Snapped chain troubleshooting

MattDB

Senior Member
I was pulling out onto a main road today and stood up on my pedals to accelerate and my drive train seemed to lock and I had to freewheel back to the pavement. I thought my chain had come off the rings but one of the links had completely sheared in two.

I've not had this happen ever and got quite a shock. The chain has done a max of 700 miles and been cleaned and lubricated religiously every week. Where it's parked up at work it can get a bit of rain on it (through a gap in the roof above it) and there was some light surface rust.

I'm quite a heavy cyclist (15st) and lots of my riding is up fairly steep hills.

I'm obviously going to replace the chain as soon as possible and get going but just wondering if there's something I should be looking at adjustment-wise that could cause a chain to stick-lock-snap?
 

I like Skol

Hold my beer and watch this....
Is this the original chain on a new bike? I only ask because the original chain on my hybrid did a similar thing at only 400 miles and I have a theory that bike manufacturers buy up lots of grey market stock on stuff like that and sometimes get stiffed with counterfeit items. I would fit a new quality chain (I am very happy with the lower mid-ranged KMC chains) and see if everything is running straight and smoothly then put it down to bad luck.
 

ColinJ

Puzzle game developer
A friend of mine snapped the chain on her new mountain bike in less than 100 miles of riding. She only weighs about 7 stone and hadn't ridden in gnarly conditions. I fixed it for her. One of the outer plates had snapped. I just assumed that it was a manufacturing defect.
 
OP
M

MattDB

Senior Member
Is this the original chain on a new bike? I only ask because the original chain on my hybrid did a similar thing at only 400 miles and I have a theory that bike manufacturers buy up lots of grey market stock on stuff like that and sometimes get stiffed with counterfeit items. I would fit a new quality chain (I am very happy with the lower mid-ranged KMC chains) and see if everything is running straight and smoothly then put it down to bad luck.
Yes! Original on a new bike. Have just ordered a replacement. I went for a Shimano one.
 

Klassikbike

Well-Known Member
Shimano chains are good but only if the pins are pushed in just right. Also many open and close links again on Shimano chains which is not recommended to do and highly increases the risk of snapping a chain. been running my current Shimano chain for over 12.000 km and still looks good (Used for hard XC riding).
 

sgl5gjr

Senior Member
Location
Huntingdon
The Teen's MTB had a new chain after 200 miles or so..... it was marked "kmc" but clearly it wasn't an original as I replaced with a KMC Z series and the quality difference was noticiable. I'm guessing the bike factory were either hoodwinked along the supply chain (excuse the pun) or knowingly were complicit by using poor chains.
 

shadow master

Well-Known Member
Depending on the system you have you may experience slipping under load in the smaller sprockets,after fitting the new chain,8,9,10 speed systems are very sensitive to this as the chain and rear sprocket very quickly become a pair,and changing one without the other can lead to trouble, agree with avoiding the shimano pin system if your not experienced,shimano chain is fine but use a quick link from some one else,it was probably only the factory pin that failed on your old chain,and a link for 99p would have been fine....but hay ho.
 

I like Skol

Hold my beer and watch this....
,it was probably only the factory pin that failed on your old chain
Not sure what you are basing this statement on when the OP clearly states this....
but one of the links had completely sheared in two.
IMO, the OP's failure and the one I experienced are purely due to the chains being constructed from such a poor grade of steel that they are not fit for purpose. This is a totally separate problem to chains that come apart due to poor installation and the visual differences between the two types of failure are usually obvious. My chains short life of only 400 miles was characterised by atrocious rusting tendencies, premature wear (it was approaching 1% when it failed) and catastrophic failure.
brokenchain.jpg


I have not had similar poor performance from any chain I have used before or since this one and I am not talking about high end chains, just the normal low-mid price grey chain.
 
Location
Loch side.
A couple of issues here require clarification.
The word snap doesn't help to explain the mode of failure. A twig snaps when bent, but a chain rarely breaks like that. It usually fails by separation of the plate and pin or in rare cases like the brilliant photo above, breaks from embrittled material or in the case of chains with performated sideplates, break at the narrowest section of the sideplate and then usually just on the one side. The latter just about always happens because the user powered the crank whilst shifting up. This typically happens when the rider unexpectedly encounters a steep hill and is caught off-guard in the wrong gear.

Chains never break in the shear mode i.e. they never shear. This is because shear is a very specific term and the only place a chain can shear is for the pin/rivet to be cut off flush between the outer and inner plate from the force of the two sliding away from each other. This never happens, other things fail first.

So, this means we still don't know how the OP's chain broke and it is no use speculating. Only a photo or a clear description will help. Weight has nothing to do with it unless the weight was used to power the crank when caught in the wrong gear as described above. However, you don't have to be very heavy to break a chain in this way.

The eye cannot tell if a chain still "looks good" unless you are talking about beauty. You cannot gauge wear by looking at a chain, you need an instrument for that.

A chain that is worn to 1% elongation is past it's useful life by 100%. It should have been changed at 0.5% elongation.

KMC and Shimano don't make rubbish chains. Even the cheapest ones in their range are excellent. They are all made from the same steel, only the surface plating and thus looks differ. They are all prone to rust where it matters - inside, since the insides cannot be protected with plating or coating.

Chains comprise hundreds of small components, each which has to be made fast, precise and perfect, for the chain to be reliable and affordable. Individual components like the sideplate in the picture above, can fail. There is an adage about the weakest link and the weakest link in the picture above is a sideplate that embrittled, either by incorrect heat treatment or by electroplating. I cannot see what plating the chain has because the chain is covered in mud. This happens but it doesn't mean that all chains in that brand are bad. It warrants a supplier replacement.

The OP asked whether he/she can adjust something to prevent this happening again. Yes, make sure your gears run smoothly and quietly, that the limit screws are set correctly and that you are not shifting under power.
 
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DWiggy

Über Member
Location
Cobham
I have been using Shimano Ultegra chains, with a quick link to join and so far they have been trouble free and reliable so far(+100 mile per week in all conditions) I have also noticed that the Ultegra chains don't rust anywhere near as much as the 105, and only about £15 from Wiggle
Just clean and oil once a week, works for me.
 
OP
M

MattDB

Senior Member
Thanks everyone.

Apologies for incorrect use of the term 'shearing' I do recall a physics lesson from years ago now. I can take a picture when I get home but it's not dissimilar from the above i.e. it is the sideplate that has snapped (on both sides i.e I have 4 jagged pieces), but I don't think it was near the pin, rather in the middle of the figure 8 part.

It has been shifting pretty well, it's Sora and I seem to have to adjust cable tension quite regularly - very occasionally I think it's in gear and it will slip up or down on the cassette when I apply more pressure so perhaps this is what happened here.

I wasn't planning on changing the cassette as well this time but will look at this if it slips with the new chain. I will def not be cycling up any hills until everything is smooth with the bike on the stand.
 
Location
Loch side.
Thanks everyone.

Apologies for incorrect use of the term 'shearing' I do recall a physics lesson from years ago now. I can take a picture when I get home but it's not dissimilar from the above i.e. it is the sideplate that has snapped (on both sides i.e I have 4 jagged pieces), but I don't think it was near the pin, rather in the middle of the figure 8 part.

It has been shifting pretty well, it's Sora and I seem to have to adjust cable tension quite regularly - very occasionally I think it's in gear and it will slip up or down on the cassette when I apply more pressure so perhaps this is what happened here.

I wasn't planning on changing the cassette as well this time but will look at this if it slips with the new chain. I will def not be cycling up any hills until everything is smooth with the bike on the stand.
No need to apologise. It is just that when troubleshooting, diagnoses requires precise references. I would like to see a photo of the broken link. That would be interesting. It may even tell you something about your shifting habits, should it turn out to not be a faulty chain. Don't bother with the new cassette until your new chain tells you it is time for a new one. It will communicate this to you quite clearly by skating in the gear/s you use most often.
 
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