Shifting By Itself
 

Hi,

I am having 2 problems with my drive train. I am not sure if they are related.

1. The chain has repeatedly come off my inner chain ring when I am one of by largest cogs in the back. This has happenned a few times when I was climbing a hill. I checked and the front derailler appears to be adjusted properly.

2. The chain keeps wanting to move from the largest cog to the next largest, if you pedal backwards and, when you pedal foward, it goes back to the largest cog. If you keep pedalling backwards the chain catches on the derailler. When pedalling fowards, it sometimes goes goes to the next largest and then jumps back up to the largest.

My bike has been into the LBS several times and no fix. At first, I thought that a link was jammed on my chain or that the chain is twisted. LBS claims not.

I am riding an old Cannondale 3.0 frame with a 20-year old 8-speed Dura-Ace derailler. The chain is an 8-speed chain. The frame needed to be spread to take the 130mm hub, but I have never had a problem with it.

I am not sure what is happenning but would appreciate any help.

Thanks,

David

The first problem may be that you are cross chained. Your chain is traveling too far from inner ring to largest ring in the back. Maybe try shifting into your "granny" gear but shift up into a smaller cog in the back to help even out being in your small in front.
As for the chain jumping perhaps the derailleur hanger is bent slightly so when pedaling backwards the chain doesn't come off the casette straight and catches the rear derailleur. This should be an easy fix for a good bike shop that can straighten your hanger and tune up your shifting which would straighten out the chain and it wouldn't jump.

1 The low limit stop on the front derailler should prevent the chain coming of the granny gear when you are in the lowest gear at the back. However you can attach a dog-fang device to the seat tube that will help keep the chain on the granny gear (it works for me). http://www.probikekit.com/display.php?code=K1108

2 It is normal for the chain to come off the big ring when you pedal backwards because the front derailler guides the chain onto the ring at the top, only working when you pedal forwards.

I believe you've got this backwards--Largest ring in the back is directly behind inner ring in the front.

Don't pedal backwards.

since they have worked on it twice and you still have the problem, perhaps you could set up a time to have the mechanic go over it with you. Perhaps they are right and you will discover something new about a bicycle or maybe they will be able to zero in on the problem.