The bottom bracket is the part of the frame around which the pedal cranks revolve, also the bearings and axle assembly that runs through the bottom bracket shell of the frame.

The bottom-bracket shell may be unthreaded, like the one at the left (photo credit: ConnollyB, Wikimedia Commons), with bearing parts press-fit or clamped in place inside the shell. On the other hand, the bottom-bracket shell may be threaded internally at each end, so the bearing parts thread in. The bearing parts may be pre-assembled bearing cartridges or traditional cup-and-cone assemblies.

An old American term for "bottom bracket" is "hanger". This is usually used in connection with one-piece cranks.

Bottom brackets come in different sizes, according to the nationality of the frame.

What happens if you try to mix different sizes:

Bottom Bracket Shell Threading (Below)	BritishI.S.O. 1.37/1.375" x 24 tpi CUPS R-L (34.8/34.9 x 1.06 mm)	Italian 36 mm X 24 tpi CUPS R-R (1.417" x 1.06 mm)	French 35 mm X 1mm CUPS R-R (1.378 x 25.4 tpi)	Swiss 35 mm X 1mm CUPS R-L (1.378 x 25.4 tpi)	Raleigh 1 3/8" X 26 tpi CUPS R-L (34.9 x 1.06 mm)
British/I.S.O. 1.37/1.375" x 24 tpi (34.8/34.9 x 1.06 mm)	Made to Fit	36 mm Cup diameter is too large, thread won't even start.	Cup diameter is slightly too large, usually won't start. Right (fixed) cup threaded in the opposite direction.	Cup diameter is slightly too large, usually won't start.	Diameter matches, but thread pitch does not. Will bind after only a few threads are engaged.
Italian 36 mm X 24 tpi (1.417" x 1.06 mm)	British/I.S.O. cups fall through	Made to Fit	Italian shells are larger diameter, all other size cups fall right through, threads will not engage.
French 35 mm X 1mm (1.378 x 25.4 tpi)	35 mm = 1.378". Shell is slightly larger, thread pitch slightly finer. Left side may seem to fit, but will be loose.	36 mm Cup diameter is too large, thread won't even start.	Made to Fit	Left (adjustable) side is interchangeable. Right (fixed) side is threaded in the opposite direction, won't fit.	35 mm = 1.378". Shell is slightly larger, thread pitch slightly coarser. Left side may seem to fit, but will be loose.
Swiss 35 mm X 1mm (1.378 x 25.4 tpi)	35 mm = 1.378". Shell is slightly larger, thread pitch slightly finer. May seem to fit, but will be loose.	36 mm Cup diameter is too large, thread won't even start.	Left (adjustable) side is interchangeable. Right (fixed) side is threaded in the opposite direction, won't fit.	Made to Fit	35 mm = 1.378". Shell is slightly larger, thread pitch slightly coarser. May seem to fit, but will be loose.
Raleigh 1 3/8" X 26 tpi (34.9 x 0.977 mm)	Diameter matches, but thread pitch does not. Will bind after only a few threads are engaged.	36 mm Cup diameter is too large, thread won't even start.	Cup diameter is slightly too large, usually won't start. Right (fixed) cup threaded in the opposite direction.	Cup diameter is slightly too large, usually won't start.	Made to Fit

Chainline Standards:

Application	Dimension	Notes
Track/Coaster Brake Traditional One-Speed Most internal gear hubs	40.5-42 mm	Older bikes with 110 spacing would be on the smaller end of this range Newer bikes with 120 mm spacing normally use 42 mm
Road Double	43.5 mm	Shimano spec, measured to the midpoint between the rings. with typical 5 mm chainring spacing, this puts the inner at 41 mm, the outer at 46 mm.
Road Triple	45 mm	Shimano spec, measured to the middle ring.
Singlespeed MTB Alternate	47.5 mm	White Industries ENO hubs use this chainline, which lines up with the middle position of a typical MTB triple. It's also fairly close to the outer position of a typical "road" double.
MTB Triple	47.5-50 mm	Shimano spec, measured to the middle ring. 47.5 preferred, but for frames with oversized seat tubes, the longer dimension may be needed, because the fat tube places the front derailer mechanism farther to the right. SRAM standard chainline is 49mm with 142mm OLD and thru axle
Singlespeed and Single-chainwheel MTB, "Boost 148"	52 mm	Wider chainline need for chainstay clearance on newer MTBs, typically with disc brakes and 148 mm OLD spacing This is close to the chainline of the outer ring of a typical MTB triple.
Rohloff Speedhub	54 mm (58 mm w/13 tooth)	Sprockets are proprietary, threaded. All except the 13-tooth may be flipped over to double wear life.
Freeride and Downhill	55 mm	Newer Freeride and Downhill bikes with 150 mm spacing
Tandem	about 60 mm	Tandems with 160 mm spacing
Fatbike	66 mm	Fatbikes with 170 mm spacing
Fatbike	76 mm	Fatbikes with 190 mm spacing -- needed with 4.8" (125 mm) wide tires.

Standard bottom-bracket bearing assemblies are specified in terms of width of the bottom bracket shell (generally 68, 70 or 73 mm) and the length of the spindle (102 - 130 mm.)

Current usage generally assumes that 68 and 73 mm bottom bracket units are for British/ISO threaded frames, and that 70 mm units are for Italian threaded frames.

The spindle length mainly depends on what kind of crankset you'll be using, and doesn't have much to do with the frame. New cranks come with a spec sheet that lists what length spindle they are intended to be used with. Sometimes two lengths are listed. When this is the case, the longer size is for use on frames with fat seat tubes, because such frames put the front derailer mechanism farther to the right than on a frame with a standard seat tube.

See also my article on Bottom Bracket Sizes for information on matching spindles to cranks.

See also my article on Bottom Bracket Tapers.

My Tool Tips series includes related articles on Cottered, and Cotterless crank removal, as well as tools for disassembly and adjustment of cup-and-cone bottom brackets.

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