Schlumpf Speed drive with a 34t chainring acts like a 54t when you kick it into overdrive , and it shifts instantly.
at any time even bogged down on a hill (I run a mountain drive - Brompton, bigger reduction gear 2.5:1_54t /'21.6't)

your BB shell edge has to be Chamfered as the grip is there to transfer the Torque.

but once done its a low maintenance setup.

their High speed drive is a 2.5x overdrive, the speed drive is 1.65x..


it is certainly possible to put a 110 bcd double crank on It .. needs you to be able to get all the chain tension slack
to make that very big up shift.. so down hills is where I get out if granny gears..

without a FD you use your fingers and stop the bike .. I got a custom FD bracket , to put a double crank on my brompton

the MD works better .. will sell the Brompton clamp on FD bracket .

Read (here) that There is a similar add on for DaHons. then you get a braze on FD, & a thumb shifter..