The bottom bracket is one of the most critical components of a bicycle. It connects the crankset to the frame, allowing for smooth rotation of the pedals. Over the years, various designs and standards for bottom brackets have been developed to improve performance, durability, and ease of maintenance. This guide will introduce you to the different types of bicycle bottom brackets, explaining how they work and what they look like.
1. Square Taper Bottom Bracket
- Description: The square taper bottom bracket is one of the oldest and most common designs. It consists of a spindle with a square-shaped end that fits into square holes in the crank arms. The crank arms are bolted onto the spindle, and the bearings are housed either internally or externally.
- Appearance: The spindle has a square cross-section, and the crank arms slide onto this square taper. The bottom bracket shell often contains sealed bearings that are not visible externally.
2. Octalink Bottom Bracket
- Description: Introduced by Shimano, the Octalink bottom bracket features a spindle with eight splines instead of a square taper.
- Appearance: The spindle has eight splines (ridges) that interlock with matching splines on the crank arms. The bottom bracket shell houses the bearings, and only the splined spindle is visible when the crank arms are removed.
3. ISIS Drive Bottom Bracket
- Description: ISIS (International Splined Interface Standard) Drive is a splined bottom bracket similar to Shimano’s Octalink but designed as an open standard. It features a 10-spline spindle and is commonly used on higher-end bikes due to its stiffness and performance.
- Appearance: Like the Octalink, the ISIS Drive bottom bracket has a splined spindle. However, it features 10 splines instead of eight. The bearings are housed in the bottom bracket shell.
4. Hollowtech Bottom Bracket
- Description: Shimano’s Hollowtech system is an external bottom bracket system. Unlike older systems where the bearings are inside the frame, Hollowtech places the bearings outside the frame. The spindle is integrated with the crankset.
- Appearance: Hollowtech bottom brackets feature two external bearing cups that sit on either side of the bottom bracket shell. These cups are visible on the outside of the bike frame.
5. BB30 Bottom Bracket
- Description: BB30 is a press-fit bottom bracket standard that was originally developed by Cannondale. In this system, the bearings are pressed directly into the frame.
- Appearance: BB30 bottom brackets do not have external cups. The bearings are pressed directly into the frame, so only the ends of the spindle are visible when the crank arms are removed.
6. PF30 (Press-Fit 30) Bottom Bracket
- Description: PF30 is a variation of the BB30 system but uses plastic cups to house the bearings. These cups are then pressed into the bottom bracket shell.
- Appearance: Similar to BB30, but with visible plastic cups that hold the bearings inside the frame. The cups are flush with the frame, giving a clean look.
7. BB86/BB92 Bottom Bracket
- Description: BB86 (for road bikes) and BB92 (for mountain bikes) are press-fit bottom brackets where the bearings are housed in plastic cups and pressed into the frame.
- Appearance: Like PF30, BB86/BB92 bottom brackets have plastic cups that are pressed into the frame. The cups are internal, so there are no external bearing cups visible on the outside of the frame.
8. T47 Bottom Bracket
- Description: T47 is a relatively new standard that combines the benefits of threaded and press-fit bottom brackets. It uses a large-diameter threaded shell with external cups that house the bearings.
- Appearance: T47 bottom brackets look similar to external cup designs like Hollowtech II, but they use larger diameter cups and threads to attach to the frame.
9. Italian Threaded Bottom Bracket
- Description: Italian threaded bottom brackets are similar to traditional English threaded bottom brackets but with different threading. This design is mostly found on older Italian frames. The cups are threaded directly into the frame.
- Appearance: Like other threaded bottom brackets, Italian threaded systems have external bearing cups that are visible on either side of the frame.
10. GXP Bottom Bracket
- Description: GXP (Giga X Pipe) is SRAM's proprietary system that uses a two-piece crankset with a spindle integrated into the drive-side crank arm. The non-drive side has a bearing preload adjuster.
- Appearance: GXP bottom brackets typically feature external bearing cups for the threaded version and internal bearings for the press-fit version.