How long should a chain last?
That's nearly the same as saying 'how long is a piece of string'? And the answer is it all depends on your riding style and how much grit and dirt has accumulated on the chain. Some cyclists may get 10,000km from the life of a bike chain, for others they may be lucky to make it to 2000km.
The chain usually needs replacing when the bike no longer shifts smoothly, or chain slip while pedalling is experienced. (If this occurs while standing, it can be very painful as you come crashing down on the top-tube of the bike).
Often, you can get away with ignoring your chain except for a little oil now and then, but you will come to regret this.
When chains wear, there is often no easily visible indication that they are doing so. They seem to look just the same as when new, just a little dirtier. But chains wear in an odd way: they 'stretch'.
Chains don't actually stretch in the normal sense of that word. If you measured an old and a new chain, each link of the old worn chain will be the same length as a link on the new chain, yet the old chain will be longer. You could painstakingly count the number of links to make sure that there was the same number of these same sized links. Yet, the old chain would still be longer.
The secret: Chains wear out the connecting pins that hold the links together. The pins get ground down by all the grit your chain picks up from the road.
A worn chain measures just a tad longer between the pins, and does not mesh well with the gears on the bike, all of which are carefully made to mesh perfectly when the pins are half an inch from each other. As the pins move further from each other they come to rest on a different part of the gear tooth. Instead of nestling smoothly in the hollow between gear teeth, they land on the face of the following gear tooth, and start to wear that tooth down.
Putting a new chain on worn chain rings will yield worse chain skip. So neglecting the chain means you not only get to replace the chain (fairly cheap) but now you also have to replace the chain rings and probably a few cogs on the rear gear cluster as well. This can get expensive, especially if your rear cogs need to be replaced. It's much cheaper to replace the chains in a timely manner.
The rule of thumb is you should replace your chain before a foot of chain has stretched to the point where it is a foot and one eight of an inch.
Checking this is simple:
-
Shift to the big-big gear combination. (Gives a nice long place to measure).
-
Lay your bike on its left side.
-
Lay a ruler or tape on the top run of chain such that the zero mark is dead centre in one of the chain pins. Hold it there very steady.
-
Check the other end of the ruler. On a new chain it will be dead centre on another chain pin.
-
If the chain pin centre is less than 1/16th beyond the end of the ruler it's OK.
-
Between a 16th and an 8th, you should think about changing the chain at the earliest convenience.
-
If the chain pin centre is 1/8th of an inch beyond the end of the ruler it is past time to change your chain.