After receiving the 36T inner ring a few days ago, I've been fettling parts today. The mount for the front derailleur has a couple of issue, but they have been overcome. It changes between the ring in the workstand, and a road test will follow in due course. With the chain on the big ring and small sprocket the chain causes no issues when the bike is folded or unfolded. I fitted a suspension lock-out control for cable control of the front derailleur, and although this feels tough to move when the bike is in the stand it may be ok when I'm on the bike - I may change it to a Sunrace shifter. Some photos follow. 1) There is an angled face to the finger which means the clamp bolt/washer of the front mech isn't flat to the surface. 2) I cut and filed 2 grooves so the finger goes into the slot of the mount - the slots prevent it from turning and give more security. 3) Using a dremel I tried to spot-face the angled area of the finger to make it parallel - this was a mistake, so I then used a file. Not pretty, but it did the job. 4) two pieces of window-fitters plastic shims were cut to length (60mm), put in boiling water and then moulded around a 30mm tube to give them a curve which made fitting easier. 5) Because the clamp is 40mm (tube is 38mm) the 1mm shims are ideal, don't damage the frame and the mount is secure. 6) a view of the set -up 52T and 36T rings. 7) with the Brompton folded the chain mostly stays on the big ring, and doesn't fall off when the bike is unfolded. Good points about the derailleur mount..... a) It's available. b) Relatively cheap. c) there is good adjustment up-down fore and aft of the finger and the derailleur clamp position, and of course it's possible to move the clamp up and down on the frame tube. This is probably better than having a braze-on already on the tube (unless the position is decided prior to spraying)
) a 73mm wide BB unit can be fitted instead of the usual 68mm, this gives the ability to space the driveside as required and a 'traditional' lockring from a cup& cone BB can be used on the non-driveside, which will also give the benefit of accurate adjustment to eliminate any bearing preload!
. A larger front ring and triple setup could, of course, give more range/a higher top gear - I acknowledge other owners have fitted triples, but the chainline to the large ring must move out even more to allow an inner ring and ability to fold.