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Understanding Bottom Brackets and Cranksets

The last few years have given rise to a frankly bewildering amount of bottom bracket “standards”—such that to call them standards is almost any oxymoron. With so many different types of bottom brackets and cranksets out there, it can be difficult to tell which crank and bottom bracket systems are compatible.

 

BOTTOM BRACKETS

The bottom bracket (abbreviated BB) is a set of bearings that attaches to either side of a bicycle frame’s bottom bracket shell. The bottom bracket is essentially the interface for the crankset and the frame, and provides a tight fit that does not allow the crankset to wobble as it turns. The bearings inside of the bottom bracket help the cranks spin smoothly and efficiently inside the frame. Bottom brackets come as either threaded or press fit types.

Threaded bottom brackets must be screwed into the frame, and can only be used with frames that have a threaded bottom bracket shell. Threaded BB’s have a set of external cups that contain the bottom bracket's bearings. Threaded bottom brackets and frames come in either Italian or English threaded versions, which have different thread patterns, so they are not interchangeable. Threaded BB’s are very common, and are relatively simple to install.

Press fit bottom brackets are essentially cartridge bearings that are pressed into the frame’s bottom bracket shell. They have the benefit of allowing for a thicker, stiffer crankset spindle, which can make pedaling more efficient. They are very easy to install, but can sometimes be more difficult to maintain over the long run.

An older bottom bracket type that is found on vintage bikes and some modern bikes is the square taper bottom bracket. Square taper bottom brackets combine the bottom bracket bearings and the crank axle into one unit. Square taper cranks do not have an integrated spindle, and fit onto two square tapers on either end of the bottom bracket. Square taper bottom brackets are always threaded, and come in both English and Italian threaded version.

 

CRANKSETS

The crankset is the primary interface between the rider and the bike. It consists of two arms that attach to the pedals, as well as the chainrings that drive the chain. It consists of the driveside assembly, which includes the crank arm, spider, spindle, and chainrings, and the non-drive side crank arm.

Most modern cranksets will have a steel or titanium axle called a "spindle" affixed to the driveside assembly. The axle is inserted through the bottom bracket shell of the frame, and the non-driveside crank arm is attached to it with a fixing bolt. Older cranksets typically do not have an integrated axle spindle, but instead have a socket for the two ends of the bottom bracket to insert.

 

 

Crankset-Bottom Bracket Compatibility By Type

Bottom Bracket Installation Type Crank Compatibility
68mm English (BSC) Threaded Road/MTB Standard 24mm external type (Shimano Hollowtech II, SRAM/Truvativ GXP, Campagnolo Ultra-Torque, FSA MegaExo, Race Face X-Type, etc), square taper, ISIS, BB386 EVO
70mm Italian Threaded Road Standard 24mm external type (Shimano Hollowtech II, SRAM/Truvativ GXP, Campagnolo Ultra-Torque, FSA MegaExo, Race Face X-Type, etc), square taper, ISIS, BB386 EVO
BB30 / PF30 Press fit Road/MTB BB30, BB386 Evo, Standard 24mm external type, BBRight
BB86/BB92 Press fit Road/MTB Standard 24mm external type, BB386 Evo
BB386 Evo Press fit Road BB386, standard 24mm external type
BBRight Press fit Road BBRight, BB386 Evo, standard 24mm external type
BB90/BB95 N/A Road/MTB Standard 24mm external-type only
Square Taper Threaded Road/MTB Square taper cranksets with no integrated spindle

 

Crankset-Bottom Bracket Compatibility By Manufacturer

FSA MegaExo FSA MegaExo cranks ( BB30 versions also available for use with BB30 frames)
SRAM GXP SRAM GXP cranks (BB30 versions also available for use with BB30 frames)
Shimano Hollowtech Shimano Hollowtech cranks
Campagnolo Ultra-Torque

Campagnolo Ultra-Torque cranks

*Campagnolo Ultra-Torque cranks are compatible with all modern BB sizes with the correct Ultra-Torque cups

RaceFace Turbine RaceFace Turbine cranks
Chris King MTB Shimano 24mm external MTB cranks

 

 

 

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