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New cassette & chain: Now highest gear skips
#1
I just replaced my very-worn-but-still-functioning 12-26 tooth cassette and chain with a brand new SRAM PG-950 11-28 tooth cassette and PC-991 chain. Everything works perfectly except it skips on the smallest sprocket (with the biggest chain ring) even under the lightest load (just turning the cranks with my hand while on a stand). Note that it does not "autoshift" to another sprocket -- it just jumps a tooth approximately once per revolution of the sprocket and remains in the same gear.

I checked and I'm certain that the pulley is properly aligned with the 11 tooth sprocket. So, for example, the limiting screw isn't preventing it from fully engaging. I've adjusted the cable tension barrel and B tension (99% sure I got those right). The chain length is correct (pulley centers at right angle with ground). As an experiment I tried removing links to make the chain extra short, but that only made a very slight improvement. I measured the chain to make extra sure it really is new. When I put the bike on a stand and turn the cranks slowly, I can see the teeth at the bottom having trouble engaging -- the chain rides up near the tops of the teeth for a moment before finally slipping all the way down into the proper position. What would cause this?
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#2
Maybe a stiff link in the chain where you riveted it?
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#3
I'll second Joe's thoughts, might be a stiff link. Check this video for some tips on dealing with it...

http://bicycletutor.com/stiff-chain-link/
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#4
Turn the pedals backwards and this usually shows up a stiff link.
You may see the dérailleur jump as the stiff link goes through jockey wheels.
[font=Trebuchet MS]Ride hard or ride home alone![/font]
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#5
Thanks for the replies. Well, it would have to be about 10 stiff links since it skips about once per revolution of the sprocket. The entire chain actually seems a bit stiff to me, but maybe I'm just comparing it with the old worn chain.

If I had accidentally mounted the 11 sprocket upside down it would be obvious, right? The spacing is the same as the others, so I was thinking that it couldn't be that, but....?

If I got the barrel adjustment wrong then the pulley wouldn't be lined up, correct? And the B tension adjustment shouldn't matter much for high gear, yes? What could this be? It's driving me crazy!
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#6
- You should not be able to mount sprockets wrong, the transition from Uniglide to Hypergliede fixed that (for Shimano + compatible, Campa: I don't know).
- The new chain will feel a bit different than old one, that is ok.
- The barrel adjustment should not affect the lineup of the deraillleur pulley vs. smallest or highest sprocket, the limit screws are used for that. A slightly misaligned derailleur can show these symptoms. Try adjusting the limit screw for highest gear...

Another source of these problems might be a bent derailleur hanger, though I wonder why the other gears seem to work...
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#7
(12-01-2009, 11:24 AM)Joe_W Wrote:  Another source of these problems might be a bent derailleur hanger, though I wonder why the other gears seem to work...

I haven't had time to check yet, but I think you nailed it, Joe. I've had crashes that have scarred the derailleur and I remember noticing that the pulley assembly was leaning in a bit. I think it's only a problem with the 11 tooth sprocket because there are fewer teeth to "hold on" to the chain. But, given how extreme the problem is with the 11 sprocket, it does seem like some of the other higher gears would exhibit at least some signs of the same problem. I'll try to check it tonight.

Is it common for local shops to have the correct tool for fixing this or is that something I should ask? If I pay for it to be done then I don't want them to use some funky method that only improves rather than corrects. I could clamp a vice on there and guess at it myself :-)
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#8
There is a do-it-yourself-method using the front wheel, though I have not tried it. Maybe somebody can comment on that. You put the bike in a repair stand, remove the rear derailleur and put the axle of the front wheel in the hanger. The wheels should now be parallel (check the correct alignment of the rear wheel before!).

I'm not sure how common the alignment tool is in bike shops. I think every shop should have one, I even thought about getting one for myself, but since this is nothing you have to do regularly and a good mechanic can check and realign in a few minutes (so it will be inexpensive), I decided against buying it.
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#9
It turned out to be the lockring! I must not have tightened it enough the first time -- Doh! Thanks everyone for your replies!
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