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Why does my new Dura-Ace chain skip only on one cog (21t)?

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Why does my new Dura-Ace chain skip only on one cog (21t)?

Old 08-16-10, 03:47 AM
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Autobus
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Why does my new Dura-Ace chain skip only on one cog (21t)?

Installed a 2011 Dura-Ace 10 speed chain ) to replace my stretched-out SRAM PC 1090-r, which was a very good chain, and now I get chain skip only when I'm in my 21t cog (Dura-Ace 12-13-14-15-16-17-19-21-24-27), which is a cog I rarely use, so it's not worn.

Very annoying. What could be the culprit? The Sram was the same length and did not skip.

Last edited by Autobus; 08-16-10 at 11:22 AM.
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Old 08-16-10, 11:22 AM
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Old 08-16-10, 12:18 PM
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Your 21T cog is most likely worn out, if the skipping occurs when pedaling with a lot of torque and the 21T is a frequently used cog.

There will always be one or two cogs that wear out first and it's only revealed when a new chain is installed. It's new-chain skip.

If that cog isn't frequently used, then is could be something else. Some people put up with the skipping since it can quit after the chains sees 2-300 miles of use. The other alternative is to install a new cassette and use it for only a few hundred miles and then put the old cassette back on. It will last the life of that chain and there's not need to change the chain at only .5% elongation, since the cassette will be trashed after using that chain.

Last edited by DaveSSS; 08-16-10 at 12:21 PM.
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Old 08-16-10, 12:37 PM
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Originally Posted by DaveSSS View Post
Your 21T cog is most likely worn out, if the skipping occurs when pedaling with a lot of torque and the 21T is a frequently used cog.

There will always be one or two cogs that wear out first and it's only revealed when a new chain is installed. It's new-chain skip.

If that cog isn't frequently used, then is could be something else. Some people put up with the skipping since it can quit after the chains sees 2-300 miles of use. The other alternative is to install a new cassette and use it for only a few hundred miles and then put the old cassette back on. It will last the life of that chain and there's not need to change the chain at only .5% elongation, since the cassette will be trashed after using that chain.
Thanks. Your theory is substantiated by this fellow Jobst Brandt who I dug up. This guy really knows alot of stuff!
Chain Sprockets do not change pitch when they wear, only their tooth
form changes. The number of teeth and base circle remain unchanged on
a worn sprocket. What changes is the diameter at which the lengthened
(worn) chain bears on the sprocket teeth, making wear pockets at a
larger diameter than a new chain requires. In practice, this amounts
to a change of pitch (or pitch circle), because the chain will no
longer ride in the original valleys between the sprocket teeth.

A new chain often will not freely engage a worn rear sprocket under
load even though the sprocket root diameter has the same pitch as the
chain. Under tension, the incoming chain rides in the pockets worn by
an elongated chain that are higher on each tooth (a larger pitch
diameter) than a new in-pitch chain requires. This wear occurs
because a worn chain rides high on the teeth. A new chain with
correct pitch cannot enter into engagement because its rollers ride in
the wear pockets made by a chain with a larger pitch.

.
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Old 08-16-10, 01:45 PM
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I should also add that Ti sprockets last half as long as steel, at most. I learned my lesson and never bought another cassette with Ti sprockets after wearing out two of them (19T and 21T) in only 4,000 miles.
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Old 08-16-10, 01:49 PM
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You wore the cassette too, but didn't replace it in spite of what you think
Gear [2.523:1 ]
where it sits is lowest practical chainline gear, on the big 53t ring...

but it is well off the chainline of the big front ring.

look for replacement parts.

Last edited by fietsbob; 08-16-10 at 02:53 PM.
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Old 08-16-10, 02:10 PM
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Originally Posted by DaveSSS View Post
I should also add that Ti sprockets last half as long as steel, at most. I learned my lesson and never bought another cassette with Ti sprockets after wearing out two of them (19T and 21T) in only 4,000 miles.
Wow, thanks for clarifying DaveSSS. I thought titanium lasted longer than steel while being lighter!

The Dura-Ace is the only cassette in the Shimano group that has titanium climbing sprockets and it's sooo much more expensive than Ultegra or 105.

Makes me mad that I shelled out extra $$ for something that only lasted two seasons....
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Old 08-16-10, 02:11 PM
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All these answers make sense except for the fact that the OP hardly uses this gear and it's unlikely to be the first one to wear out.

Frequently a new chain means that there might be a protruding pin or a damaged plate that rubs against the ramps on the adjacent gear... it sounds and feels like a bad derailleur adjustment and might only happen in one cog (at first, then it gets worse), especially under a lot of tension (which makes it hard to diagnose on a workstand). Could be that you just buggered up the install or got a defective chain.
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Old 08-16-10, 07:44 PM
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Originally Posted by Autobus View Post
Wow, thanks for clarifying DaveSSS. I thought titanium lasted longer than steel while being lighter!

The Dura-Ace is the only cassette in the Shimano group that has titanium climbing sprockets and it's sooo much more expensive than Ultegra or 105.

Makes me mad that I shelled out extra $$ for something that only lasted two seasons....
Ti would be more durable if it had a harder surface-coating like Ti-nitride. Unfortunately that may not work well due to titanium's higher flexibility than steel (the hard coating would chip/flake off when the underlying metal flexes).
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