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I replaced the chain recently - the new chain unships violently when I apply a moderate amount of torque, such as when setting off standing. As in, it comes off the big ring to the outside (i.e. away from the small ring). If I set off seated in the saddle pedalling very gently it's ok, but if I accelerate, it comes off. Similar to this, Chain slips off biggest cog under torque, but in my case the 'and then it's back to normal' doesn't manifest itself - the chain actually comes right off outboard of the largest chainring. Notes:

  • Have recently regreased bottom bracket, but is now back on correctly and no significant play
  • Old chain was at (or maybe past) the 1% stretch mark before I took it off
  • Chainline looks no worse than it was before
  • Chainset is an Ultegra R8700 34/50
  • Have checked the front mech is definitely not touching the chain (so it's not that that's throwing it off)

The chain appears to be meshing correctly on the chainring in the bike stand, it's just when I apply torque that it violently unships, which is obviously unacceptable from a safety perspective if nothing else.

The chainring looks quite worn, see below (it hasn't been changed in the 5 years I've had the bike)..... and the only thing I can think is that the old chainring wore along with the chain, and the fact I let the old chain get past 1% stretch has affected the chainring to the point where it can no longer support a new chain.

Nothing else has really changed. Yes, I took the bottom bracket off to regrease it, but have put it back on - as far as I can tell - correctly, and thus should operate just as well as it did before.

Will replacing the chainring fix the problem, or is this a symptom of a deeper malaise / different problem?

Old chainring

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    Is your new chain definitely the same lengths as the old one (in links - it will be shorter in normal measuring units)? Have you touched the limit screw? I'm thinking the cage might be too far out; combined with a marginal chainline that could lead to dropping. When you start out in the big ring, whereabouts in the cassette are you? Middle is fine but if cross chaining into a big sprocket the chain is pushed sideways.
    – Chris H
    Sep 28, 2023 at 9:39

1 Answer 1

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Put a fork in it - that chainring is cooked.

https://www.sheldonbrown.com/chain-wear.html

Your chainring has the classic "shark fin" tooth shape

enter image description here

(note the slope of the teeth on the sprocket in front matches the slope seen on the OP's chainring)

Your chainring needs to be replaced.

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    Yep concur. OP will very likely need a new cassette now, and possibly even jockey wheels too.
    – Criggie
    Sep 28, 2023 at 10:06
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    Interestingly the top jockey wheel is showing 'shark fin' characteristics, but the bottom one has got some definite flat tops left. The cassette is mixed - there are still some tops left, but they are definitely narrower on sprockets 2, 3 and 4, which makes sense as they will be the most used ones.
    – BenTaylor
    Sep 28, 2023 at 10:34
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    @BenTaylor I think that makes sense because the top jockey has some guiding responsibilities and potentially more stress than the bottom one which just takes up the chain slack. Personally, I wouldn't mess around and do one clean swap front to back, even though you could even get individual cogs if you only ever ride them and the others are really pristine.
    – DoNuT
    Sep 28, 2023 at 11:07
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    Is that one of the cranksets under recall? Might be happy days for OP.
    – throx
    Sep 28, 2023 at 22:29
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    It would seem it is one of the ones covered by the recall. I will consider getting an inspection from a local dealer - which if it fails and gets replaced, I will be make sure I keep the new chainring as a spare!
    – BenTaylor
    Sep 29, 2023 at 13:31

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