Brompton Bottom Brackets / Cranksets

Brompton Bottom Brackets and Chainset / Crankset Compatibility

As you can see - we like a bit of alliteration in our headings!

Let's start with recent history .....

In October 2017 Brompton bicycle made a recall on factory installed bottom brackets

Note: Non-factory fitted bottom brackets were not affected as they were of a different type

We're not going to write about that here - you can read all about it on the Bompton recall page
(It affected bikes in the serial number range  1403284144 to 1705150001 - all models)

What we are going to write about is the different types of bottom bracket supplied with/by Brompton and the compatibility with the different chainset/cranksets

Pre-2013 there was the fixed crankset

Brompton 44T fixed crank

The crank was permanently fixed to the chainring, and it looked like this (this is the 44 tooth chainring version). They are identifiable as they are shiny silver (depending upon how dirty yours is of course) and have a plastic chain guard (assuming it hasn't fallen off) which was held on with plastic lugs - so quite liable to falling off

Very, very infrequently the crank arm would work loose from the chainring (it didn't fall off - just became slightly flexible after years of use  - generally the teeth were worn out by then anyway!) - if that happened then the whole thing needed replacing

If you wanted to change gearing (e.g. reducing gearing from the standard 50T to 44T to make hill climbing easier) then again it was necessary to change the whole chainring / crank combination (and chain, and if changing chain it's also best to change the rear sprockets)

Brompton cranks fit to square taper bottom brackets - these get their name from the fact that they have square spindles, which taper towards the end. But unfortunately there are two (main) types in use JIS and ISO. The main difference is in the tapering

The fixed crankset fitted to a ISO bottom bracket (ISO stands for International Standards Organisation) - this is a type of square taper bottom bracket (and can easily be confused with a JIS version - but should not be fitted to that!)

ISO square taper bottom bracket (22 spline version)

The Bottom bracket used was mostly 119mm long but there were some Miche 122.5 mm ones used for a short time

The FAG ISO bottom bracked (shown here) uses a 6 notch tool for installation / removal

The FAG ISO bottom bracket used by Brompton could be itentified as it had notches cut out at the end of the spindle. The easy way to make sure is to replace the bottom bracket at the same time as replacing the crankset (bottom brackets are not overly expensive and do wear out too)

So remember - old fixed cranksets only fit to the ISO bottom bracket. If used in conjunction with a JIS bottom bracket they can work loose - this is because it will sit approx 4.5mm further out (and also affect the chain line)

The fixed crankset was discontinued on new bikes in 2013 and phased out as an after-market item in 2016

Next up came the ....

Spider Crankset / Chainring 

In 2013 came the spider crankset - here the chainring can be removed from the crank by undoing the 5 chainring bolts

The plastic chain guard is also better as it is fixed to the chainring using screws

Brompton spider crankset

Having a detachable chainring means that it can be replaced if it wears out or if you want to change the gearing of the bike

It has a more modern, brushed metal look and the cranks are held on with bolts done up using an 8mm allen key - which is easier than the rcessed bolts on the older cranks

In 2016 the BLACK edition (black instead of silver) was also made available as a general upgrade to the after-market

The spider crankset is available in 44 / 50 and 54 tooth options - and is the current (as of 2017) crankset un use by Brompton

The  spider cranks fit to a JIS square taper bottom bracket

Stronglight bottom bracket showing 22 spline fitting

Initially Brompton fitted to a FAG at the factory  - the FAG bottom bracked uses the Park Tool BBT-4 tool for installation and removal

The Stronglight  bottom bracket was offered by Brompton as the after market version instead of the FAG which was being fitted at the factory

The Stronglight bottom bracket uses the Shimano style 22 spline tool - BBT-22

Thun IBEX bottom bracket

Following the Brompton recall - they have moved to a Thun bottom bracket - as shown - this was also the bottom bracked used to replace the FAG ones during the recall

In late 2016 / early 2017 this same Thun IBEX bottom bracket was also used as the after-market version

Two things to look out of when removing bottom brackets:
1/ The bottom bracket cups (ends) are predominantly plastic - so treat them with care, else they can become rounded and difficult to remove
2/ The drive side (where the chainring fixes to), uses a reverse thread - so you do the normal tightening direction (clockwise) to undo it

FAG 6 notch bottom bracked installed in bike

You can see the bottom bracket fitting without removing the cranks as shown, showing the 6 notch FAG type, but not whether it is the square taper type (ISO or JIS) - for that you need to remove the cranks

We discovered (while changing bottom brackets as part of the recall) that sometimes there was a 6 notch tool and a 22 spline tool required (the drive side used a FAG type tool and the non-drive side used a Shimano style tool) but only on a small small sample of the bikes we worked on

Useful external resources:
Sheldon Brown - Square Taper Bottom Brackets
Park Tool - Crank Removal
Park Tool - Bottom Bracket Removal
Brilliant Bikes - Brompton Bottom Brackets
Brilliant Bikes - Brompton chainsets/cranksets