Am I right that the biggest I can easily go at the front would be 54 with Brompton cranks?
If you stick to the ones Stronglight makes for Brompton, then that is true.
If the hub was a BWR, the cog could be a 12t, as the spline is smaller.
tcs :'13-24, actually'.
I could not fit larger than a 15 on my Mk2 ,[UK AW3 hub]
I had the sprocket teeth into the rear frame tube, with a 16t.
I have no experience with Mk3[sachs/sram hub] or the current Mk4
The end of the right rear triangle could be made to provide more clearance.
my gear : 54:15 , I use a Schlumpf mountain drive 2 speed crankset,
it's planetary low is a '/, 2.5 reduction gear or a 54/21.6t.
6 gear ratios, high in low range is #3, low in high range is #4.
Because the town is built on a Hill.