Rear derailler "too small" for new sprocket set

[eddie_matos]

Hi all,

I decided to have a go at replacing my sprocket set this week, but I'm quite inexperienced in some of the finer intricacies of bicycle maintenance and have stumbled across a problem.

The new sprocket set is significantly larger then the old one and it appears that the rear derailleur (Shimano Sora) is struggling to cope. Basically the guide pulley (as Wikipedia told me it was called!) is so close to the larger sprockets that it is pretty much resting on them. Even though the gear changes are fine, the result of this is that the rear derailleur bumps up and down with the teeth of the sprockets.

It seems to me that the derailleur would be able to cope fine if it could just hold the chain out with a bit more 'strength'. So I spent some time looking for some way perhaps to tighten the spring inside it, and found a screw which looked promising. Unfortunately I tightened it fully to no avail.

I'm certainly one for opening things up and trying to solve problems from first principles but a mistake early today meant I spilt hub ball bearings all over the floor - so I'm looking for a bit guidance on this one if anyone could be so kind!

Thank you very much for anything anyone can do to help.

Ed

[cyclerUK]

Sora derailleurs are designed for "road" cassettes with a limit of 28 teeth on the largest sprocket. If you have anything bigger than this then you will struggle to get it to work properly. The screw which sets the guide pulley height is known as the "B" screw and is found at the back of the derailleur.

See Sheldon Brown site for info:
http://www.sheldonbrown.com/deakins/lowgears.html

If this is the screw you have adjusted, and it's screwed completely in, then 2 options. Revert back to a smaller cassette or fit an MTB range rear derailleur. MTB derailleurs will go as high as 34 tooth sprockets.
(If you have fitted a larger cassette then you may also have to have a fractionally longer chain as well.)