Hollowtech 2 BB MTB/Road?

cooper.michael1

I currently have an XT Hollowtech 2 chainset on my cross bike, if I was to replace this with a shimano tiagra/105 compact, could I just leave the XT BB in place? Essentially are the MTB and road Bottom brackets the same?

Cheers, Mike

maddog 2

AFAIK the road cups are slightly narrower, but only by a mil or so. The main issue is the central sleeve, and on the mtb BBs it is often wider (as the cups run spacers whereas the road BBs often don't).

If you simply swap the chainsets then I suspect the MTB BB would be too wide, and not leave enough axle showing to securley mount the NDS crank arm. However, if you remove the spacers then it may be fine, but as i said above, the central sleeve may then be too wide to allow all the spacers to be removed.

Suck it and see. I recently tried to fit an Acros MTB BB to a road setup but no amount of bodging could get it to work. I tried allsorts of sleeves - Shimano, Hope, Acros, GXP - but nothing quite worked.

whyamihere

The central sleeve is removable, just pull it. Then it will work fine.

maddog 2

yep that's always an option. Depends how wet the bike gets I guess.

JohnnyAllez

Just fitted both types this week, Shimano MTB BB is set up for 73mm width shells, so if fitted into 68mm sheel you add the plastic spacers they supply to space it out to 73mm, Shimano road BB shells are 68mm width so the difference looks to just be in the plastic sleeve between the cups. The cups look to be exactly the same.

I`ll stick a vernier on `em and check.

JohnnyAllez

Maddog is correct, as usual MTB cups are 10mm wide, road cups are 11mm wide. So I suppose with some judicious use of the plastic spacers available you could make it work, depending on what width your frame BB shell is. 68mm shouldn`t be a problem.