Author Topic: Brompton derailleurs differ?  (Read 167 times)

LittleWheelsandBig

  • Whimsy Rider
Brompton derailleurs differ?
« on: July 26, 2020, 05:09:38 pm »
Apparently Brompton changed the derailleur dogleg connector when they swapped to the underbar shifter in 2017. Does anybody know if the derailleur springs are interchangeable between the two?

I have purchased an aftermarket Ti spring to replace the somewhat corroded decade-old original but it is a slightly larger overall diameter and does not look like it would fit into the plastic cable casing stop.
Wheel meet again, don't know where, don't know when...

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton derailleurs differ?
« Reply #1 on: July 27, 2020, 09:26:07 am »
A reply on the Brompton e-list suggests that there is no functional difference for the derailleur spring on pre-2017 and current derailleurs. In either case, a Ti Parts Workshop derailleur spring is a larger diameter than the original and needs some bodging to fit within the plastic cable stop. I am inclined to drill a larger recess to allow the spring to fit comfortably.

The major difference seems to be the cable fixing location. Old derailleurs/ cables use a tiny grub screw within the lever. New ones are secured at the derailleur.
Wheel meet again, don't know where, don't know when...

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton derailleurs differ?
« Reply #2 on: August 03, 2020, 08:41:29 pm »
I have fitted a stiffer and shorter aftermarket external spring and a Brompton gear cable and casing in accordance with the instructions. The derailleur does not shift into the bigger cog and my guess is that the internal spring is merely compressing when the lever tightens the cable.

The original external spring is about the same stiffness as the internal spring, so the derailleur moves when the lever pulls the cable. The only way the stiffer spring would compress would be if the internal spring is fully compressed at all times.
Wheel meet again, don't know where, don't know when...