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Old 09-26-06, 02:53 PM   #1
mikemck
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Chain skipping on the casssette

Here's my setup...

I have Shimano SIS 6 speed shifter, Shimano 7 speed hyperglide cassette, and a specialized 8 speed hub.
The bike shifts great and everything is good there, but it skips in every gear, some being worse than others.

I'm thinking it's the 6 speed chain and I need to change it out for a narrower 7 speed chain. Does that sound correct?
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Old 09-26-06, 03:05 PM   #2
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You've got a number of issues here. First is that 7-speed cassette has the cogs spaced more narrowly than a 6-speed freewheel or cassette, and 6-speed Shimano shifters are designed for a 6-speed freewheel or cassette. Which means that if you line the derailler's increments up so that the chain is perfectly in-line while on the middle cog of the cassette, it won't be perfectly lined up as it gets toward the largest and smallest cogs. This may cause the chain skipping, although it would make more sense if your chain skipped on the larger or smaller cogs but not in the middle.

(I assume you're using a spacer to mount the 7-speed cassette on an 8-speed freehub body. Otherwise the cassette can't be fastened down tightely and could slide from side-to-side.)

Current chains designed for 5/6/7/8 speeds are fine for your situation. Back in the 80's there were "narrow" (for that era) chains designed for 6-speed sprockets. These would be slightly too wide for a 7-speed cassette where the cogs are more closely spaced.

Another possibility is that the chain and/or cassette are worn-down. Teeth get shaped like shark-fin profile when worn down, and the chain starts to skip. Get a chain checker (I trust this a lot more than measuring with a ruler) and see if the chain is worn. If so, you may need to replace it, and will probably need to replace your cogs if the chain is worn. If you're going to replace cogs and chain, you may as well get 8 (or 9 or 10) speed cassette and shifters to match. They'll fit on the hub that you're currently using.
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Old 09-26-06, 03:17 PM   #3
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I read a recent rec.bikes.tech posting by jobst brandt and he explained that chain skipping is caused by using a new (unstretched) chain on worn out cogs... good explanation of the mechanics involved.

If you're having shifting problems, that's a different kettle of fish.

I know chains don't actually stretch, but let's just pretend for this one post, OK?
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Old 09-26-06, 03:28 PM   #4
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Actually, chains do stretch, at least functionally-speaking. It's just that no individual pieces in the chain's system stretch; the pins wear down by friction, but that contributes to the chain's elongation on a link-by-link basis.
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Old 09-26-06, 04:23 PM   #5
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I picked up a cheapo Bell chain tool at Walmart, and removed one link from the chain. It stopped skipping on all gears exept 4, which is the third cog on the rear.
I went to remove one more link, to see what that would do, and the chain tool broke completely in half....
The LBS is closed or I would have bought a real tool to begin with.

I'll go to the LBS tomorrow, and return the Bell tonight.

I plan to eventually change out the 6 speed SIS shifters, just have not got around to it yet.
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Old 09-26-06, 04:34 PM   #6
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Quote:
Originally Posted by mikemck
I picked up a cheapo Bell chain tool at Walmart, and removed one link from the chain. It stopped skipping on all gears exept 4, which is the third cog on the rear.
I went to remove one more link, to see what that would do, and the chain tool broke completely in half....
The LBS is closed or I would have bought a real tool to begin with.

I'll go to the LBS tomorrow, and return the Bell tonight.

I plan to eventually change out the 6 speed SIS shifters, just have not got around to it yet.
Your chain needs to be long enough that the rear derailleur does not reach it's forward limit of travel when the chain is on the largest chainring and largest cassette cog. Not that you would intentionally use that "cross-chain" gear combination but if you were to accidentally put it there you could seriously damage the derailleur and even the frame.

Al
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Old 09-26-06, 05:56 PM   #7
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Quote:
Originally Posted by Al1943
Your chain needs to be long enough that the rear derailleur does not reach it's forward limit of travel when the chain is on the largest chainring and largest cassette cog. Not that you would intentionally use that "cross-chain" gear combination but if you were to accidentally put it there you could seriously damage the derailleur and even the frame.

Al
Ah, excellent point. Did not even think of that.

I'm looking now to buy a 7 speed shifter to see if that cures the problem for me. I'll replace the chain as well, since I've screwed with it now. It was pretty old anyway.

Thank you everyone for all the help, it is much appreciated.
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Old 09-26-06, 07:16 PM   #8
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Quote:
Originally Posted by mikemck
I'm looking now to buy a 7 speed shifter to see if that cures the problem for me. I'll replace the chain as well, since I've screwed with it now. It was pretty old anyway.
Seriously, if you're going to buy new shifter, why not just go with 8-speed? You can pick up an 8-speed cassette from Nashbar or Performance for around $20. And it'll fit on your hub, certainly.
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