Tuesday, 12 November 2013

Bottom Bracket "standards"

Over the past few years there have been numerous attempts to replace the threaded BSA bottom bracket shell, here are the current standards on the market for road and MTB bikes.

BSA: this is the ENGLISH threaded standard bottom bracket. 68mm or 73mm wide, with variants of 83mm and 100mm. Less common now is the ITALIAN threaded standard that is 70mm wide and has right threaded cups on BOTH sides.
BSA threaded, external bearing type
BB30: Initially designed as an open standard utilizing direct fit bearings and an over size alloy 30mm axle, 68mm shell and 41.96mm ID. BB30 has a lot of issues, the main being the tolerance of the frame's b/b and the fact that the bearing gouges the shell, meaning when you replace the bearing the fit is never as good.
BB30's direct fit bearing is very simple
PF30: is a direct evolution of BB30 and uses the same oversize axle and shell width (68mm), but the shell ID changes to 46mm. This shell size increase is to accommodate a complete press-in unit.
PF30 cartridge
BBright: introduced by Cervelo is similar to BB30 but has a wider (79mm) asymmetrical BB shell, but retains the shell ID of 41.96mm (of BB30). Instead of having a BB shell that is symmetrical 68mm wide, the non-drive bearing is moved outward from the centre plane of the bike by 11mm. BBright requires a specific crank set with an 11mm longer axle. The bottom bracket is a standard BB30 item.

BBright is also offered in a PF30 variant using the the same asymmetrical 79mm shell but with the PF30 ID of 46mm.

BB86: is a press fit ROAD standard developed by Shimano. The BB shell is 86.5mm wide with an ID of 41mm. This standard is compatible with 24mm or 19mm axle chainsets.
BB86/ 92 cartridge unit
BB92: is a press fit MTB standard developed by Shimano. The BB shell is 92mm wide with an ID of 41mm. This standard is compatible with 24mm or 19mm axle chainsets.

BB90: is a direct bearing press fit system used by Trek bicycles, easily identified by the 90.5mm BB Shell width. This standard is compatible with 24mm axle chainsets.

All press fit designs can suffer premature bottom bracket ware if the shell is not correctly reamed. If the shell is slightly out of round it can deform the BB creating fast bearing ware. If a fame has an ovalized BB shell and it is not correctly reamed bearing failure will be frequent.
BB standards made simple...

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