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Thursday, 6th Aug 2020

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Cycle Clinic

Chain Stretch

Q Someone has told me that I need to replace my bike chain because the old one has stretched. Is this true, because I don't see how a chain can stretch, after all it's steel, not rubber.

A  You are both right !

Steel bicycle chains don't stretch in the literal sense, but because they get longer with use it seems as though they have 'stretched'.

What actually happens is that the little joins between each link in the chain flex backwards and forwards each time they go round the gear wheels and the big wheel by the pedals (the chain wheel).The chain goes from straight to curved and back again, and sometimes bent the other way, too.

This movement, hundreds of times, gradually wears away at the steel pivot pin a tiny bit, and over the length of the chain it adds up to more than 100 times that tiny bit.

This still doesn't amount to a great length overall, but our bikes are precision pieces of equipment and we rely on the chain to drop exactly into the slot between each tooth on the gear wheel.

If it doesn't fit exactly two things can happen.

Firstly we start to notice a tiny noise each time the chain and the tooth come together, and the tooth on the gear wheel starts to wear away so that both the gear wheel and the chain are now worn.

Then it becomes harder for the chain to move between gears when you want to change gear, especially when you are riding uphill and the chain is tight.

If the teeth on the gear wheels get very worn it is time to replace them.

A complete gear block, or cassette is more technical to replace, and of course a set of gears costs more than a chain.

The 'chain wheel' also wears if the chain has stretched, and this usually means the whole pedal assembly on both sides of the bike have to be replaced as well.

Add in the labour costs if it is not something that you feel competent to do yourself and it is certainly wise to replace the chain whenever it is showing signs that it has 'stretched'

By: 
Mike Skiffins, Portsmouth CTC http://www.portsmouthctc.org.uk
Posted on Thu, February 20 2020