Bicycle Bottom Bracket

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The bottom bracket is a very essential part of the bike design. At the bottom of the frame there is a hollow piece of tubing which contains a part of the crank arm, extends from one side of the frame to the other and connects the crank arms together. The bottom bracket holds the crank arm securely and also allows for the crank arm to move freely. There are a few different types of bottom brackets. The names of them are: three-piece, eccentric, Ashtabula, and external bearing. The three-piece bottom bracket is when the axle is separate from the cranks and is able to rotate. An eccentric bottom bracket is when there is a cylindrical plug that fits into the shell and allows for the crank arms to rotate. An Ashtabula bottom bracket is a bracket in which the axle and crank arms are a single piece. The external bearing bracket is a relatively new design. The bottom bracket is connected to one of the crank arms and the bearings are placed on the outside of the bottom bracket which is inserted into the hollow part of the frame.

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How it works


The bottom bracket is a hollow part in the bottom of the frame that allows for the crank arms to move smoothly and be held to the frame. Previously stated, the bottom bracket contains threads which enable the bracket to be held to the frame. In order to remove the crank arm you have to turn the crank arm counter-clockwise because as you pedal, you are tightening the piece that holds the crank arm in. Inside the hollow part of the tube are ball bearings which enable the crank arms to be rotated easily. This is on either side of the hollow spacing of the tube and this allows for the crank arms to be held tightly and spin freely.

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Types of the bottom bracket


Three piece - contains a cartridge which holds the bearings and enables the crank arms to rotate. This design was seen as the standard.

Eccentric - consists of a cylindrical plug that is able to fit in a bracket shell which is larger than normal. This design is often used in tandem bikes.

Ashtabula - design which consists of the axle and crank arm as one piece. The bottom bracket shell is larger so that the removal of the crank arm is easy. This design is often used in low-end mountain and road bikes.

Thompson - this design is much like the Ashtabula but the difference is that the drive side crank is removable. These designs are not very common because this design allows for water and dirt to get into the shell.

External bearing - many new designs are now using this type of system. The bearings are now being placed on the outside of the bottom bracket. Examples of this design include: Shimano's Hollowtech II,and RaceFace's X-Type.

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3D parts

The table belows lists the Bill of Materials for The Bottom Bracket:

Bottom Bracket Bill of Materials
Part # Part Name # Req'd CAD File Image
1 Pedal 2 Pedal
Bike pedal.JPG
2 Front Sprockets 1 set of 3 Front Sprockets
Sprockets front.JPG
3 Bottom Bracket Bearing 2 Bottom Bracket Bearing
Bearing bottom bracket.JPG
4 Bearing Cap 2 Bearing Cap
Bearing cap.JPG
5 Round Washer 2 Round Washer
Round washer.JPG
6 Hex Washer 2 Hex Washer
Hex washer.JPG
7 Crank Shaft 2 Crank Shaft
Crank shaft.JPG
8 Pedal & Crank Shaft Assembly 2 Pedal & Crank Shaft Assembly
Pedal crank assem.JPG
9 Bottom Bracket Assembly 1 Bottom Bracket Assembly
Front crank assem.JPG

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Bottom Bracket