a.twiddler
Veteran
Modifying the gearing on the Brompton
After my recent trip to Welshpool and back I'd been considering a quick way to get some lower gears. I’d already set up the original 50T Chainwheel with a 28T inner ring before I’d had all the fun and games with changing the FAG bottom bracket for the Shimano one. Unfortunately there was then just not quite enough clearance with the rear triangle when folded, so I refitted the single 44T ring. This had the virtue of simplicity with gears of 1st 38.4”, 2nd 51.2”, 3rd 68.3” (44X14T). To some of the Herculean thighed Titans out there 38.4” as a bottom gear is probably fine, and for the short sharp hills round here it’s manageable but for longer drags, for me at least, something lower is needed.
I had a go at the chainring bolts with a view to putting the smaller ring on the outside. Having read some of the vintage Brompton touring stuff, if using a greasy finger to change the chain between chainrings the system doesn’t care whether the smaller ring is on the inside or the outside unless a front derailleur is involved. The chainring bolts proved stubborn. When I assembled them I was determined that they wouldn’t vibrate loose so I used threadlock. Despite using solvents, heat and buying a humungous screwdriver as well as using a standard chainring tool, I only managed to shift one set of bolts. In the end, In a fit of exasperation, I ended up drilling the rest out. Fortunately I had enough spares to reassemble it again.
I set it up with the small ring on the outside, added two links to the chain and refitted the crank, pedal and chainwheel. It went on OK, and the chain runs freely on both rings. I thought it advisable to put the chain on the big ring before folding to avoid having too much slack, and there was adequate clearance with the rear subframe folded. Everything runs freely. I haven’t been up the road on it to try it out in anger yet but it looks promising. The gear range is 1st 24.66, 2nd 32.88, 3rd 43.47, 4th 43.6, 5th 58.2, 6th 77.6.
A smaller ring, such as a 26” would give 2 distinct ranges with no overlap but as it is with a 28 + 50 X14 the highest gear on the low range is virtually identical to the lowest on the high range. With no instant gear changer between rings, it would be difficult to make it work with the overlap as in the Brompton 6 speed system, so I’ve set it up to work as a separate high set of ratios and low set of ratios. Just got to suck it and see. Small ring:-
Big ring:-
Folded:-
After my recent trip to Welshpool and back I'd been considering a quick way to get some lower gears. I’d already set up the original 50T Chainwheel with a 28T inner ring before I’d had all the fun and games with changing the FAG bottom bracket for the Shimano one. Unfortunately there was then just not quite enough clearance with the rear triangle when folded, so I refitted the single 44T ring. This had the virtue of simplicity with gears of 1st 38.4”, 2nd 51.2”, 3rd 68.3” (44X14T). To some of the Herculean thighed Titans out there 38.4” as a bottom gear is probably fine, and for the short sharp hills round here it’s manageable but for longer drags, for me at least, something lower is needed.
I had a go at the chainring bolts with a view to putting the smaller ring on the outside. Having read some of the vintage Brompton touring stuff, if using a greasy finger to change the chain between chainrings the system doesn’t care whether the smaller ring is on the inside or the outside unless a front derailleur is involved. The chainring bolts proved stubborn. When I assembled them I was determined that they wouldn’t vibrate loose so I used threadlock. Despite using solvents, heat and buying a humungous screwdriver as well as using a standard chainring tool, I only managed to shift one set of bolts. In the end, In a fit of exasperation, I ended up drilling the rest out. Fortunately I had enough spares to reassemble it again.
I set it up with the small ring on the outside, added two links to the chain and refitted the crank, pedal and chainwheel. It went on OK, and the chain runs freely on both rings. I thought it advisable to put the chain on the big ring before folding to avoid having too much slack, and there was adequate clearance with the rear subframe folded. Everything runs freely. I haven’t been up the road on it to try it out in anger yet but it looks promising. The gear range is 1st 24.66, 2nd 32.88, 3rd 43.47, 4th 43.6, 5th 58.2, 6th 77.6.
A smaller ring, such as a 26” would give 2 distinct ranges with no overlap but as it is with a 28 + 50 X14 the highest gear on the low range is virtually identical to the lowest on the high range. With no instant gear changer between rings, it would be difficult to make it work with the overlap as in the Brompton 6 speed system, so I’ve set it up to work as a separate high set of ratios and low set of ratios. Just got to suck it and see. Small ring:-
Big ring:-
Folded:-
