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After my chain has broken what should I know about buying and maintaining a new chain (apart from lubing it and replacing when it 'stretches')? Do you keep a spare chain, or a spare link?

I have a commuter bike (a Kona Dr Dew) with Shimano dérailleurs.

The first chain was replaced "worn to at least 100%" after the first 6 months/5000 km. The cassette seems OK and not worn. This was the second chain, replaced two months ago. I noticed occasional slipping gears but don't know whether that was the chain, or the indexing of the gear shifter.

The chain (the replacement chain, which broke) is an SRAM PC-951 which Google says is "our most economical chain".


Update - it wasn't the removable SRAM link, it was one of the factory links that broke. Because it had been skipping and the previous chain worn more than 100%, I asked to replace the chain and the cassette (even though he agreed that the cassette didn't look really worn, he said 5000 km is too far for a chain); and got a chain stretch measuring tool, if replacing the chain every 6 months isn't often enough. Anyway it's tight again now. The new chain is a more expensive SRAM PC-991 (which cost more than the new SRAM 950 cassette). The original parts I think were all Shimano.

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Slipping gears would suggest worn cogs, and that can cause a chain to get sucked into a knot. –  Daniel R Hicks Nov 2 '11 at 15:29
    
@DanielRHicks - I've changed the cogs now. –  ChrisW Nov 3 '11 at 1:44
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7 Answers

up vote 4 down vote accepted

To keep your chain from breaking, keep the derailers properly aligned, ease off of pedaling when you shift, and don't "stomp" on the chain when it's not fully in gear. (Folks who don't go back to the non-indexed days probably never got into the habit of coasting while shifting, but it wouldn't be a bad thing to practice a bit.)

Broken chains are exceedingly rare, and are almost always the result of putting load on the chain when it's not fully in gear. The other likely source of damage is having sprockets that are so badly hooked that the chain gets sucked into a knot.

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I've broken chain only once. When I install it badly. –  Crowley Nov 2 '11 at 11:30
    
Is it possible that there is/was a loose link on the chain? Which I was supposed to notice and fix during maintenance? –  ChrisW Nov 2 '11 at 13:37
    
Does "exceedingly rare" mean that you don't carry the means to fix it on the road, but count on it not happening? –  ChrisW Nov 2 '11 at 14:52
    
"Exceedingly rare" means that it's never actually happened to me, in tens of thousands of miles, but I have seen it happen several times to others in my group (who generally were behaving like gorillas and pedaling hard as they shifted). I did come close this past summer when a worn chain and front cog combined to suck the chain into a knot. But luckily I stopped pedaling instantly and avoided further damage. –  Daniel R Hicks Nov 2 '11 at 15:38
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Yep, when it jammed the chain had probably been sucked by a hooked sprocket or some such and gotten itself into a knot. Or else you got the chain crossed in the derailer. Chains can bend up/down but they do not like to bend left/right. –  Daniel R Hicks Nov 3 '11 at 11:44
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I've snapped a chain twice.
Once the chain was very heavily worn and it snapped at one of the factory links, the second time it was also heavily worn but it snapped at the Connex Link which I may have re-used (I can't remember if I did or not). Both times I was heavily on the pedals, taking off from the lights, and it is not a pleasant experience.

Based on this I'd recommend
1. Get a chain wear indicator. (Sounds like you have one now) and check the chain regularly and definitely replace it before the chain wears out.
2. Don't re-use master links. Replace them when you replace the chain. Also note that some can be installed the wrong way (The Connex link is like that, make sure it is installed correctly).
3. For chains with a connecting pin, don't break the chain in the same spot twice.
4. Use a proper bicycle chain lubricant. Things like WD-40 can just dry out and prematurely wear your chain.
5. Don't continually break and re-connect the chain. Use a chain scrubber if you want to clean it, rather than breaking the chain too often. Although I've given up on the chain scrubber now, I just wipe the chain down with a rag, lube the chain, then wipe off the excess and ride. Doing this after rain helps keep rust off too.

I carry a compact chain tool and a Connex connecting link, just in case. But I'm more vigilant about checking my chain wear and replacing the chain now, so I shouldn't need it.

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Bump for saying wd-40 isn't the way to go for lube. The stuff just doesn't last. –  krs1 Nov 3 '11 at 13:16
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Above all else, if you've broken multiple chains in a short period of time, it might be your riding style.

Rarely does a chain break in a straight line, being pulled. The energy is fairly dispersed across the links on the top of the chain while pedaling normally, and even under all out efforts, this is the case. When it is NOT the case is when you are shifting, or using extreme gear combinations like big front big back or little front little back. While shifting, the force exerted is temporarily focused to a few links - especially as the chain moves from one cog to another. In extreme "cross chaining" (extreme gear combinations mentioned above), the force is sometimes unevenly distributed to the links at extreme bending angles left to right, a recipe for snapping chains.

It's a little hard to describe but when an experienced rider shifts under hard effort (hill climbing, sprinting) they will either intentionally or habitually "let up" on the pedals during the shift. This has two effects, the first being lack of snapping chains, the second being more responsive shifting. The best shifting setups, adjusted perfectly, don't shift awesome under load, but letting up a bit always helps.

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Before anything else, ensure that you are properly securing the master link as this is the most common point of failure.

You should replace your cassette when you replace the chain. The chain and cassette wear fairly evenly. If one is worn out so is the other, and running a new chain on a worn cassette will wear the chain faster.

If the gears are slipping then the cassette may be worn out and/or the derailer(s) are broken and/or in need of adjustment. The derailer hanger might also be an issue.

The SRAM pc-951 is an extremely common chain and in the dozens I've put on I haven't seen any snap on their own after such a short period. It's probably not quality of the chain, and something in your drivetrain is damaged or in dire need of adjustment.

There are a lot of points of failure in a drivetrain; go to a shop, a good one will take a quick look for free.

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You're right. I meant chain, fixed mistake. –  krs1 Nov 2 '11 at 18:20
    
Replaced the cassette now. –  ChrisW Nov 3 '11 at 1:44
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Right now I am doing the opposite: wearing a chain out until it snaps. This is on a 'beercycle' bike that is kept indoors so rust is not a problem. What I know from the experiment is that, so long as the join link is properly assembled and the chain kept rust free (although not extensively oiled) you can go many thousands of miles past the 'stretch' stage even if the sprockets end up as a 'box of neutrals' (to use an automotive term) with only selected gears actually working.

So, in summary, make sure the link is assembled and the chain is rust-free. Oil really is an optional extra, as half the bikes on the road demonstrate.

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You said "the link": so do chains have one link, which must be kept assembled? So on a chain there's one link in particular to look out for? –  ChrisW Nov 2 '11 at 13:16
    
This is for a new commuting bike. I want it to run well all the time. –  ChrisW Nov 2 '11 at 13:25
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Yep, the chain has to get on there somehow. Research chaintools and connector pins (for Shimano chains) and the SRAM Powerlink (for SRAM). –  ʍǝɥʇɐɯ Nov 2 '11 at 15:27
    
What do you mean by "even if the sprockets end up as a 'box of neutrals' "? Am I missing something here? I assume you're not advocating leaving the worn chain on a bike. –  Neil Fein Nov 2 '11 at 18:35
    
It was one of the factory links that broke. –  ChrisW Nov 3 '11 at 1:50
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All the other answers deal with old, worn chains. I'd like to talk about another angle.

I've only had it happen once, while I was stomping up a really steep hill. It was a brand new 9-speed SRAM chain. It snapped at a link that I took apart and reassembled while installing it; I guess that I didn't put the pin in correctly.

Somebody told me that this kind of mistake is easy to make with the modern narrow chains - the plates are so thin, that there's very little room for error when installing the pin. And for this reason, you should never try to reassemble the links; when installing the chain, just keep removing links until it's the right length.

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Whatever it is I do I guess. I've never had one break. I don't even worry about stretch. If it's dry, I oil it, but otherwise I leave well enough alone. I don't even bother to clean my chains.

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Of course, not replacing your chain when stretched will destroy the sprockets, and eventually your shifting performance will deteriorate or chain suck will become a problem and you'll have to replace the entire drive train. It's a choice. –  Daniel R Hicks Nov 2 '11 at 11:50
    
I figure by that point that I'll want a whole new drive train anyways. Of course, with my luck, that's not been an issue yet either. The newer bike has a nice expensive drive train that's lasting forever, and my dirt cheap beater bike that's a billion years old and with about as many miles on it is also lasting forever. Obviously if you've got something broken it should be fixed, but I really question a lot of the tribal wisdom in regards to preventative maintenance. I wonder how much of it is really just to make the rider feel better for having done it? –  Brian Knoblauch Nov 2 '11 at 11:57
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It's up to you. But a bad drive train is like the proverbial frog in a pot. It slowly gets worse and worse without you really noticing until you're suddenly left in the middle of a 100 mile ride with no gears. –  Daniel R Hicks Nov 2 '11 at 12:37
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This is a new bike. I commute on it daily and want it to run well. –  ChrisW Nov 2 '11 at 13:23
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