Now thats what I call yet another "Rear Derailleur problem"

[wev]

Looking around, seems there's a lot of issues with Rear Derailleurs.

Anywho, I recently had to change the rear wheel on my partners bike, after having done so I took it for a test ride last night only for the rear wheel to completely lock up on me. Somehow the Derailleur had caught in the spokes and span round. I fixed that by twisting it back and bending the hanger/derailleur back into place. However now when I change gear the derailleur slips forward and the lower cog presses against the frame losing all tension in the chain which slips off onto the axle.

I've read various guides all of which mention a H and L screw and some mention a B (?) screw, plus the little plastic twist grip thing attached to the cable. The derailleur is a Shimano, beyond that I can't find a model number, but the bike is a cheap "Argos" job. Tightening/Loosening either the H or L screw makes no difference nor does altering the tension in the gear cable with the plastic twister and I can't find the third screw on the derailleur.

Any ideas? (I believe my other half took pictures if they'd help).

[X-RAY]

what do you mean by "changing the rear wheel"? pleas post pics also.

[wev]

The bike was second hand when we bought it, the rear wheel appeared to be buckled when we got it home so we replaced it.

Pic of the derailleur

[trevgbb]

The Rd is completely mis aligned, using your pic as a reference, loosen the allen headed screw which attaches it to the drop out and pull it back until it is nearly vertical. there should be a small nib of metal on the back of the mounting plate which has to align with a similar nib on the drop out, do this then retighten, you will then need to re adjust the RD as shown in the videos, but, it looks to me as though the mounting plate for the barrel adjuster is bent, you may be able to straighten this if you are careful.

You will also need to assess if the RD is bent, if it is, can it be straightened or do you need a new one?
Oh, and have you damaged the wheel?
And is the gearing the same as the previous set up?

[wev]

Yes, the derailleur is misaligned, the bit with the Shimano logo keeps springing forward when the gear changes, I've literally realigned it (although not matched the nibs up as I'd forgotten about them, will rectify that though) and tightened the allen headed screw (head to go out and buy a new set of allen keys as the one I needed has seemingly vanished into thin air and my local DIY store doesn't sell them individually) back to as tight as I can get it and that seems to stop it springing forwards.