Hi all,

My chain failed yesterday on a bike I bought just 3 months and 200 miles ago!

I cleaned it up and found that aside of the busted link, there were SIX other links with easily visible fractures. It is a budget Decathlon mountain bike (ST530), 9 speed with a single chain wheel. I've looked into the Z9 KMC chain and it gets on the whole good reviews so I guess they aren't failing every 200 miles for everyone!

I've emailed pics to Decathlon and KMC but just wondered what your thoughts are. I don't want to just stick a new chain on it - certainly not a KMC - without understanding what could have caused it.

Thanks in advance. Matt.

Sounds like a warranty claim before you go and buy a replacement. Cracked links seem to come up every once in awhile with different mfrs. I can't answer why but KMC are as reliable as anything out there but sometimes they let a stinker get through QC.

There is nothing systematically wrong with KMC chains. I've had good service from them and they also make some of Shimano's chains. Apparently you got a, presumably rare, defective one.

If I was Decathlon I would be sending you an apology, a new chain cut to the proper length and the quicklink most rikki tic. Every KMC I've used has been great.

Here is a thought. The bike has a cheap Miranda narrow/wide chainring, and you didn't put the chain on correctly, and it got chewed up.

I cannot think of anything on the bike which would have caused cracked chain links. I would not hesitate to put a new chain on and ride it.

Back around 2005 Wippermann had a bad batch of stainless chains that developed cracks in the sideplates around the rivets. I learned about it because a friend of mine had one, and the bike shop replaced it under warranty.

https://www.mtbr.com/threads/wipperm...e-junk.373092/

https://forum.slowtwitch.com/forum/?post=557143

Yep, you aren't going to "get to the bottom" of this, stuff happens. Put on another chain and keep an eye on it.

Yes, a definite possibility; however I'd think it would be obvious that the chain was not meshing with the chain ring. Perhaps the assembler forced the chain on spreading the links and the chain went downhill quickly with use. But then, why were only a few links damaged?

How badly would you have to get it wrong to do that kind of damage? The off road hammering I have given my chains when short of lube and adjustment in my time, and never have they broken that quick.

The wide pin on a narrow-wide ring could easily spread the narrow chain link,

I've only seen anything like that one time. My wife's bike that her sister was riding (so plenty of blame and shame for me!). Bike was bought used and sat in the garage for several years. When the chain broke, there were many other side plates which were cracked and ready to fail. New chain (whatever was in the nearest bike shop) fixed the problem.

Did you clean the chain in aggressive degreaser. Some may cause embrittlement and several chain manufacturers warn against certain degreaser.

A 9-speed chain is about the widest chain one would be running with most 1x setups . Most narrow-wide chainrings are stated to work with 9/10/11/12 speed systems, so I'd think a 9-speed chain would be the safest chain, at least in terms of the risk of a wide tooth spreading chain links, right?

Thanks for all your replies. To address a couple of them, I used a popular bike specific lube on the chain manufactured by WD40 - no deep cleans or anything out of the ordinary. The chain felt like it was meshing fine.. as I type this I'm wondering could there be a mis-match between the chain wheel and the chain? If the chain wheel is steel and the chain links are softer stainless, and the teeth of the chain wheel are fractionally too far apart for the chain...?


t's just a thought. I really don't know but I'd rather not just accept it as a freak occurence. If I just stick a new KMC Z9 chain on there I'm basically saying that I put the failure down to a rouge chain - but why so many links over so few miles?


No answer back from vendor or KMC as yet but I'll post on here if I hear anything. Thanks again for your input.

KMC will point you in the direction of a UK Distributor (Raleigh, Bob Elliot, MooreLarge or Chicken Cyclekit are some) and they will handle the warranty

Pretty sure the chain is not stainless.

I understand and sympathize with wanting to know what went wrong. The reason most companies and individuals just replace the chain instead of performing a detailed failure investigation is cost. Unless you or a close friend are a metallurgist with access to a failure analysis lab you can use after work hours, that failure analysis is going to cost big bucks -- five or six figures.

On the other hand, you can buy a super-duper great replacement chain for double digits.

KMC makes all the Shimano chains, except Dura Ace. They don't want some other company to have their name on the best chain they make.

What happened to you is very, very, rare. Just replace the chain and go ride. Chains are cheap.

Any pictures of this? post the picture online and then post the link here.