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Two answers to this question gather that after 14335 km (8907 mi) of cycling with a single chain, the cog is at risk of being damaged.

I haven't noticed anything unusual apart from the chain being too long. How do I tell if my cog needs replacement?

My rear cog on a Rohloff Speedhub

different view
Different angle, close-up

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It would be a bit easier to tell the shape of the cog if we had a straight-on picture. – Daniel R Hicks Feb 10 '13 at 23:39
@DanielRHicks - Agreed, and a closeup of the teeth would be even better. – Neil Fein Feb 11 '13 at 0:19
I'll upload another photograph as I get home tonight – gerrit Feb 11 '13 at 12:14
It's definitely worn, and there is some barely-perceptible hooking. But it's probably fine, with a new chain. – Daniel R Hicks Feb 11 '13 at 22:59
I'm going to get a new chain soon in any case, but am hoping to postpone bigger maintance to autumn, when I will be much closer to specialists that I trust to work on my bicycle and do things I can't do myself. – gerrit Feb 11 '13 at 23:01

3 Answers 3

The sprockets (apart from the 13 tooth) are reversible, so you can get twice the life out of one by taking it off and using the other side of the teeth. From the photo it looks to me as if it's only worn in one direction so far, or at least significantly more worn in one direction.

(From my experience with a 3 speed hub, a hub gear sprocket can be badly hooked without causing problems when run with an equally worn chain, long after the point where a derailleur wouldn't shift properly. I'm not recommending that you let it get that worn though.)

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Actually, on the rear a new chain does not present a problem with a hooked sprocket. In fact, much the opposite -- a new chain can "rescue" a hooked sprocket. It's the front where, with a new chain on a hooked sprocket, you can get "chain suck". – Daniel R Hicks Feb 12 '13 at 16:18

A quick way, is to just have a little look.

If a new chain meshes nicely and there are no funny sounds or any skipping when it turns, then it's good to go.

Sheldon Brown has some examples of worn cogs and how to spot them here.

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That cog is barely worn at all. I wouldn't bother flipping it until the tip of the teeth is about 2mm, and then I'd ride it until the tips were breaking off. Specifically, that's what I actually do and I have a cog bolted to my workshop wall to remind me what "slightly too worn" looks like. The problem is that the chain starts to skip or hang, because the teeth are so short the chain will easily ride up on or over them.

worn rohloff cog

When I replaced that cog I did not replace the chainring, just the chain.

The problem with leaving it until too many teeth have broken off is that it can be hard to get a chain whip on to remove the cog.

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Yep, that's slightly too worn alright! – andy256 Oct 9 at 9:00

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