Brompton Bottom Bracket

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)

Which bottom bracket?

Older / A Line / C Line / P Line

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

T Line

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