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On a ride yesterday I changed from my large to small cog on the front gear set and my chain instantaneously snapped. The snap occurred right on the point where the mech would have applied pressure. The previous day I had a gear service from a mechanic who now swears that this is pure coincidence but I would prefer opinions from people who aren't trying to save money and reputation. I was already considering returning my bike for a re-service as the outside of the front derailler was still rubbing the chain when in the smallest back cogs. Could it be a case of poor adjustment work that placed too much strain on my chain during the gear change (I was only in gear 3 on the back)? This was also the first time I used the small cog since the service

  • Where were you pedaling at the moment you changed cog? Where you going uphill and pushing hard, or were you on flat road without too much effort? – L.Dutch Sep 18 '17 at 15:16
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    A less-than-perfect derailleur adjustment combined with shifting under load can cause unusual/sudden forces on the chain. Worst case it can try to engage two chainrings at once and stick. How was the shifting before you snapped the chain? Was the chain changed at the service (it probably should have been but it's not clear)? – Chris H Sep 18 '17 at 15:28
  • The age of the chain (distance covered), the maintenance (greased/dry), the appearance (rust!) are certainly key parameters in this case. – Carel Sep 18 '17 at 15:51
  • The scenario you describe -- chain snapping during a front cog change -- is probably the most common case for chain breakage. As to what caused it, hard to tell -- could be an overly clumsy gear change with too much force on the crank, could be a worn chain, could be a worn cog, could be a botched splice in the chain. The service could only be a factor if they had badly screwed up the derailer adjustment or they had for some reason "broken" and rejoined the chain. – Daniel R Hicks Sep 19 '17 at 0:22
  • Have you ever used a chain tool to break the chain and then reinserted the same pin? Used to be okay with older thicker chains, but modern chains are thinner with less material outside the outer plate. Use master links or disposable pins if you have to break 9-10-11 speed chains for any reason. – Criggie Sep 19 '17 at 1:02
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A chain is in danger of snapping when it develops a weakness. It will fail when the force on it exceeds the strength of the weakest park.

The rider might exceeds the strength of a weakened chain through high-force pedaling stokes, when accelerating from a standstill perhaps.

Another way the strength might be exceeded is through a short but high 'shock' load on the chain, that might be induced by a gear change, but without very high pedaling force. Think of the 'clunks' you sometimes get when changing to a smaller sprocket in the rear.

What I suspect happened in your case is that the front derailleur imparted a lateral bending force on the chain at a weak point which in conjunction with pedaling force caused it to fail.

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There are a couple things that can be going on here.

It's possible to adjust a front derailer, or more specifically some derailer/crank/chainring/chain/shifter combinations, in a way where it can seem fine most of the time and doesn't have any obvious problems with the derailer position, but is prone to jamming the chain, which can in turn cause the chain to get damaged or break. It's a problem that for the most part has to be uncovered by test-riding post-adjustment and making further adjustments, usually to the derailer positioning, as needed on a case-by-case basis.

Another thing that can cause what you experienced is the chain breaking due to prior damage, which can include being rejoined in a manner that weakened it. When chain manufacturers say to only use their special connection links/pins and not re-press pins on modern chains the traditional way, what they're trying to prevent is the exact kind of breakage you experienced. I had my only chain breakage ever when early on in my cycling life I re-pressed a pin on my SRAM PC48 instead of using the Powerlink for some bad reason, and it broke very shortly thereafter during a front shift.

Finally, there is a limit to how much load a chain can endure while shifting, both as a habit and during single incidences. Pushing that limit can result in a break even without any other issues and regardless of what any mechanic did.

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