So is the bottom bracket spidle length dependant of the crank arms or the frame? I have raceface turbine cranks with an xt(square taper) 113mm BB spindle. I am looking at a new crank that comes with the BB and has a 118mm(isis) spindle. Is it my frame that determines the 113 BB I have now and I need 113mm for any crank? Or is it the cranks that determine the spindle length, and therefore the new 118 would work on any frame(provided the shell length matches) with the new cranks? I hope this makes sense. It's kind of hard to explain.

the 118 mm spindle will work with the frame providing the bb shell is the same (68 or 73mm) although it may affect your front deraillers operation - it may no longer be able to move far enough to push the chain onto the outer cog.
The cranks will however work with the 113 mm spindle, so ask the shop your buying it from to swap it (it wont cost them).
Alternatively, if you can stretch to it, get an external bottom bracket (hollowtech II, X-type etc) because they are way stiffer and more reliable.
hope that helped.
Dave

Well yeah. I guess it shouldn't make much difference anyway now that I think about it. If anything, the cranks will just be 2.5mm further out on each side. Shorter would be a different story. I was wondering though if the cranks being thicker material or angles differently if it could hit the frame. I'll be doing it myself. I've never used anything but a square taper before. Is isis not good? It is an isis raceface evolve DH BB.