UKH

I've just taken my bike to be serviced. The chain broke just before I took it so asked the shop to replace but they said it was better to repair than replace as the now stretched chain will  fit the cassette better (a new chain will sit on top of the cogs and lead to slippage, he said). But surely if the cassette is worn (I've not ridden it that much) then a new chain would be better anyway? 

As a new cassette is only £20 and a new chain £16 I'm tempted to have both done.

If the chain has grown a lot, it can wear the chainrings as well as the cassette to the point you need to replace the lot. The shop should be able to measure this and advise if it is at that point or not. If not then I'd just change the chain, and at those prices it is tempting to do the cassette also. Just to be aware that you might get chain and cassette done to find the new chain doesn't fit the worn chainrings!

By chain rings, you men's mean the cogs on the front? He did say they tended to wear less than the rear cassette. He also told me you can't tell how badly the cassette is worn but then said "it's really hard" which might mean he can't be arsed which is fair enough!

I just put a new chain on a 5 year old bike (reasonable but not high mileage) and haven't had any problems. What I read regarding replacing the cassette and rings was that you should do so after several replacement chains.

Chains can break (doesn't seem to happen that often but it can) and maybe it's not that worn and that's why the shop is just saying they'll put it back together again.

Cassettes tend to wear with the chain, so it does seem the normal advice that you replace them together, some people will say the chainset too (or chainset rings if they replaceable separately). In the past I have replaced the cassette and chain, and other times I've only changed the chain. It hasn't seemed to have made that massive a difference either way - perhaps it depends on how perfect you expect the shifting on your bike to be! If you are racing, fair enough, it should be perfect but for most of us going to work by bike or out for some exercise at the weekend, a bit of clunk clunk and the occasional curse when it doesn't shift probably isn't the end of the world and it settles down as the new bits wear in. 

If your drive train does get really worn though it can shift unexpectedly, and that can literally throw you if you are, say, standing up and powering through traffic it is not good. I got the biggest bruise I've ever had once on my inside thigh after a mis shift like that threw me forward and the bike's stem went into my thigh. I fell off, but fortunately wasn't in traffic at the time!

Yes, get them both done . . 

There's a VERY interesting discussion on chains here: https://www.cyclechat.net/threads/chain-snapped.247687/

If I'd repaired a broken chain with the correct quick link then I wouldn't bother replacing it unless there was very obvious damage/stretching. So long as it's a peacefully sleeping dog, let it lie.

Chains don't stretch, they wear, so that the internal distance between links increases.  Cassettes and chain rings also wear but at a lesser rate than chains.  If you are a purist you can replace both at the same time, but most people go 2 or 3 chains per cassette.  The usual cue is that shifting is poor.  On inspection, a worn cassette will have somewhat crescent-shaped teeth.

If a chain has worn to the extent that it has broken, it's time for a change.  A new chain will fit a worn cassette badly, so best to change them both by the sound of it,

Replace both the chain and cassette.

Get a chain wear tool, this is the one I use

https://www.wiggle.co.uk/cycle/chain-tools/?g=25

When the chain stretches to 0.75mm replace the chain. You should get about 1000 miles use out of a chain so long as you clean and oil it after every ride. I typically replace the chain on my summer bike once a month. You’ll need a chain tool, I have one of these

https://www.wiggle.co.uk/park-tool-5-to-11-spd-and-singlespeed-chain-tool/

The best way to clean your chain is to use one of these with a bit of mild detergent or degreaser

https://www.wiggle.co.uk/park-tool-cyclone-chain-scrubber-cm-52/

If you do all of the above your cassette and chainrings should last 10,000-15,000 miles maybe more

if you’ve never replaced a chain before google how to do it there are plenty of YouTube videos. It’s a pretty simple thing to do but different manufacturers have slightly different coupling methods. It’s important to get the chain length right and the easiest way is to measure the new chain against your old chain and cut the new one to the same length. After you’ve done it a couple of times you should be able to change a chain in a few minutes and you would of learned some new skills which is good for the soul and saves you money at the bike shop 😀

You could do it in stages, replace chain, if no issues stop. Else replace cassette, if no issues stop, else replace chainrings. Stop.

1000 miles?! Bloody hell, I’d go through one a month some months!

Chain wear depends hugely on how you care for your bike and where and how much you ride.

For me over the last 26 years of MTB riding i've tended to find Chain-rings last around 4 cassettes, Cassettes last around 4 chains (this is for my XT stuff but this varies by how fancy materials are).

I live in an area with chalk in one direction, sand in another and clay soil in a third direction. If I ride on sand I always clean the chain and relube after - it kills them. Chalk and clay (commute) I tend to leave it if the rest of the bike is not too filthy but probably wash every third time out. Doing in excess of 2000 miles a year, generally I go through 4 chains (Normally 3 in winter, 1 in summer) and 1 cassette a year, but I keep a close eye on the chain and change them early if in doubt - I'm probably a bit cautious with it and spend a bit too much, but then I always have a reliable drive chain

Thank you all, it seems like a new chain is the way to go

Hey Lee, I'm guessing with that sort of mileage the climbing is still very much on a back burner. Give me a shout if that changes and you fancy a trip to Yorkshire. 

Depends on your definition of stretch but I took a worn chain and placed it next to a new chain of same no. of links and it was quite a bit longer.

Hi Ash, the last climb I did was nearly 5 years ago back in 2014 that happened to be a 7c FA, a kind of go out with a bang sort of thing! I switched from climbing to road cycling and now have weedy arms and massive legs. 

I normally ride between 250-350 miles a week however I smashed my collarbone into three bits at the end of Feb so normal activities have been put on hold. I had it screwed and plated back together 4 weeks ago and now have normal full range movement. I’ve been training pretty hard indoors but i’ll be back outside in May once the bone has meshed together a bit more.

Yorkshire sounds great but only on a bike. Maybe I could come over, fix your chain, and we both go for a ride around the Dales 😀😀

Cogs at the front, yes. You could always get the new chain and then put it round the chainring before taking the old one off. Providing it fits and doesn't sit on top of the teeth you will be fine to just change the chain/cassette!

Only mentioned it as I had exactly this happen to me, changed the chain and cassette and went from a usable bike with a jumpy chain to a bike where the chain didn't fit the chainrings and was useless until they were replaced...

A few years ago I seemed to be in the habbit of snapping chains, so I always carry a chain repair tool and a quick link, it's a 10 minute job with a youtube tutorial to help.  I wouldn't replace anything, just fix the chain, if it snaps again, replace the chain.

I'd say it depends on the actual definition of stretch not an individual's definition. What chains do definitely does not meet the definition of stretch. Stretching is a property of elastic materials. 

From my experience I change chains when the indexing on the gears starts to get worse and no amount of fiddling with the cable tension resolves it. Generally that means wear somewhere in the drive train or the rear derailleur has had its day. I have a chain wear measurer which is useful. I change the chain first. If the indexing is still off I'll change the cassette. If it's still off I'll change the shifter cable. Then I'll look at the jockey wheels. They wear as well (they go pointy and affect the running of the chain), but they're dead cheap and easy to change. Then I'll look at the rear derailleur. From my experience this seems to be the order of wear in the drive train. I've changed more chains then cassettes, more cassettes than jockey wheels, more jockeys wheels than rear derailleurs.

I've never had to change my chain rings or front derailleur. They wear at a far slower rate to chains and cassettes and it'd be the last thing I look at. And this on a cyclo-cross bike that is used a lot off-road. The front part of the drive-train is pretty solid in my opinion (I'm running Shimano Tiagra on that bike).

In your case I'd be changing the chain and cassette and then seeing of the indexing is ok. If not, moving on as above.

Good luck.

Eh?  Never heard of plastic deformation, or creep?

Neither of those things apply to a chain in this context though. It's wear in the pins etc that cause it to lengthen.