Bicycle Bottom Brackets
Bicycle Parts • Bicycle Parts Makers • Bicycle Bottom Brackets
A bicycle bottom bracket is the shaft and ball bearings combination
connecting the two crank arms together. Bearings mounted on the outside
of the shaft allow the cranks to rotate the pedals. These bearings are
housed in a shell with threads on the outside. The threads screw into
the bottom of the bicycle frame. One set of threads screws in a
clockwise direction while the other side screws in counter-clockwise.
These threads prevent constant pedaling from unscrewing the bottom
bracket.
Older style bottom brackets have solid shafts with square tapered ends. Both ends of the solid shaft have threaded holes allowing the cranks to be tightened down with a crank bolt. Older cranks must be removed with a crank remover.
Newer style bottom brackets often have larger diameter hollow shafts that have a number of splines (teeth-like grooves) on the end. The crank has splines to match the bottom bracket. Newer shafts are much stiffer and thereby more efficient at transferring energy from the left side to the right side drive train. They are also substantially lighter than older systems.
Because bottom brackets have different shell sizes for different types of bicycles, it is important fit a bottom bracket to allow for sufficient distance between the crank set and the frame. Note, too, that triple chainring cranksets use a longer bottom bracket spindle than double chainring cranksets.
Shop for new and used bottom brackets and cranksets at eBay auctions. The mountain bike bottom bracket category is listed first, followed by the road bike bottom bracket category.
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