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Bottom bracket - Definition |
| Related Words: Advocate, Ally, Analyze, Arm, Assemble, Back, Backbone, Band, Bearer, Blood, Bond, Bra, Brace, Branch, Brassiere |
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The bottom bracket on a bicycle contains the parts to which the cranks and chainrings and pedals are attached. The cranks are connected to a bottom bracket spindle, rotating on bearings inside the bottom bracket shell. (The shell is part of the bicycle frame).
Older bicycles use loose bearings, with separate cups and spindle; overhauling requires removing at least one cup, installing new ball bearings, replacing the spindle and adjusting the cups. Most modern bicycles use a one-piece cartridge bottom bracket instead, with the spindle and bearings sold as a single unit. This makes servicing the bottom bracket a simple matter of removing the old cartridge from the bottom bracket shell, and installing a new one in its place.
| Bottom Bracket Thread Name
| Nominal Thread Description
| Cup Outside Diameter
| Shell Inside Diameter
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| ISO/English
| 1.37" x 24 TPI
| 34.6-34.9 mm
Left-hand thread
drive side
| 33.6-33.9 mm
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| Italian
| 36mm x 24 TPI
| 35.6-35.9 mm
Right-hand thread
both sides
| 34.6-34.9 mm
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| French
(obsolete)
| 35mm x 1 mm
| 34.6-34.9 mm
Right-hand thread
both sides
| 33.6-33.9 mm
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| Swiss
| 35mm x 1 mm
| 34.6-34.9 mm
Left-hand thread
drive side
| 33.6-33.9 mm
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| Whitworth
(obsolete, found on
older English 3 speeds)
| 1-3/8" x 26 TPI
| 34.6-34.9 mm
Left-hand thread
drive side
| 33.6-33.9 mm
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