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Old 05-17-12, 07:00 PM   #1
diverguy
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Chain jumps off chainring. Thoughts on the cause?

First time posting in this forum; I usually hang out in the road biking (41) forum.

I've been riding a Cannondale Synapse 5 for about three years. It has about 10,000 miles on it and is equipped with 105s from front to rear with a triple chainring on the front. I've changed the chain before, adjusted it properly, and it always shifted as smooth as butter afterward.

The last time I changed the chain (same brand as before), I followed the same procedure as always to adjust the Shimano RD. The only difference this time is that the chain will sometimes jump from the middle gear on the front down to the smaller gear. Typically, it will only do this when I'm shifting the rear from a small gear to a larger one, though sometimes it will happen when shifting in the opposite direction. The chain only jumps gears on the front, never on the rear, and only does this when shifting. Once it has settled into a gear, it stays there no matter what the load. I have noticed that when shifting the rear from a smaller gear to a larger one, the chain seems to flex more than it used to and I'm sure that's related to the problem.

I'm hoping someone here has some ideas on why this is happening. Is it possible that the last time I changed the chain, I could have miscounted the number of links and may have one more link in the chain than is optimal? Could that be causing the problem? Any other ideas?

Thanks. John
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Old 05-17-12, 08:01 PM   #2
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Sounds like your FD cable has "stretched". Try twisting whichever barrel on the downtube (usually it's the left one, unless the cables cross below the down tube) has the housing from the left brifter about half a turn to the left.

For the record, cables don't really stretch, it's more a function of things like ferrules seating fully on housings and maybe the cable slipping a tiny bit in the clamp at the derailleur.
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Old 05-17-12, 09:05 PM   #3
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Originally Posted by IthaDan View Post
For the record, cables don't really stretch, it's more a function of things like ferrules seating fully on housings and maybe the cable slipping a tiny bit in the clamp at the derailleur.
They do stretch, at two points in their lifetime; when they are new and getting adjusted, and when they are old right before they snap. So OP, for some cheap insurance, it's not a bad idea to get new shifter cables.
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Old 05-17-12, 09:14 PM   #4
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10,000 miles, maybe the chainrings are worn out. What does the tooth profile look like?
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Old 05-17-12, 11:50 PM   #5
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You may also have one tooth on the middle ring that is bent inwards. I've seen this on a couple of Shimano triple cranks lately (105 and Ultegra), and it causes the chain to derail from the middle to the inner ring.
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Old 05-18-12, 12:01 AM   #6
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I've seen this pattern on Shimano triples at the bike shop. Replacing the middle ring is the solution. Certainly go ahead and check for bent teeth first. As for the derailleur-related advice, it's a common misconception that the derailleur is responsible for preventing chain drop from the middle ring to granny, but except when you're in the process of making a shift, that task actually belongs to the chainring teeth.
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Old 05-18-12, 03:56 AM   #7
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Hmm... calstar, Chris_W, and mechBgon all seem to be heading to the same place. I do use the middle ring 95% of the time so if any ring is worn or has the potential to be bent, it would be that one. From a cursory visual inspection, it looks fine. Should I be able to visually tell? Is there a technique or test which will confirm this is the problem? Is replacing it just a matter of taking off the crank arm on that side, unbolting the large ring and swapping it out?

Thanks for the advice.
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Old 05-18-12, 10:16 AM   #8
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Quote:
Originally Posted by diverguy View Post
First time posting in this forum; I usually hang out in the road biking (41) forum.

I've been riding a Cannondale Synapse 5 for about three years. It has about 10,000 miles on it and is equipped with 105s from front to rear with a triple chainring on the front. I've changed the chain before, adjusted it properly, and it always shifted as smooth as butter afterward.

The last time I changed the chain (same brand as before), I followed the same procedure as always to adjust the Shimano RD. The only difference this time is that the chain will sometimes jump from the middle gear on the front down to the smaller gear. Typically, it will only do this when I'm shifting the rear from a small gear to a larger one, though sometimes it will happen when shifting in the opposite direction. The chain only jumps gears on the front, never on the rear, and only does this when shifting. Once it has settled into a gear, it stays there no matter what the load. I have noticed that when shifting the rear from a smaller gear to a larger one, the chain seems to flex more than it used to and I'm sure that's related to the problem.
I'm hoping someone here has some ideas on why this is happening. Is it possible that the last time I changed the chain, I could have miscounted the number of links and may have one more link in the chain than is optimal? Could that be causing the problem? Any other ideas?

Thanks. John
Your derailleur shouldn't need adjusting if you are only replacing the chain.
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Old 05-19-12, 12:47 PM   #9
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Turns out you guys were right on target. Today's ride took me right past a well-respected bike shop in Lakeland so I swung in. The rear was not adjusted right so he straightened that out. The center ring on the front is the one I use 95% of the time and it's pretty worn down. I ordered a new one and it should arrive in about a week.

Appreciate everyone's insight. Nice to have confirmation on what this guy said.
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Old 05-19-12, 02:29 PM   #10
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I see a lot of triples with the mid ring wasted and double 42/52(or 53) with the little ring wasted.

Use that big ring+ 3rd biggest cog some more. Big rings and cogs are harder to kill.
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