Bicycle Tutor

I recently acquired an older mtn bike with a very light aluminum frame that has 8spd XTR shifters and derailleurs in good condition but with a trashed STX crankset and BB. I'm wondering if an '09 9spd XT crankset would work smoothly with an 8speed cassette and if so, what would be the best chain to use.

Sheldon brown says, that it could work
http://sheldonbrown.com/speeds.html

"Going the other direction, using wider chains with chainrings intended for narrower chains is not generally a major problem if there's only a one or two generation difference. The only problem you might run into is that the chain will be more liable to rub on the inside of the bigger chainrings in the small/small crossover gears, gears you shouldn't be using in any case. "

But: you have to look whether the BB / crank set matches the chain line of your front derailleur, other than that I see no problems. Use an 8speed chain, as the rear cassette will most probably not accept a 9spd one. Go for a good one, the cheap chains will be worn out quite fast, so you end up paying more, I think the HG70 is ok. Oh, and if you buy a new chain, go and get a new cassette.

I figured I'd need a new cassette. The question is which one? I read the compatibility info by Sheldon Brown, but it didn't quite clear it up for me. Can I use a 9-speed cassette with an 8 speed derailleur (and shifters)? If so, this would be ideal as I plan to upgrade to 9-speed shifters and different brakes down the road.

Well... Sheldon writes about "8 of 9", where you use 8sp shifters with 8 gears of a 9 sp cassette. I think this is not only difficult to set up correctly but also prone to automatic misadjustments, as the cable stretches, the RD gets knocked at etc. So I would not do this. If you want to upgrade, this is the time. Or next time, when you change chain and cassette. So if you don't want to invest the money in the new shifters at the moment, go for an 8sp cassette / chain and drive them some 1000 km. I think, the 30€ (or so) that you spend now on an 8sp drivetrain is money well spent, if that means you have less problems in the near future.

Thanks, Joe. I appreciate your input.