I’m building a new road bike which has a BB86 bottom bracket shell. My initial plan was to fit an ultegra chain set which has a 24mm spindle and are designed for BB30. Doing so requires a specific shimano bb (BB72-41B) which I guess downsizes the larger BB86 to accommodate. Shimano don’t seem to believe that BB86 cranks with the 30 mm spindle are necessary. Any thoughts on this? Any pros or cons of either approach? I’m thinking that BB86 should result in a stiffer crank / frame system?

If it’s got a 24mm spindle it’s not a BB30 crank, as these have a 30mm spindle.

BB86 and BB30 aren’t compatible, they’re two different standards. I have two frames with BB86 and they’ve been creak-free and have decent bearing life. There’s good reason that Shimano have never made a BB30 crank – it’s a pile-o-poo!

I find this really useful

http://www.bikeradar.com/gear/article/complete-guide-to-bottom-brackets-36660/

Thanks I seem to have misunderstood the difference between bb30 and BB86. I just stumbled across a chart that explains it is related to the BB width (bb30 If I understand correctly on a BB30 the bearings sit outboard of the frame…the BB86 seems to have the inboard within the shell?

No, BB30 has a 30mm spindle and the bearings press into the BB shell – overall width is narrower than a threaded external BB.

this is very helpful

http://problemsolversbike.com/files/tech/Bottom_Bracket_Standards_Reference.pdf

I am still not clear how the same chainset can be fitted to a bike with 68, 73 or 86 mm wide BB

I am still not clear how the same chainset can be fitted to a bike with 68, 73 or 86 mm wide BB

Spacers.