Lets start with chain wear and what it is?
Here is a diagram of a chain and its parts.
You may have gone into a bike shop and heard the mechanic talk about chain stretch. This term is a little misleading as the chain itself doesn’t actually stretch but it does however get longer over time and this is due to the wear in the links. The chain is made up of metal parts and if you rub 2 pieces of metal together for long enough they will start to get smaller. As can be seen in the diagram below this is the case with the chain pins and rollers in the chain. As the pieces of metal rub together the actual roller and pin get slightly smaller or holes in links get larger.
Each chain pin and bush getting fractionally smaller, multiplied by the number of pins and bushes in the chain means the chain ‘stretches’.
This wear can be measured by popping into many bike shops and asking them to check your chain. We use a specific tool that measures the gap between links and know when to replace it. We normally replace it at 1mm stretch as this is the last time you can before the wear starts affecting the rear cassette and finally the front chain-rings.
Why is it important to keep an eye on the chain?
Below is a scale of chain wear and when we, as mechanics, decide to change the chain.
1mm “stretch” regular chain wear only with minimal wear on cog teeth, usually only the most used cogs will show signs of wear.
starting to wear at the rear cassette
front chain rings are starting to wear badly and slipping happening in the rear
slipping all over the place and gears not behaving at all
As mechanics we would replace a chain that looks any where close to being a 4 on the scale.
Once it goes past the 4 it starts to wear at cogs in the cassette. Below you can see worn teeth on a cassette or chain-ring. Once these are worn you have to replace them with a new chain.
“Why can’t I put a new chain on a worn cog”
Remember the chain links and pins have become smaller and effectively further apart. The cogs have now been worn in a way to reflect this. Take the chain back to original measurement between pins and they are too close together to sit in the cogs properly. More slipping and general annoyance.
Remembering that, as mechanics, we only see the bike once every 6 months we have to make the call on whether to replace earlier than someone who was checking it weekly.
When we quote a bike (call with a quote before doing the work) we normally try to establish how much riding is being done on the bike before we suggest a new chain.
Asking the right questions can avoid expensive repairs in the future.