I have made quite a few mods to my Brompton S3L, which I have ranked in terms of technical difficulty, cost and benefit:
Third-Eye bar end mirror - easy mod, inexpensive, essential for riding in traffic
new seat - easy mod, cost depends on choice of seat, big improvement (imho)
Ergon grips - easy mod, grips are somewhat pricy, but well worth it for reducing strain on the hands
Sturmey-Archer 8-speed hub - complicated mod, expensive (new rear wheel), greatly improved gearing (my range is now 32 to 98 gear inches)
hub dynamo lighting - complicated mod (build new wheel), very expensive, on a par with good battery lighting but without the batteries
MKS removable pedals - medium easy mod (left side crank requires special spacer washer), medium cost, much nicer than the inelegant standard folding pedal but you must then deal with the pedal once it has been removed
07 Schwinn Voyageur GSD, Next Avalon, 2007 Dahon Yeah
Originally Posted by maurice74
any guide to gear improvement?
I have an M3L. I think the stock M3L is too high geared (great at helping you reach hi speeds, but not so good at tackling hills). The included SA hub does have great range, though. Because of this, I (my LBS) swapped out the front chainring for a smaller one. Now, I do much better in tackling those hills. Some have been able to stack two chainrings up front (kinda like pancakes), but I didn't go that route. The price for this upgrade was pretty reasonable, too. I think I payed a little over $100 USD (parts included). Then again, my LBS is great and I have developed a long term relationship with them.
The stock m3L may work if you live in a flat city, though.