Bicycle Mechanics - Rear derailleur

Please could someone help me? I'm having trouble with the rear gears (Deore) on my Scott Tampico mountain bike. Basically I just can't get them to index properly. If i tighten the cable enough so that the chain will wrap the biggest cog, then I can't get the chain to wrap the smallest cog - and if I loosen the cable so that the chain will wrap the littlest cog, then I can't get it to wrap the biggest. I've tried slackening the cable and tightening it bit by bit, but I still have the problem just described. And also, more generally I just can't seem to index the larger cogs. If I tighten the cable so that I can get into a low gear (i.e. big cogs) then I have to do two or three clicks on the gear changer before it'll change down.

I've tried replacing the chain - didn't help.
The limit screws are set correctly.
I don't think anything is bent.

Any suggestions anyone? I'd be very grateful for your help.
Thanks.
Danny D

Investigate the cable housings. Perhaps grime is in them and causing hesitation. You can usually shift into larger rear gear and then without turning the crank shift into smaller (higher) rear gears. That will put slack into the cable so you can slip the cable housings out and slide them down to see what the cable looks like where it was inside the housing.

Often rust and muck (and water) can get into these spots. I ususally force WD40 into the cable and housing to flush out the water and then rub in a lube into the part of the cable that would be inside the housing.

If you find lots of rust then you will have to replace the cable (and the housing-segment where the rust was found).

Also clean and lube the pivots and spring of the derailleur.

If you don't have the right tool, have a shop check the derailleur hanger alignment. My guess is that's what's causing your shifting problem. If it's a little bit off, no amount of adjustment will make the derailleur work right. If it turns out to be OK, you have at lease eliminated that as the source of your problem so you'll know that you have to look at other things.

Please could someone help me? I'm having trouble with the rear gears (Deore) on my Scott Tampico mountain bike. Basically I just can't get them to index properly. If i tighten the cable enough so that the chain will wrap the biggest cog, then I can't get the chain to wrap the smallest cog - and if I loosen the cable so that the chain will wrap the littlest cog, then I can't get it to wrap the biggest. I've tried slackening the cable and tightening it bit by bit, but I still have the problem just described. And also, more generally I just can't seem to index the larger cogs. If I tighten the cable so that I can get into a low gear (i.e. big cogs) then I have to do two or three clicks on the gear changer before it'll change down.

I've tried replacing the chain - didn't help.
The limit screws are set correctly.
I don't think anything is bent.

Any suggestions anyone? I'd be very grateful for your help.
Thanks.
Danny DCable mounted in the correct position at the rear derailer?

Hi there.
Thanks very much for your response. Would you mind explaining what is meant by derailleur hangar alignment? (another question is - do the springs in the derailleur lose their springness over time? and should they be replaced periodically?)

Hi there.
Thanks very much for your response. Would you mind explaining what is meant by derailleur hangar alignment? (another question is - do the springs in the derailleur lose their springness over time? and should they be replaced periodically?)

Look at where the derailleur bolts onto the frame. Your whole rear shifting system indexes from that point. If that's off, even by a little bit, it's obviously going to effect your shifting. Now see how it hangs down from the rest of the frame? So if you ever fall down or drop your bike or bang the rear of your frame or your rear derailleur, that's the part that's most likely to get bent.

Regarding the spring thing, I've never done anything with derailleur springs. My experience has been that the bushings wear down and the derailleur gets real sloppy before the springs wear out.