Some of you may remember that, way back when, I used to build frame/fork combos that used the Actiontec suspension systems (a single piston, with the internals inside the steerer/head tube). They weren't the greatest suspension (50-ish mm of not-so-buttery travel) but they did have a big advantage in the customization (any length/rake/style you want!) and weight (~3 pounds) departments.
I stopped building those forks because I had a lot of problems with the bearings (the AT system uses a sort of splined piston with bearings running in the grooves) wearing out their grooves - the forks would develop horrible *angular* play, which was *really* disconcerting. They had to be sent in constantly for rebuilds, and I eventually decided that the hassle was not worth it.
Fast forward a few years and I'm now playing with a slightly different option which I think will work out better - using Cannondale Headshok suspension units with custom lowers to build a custom suspension fork. I did one earlier this year and it went well, but required a lot of extra effort to tear down/cryofit/rebuild the suspension unit. I thought I'd see, with this project (experiment/prototype/dumb idea) if I could build a clamp-on set of lowers that would hold up to serious mountain bike use.
And, to be honest, I think I actually have. The suspension unit here is from an old Fatty Ultra (thanks to Cory for the donated old headshoks that I've been playing with!) which, unfortunately, does not seem to hold air. So for actual testing purposes, I'll need to deal with that. Regardless, I turned down some 1.5 x .120" 4130 on the lathe to make the steerer sleeve (and make a lip for the shock boot to fit over as well), welded a ring into the bottom as a stop, then split it and brazed on 2 beefy M6 pinch bolts. From there, it was just a matter of building a standard (albeit shorter than I'm used to) 29er fork.
I did manage to outsmart myself a couple of times and learned a couple of lessons about welding fork blades to split rings, as well as figuring out that boring out a Paragon hooded dropout does NOT work well for a through axle (after some futzing around, the fork now takes a 9mm QR axle instead, c'est la vie) but the end result is still pretty good - a 1400g suspension fork with 80mm of relatively functional (if I can get it to hold air...) travel.
And of course, you can build around any size wheel you want. 650b? No problem. 24"? Sure! 36er? Just wait!
The downsides, though, are numerous. The fork requires an oversized head tube (a 44mm ID one will work, which is nice) and the axle to crown length is quite ridiculous for 80mm of travel - 520mm or so. That's equivalent to a 120mm travel Fox, for example, and it means that these things are really only practical for very tall people who want something really light without much travel. Pretty limited group, there. And of course, acquiring the headshok unit to begin with can be tricky. So long story short, I'm not really expecting to sell a lot (or any?) of these, even if I decide this one is a worthy fork. But it was a fun project to build when I was sick on a weekend.
Unfortunately, now I need to build myself a frame to actually ride it...