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I have a problem where my chain is not engaging on the big chainring. The chain is not touching the FD so, that is ruled out.

Is it possible that my chain is not compatible with my crank?

My chain is a 9 speed Shimano and my crank is an entry-level Shimano 3x crank. This only happens when I shift to the biggest chain ring, it does not happen on the smallest and mid chainring.

The chain and cassette are brand new, while the crankset has had 6 months daily use.

enter image description here

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    The chainring is worn out, possibly due to running it with a "stretched" chain for a long time. Commented Jun 8, 2019 at 3:48
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    Whats happening on the cassette (back cogs)?. Likely you need new chainrings, chain and cassette
    – mattnz
    Commented Jun 8, 2019 at 4:20
  • the cassette and chains are brand new....but my crank isn't....a little over 6 months daily use to and from work.... Commented Jun 8, 2019 at 4:42
  • @DanielRHicks I agree that the interaction between the chain and chainring looks a lot like what you'd get from an old chain wearing the teeth of the chainring. But the chainring itself doesn't look all that badly worn and it's only six months old. Is there something else it could be? Commented Jun 8, 2019 at 9:23
  • How many miles on the chainring? How many miles on the old chain before you replaced it? Which chainring do you run on the most? Commented Jun 8, 2019 at 11:53

2 Answers 2

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Based on your photo, thats a new chain on an old chainring.

The gap between adjacent rollers in the chain is called the "pitch" and is exactly half an inch for a bicycle chain.

Your chainring's teeth have worn so they're effectively further apart than a half-inch, so the new chain cannot mesh with it for the full length.

The proper answer is to replace the big chainring, but given you've called it entry-level then there's a good chance its rivetted together, not fastened with chainring bolts like a higher-level chainset. You may be up for a whole new crankset.

The other option is to ride it as-is. Drawback is that your chain will wear quicker and chainring has fewer teeth in contact, so it will also wear quicker. Eventually you'll suffer from chain slip, where you stomp on a pedal and the whole thing will spin forward potentially dumping you if not seated.

I'm guessing this is an entry level bike and you've worn out a cassette and chain on your commuting (Great work there btw) You might be able to pick up a used crankset on ebay/etc for cheap, or consider this a sign that you're now a committed commuter and plan/budget towards the next bike, based on what you know.

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    But if the chainring teeth just worn out, wouldn't the chain still fit but have an unacceptable amount of slack? Or are you suggesting that the teeth's structural forces have been so compromised that they actually 'bent' out of position? Commented Jun 8, 2019 at 20:31
  • @GregoryLeo Simple test - do you have this same "out of synch" look if you put the chain on another chainring? Generally, it takes a lot of riding to wear out a chainring, and it may work fine for years on the worn chain too - they bed-in together. But when you replaced the chain/cassette, it showed that one of them was out of spec. Visually looking at the chainring photo you can see they're curved like an old-time car wheel-well, the ones right at top of photo show it best. Try the chain on another (less worn) chainring, and see how that looks.
    – Criggie
    Commented Jun 8, 2019 at 22:33
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    So the teeth did shear out of position. It looks slightly like an escapement sprocket of a mechanical clock. Commented Jun 9, 2019 at 0:16
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Thanks for the very detailed inputs guys!

I did some further research into Shimano's official website and I found out that it really boils down to chain and crankset compatibility:

enter image description here

It turns out that I am running an HG 9-speed chain on my crankset which is only compatible on an HG/UG for maximum 8 speed chain.....guess i need to replace my crankset to an HG 9 speed compatible one.

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  • You didn't tell us you had upgraded the cassette from 7/8-speed to 9-speed. That's likely to cause problems if you have indexed shifting (which you almost certainly do). Commented Jun 10, 2019 at 1:25
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    The spacing of the links is standard for all numbers of speeds. The difference between 7/8 speed and 9 speed is the width of the chain, which will affect shifting but should not affect the chain fitting around the chainring. The diagnosis of worn chainring still holds. Commented Jun 10, 2019 at 2:53
  • Chainring doesn't really care about "speeds" at the back nor about the width of the chain. A replacement of chainring is required, but only cos its worn not because of the added gear in the back. Downside, since its riveted you can't just buy a chainring this time. Could be expensive, so shop around.
    – Criggie
    Commented Jun 10, 2019 at 10:44

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