When To Replace a Bike Chain


20.07.16 at 8:32 pm

“Spend a few quid now and save yourself a lot of money and bother in the long run”

To most people replacing the chain on a bike is much like getting the boiler serviced. They don’t bother until it’s too late and then it ends up costing them a whole heap of money. And I put myself firmly in that camp! Up until a few months ago I’d never bothered checking my chain. Sure, you may not care about performance, you may not be worried about the occasional gear slip but we all care about a bit of extra money in our pockets!

Lightbulb Moment

For me the moment I realised I needed to replace my chain more often was when I bought my first decent quality bike. My amazing Cannondale CAAD8 road bike was the first bike I felt really proud of. That, and it was pretty expensive!

When I took it in for its first service after 6 months I ended up having to pay not just for a new chain but for a new rear cassette too. After a few minutes with the mechanic he said I could have got as much as 5 times the life out of the cassette just by switching the chain over more often. This would have saved me loads of money in the long run.

My learning here was the more expensive the bike the more expensive the components. Whilst the positives of having this new bike were amazing it was clear I was going to have to do a bit more to look after it.

Why Did This Happen?

On a very simple level, a chain stretches when it ages. I’m not going to cover what happens to the inner workings of a chain when it wears, more the damage it causes and how to prevent it. If you love getting into the detail then I suggest a read of this amazing article on Sheldon Brown’s site. This guy really knows his stuff when it comes to everything bike related!Worn Cassette

At the point your chain stretches, it no longer fits properly into the teeth of your rear cassette and chain ring. This means it then starts wearing away the cogs. When you see the teeth looking like waves on the sea rather than straight up mountains you know this has started to happen. After a while, the teeth can no longer hold the chain in place so it starts to slip either when changing gear or you put a lot of power down.

No problem, you say. I’ll just head out and buy a new chain…

The challenge is that by this point you’ve done so much damage to the cassette and sometimes even the chainring that a new chain won’t fit. The teeth are now trying to accommodate a chain that has stretched, a new one simply won’t fit. This is when you have that painful realisation that you should have acted sooner.

The Maths

There are some situations where it’s just not worth changing your chain. If you are riding a cheaper bike with cheaper components then a cassette may cost very little. So you may actually be better off leaving the chain on for longer and getting the most wear out of it you can. When it starts to slip then replaces the chain and cassette together and go again.

When you start getting to the more expensive end of the scale this is definitely not the case.


I have always used KMC chains having had them recommended by a local bike mechanic. The KMC 11.93 is my chain of choice and retails at around £22.


Let’s take the Shimano groupsets as an example. My first bike was fitted with Claris on which the rear cassette would set you back about £20. At the other end of the scale you have Dura Ace. A new rear cassette on this will cost you nearly £150! So you can see here how the numbers really start to stack up. If you fail to replace the chain on a Dura Ace groupset you could be looking at £150 plus a new chain at a service. If you regularly replace a chain it will cost you £20 or so, but if you get 3-5 times more use out of the cassette you are postponing a much bigger cost.

So When Should I Change My Chain?

Some people will tell you they replace their chain once every ‘x’ miles or ‘x’ number of months. Good on them, but this is not really based on any science. If they’ve ridden for a long time they may have a pretty good idea but the problem is there are so many variables that wear a chain. Everything from humidity to maintenance to riding style and number of gear changes you make can be factors in how worn a chain gets.

If you are serious about keeping your chain in good order then I would suggest getting the tools you need to measure the wear. If you don’t you will either be throwing money away by changing it too frequently or leaving it on too long causing potentially expensive damage to other parts of your bike.

I use the following theory to judge when to replace a bike chain….

  1. Get a chain wear indicator (see below)
  2. When the wear hits 0.5% then order a new chain
  3. When the wear hits 0.75% then replace the chain

Change Wear Indicator

Park Tools Chain Wear Indicator

To show you how to check your chain for wear I am going to be using the Park Tools CC 32 Chain Wear Indicator.

This is a very simple tool that you slot into a chain which will show you exactly how much wear there is and when to replace a bike chain. You can check this out using a ruler, but I find it far easier using one of these cheap chain wear indicators. One side slots in to show you when the change is worn by 0.5% the other side when it is worn by o.75%.

How To Use The Chain Wear Indicator

I am now going to give you two examples. The first is my brand new bike where the chain hasn’t been used for long. The second is my winter bike where the chain has been on a while.

0.5% Wear


First the new bike. The wear indicator is almost slotting in on the 0.5% mark. Whilst this bike is the newer of the two I have been riding in for a couple of months, so you can see that the chain has already stretched as the indicator almost slots in on the left.


Next up the winter bike. You can see the indicator slots straight into the chain meaning this chain has hit 0.5% wear. It is at this point I would order a new chain.

0.75% Wear

When To Replace A Bike Chain

Back to the old bike and you can see how close this is to being 0.75% work. The indicator is right on the edge of dropping in and I would say it’s not a lot longer until I end up changing over this chain.