need help with rear derailleur

dylbert

Hi all, hopefully someone can help me.
When I have the chain on the small chainring and two largest cogs the derailleur is rubbing and skipping off the rear cogs.
I had a read about this and found out I need to tighten the B screw but this is not solving the problem. Is there anything else I can check before I head to the bike shop?

Cheers.

cyclopath2001

dylbert wrote:

Hi all, hopefully someone can help me.
When I have the chain on the small chainring and two largest cogs the derailleur is rubbing and skipping off the rear cogs.
I had a read about this and found out I need to tighten the B screw but this is not solving the problem. Is there anything else I can check before I head to the bike shop?Cheers.

Adjustments vary from derailleur to derailleur, but you should look for a screw that governs the travel towards the spokes. Too loose and the chain goes into the spokes, too tight and you won't be able to engage the larger cogs.

Other possibility is with the control cable tension, maybe.

This all assumes nothing is actually broken or bent.

Did these positions work before & when did they stop working?

dylbert

It's not the limit stop screws that are the problem and the derailleur moves through all the gears. When I use the two lowest gears (small chainring with the two largest cogs) the top guide cog on the derailleur is contacting with the theeth on the large cog, if that makes scence.....

BeardyGit

"
Angle adjustment ("B-tension")


Modern derailers have two spring-loaded pivots. The lower pivot, sometimes called the "a pivot" winds the cage up to take up slack as you go to smaller sprockets. The upper "b pivot" adds additional slack take-up ability by pushing the derailer's parallelogram backwards. The tension of the two springs needs to be balanced for best shifting.
Most derailers have an angle adjustment screw (Shimano calls it "B-tension adjustment"). This adjusts the tension of the upper ("b") spring of the parallelogram, and thus the height of the jockey pulley. The looser this screw is, the closer the jockey pulley will be to the cluster.
The angle adjustment will need to be set according to the size of the largest rear sprocket. If you change to a cluster with a larger or smaller low-gear sprocket, you will need to re-adjust this setting. You will also need to adjust this if you change the length of your chain.
If the angle adjuster is set too loose, however, the jockey pulley will bump into the largest sprocket when the bicycle is in the lowest gear (large rear, small front). This is the gear you should check the adjustment in.
Since a derailer shift is caused by forcing the chain to run at an angle, the greater the angle, the sooner it will shift. The closer the jockey pulley is to the cluster, the sharper the angle will be for a given amount of sideways motion of the derailer. Thus, the looser the angle adjuster screw is, the better the shifting will be."



See the pictures etc here:
http://sheldonbrown.com/derailer-adjustment.html

dylbert

I've tried this already, have the B-tension screw tightened fully but this wont solve the problem. Might just take a trip to the bike shop...

BeardyGit

If it's a new chain, it could be too long. That'll do the exact same thing.