Bicycle Mechanics - chain jumping on cassette (before + after new cassette + chain)

I just bought a used Jake the Snake - 9sp 105 STI shifters plus Deore XT rear derailleur. The chain was jumping quite a bit on the cassette especially around 5th + 6th smallest rings. I replaced the cassette and chain but it hasn't made much difference. The front shifter looks well used. Is it possible it's not holding the tension properly?

Suggestions?

cheers
Euan

Well it sounds like adjusting the derailleur is in order. Directions for setting the above and below limit screws - which in turn sets the chain location for each gear on indexed systems - are an easy search on the web in general and this forum in particular. And if the cassette and chain needed replacement your derailleur cable should be changed out as well before you bother adjusting the derailleur. It's not as hard as you might think and is a good thing to know. Once your new cable stretches you'll be doing it again. Good luck.

DanO

Jumping how? Is it jumping on the same sprocket (skipping teeth) or jumping between gears?

If it is jumping between gears you generally don't need to touch your low and high limit screws for indexing adjustment. That only adjusts the furthest towards the wheel (low) and furthest out (high) the derailleur will travel. Instead there is generally a barrel adjuster on the derailleur that aligns the indexing. Get the rear wheel of the bike up off the ground and adjust the indexing barrel until it shifts into each cog smoothly without jumping out.

That solution assumes that it is a problem with indexing though. I had a problem with my Jake the Snake jumping between gears occasionally, and usually only under hard load. If the derailleur cables aren't run right you may run into problems with cable friction pulling on the derailleur as the frame flexes. One of the first suggestions I found on Sheldon Brown's site was to try lubricating the cable guides on the bottom of the BB shell. I washed the guide and dropped some lube in there and haven't had the problem again.

You may also want to have your derailleur hanger inspected, even a slight bend can throw off the indexing and make it extremely difficult to get it adjusted right. Your indexing might be spot on for some range of the cassette, but be thrown off by the hanger bend for another area. Your LBS can bend the hanger back for a few bucks, or you can spend ~$15 or so for a new hanger. After seriously trashing my hanger once I am planning to buy a spare to throw in my saddle bag. I bent it so much it prevented me from getting into gears low enough to get up any hills and had to find a shop that was still open at that time of night.