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Bottom Bracket



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There are two main types of bottom brackets: open-bearing bottom brackets, which are exposed to water and grit, and must be regularly inspected for signs of wear and damage; and cartridge-bearing bottom brackets, which are usually sealed from the elements.

How they work

The bottom bracket joins the crankarm of each pedal with an axle, which rotates in the bike’s frame. Each type of bracket consists of an axle, two bearings, two threaded cups (the free cup and fixed cup), and, in some types, a sleeve. The axle rotates freely on the bearings, which are held at one end of the axle within the free cup and at the other, drive-side end, by the fixed cup. The crankarms are screwed into the internal threading of the axle. The axle of a third type of bottom bracket, the BMX bracket, is threaded externally -- the whole bracket is held in place by a locknut that screws onto this thread on the left-hand crankarm side of the axle.

Parts of a crankset

Above: Providing strength: The axle and bearings of the bottom bracket need to be both strong and reliable enough to bear the weight and power of the rider. Cartridge-bearing bottom bracket: Allows axle to rotate. Crankarm: Turns the axle.

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Cartridge-Bearing Bottom Bracket Mechanism

Each of the cartridge bearings is composed of ball bearings, which are sandwiched between an inner and outer race by plastic seals. The cartridge bearings are located close to each end of the bottom-bracket axle. A tubular sleeve fits over the two bearings, filling the space between them. The fixed and free cups fit over this sleeve to create a totally sealed unit.

  • Fixed cup: Holds the bottom bracket in place
  • Bearing surface: Ensures that the axle rotates smoothly on the bearings
  • Crankarm: Turns the axle
  • Axle: Connects the crankarms and rotates in the bearings
  • Lockring: Fixes the bottom bracket in position
  • Ball bearing: Lets the axle turn
  • Free cup: Screws into the bike’s frame
Each of the cartridge bearings is composed of ball bearings, which are sandwiched between an inner and outer race by plastic seals. The cartridge bearings are located close to each end of the bottom-bracket axle. A tubular sleeve fits over the two bearings, filling the space between them. The fixed and free cups fit over this sleeve to create a totally sealed unit.

Open-Bearing Bottom Bracket Mechanism

The axle of an open-bearing bottom bracket rotates on ball bearings set in a cage that butts up against a raised bearing surface. The cups hold the axle/bearing assembly in place. The fixed cup screws all the way into the frame. The free cup screws in until the axle is held without play, but still rotates freely. A lockring holds the free cup in place.

  • Fixed cup: Holds the bottom bracket in place
  • Outer race: Houses the bearings
  • Ball bearing: Supports the axle
  • Axle: Connects the crankarms and rotates in the bearings
  • Free cup: Screws into the bike’s frame
  • Crankarm: Turns the axle
The axle of an open-bearing bottom bracket rotates on ball bearings set in a cage that butts up against a raised bearing surface. The cups hold the axle/bearing assembly in place. The fixed cup screws all the way into the frame. The free cup screws in until the axle is held without play, but still rotates freely. A lockring holds the free cup in place.
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