Chain Problem

bazmcg

I'm in the process of rebuilding an old DH bike(Raleigh DHO). I've put on a new rear derailer (LX) a new shifter (LX) and a new chain (Sram PC991). When the bike is in the workstand the gears are working fine and the indexing is spot on but when I sit on the bike and pedal the chain jumps straight over to the largest sprocket and stays there, moving the shifters won't change it. At first I thought the chain might be too short so I added some more links but I still have the problem .It's bugging the hell out of me. Anyone got any ideas what the problem could be?

supersonic

What cassette is it, and how old? What speed shifter, do they match? Any play in the rear sus pivots? Mech hanger straight?

Just a few to get on with lol ;-)

ride_whenever

which type of derailleur? Could it be to do with cable getting stuck?

bazmcg

It's a LX derailer and shifter both 9 speed. The cassette is an old one but it looks ok.Everything is grand when the bike is off the ground, derailer moves fine. If the mech hanger was bent would the gears still work ok with the bike in the workstand? I'll check the rear pivots and see if there's any play in them.

supersonic

Often new chains do not like old cassettes, and is only apparent when pressure is on the cranks - however this usually causes skipping rather than jumps to the low gear. Is it a rapid rise rear mech? Could be losing tension in the cable - often the case when they are not bedded in fully. Check cable is long enough too through the sus movement.

bazmcg

It is a rapid rear mech. I'll check the cable length. It did seem quite short when I was fitting it.