My friend B just installed a chainguard on his wife's Dahon folding bike which she uses as her commuter. As folding bikes present their own challenges he was kind enough to document the process for us all.
Bike: Dahon Mariner
Problem: Pants in the chain.
Inspiration: Commuting Cyclist's chainguard
First, I ordered the chrome chain guard from Velo Orange for $10. Next, I picked up the Metal Strapping Kit (under $3 at the local hardware store), though it turns out it wasn't needed. After playing around with the chain guard, I realized I'd have more room by removing the plastic protective front chain ring, so off it went.
The chain guard is about 4" longer than the Dahon chain stay, and without modification the upper edge (parallel to the ground) would push it ~1/2" out, causing the chain guard to interfere with the pedal stroke. So, I clamped the chain guard to the kitchen counter and hacksawed its upper edge so that it could be bent up and around the seat stay later on.
The chain guard comes with two clasps, the larger of which does fit around the seat tube and position the guard in the right position. The clasp rests on the rear derailleur cable hanger and pinches the rear derailleur cable a bit, but the shifting is still smooth. No need for the metal strapping per the other website. Lining up the clasp, screw, washers, chain guard and nut was a pain, but brute force triumphed in the end.
With the front end attached, I needed a way to secure the rear end in a way that wouldn't rattle. I thought about using an inner tube to pad the seat stay but settled on using an old mountain bike grip that had been looking for a purpose for about a decade. Duct tape holds the grip in place (and matches the frame color), and a zip tie snugs the guard against the padded seat stay. Some nudging here and there, and the entire chain guard is out of the way of the pedal and doesn't rub the chain. In retrospect it would have been easier to do most of the bending and sharp edge buffing prior to securing the front, but I got it pretty well folded away and smoothed out in place. I was tempted to take it off, but the screw was enough of a pain that I decided to live with it as is.
My wife took it for it's first ride yesterday, and she did say that she felt her pants start to get chomped in the chain once during the ride, but they didn't get sucked all the way in. We'll see how it fares.