Chain seems to jump in highest gear

When I am in any gear but the highest (i.e. the hardest to pedal) this doesn't happen but...

when I am on the smallest cog at the back and I am pedalling hard, something seems to jump every second or so - it feels like a tooth is missing on one of the cogs (although they are all there).

Can anyone suggest what might be causing this?

Many thanks in advance - this is my first ever post and I found this forum by googling for "cycle forum" - it was the #1 hit.

UK newbie

 

I had this problem too. I believe it is caused by a worn down cog and so when you are on your highest gear you are putting a lot of force on it and so when it gets to much for the worn out cog the chain rides up out of the teeth and so the cog just spins beneath. The more this happens the more the cog wares down and so the only thing is to replace the cog. However, in reality it probably means replacing all the cogs and the chain as the previous cogs and chain will all be worn into each other and so will not like the new cog.
This is what I did and it worked but if any one knows any better feel free to tell me.

Thanks
Calum

 

Highlandcyclist has you looking in the right direction if you have a well used driveline - either one that has many miles on it or one that hasn't been well maintained and is prematurely worn from misalignment or sand/grit. Do you run on your smallest cog more than the others? If so, could be more worn.

Has the smallest cog ever run smoothly? If it did and it has been a while since the system was tuned up, then it could be as simple as a rear dérailleur adjustment. You may need to adjust the "H" adjuster screw. The high-side adjustment screw sets the travel limit for the chain as it is moved onto the smallest cog. The objective is to center the chain on the smallest cog. If the limit screw is set too tight, the dérailleur cannot travel far enough to fully center the chain on the smallest cog. The chain can then "skip" between the smallest cog and the one next to it. If the limit screw is set too loose, the opposite can happen and the chain can jump the smallest cog and jam between your cassette and the frame.

How many front chain rings do you have? If you have a double or triple crank, and cross-chain (run smallest front ring to smallest rear cog) it puts a lot of lateral stress on the system. The extreme angle could also interfere with the chain seating properly on the rear cog. Cross chaining - either small-to-small or big-to-big - is very hard on the chain and driveline components ... not a good thing to do.

Hope it is just a quick and easy adjustment. Good luck!

 

I would assume that the lowest cog is probably the least worn. the other cogs would be likely worn more as they are ridden more and the chain has worn to them. When it gets to the less worn cog it has trouble seating properly. Anyway, that is my thought. In this case all you can do is get a new chain and cassette (these do wearout eventually).

But you can play with the barrel adjustment on the rear derailleur as well to see if you can find a good compromise. Here is a link to how to adjust it..

Park Tool Website

 

Also check for debris, chain wear, bent derailleur and lubrication.

 

I'm going to second (or is it third...?) the suggestions regarding the worn cog. Had an old chain that was skipping and had several tight spots. Replaced it, skipping disappeared in all but one scenario: when I was cranking hard whilst in my highest gear. I since purchased a NOS freewheel, installed it, and haven't had a problem since.

Happy tinkering.

 

Mine will do it when the cogs on the derailleur get dirty. Clean them and it stops.

I seem to get fuzz from some where, haven't figured out where yet?? Socks?

 

Wear on the cog
Chain too long/loose
Dirty cassette
Bent derailleur/bent mounting bracket

 

I had a similar problem a few years back, the problem was the front chainring which was seriously worn so when sprinting out of corners it would skip

 

Since it only happens when you're on the small cog in back, you should look at your rear derailleur limits. It sounds like your outside limit isn't, uhm, outside enough. It sounds like the limit is set such that the chain is trying to shift up to the next bigger cog.

 

What he said.

All good suggestions and possibilities, but you want to tick them off in a systematic way – starting with most specific and least expensive.

Cleaning a dirty system is super easy and super cheap. More of a general issue than small cog specific, but something that should be done anyway.

The “H” limit screw is a free fix (or very inexpensive @ LBS) and is almost 100% a small cog related adjustment. If that’s what it is – you’re done.

Barrel adjustment could be needed too, but you can change the indexing interface of all the cogs with that adjustment, so go easy. If your driveline is heavily worn, good chance adjusting the barrel adjuster will throw off the “wear pattern” of the chain and other cogs. You may end up with a smooth running smallest cog and start skipping on the others. If so, smile and take it in stride … it could be time to replace some stuff.

You didn’t say how many miles you have on the existing driveline. If not at least a thousand or more, then you shouldn’t have a lot of wear in the system (unless you’ve let it get gritty). As noted, it isn’t normal for the smallest cog to be most worn, but it can happen if you run that cog often and the chainline has been out of adjustment. On many cassettes the smallest cog is separate – ease of replacement will depend on the cassette you have now. The manufacturer might offer replacement cogs, or you might be able to salvage one from a bike buddy with a parts bin. New cassettes run the gamut - $20 to $200+

If you have the tools to do your own maintenance, it never hurts to have an extra chain or two in your parts bin, a couple of cassettes, and some chainrings. These parts will wear eventually and need to be replaced … just a matter of time. Or take it to your LBS, get professional service and help the cycling economy.