I always feel a little unsteady when I'm riding my KHS Mocha because of the stem flex. I've converted it into an electric bike which probably pushes it's limits since I'm frequently traveling around 15+ mph with it--the bumps and cracks come in much quicker succession. The folding hinge actually feels pretty solid and I don't notice any play there so I feel like I can rule that out. The handlebars look like they sit a lot higher because of the upright riding style and I'm thinking that contributes to the flex.
What about cutting the stem and the adjustable post a bit so the handlebars sit lower? Also, are there ways to reinforce the stem?
Gazelle Trim Trophy, EG Bates Track Bike, HR Bates Cantiflex bike, Nigel Dean fixed gear conversion, Raleigh Royal, Falcon Westminster.
Is it flex in the shaft, or is it looseness in the catch/hinge mechanism. If it's flex in the shaft, then shortening it might help. If it's loosesness, you can try shimming the hinge. 15 mph shouldn't be pushing the design constraints at all; my cruising speed on my Downtube IXFS is 17-20 mph, and I regularly spin it over 30 on downhills; it doesn't have a problem with that.
Wanted - SRAM/Sachs Elan 12 shifter and clickstick
I think "wobbly" is the right word, not just forward/backward flex but some side-to-side when I hit bumps in the road. It's hard to say exactly, but from looking at various Brompton and Downtube pics (and the Dahons I own) the KHS stem is smaller/thinner-walled.
Perhaps the shape of the frame (v-shaped) lends to this, as well?
I just got my bike back from the LBS and here was the solution: My folder was neutered. Partially. We decided that because I use the KHS with a BionX motor and that because the stem fold wasn't imperative for my usage, we should just go ahead and replace the stem so that it doesn't fold down. I'm still able to fold the bike in half and make it fit in the car, though.
Heresy, I know, but at least it feels rock solid now.