I have a road bike, 10 speed 11-25t cassette, and I just have a question in regards to replacing the smallest cog.

I am attempting to replace the 11t cog on my shimano 10 speed cassette 11-25t, because when I'm on highest or smallest cog it skips. It makes that sensation or it feels like your shifting from the 9th to the 10th gear(from the 2nd smallest to the smallest cog) every so often, like the chain is getting caught on one of the teeth and lifts up and then lets it go.

I got my bike just recently from a guy who was selling it, I had it tuned up from a local bike shop and they replaced the chain they said that it stretched already and needs replacing. Could that have been the problem, a different chain? The chain that they replaced it with came off from one of the 10 speed bike, that they said was a take off and was hardly used.

should I replace the chain with a brand new one or should I replace the 11t or the 12t cog or both? And if I do replace the 11t or 12t cog, can I replace it with any 11t or 12t cog that I see on e-bay, whether it be for a mountain or road, shimano or not? Or is there a specific 11t and 12t cog for the shimano 105 10 speed cassette 11-25t? If there are specific 11t and 12t cog for the shimano 105 10 speed cassette 11-25t, then can anyone tell me which specific ones I should buy or which ones are compatible for it?

Is it slipping under load or just during normal pedalling or both. Does it slip on any of the other gears? normally when you replace a chain you are up for a cassette as well otherwise it slips/skips in certain gears, if the chain is replaced in time the theory is you wear out 2 chains before you replace the cassette. Considering you dont know the history of the bike you might be better off getting a new cassette and starting the cycle afresh, to your original question if you decide to purchase individual cogs(i wasnt aware you could do this although miche used sell them) then they must be ten speed shimano or sram, mtb or road no dif

I think a more likely issue is that the der. limit screws are set incorrectly or a cable tension problem. I would check those first before doing any part swaps.

First thing to check on any cassette with an 11 tooth small cog is that the lockring is made for 11 tooth cogs. This is easy to miss. A lockring designed for 12 or larger tooth small cogs is too large in diameter for an 11 tooth cog and will cause exactly the symptoms described

^^THIS^^ Don't ask me how I know. The cassette lockring should have either '11T' or '12T' or something very close to that printed on it to tell you which small cog it's designed to be used with.

As stated several reasons why chain/cog skip can happen. Go through each possibility. Then report back for the next guy. Andy

Thank you for replying. They just slip only on the smallest gear and just on normal pedaling when riding the bike, if its upside down when I'm working on it, it does not slip at all ... I have a mountain bike 8 speed that have a cog that is 11t I switch it and it did not work, the sprocket was a bit big and it maid the problem worst...

Thank you for your reply, I will check on that and I will let you know what happens

thank you for your reply, I think it has the proper lock ring for it, because I have tried a lock ring that was for a 12t cog and it just kept on skipping.. the skipping that it does on my bike is not consistent like every revolution when pedaling it does it after 3 rev. or 5 or 7 but never pass 10 rev. of pedaling.

Thank you for your reply, I am constantly checking what could be the problem. Just a while I took the wheel off and checked the cassette and notice on the smallest cog that two of the tooth were rubbing against the chain, so that could be the problem..

I agree with this. It could also be a bent derailleur hanger, I think.

It's a very rare rider who manages to wear out the 11T cog while the rest of the cassette is OK. The cog wears faster, but most of us use it a lot less often.

UPDATE: Just awhile ago I took off the wheel to inspect the cassette and noticed on the smallest cog that two of the tooth were rubbing on chain more than the others. Two of them were cleaner than the others so it seems like they are miss aligned. I will to replace the 11t cog with a new one and see what happens. My only concern is that I buy most of my stuff from ebay and i don't want to buy the wrong part.

Thank you for your reply, I got the bike used so i have no idea on what the history was. All i know is that i had it tuned up after i got it, and the only thing that they replace was the chain, the chain that they gave me was off another 10 speed bike they said was still in great shape. So i really don't have any clue on what the bike's history is.

This is how the bike looks when I got it, 105 frnt derailleur and ultegra rear d. I got the bike for $275, and now it has mavic ksyrium elite wheels silver that i got for $125.