Having recently built up a 456 (using up some hardware from the parts bin) I have done away with the front derailleur, but I now have a rear derailleur that knocks up against the frame when I hit the bumps. (This seems to happen irrespective of the gear selection) I have placed a bit of spare tube to protect the frame. Question is:
a) Is this user error, due to not having setup the derailleur correctly?
b) My chain length is wrong?
c) I need to get a short cage derailleur?
d) This derailleur belongs in museum/the skip..

IMG_0160 by davidpday, on Flickr

IMG_0159 by davidpday, on Flickr

Adjustment screw on the back of the mech to make it hang more vertical

Shadow rear mech is what you need to pull it away from the frame…..

I’m looking just ahead of the derailleur mount bolt and I’m thinking I see the bit of metal that the adjuster screw (called the B screw, isn’t it?) pushes against. You need to unbolt the mech, turn that metal ring round until the screw is in contact with it, then mount the mech back on the frame. That’ll stop it being pulled so far forward and fouling the chainstay.

FWIW here’s my LX mech on the same sort of frame.

That looks very close, too, but I don’t remember it bumping. Good luck!

Makes sense. Looking at it again, it looks like that ring might be pushed around anti-clockwise, with the screw holding it in place, may well pull the mech back. Ill give it a try. Thanks.