I'm on an old Muji 3-speed commuter bicycle which uses an internal Shimano gear hub, and I recently (3 months ago) got my bike chain changed by my LBS.

For about 2 weeks I felt the chain would "catch", the way it feels when you're trying to change gears on a bike with derailleurs, and last night on my way home, the chain broke. Is this because the chain didn't fit well?

  • Why was the chain replaced? Commented Nov 20, 2014 at 1:58
  • Three months?! The chain was either incorrect type, faulty, or incorrectly installed. LBS owes you a new chain. Commented Nov 20, 2014 at 2:04
  • Can you post a picture of the failure? Did it fail, for example, at a quick link?
    – Batman
    Commented Nov 20, 2014 at 3:41
  • The chain was replaced because at the time, the tension of the chain was quite loose and I asked the guy if he could increase the tension, and he said the chain needed to be replaced because it couldn't be tightened any further. But I managed to get my bike back to the LBS just now, and they fixed the chain for me. I think the guy replaced the part which connects the 2 ends of the chain (sorry, I'm not very well versed in this).
    – huijing
    Commented Nov 20, 2014 at 6:19

3 Answers 3


3-speed drive trains should naturally have a straight chain line and lots of room for the chain, which is typically wider. So I doubt that the chain is too wide.

I suspect the LBS did not correctly insert the chain pin, when reassembling. If this was the case the pin could be protrude on one side and potentially catch on various drivetrain components. Continually catching will stress the chain. If the protrusion is significant enough to catch, on the opposite side of the protrusion the pin may not be sufficiently inserted into the outer link plate. Eventually the catching could cause the partially inserted pin to pop out of place, breaking the chain, leaving you stranded. Check where the chain broke to see if the pin at the break point is reasonably flush to the outer link plate or protruding by a large amount on the side where the pin is still in place.

Speculation Warning

This is of course rampant speculation, as I have no pictures to go off of, but is the only logical mechanism that comes to mind. (And it is something I have actually done and can verify can happen!)

  • Based on what the LBS did for me to fix the chain, I think what you described is close to what happened. The guy at the shop couldn't give me a direct answer when I asked him why the chain broke, but he did fix it by replacing the part of the chain with the chain pins (I think that's what he did).
    – huijing
    Commented Nov 20, 2014 at 6:22

When you change the chain you have to change the sprocket also. They wear at an equal rate and a new chain will not fit an old sprocket. The front sprocket is usually ok because the load and wear is spread over a greater area.

  • Mismatched wear between chain and cogs/chain rings will not cause a chain to break.
    – mattnz
    Commented Nov 20, 2014 at 23:03
  • Yes actually it will. If you pedal hard and the chain skips a link it puts an excessive load on the chain and it can pop a pin.
    – user19614
    Commented Nov 20, 2014 at 23:50

Chains usually only break when lateral force is applied to them. To have one break on a single speed drivetrain is fairly rare. I'm gunna give your LBS the benefit of a doubt and assume they installed the chain correctly. The problem with changing just your chain is that it's only one piece of a system that wears as a unit. As your chain wears it actually gets longer, this is a result of the friction from the pins taking metal off of the inside of the bushing in each link, which lengthens the measurable distance between each pin.

enter image description here

As a result, the cog starts to wear on the pull side of each tooth.

An example of an excessively worn cog:

enter image description here

When you put a new chain on a worn cog they won't necessarily mesh. The new chain will have some "wiggle room" to slide around in and possibly even slide laterally enough that one of the sharpened teeth can pick up a link, which on a single speed can put enough force on the one side of the link to pop the pin out.

My question for you would be, how worn was the cog? If the cog is fine, then how loose was your chain? A loose chain can just as easily jump the teeth and break.

  • 1
    Then the LBS is still negligent for putting a new chain on a worn drivetrain.
    – Rider_X
    Commented Nov 20, 2014 at 17:37
  • @Rider_X - I totally agree, but I wouldn't be surprised if the LBS didn't have a replacement part. Old 3-speeds weren't exactly standardized. I worked in one shop that had a lot of old parts from back in the 40's-50's. They mostly just accumulated dust, but every other shop knew we had them, so if anyone brought them an old bike that you couldn't order parts for anymore, they always sent them to us. Some 3-speed hubs had a cog which integrated with the internals, and the only way to swap it was to disassemble the entire unit. Replacing the whole hub may be the only option for fixing this bike.
    – ShemSeger
    Commented Nov 20, 2014 at 18:23
  • Mismatched wear between chain and cogs/chain rings will not cause a chain to break.
    – mattnz
    Commented Nov 20, 2014 at 23:02
  • @mattnz - Yes it will, it's not common, but I've fixed more than one.
    – ShemSeger
    Commented Nov 20, 2014 at 23:12
  • 1
    From what you guys have discussed, I think my case highly likely due to this mismatch of chain and cog, because even now there is still some slight catching from time to time. But there's also not much I can do for it because replacement parts for my old bike don't seem to be in supply these days.
    – huijing
    Commented Nov 21, 2014 at 5:34

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