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The crank on my 2016 Giant XTC Advanced (Shimano XT components) is slipping relative to the large chainring but not the small chainring. In searching for reasons many of the answers focus on chain skipping but do not believe this is the case.

I positioned the front wheel against a wall and then marked the chain and large chainring so I could see whether the chain was skipping relative to the large chain ring. I applied pressure on the crank using only my hand and the crank slipped. The marks on the large chainring and chain were still aligned and still in the same position so no chain skipping and no movement of the freehub.

I do not have the same issue when riding in the small chain ring and there is no movement between the large and small chainrings.

Any help is much appreciated.

REVISION I have to apologize, I took the crank out (twice) and realized it is almost impossible to have the crank move relative to the right chainring. I repeated the test previously performed and it showed that the chain was moving relative to the right chainring. I've included a picture showing the two black marks indicating the movement and the state of the teeth. Apologies again.

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    The bike has a bit of mileage on it but the chain has been replaced frequently and the rear wheel and chain are both brand new (never ridden). There is no play between the left and right chainrings not on the bottom bracket. The condition of the teeth on the right chainring is good. If I spin the crank there is no obvious buckling or miss-alignment of teeth.
    – Peter N
    Nov 25 '20 at 6:57
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    It was and hasn't been ridden on; as an FYI the bike hasn't been ridden since March but how relevant that is I don't know. In the original post I mentioned that I had marked the chain and right chainring. I positioned that top dead center and, after the crank slipped, it was still top dead center indicating that it wasn't the freehub. In any case the rear wheel and cassette are both brand new.
    – Peter N
    Nov 25 '20 at 7:46
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    The chainring has what is called shark teeth, very pronounced even. A classic sign of wear. Compare to a new item. Especially those around the 7-8 o'clock position.
    – Carel
    Nov 25 '20 at 12:49
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    Lacks clarity: Is the chainring slipping relative to the crank, or is the chain skipping on the chainring? I'm going to say most likely the latter, as the ring in the picture looks to be fairly worn, but it shows no scratches from slippage relative to the spider.
    – Daniel R Hicks
    Nov 25 '20 at 13:05
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    Don't be tempted to flip the chainring and "use the other side" sadly it doesn't work like that even if the ring were symmetrical.
    – Criggie ♦
    Nov 26 '20 at 3:31
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From looking up the specs I presume your crank is FC-M8000-2. If you're saying that the crank arms are moving while the big ring and chain remain stationary in space, and the rear cog didn't move, the only possible explanations are that either the interface between the spider and the right crank has failed, or the metal toothy part of the composite XT big ring is spinning relative to the fancy three dimensional carbon body of it. Do more marks and repeat your test to see which it is.

If it was the spider interface, the smaller chainring puts less torque on it so that could explain why it's not slipping on the small ring. But looking at pictures of the crank, it seems like the spider and the arm are contoured together such that the arm couldn't just spin free. So I'm guessing your chainring is all whirly.

If the chain is moving, the suspect becomes freehub slippage, which is about a million times more common.

Edit in response to picture and note that the chain is new etc:

Your chainring is worn out.

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  • Thanks for the reply. It is an FC-M8000-2 crank and the left and right chainrings are not moving relative to each other. I marked them with a pen and there was no indication of movement. In the original post I mentioned that I had marked the chain and right chainring. I positioned that top dead center and, after the crank slipped, it was still top dead center indicating that it wasn't the freehub. In any case the rear wheel is brand new. As an FYI the bike hasn't been ridden since March but how relevant that is I don't know.
    – Peter N
    Nov 25 '20 at 7:05
  • Purely from the picture on shimano's website it looks like the small ring is attached more directly as well. Combined with the lower torque and especially if the OP mainly uses the big ring, this would be much less vulnerable. It's also less vulnerable to external impacts
    – Chris H
    Nov 25 '20 at 7:32
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    I've revised the problem above and have to apologize, I took the crank out (twice) and realized it is almost impossible to have the crank move relative to the right chainring. I repeated the test previously performed and it showed that the chain was moving relative to the right chainring. I've included a picture showing the two black marks indicating the movement and the state of the teeth. Apologies for the confusion.
    – Peter N
    Nov 25 '20 at 10:07
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    Thanks so much. When I compare it to the smaller chainring, which I hardly use, the difference is glaring. I'll get it replaced.
    – Peter N
    Nov 25 '20 at 13:37
  • @Peter N These chainrings are cheap compared to even the chain. Try to replace them before they get to that state!
    – MaplePanda
    Nov 25 '20 at 19:25
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By far you have to replace the big chainring. This is waaaaay abused right now.

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Confirmed - the cause was the worn chain ring; as you all know. I got it installed today and the issue stopped.

Apologies again for the miss direction at the start; I'll do better next time. Thanks to all the contributors to this thread.

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    hey peter, on stack exchange you follow up by marking the answer that solved your problem with the green checkmark and possibly adding comments underneath it
    – Paul H
    Nov 27 '20 at 6:34
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    Thank you for the confirmation and closure. Now go riding, and wear out the replacement chainring too :)
    – Criggie ♦
    Nov 27 '20 at 11:47

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