Originally Posted by
GlowBoy
Disc braking puts quite a bit more bending force on the fork legs than rim braking, and the fork needs to be designed for it (i.e. beefier tubing in most cases). A decade ago when a lot of mountain bikers were trying to figure out how to retrofit their rigs for discs, there were a number of failures of forks that had had disc tabs welded on. When I'm descending out of the West Hills at 35mph every evening, my life depends on my fork, stem and handlebars. I take no chances with those parts.
I'll ask Bike Friday and HPM about their disc options and confirm that their forks are specifically designed for disc use. If not, I can always get a custom fork made to my exact specifications for $250. Combined with the Xootr frame (assuming I'm willing to give up the rear disc -- gotta do some thinking about that) it wouldn't be a terribly expensive route to go.
Since the fork legs are significantly shorter for 20" wheels the bending moment will be much smaller. Although I could believe that Bike Friday would use heavier tubing for their disc fork because they already use different tubing for different size riders, the Swift is a comparatively ham handed design whose stock fork is already overbuilt. I seriously doubt HPM specifically designed their fork for disc usage, but rather simply recognized that the stock fork is already substantial enough to handle all sorts of abuse. Go ahead and ask them if their disc fork is any different from the Xootr fork; I'd be very interested in their response.
Beware that the Swift fork steerer tapers from 1-1/8" (at the headset) to 1" (at the stem riser) and has an unusually large axle to crown height. Although people have upgraded to untapered forks with smaller AC heights, so if you want a stouter fork I'd check out the BMX disc offerings before getting a custom heavyweight. Even BMX race forks billed "for light riders" should be able to take more punishment than you'll ever see on the road.
But given how picky you are about everything from the components, to the geometry, to the design of the fork tubing I think you should just get a Bike Friday. They cater specifically to customers like you.