Home Brompton Maintaining the 68mm BB standard with the road bike crank on the Brompton

Maintaining the 68mm BB standard with the road bike crank on the Brompton

by Petrelli

The Brompton bottom bracket follows the BSA threaded standard, the common shell width on the BB is 68mm.  Owning to the fold-ability design, the BB area had shifted slightly to the non driver (left hand) side. This offset allows the original square taper BB and crank set to work perfectly on the Brompton. However, most of the road bike cranks wouldn’t fit so perfectly onto the Brompton with the offset in mind. Under most of the circumstances, a width of spacers are needed to compensate the offset on the Brompton’s frame BB shell design. Those added spacing would give the extra safety burden to the installing road bike crank. There are cases that the crank arm actually falls off during the ride as there wasn’t enough mounting space on the axle after the extra spacing had been added.

The owner of this Brompton S2E understood the issue and came for a solution that could perfectly fitted his road bike crank without having to give up his riding safety. 

My solution offered to him was rather straight forward. If we needed to add an extra spacing of 2mm (on the drive side) to make the crank installation work and increased the case BB shell width to 70mm (68+2mm), why not we machine 2mm away from the non drive (left) side to help maintaining the standard. The idea was big, but it would take of work to achieve it. 

The BB shall had been machined down to 66mm and to be used with an extra 2mm of spacer for the proper road bike crank installations.

We straight away strip down the Brompton and handed the frame to our machining department for the further process. The amount of thickness that needed to be removed was computer programmed into the CNC machine and in no time we have a perfectly measured 66MM BB shell for the further crank installations. 

Precision machining starts.

The work was carried out almost a month ago, so far the Brompton is riding strong with the new settings.

2mm had been machined off at the BB shell area, you can compare to the width of the rear hinge area which were having the original width.


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