Brompton rear dropout / to fit regular derailleurs... by liemnico

Last crawled date: 3 years ago

logfile:
2018-06-21 -- v0.8, initial digital beta design
2018-06-22 -- v0.9, v0.8 plus bolt thread engineered
2018-06-27 -- v1.1, v0.9 printed, thread works, all now massive for plastic!
Intro:
Have you ever wanted to mount a regular bike's rear derailleur to a Brompton?
As an owner of a T5 Brompton, I did. The reason: Older Bromptons lacked the brazed-on mount socket for the gear shifting mechanism, allowing to mount single sprocket derailleurs only. These would be featured by single gear or internal hub gear Brommies only.
Warning:
This is a beta-version! The final product should be printed in metal oder CNC machined from metal. There is a thread inserted, but I don't know how threads turn out on a 3d metal printer. Keep in mind that the derailleur thread engineered here is M10x1.00 (DIN961), which complies with usual derailleur threads and thus differs from usual machine screws (M10x1.25, DIN 936).
To do list:
Since I have to file the default dropouts to widen them for 10 mm axle compatibility, I will also have to redesign the typical axle nuts. These will be uploaded here as well.
Furthermore, the derailleur has to be modded by adding a small 'chain catcher hook', which can be made from a spoke. Pictures yet to come.
Refinements envisioned:
Change the derailleur hanger design to have a high STW ratio, since this saves weight on precious LSM and thus drops the price.
Develop a widely applicable shape of how to cut off various derailleur cages: If this was unified, so could be a future derailleur cage design to fit all derailleur cages.

Benefits: Designing the derailleur cage on your own could possibly render the 'chain catching hook' obsolete, giving you a nice leightweight derailleur cage of 95 mm jockey wheel distance (the distance of Brompton's jockey wheels, which is related to the distance of bottom bracket axle and rear triangle axle).

If a long cage was designed, the spring tension should be reduced. This will need some tinkering on the derailleur pivot axle or spring mount.