Double Chainring, on a 6 speed Brompton, Is 12 speeds..
Sorting out another way, now, myself,
have a Schlumpf Mountain drive crankset, 54t chainring.
changes: 54/15 . & 47/13 are close to the same ratio..
47/54 on the 2 speed crankset, 13t cog, also gives 12 speeds,
no duplicates.
Last edited by fietsbob; 08-16-11 at 10:50 PM.