Rear derailleur issues

[clonmult]

Occasionally have an issue with the rear derailleur on my mtb.

Hope this makes sense - instead of pointing "backwards", it will jump forwards, and loose all tension, total pita. Easy enough to push back in place, but it has invariably done this when I've been making my way up a hill, foot slips on pedal, knee hits bar .... ouch.

Any suggestions as to why? Its an old Shimano derailleur, one of their lower end models (can't remember the model #).

[mat205125]

Not sure of the cause, but I've suffered a few chain slips in my time ...... and wear the DMR imprinted shins as my badge of honour.

[beyond rational]

Sounds like there is something wrong with the spring or the B Limit screw is faulty

[rico]

To be honest, if the mech is playing up, I'd just replace it.

Chuck the old one on ebay saying "used and abused" and some moron will pay a few quid.

[mat205125]

They really are that cheap that it ain't worth messing about with, I agree.

[kieranbennett]

look here for a good replacement;

www.chainreactioncycles.com/Models.aspx?ModelID=1186

[Moose.]

Same one I've got one mine and it's annoying the hell out of me at the moment. I've taken it off twice now and cleaned it, but within one ride it goes stiff again, refusing to go down through the gears. The whole mechanism is held together with pins which cannot be removed, so it's impossible to clean fully. Perhaps I need a stronger spring inside it to pull it back down through the gears again?

Short story of the matter, I'm probably going to change it soon I think!

Edited by Moose. on Friday 9th March 14:12

[drewcole81]

After spending the last weeks riding through bog land and lakes my gears are now Focked! But taking them apart and giving them a nice bath and scrub always does the trick for me.
I found that the lower end of the market stuff will die quicker as you'd expect, i'm running with XT stuff so its pretty good.

The best thing to do is, if you only have the low end market rear mech, i'd simply replace it, a like for like will cost you £25 - £35. Go for a Deore or Deore LX. But you must replace the cable at the same time.

'WITH OLD DIRTY CABLES, COMES RUBISH BALL CRUNCHING SHIFTING!'

[Parrot of Doom]

If the mechanism is moving forward, that means the chain is pulling it forward, which means that either of the jockeywheels are jamming, or the chain is catching against something.

Are you sure the angle between the front cog and the rear cassette isn't too great? Thats usually what causes the chain to jam.