain

When you replace the chain, always replace the sprockets too – they’re much cheaper than the chain anyway. It’s a false economy not to, since putting a new chain over worn sprockets will make your chain wear out faster. It’s much easier to loosen the bolt which holds the front sprocket when the chain is still on the bike. You put the bike into a high gear and get someone to stand on the rear brake while you loosen the holding bolt a bit. Since the front sprocket is still attached by the chain to the rear spocket, it can’t spin around while you try to loosen it.