double for mtb?
 

My knees are not used to and not liking the width of cranks that are triples. Can I use a double crank with a 108 length bb? Anyone else do that? Any suggestions?

I ran a double back in my racing days, but basically just took the small chainring off, instead of replacing the crankset.

Where would one acquire a double for the rigors of MTB'ing?

I'm running a road isis BB on my raceface prodigy cranks with two rings. It works fine. Granny gears are definitly not needed around raleigh, nc, and this skinny q-factor feels more familiar (started on road bikes).

you can only run a shorter BB if you have a square taper, ISIS, or old (3 piece) XTR. Newer shimano octalinks are different between road and mountain stuff.

same thing... i commute on a mnt bike (14miles a day) but ride >10K a year on a road bike. there's about an inch and a half difference in Q between the two. the knees did NOT like the change at all. changed out the BB and threw on a Truvativ 42/52 they had in the shop and off i went.

works great.

I have a double crank set on my mountain bike. It is a Truvativ Holzfeller 2.2 RG, I don't know what the BB length is or what the q-factor is though.

You can just take the little ring off to run double, but that doesn't solve your q-factor problem. What you want to do is buy a 2x9 specific crank like the Middleburn RS8 duo, the FRM 2x9, or the new FSA 2x9 when it comes out. This way you can run a tighter BB and reduce your Q-factor to a more road-like configuration. I believe extralite makes a double crank as well.