Bicycle Mechanics - Why different B/B axle lengths ?

Hi guys, I'm slowly refitting my bike with new XT kit and noticed that the XT B/B axle is available in two lengths, differing by 5mm. Why the difference :confused:. If it's any help, the B/B shell is 68mm and the rear hub is XT 9-speed.

Axle length depends on BB shell width (68, 70, or 73mm), chainring count (1, 2, or 3), and crank design. In this particular instance, it sounds as though BB shell width is the deciding factor. Be forewarned, however, that different cranks fit differently onto the axle tapers. For example, I was surprised (and disgusted) that Shimano RSX cranks do not seat fully onto a Shimano UN-72 cartridge.

Cheers John, I guess it kind of depends on the B/B width and cranks combined with the rear hub and cassette combo. I guess I should try both and see which one leaves the middle front ring and middle rear sprocket most perfectly lined up.

Any other ideas ?

You need to vary the lateral position of the cranks to achieve a good chainline. This depends on where the centre of the rear cogs are (cog count/spacing, dropout width), whether you are running a double or triple chainring, and you need clearance for the cranks and inner chainring from the chainstay.
Some BB units are finely adjustable to give the narrowest width which works.
As John noted, crank to bb axle fit can be variable with tapered systems, but splined systems are more accurate.

Thanks guys, It's all XT 27 speed so is 'optimum drive train alignment' a situation where the front middle chainring lines up perfectly with the middle rear sprocket ?

Originally posted by Astra
Thanks guys, It's all XT 27 speed so is 'optimum drive train alignment' a situation where the front middle chainring lines up perfectly with the middle rear sprocket ?

That would be ideal, but on many bikes, the best you can do is to position the inner chainring so that it just clears the chainstay. In any event, you should avoid riding in the extreme crosschain combinations, and may find your chain runs smoothly only with something like the 7 inner cogs with the inner ring, the 7 middle cogs with the center ring, and the 7 outer cogs with the outer ring, which still gives you two more usable combinations than I have on my mountain bike.

Thanks for the replies guys. I guess I'll have to take my frame and rear whell to the BS and see how it fits with both B/B's and take it from there :) .