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Old 08-07-08, 08:14 AM   #1
dquattlebum
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Chain Slipping/Skipping when on middle gear(39) of crankset.

I hope I'm explaining this right so it makes sense. I got the Cannnondale Synapse 7 with the Crankset Shimano Sora, 50/39/30 and a rear cog of SRAM PG-950, 12-26, 9-speed, It seems when I'm on the 39, middle gear, on the crank I get a kind of slippage/skip on the chain when I'm pedaling. The chain doesn't fall off, but its almost like its missing teeth and having to re-catch the gears. If I'm on the big wheel of the front crank its perfectly fine, no slippage. I don't do much riding on the small one, but from what I've tested it works fine.

When I first got the bike it was riding fine after about 100-120 miles it started doing this action, I brought it in to the bike shop I bought it at and they did a full tune up on it and the problem went away. Now I've gone another 150ish miles and its starting to do it again.

Is this normal? I don't ride really hard, but I do ride often. I'm starting to do longer and longer rides so having this issue pop up every 120-150 miles could be annoying, unless its common and I need to learn how to adjust it.

Sorry for the long winded post, hope it made sense.

Thanks in advance.
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Old 08-07-08, 08:28 AM   #2
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try adjusting your cable to the front dérailleur. there should be a adjuster at the shifter turn it 1/4 of turn a time until it shift crisp, and that should solve your problem.
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Old 08-07-08, 10:24 AM   #3
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I don't see how a front derailleur adjustment could correct chain slippage.
Can you tell if the chain is actually slipping on the 39 ring or if it is slipping at the rear of the bike when on the 39 ring?
Chainrings on road bikes usually have very long lives, like 10's of thousands of miles, but it is possible to wear one out.
If the chain slip is actually at the rear of the bike you could try adjusting the shift cable at the rear derailleur. Try a 1/2 turn, or so, counter-clockwise.

Al
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Old 08-08-08, 08:12 AM   #4
dquattlebum
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I'm fairly certain its slipping on the front 39, at least when I'm pedaling and I'm watching it happen its looks like that's where its slipping.

I forgot to include some more specs that may help. My Shifters are Tiagra, my front derailleur is Sora, Rear is Tiagra.

Should I make the adjustment on the derailleur or the shifter?
I was going to drop by my bike shop today on the way home, they handle all my adjustments for free, if I can't figure it out.

I really appreciate the info.
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Old 08-08-08, 08:55 AM   #5
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Tiagra is fully indexed up front so adjustment can help with chain skip. if a little adjustment doesn't solve the problem, you might try a new chain. some are more prone to skip than others. Also, when shifting the front, make sure you 'pedal soft' until the shift completes. That said. i had an old cannondale touring bike that started skipping after i did my spring tune up. drove me nuts for a while. did a little adjustment and it went away. I know that doesn't really make sense, but the fd was just taking the chain to exactly the wrong place when on the 4th cog. one little tweak and no more problem.
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Old 08-08-08, 11:57 AM   #6
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If this happens only when you're in the middle ring in front and one of the smallest cogs in back (a favorite combination for a lot of new cyclists), you've probably worn the rear cog to the point where it's skipping under load. Check on your next ride to see whether the skipping happens with middle front/middle rear combinations. If not, then the problem is almost certainly a worn 12- or 13-tooth cog in back. Very common.

Last edited by Trakhak; 08-08-08 at 12:07 PM.
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Old 08-08-08, 12:10 PM   #7
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Here are some other suggestions:

It is abnormal for a front derailleur to go out of adjustment that quick -- new cables are the exception cause they stretch for a while.

Can you hear or see the derailleur rubbing on the chain when shifted to the middle chain ring ? If so, it does need to be adjusted.

It is possible that the chain has a bad link which "kinks" under the lighter load that the middle chain ring places on it as compared to the large chain ring.

You may have bent tooth on the chain ring, although that can't explain why it only starts to happen after a few hundred miles.

Most of the these 'difficult' to solve problems are caused by multiple things that by themselves are not problems, but added together stack up to be a bigger problem.

It is a very, very good idea to learn how to make adjustments on your bike yourself. I quick google search on derailleur adjustments should give you a starting point. Also, you can download tech documents from the Shimano USA website.
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Old 08-08-08, 12:33 PM   #8
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Very unusual to slip on chainring teeth. I've had rings worn down to just 2mm height and the chain still runs smoothly on it without slipping. A worn chain will slip on the rear-cogs well before it slips on the chainrings. And in that case, it'll slip using the big-ring as well.

Does sound like the FD may be pushing slightly on the chain and it may be catching on the big-chainring slightly.
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Old 08-11-08, 07:34 AM   #9
dquattlebum
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Morning - Update.

So it seems that both the FD and the cable needed adjustments. The cables was a little stretched and that contributed to the FD needing adjustment.

After working with my bike mechanic, and him showing me how to do it, he reminded me that when I had this problem before it was on my RD and the cable for that. Apparently it took longer to stretch the cable and everything. So he went through a full adjustment on the bike, since I hadn't had one since I bought it. Now its running smoothly.
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