The bottom bracket is a bit of an unsung hero in the bike world - it sits inside the bike frame (screwed in) and then has the pedal cranks bolted onto it or crank arms / spindle pushed through it
(Brompton obviously recognise the importance as their code for the bottom bracket is Q100001 - i.e. part no 1 on the bike!)
Eve...
The bottom bracket is a bit of an unsung hero in the bike world - it sits inside the bike frame (screwed in) and then has the pedal cranks bolted onto it or crank arms / spindle pushed through it
(Brompton obviously recognise the importance as their code for the bottom bracket is Q100001 - i.e. part no 1 on the bike!)
Every turn of the pedal rotates the spindle inside, keeping both cranks / pedals turning together - it has bearings inside which keep it moving smoothly (until they wear out)
The traditional Brompton (or A Line / C Line / P Line) use a sealed bottom bracket (i.e. no replaceable parts inside) with a square taper crank fitting (older cranksets use a ISO fitting and newer spider cranksets use a JIS fitting)
These Bromptons use a 119mm length spindle and a 68mm shell
It is possible to change to other styles of bottom bracket
With the introduction of the T Line came a different style of bottom bracket with external replaceable bearings
This bottom bracket (made for Brompton by FSA) can be retro-fitted to any Brompton - but also needs the new superlight crankset
Brompton JIS bottom bracket - for the Brompton spider crankset introduced in 2013 Note: Not compatible with the older fixed crankset
Thun Brompton fit IBEX-ML 68 x 119mm Bottom Bracket This is a replacement bottom bracket for use with the Brompton spider crankset introduced in 2013 Note: Not compatible with the older fixed crankset
Shimano BB-UN300 bottom bracket British thread, 68 - 118 mm
Brompton ISO bottom bracket - for "fixed" crankset (pre-2013) See here for the replacement bracket