A question that crops up frequently is with regards to Specialized’s OSBB bottom bracket and what size it is exactly.
The short answer is OSBB comes in 3 variants which are all called OSBB. 46×61, 42×68 and 46×73. A summary table is below, with the detailed drawings below that.
|Variant (Diameter x Width) (mm)||Application||Description|
|46 x 61||Road||Frame tolerances are the same as PF30. It is extremely narrow|
|42 x 68||Road||This has exactly the same dimensions as BB30 and is designed for 6806 or 61806 bearings and a 30mm axle.|
|46 x 73||MTB||This the same as PF30 MTB|
46 x 61mm OSBB Road Applications
This is a shortened version of PF30. It uses the same type of cups to encapsulate the bearings which then push into the bike frame. The bearings are usually 6806. it was designed for 30mm axles.
This is technically a poor standard because the bearing stance is very narrow. It lends itself to quite a lot of flex which would require significant material in the bike frame to regain the lost stiffness. OSBB in this guise has the narrowest stance of any modern BB standard.
42 x 68mm OSBB Road Applications
This is the same as BB30. It uses two smalley VHM 42 circlips to hold in a pair of 6806 bearings. It was designed for 30mm axles. A schematic of this is shown below
The only real quirk of this against BB30 is the use or non use of circlips. Some OSBB bottom brackets have an integrated lip in the bottom bracket and do away with the Smalley VHM 42 circlips.
42 x 73mm OSBB MTB Applications
This is the same as PF30 MTB.
Cranksets and OSBB
If a 30mm crankset is being used, 6806 bearings should be used with inboard bearings. The use of 6806 bearings in an outboard sleeve configuration is not recommended due to the small width of the bottom bracket.
If a different crankset is used such as Shimano or SRAM GXP. then a Hambini Bottom Bracket with 6805 based bearings can be used.