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Whilst riding recently the cable slipped from the clamp on the rear derailleur.

I'm attempting to affect repairs by re-clamping the cable. However by pulling the cable tight I have lost the bottom 2 gears. If I then slacken the cable just a small amount I gain the bottom gears again but lose the top gear. I can't seem to reach a balance.

This is the first time I've had to fit a cable to a derailleur, are there any tips? Am I potentially doing something wrong? Do I have to be in a certain gear before attempted to re-attach the cable to get the correct balance?

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3 Answers 3

up vote 3 down vote accepted

This might happen if there is friction when the cable moves in the housing.

The definitive "sign" of it is whan you shift to larger cogs two or three times in a row (say, three consecutive shifts to a larger sprocket), but when you shift back the derailer doesn't move. This happens because when you shift up, you stretch the cable with a rigid lever (the shifter lever itself), but when you release the cable, due to friction, the spring in the derailer is not enough to pull the cable. Then if you keep releasing shifts, eventually the cable is lose enough for the spring to pull it. But then the cable is still "in the wrong position", and if you shift up again the derailer won't move since only the cable slack will be compensated.

I don't know if this is the problem, but if you have to do "double shifts" and can't get to the smaller cog without releasing too much cable (thus losing larger cogs), then I'd bet the cable or the housing have at least one frayed spot.

Other possibilities would be a bent derailer or derailer hanger, or the cable is wrongly attached as described by Armb.

Hope this helps!

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+1 Thanks for the answer. It may or may not be the problem but it's certainly improved my understanding. –  DMK Feb 5 '13 at 9:41
    
Given that it wasn't a crash that caused the original problem, a frayed or kinked cable jamming is much more likely than a bent hanger. –  armb Feb 5 '13 at 14:09
    
After re-clamping the cable friction did proved to be one of the problems. Thanks for your help. –  DMK Feb 13 '13 at 10:47

Are you sure the cable is clamped correctly? Having that wrong can change the amount the derailleur moves for a given amount of pull (in fact it's sometimes done deliberately to use mismatched components - http://sheldonbrown.com/drivetrain-mixing.shtml#alternate). Otherwise, make sure the cable moves smoothly through the housing and that the hanger isn't bent, then work through: http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur and/or http://sheldonbrown.com/derailer-adjustment.html

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Thanks for the links. They make for good reading, I've now got a much better idea of how to go about the repair. I'll let you know how I get on. –  DMK Feb 4 '13 at 16:31

My recommendation is to dismantle the cable and housing, clean and lubricate with dense teflon grease, or replace with a new one if necessary. Then use one of the hundreds of videos on youtube on how to setup the rear derailleur, normally they all start by the smallest sprocket in the cogset...

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