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Old 07-22-08, 05:50 AM   #1
dsm iv tr
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Chain dumps off largest chainwheel in high gear

The title is incorrect. It is meant to say "largest chainwheel".
I tried to edit the title, but no luck. This is what happens when you post in the morning...

Hey, folks!

This is a bike with a 7-speed freewheel, the wheels are true, and I've readjusted both der.'s from scratch to be on the safe side. Shifting otherwise works fine on both f/r der.'s.

Shifting to the largest chainwheel from anything above 4th gear in the rear causes the chain to jump off the largest chainwheel to the outside instead of shifting properly.

This started happening after the bottom bracket was replaced. I think, though, that the BB length is not quite right and thus is causing the chainline to be way off - and consequently dump the chain on the outside of the chainwheels when it's located on the smaller sprockets (which are obviously further over than the larger ones) at the back.

Right now I have the front der. high limit screw turned in enough so it's only able to shift to the granny + middle ring. The chainline is okay for commuting use in 5 of the 7 rear sprockets. It's rideable, but a little.. slow.

Any second (third, fourth..) opinions on what could be going on?
Is there a way to measure proper spindle length for a particular frame?

Thanks!

Last edited by dsm iv tr; 07-22-08 at 03:11 PM.
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Old 07-22-08, 08:40 AM   #2
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Quote:
Originally Posted by dsm iv tr View Post
Hey, folks!

This is a bike with a 7-speed freewheel, the wheels are true, and I've readjusted both der.'s from scratch to be on the safe side. Shifting otherwise works fine on both f/r der.'s.

Shifting to the largest chainwheel from anything above 4th gear in the rear causes the chain to jump off the largest chainwheel to the outside instead of shifting properly.

This started happening after the bottom bracket was replaced. I think, though, that the BB length is not quite right and thus is causing the chainline to be way off - and consequently dump the chain on the outside of the chainwheels when it's located on the smaller sprockets (which are obviously further over than the larger ones) at the back.

Right now I have the front der. high limit screw turned in enough so it's only able to shift to the granny + middle ring. The chainline is okay for commuting use in 5 of the 7 rear sprockets. It's rideable, but a little.. slow.

Any second (third, fourth..) opinions on what could be going on?
Is there a way to measure proper spindle length for a particular frame?

Thanks!
Your limit screw is set too far out on high gear. It allows the chain to be moved too far outboard and to fall off. You may want to check the alignment of the FD also. The nose may be turned too far to the right allowing it to bump the chain off.

Check Park Tools on how to do the adjustment.
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Old 07-22-08, 10:18 AM   #3
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What he said. Here's the link to the Park Tool site for instructions on adjusting the rear derailleur.
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Old 07-22-08, 10:30 AM   #4
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Hi dsm iv tr,
I had a simular problem on a 7 speed and checked my F&R deraileur adjustment and alignment several times. A friend told me to change the chain and problem solved. After inspecting the chain it wasn't stretched very much but it had quite a sag from side to side. In the end I think the chain was whipping sideways as I shifted.
I would still find it odd that your trouble started after you did the BB.
Good luck!

Last edited by cyberpep; 07-22-08 at 11:58 AM.
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Old 07-22-08, 10:51 AM   #5
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Measure your chainline to be sure. Center of seat-tube to center of large-chainring should be around 42-45mm.
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Old 07-22-08, 02:55 PM   #6
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I'll check the the H limit screw on both der.'s again. I suspect, though, that If it were an issue with the H screw at the front, it would fall off when shifting to the large chainwheel no matter what gear I'm in at the back. Same for the back H screw, except it would fall off no matter what chainwheel I was using.

I've also readjusted both der.'s from scratch again. Same problem.

I'll measure my chainline and post when I get back from my ride.

Last edited by dsm iv tr; 07-22-08 at 02:58 PM. Reason: oops
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Old 07-22-08, 03:08 PM   #7
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I just realized that I posted "largest freewheel" instead of "largest chainwheel". My bad. Is there a way to edit the title?
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Old 07-22-08, 04:31 PM   #8
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From your current H screw adjustment, I'd slowly back it out until it just barely allows a shift to the largest ring.
Also inspect your chain for stiff links. That's probably NOT the problem, but only takes a few seconds to check.
Just slowly rotate the crank backwards and observe the RDER. It'll noticeably "twitch" when a stiff link passes through.
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