Bottom Brackets vs. Chains
Comparing 2 standards across 20 specifications. 20 key differences identified.
Bicycle bottom bracket (BB) shells connect the frame to the rotating crankset spindle. Traditional standards include BSA English Threaded (68/73 mm shell width, 1.37" × 24 TPI, reverse left-hand thread on drive side) and Italian Threaded (70 mm shell, 36 mm × 24 TPI right-hand both sides). Modern press-fit and oversized standards include BB30 (68/73 mm × 42 mm press-fit), PF30 (68/73 mm × 46 mm), BB86/92 (86.5/91.5 mm × 41 mm), and T47 (threaded M47 × 1.0 mm).
Bicycle roller chains are standardized under ANSI B29.1 and ISO 9633. Across all modern bicycles, the chain pitch (distance between consecutive pin centers) is strictly 1/2 in (12.7 mm). Drivetrain speeds (1-speed to 13-speed) differ in internal roller width (1/8" for single-speed down to 11/128" for 11/12/13-speed) and external pin length (tapering from 8.6 mm on 1-speed to 5.25 mm on 12-speed chains).
Specification Comparison Table
20 Parameters| Parameter / Spec | ||
|---|---|---|
|
BSA English Threaded Shell Width
|
68.0 (Road) / 73.0 (MTB) mm shell width | — |
|
BSA Thread Specification
|
1.37 in × 24 TPI (ISO 6698) Left-hand drive side / Right-hand non-drive | — |
|
Italian Threaded Shell
|
70.0 mm width / 36 mm × 24 TPI Right-hand thread on both sides | — |
|
T47 Threaded Standard
|
68.0 / 73.0 / 86.5 mm width M47 × 1.0 mm internal thread (fits 30 mm & DUB) | — |
|
BB30 Press-Fit Standard
|
68.0 (Road) / 73.0 (MTB) × 42.0 mm (42 mm bore, bearings pressed directly into shell) | — |
|
PF30 Press-Fit Standard
|
68.0 (Road) / 73.0 (MTB) × 46.0 mm (46 mm bore with composite bearing cups) | — |
|
BB86 (Road) / BB92 (MTB) Press-Fit
|
86.5 (Road) / 91.5 (MTB) × 41.0 mm (41 mm bore, designed for 24 mm spindles) | — |
|
Shimano Hollowtech II Spindle Diameter
|
24.0 mm chromoly steel spindle | — |
|
SRAM DUB Spindle Diameter
|
28.99 mm aluminum alloy universal spindle | — |
|
BB30 / 30mm Spindle Diameter
|
30.0 mm oversized alloy spindle | — |
|
Chain Pitch (All Speeds)
|
— | 12.70 mm (0.500 in / 1/2 in pin-to-pin pitch universal) |
|
Single-Speed (1s / BMX / Track) Inner Width
|
— | 3.18 mm (1/8 in / 0.125 in roller width) |
|
Single-Speed External Pin Length
|
— | 8.60–9.40 mm |
|
6/7/8-Speed Inner Width
|
— | 2.38 mm (3/32 in / 0.0938 in roller width) |
|
6/7/8-Speed External Pin Length
|
— | 7.10–7.30 mm |
|
9-Speed External Pin Length
|
— | 6.50–6.70 mm |
|
10-Speed External Pin Length
|
— | 5.88–6.10 mm |
|
11-Speed External Pin Length
|
— | 5.50–5.65 mm |
|
12-Speed External Pin Length (Shimano / SRAM)
|
— | 5.15–5.30 mm (e.g. SRAM Flattop / Shimano HG+) |
|
Maximum Wear / Elongation Threshold
|
— | 0.5–0.75 % elongation (replace at 0.5% on 11/12-speed to save cassette) |