Work is underway on the Skibike MkII!

The URBC is attempting to take the skis-instead-of-wheels principal to its logical extreme - full suspension. I scored a full suspension MTB frame, probably a Huffy or equivalent, that had been burned in the Cerro Grande fire. Most of the parts were completely melted, but the front triangle is intact. I pryed, pounded and cut all the melted parts off, filed off all the cable stops, stripped the burned paint and rust, and painted it with some nice "Old Ford Blue" engine block paint. Now for the fun part - suspension!

We scored an old Manitou elastomer based suspension fork that was easily converted from 2.5" to about 4.5" of travel by removing some bump stops and using hardware store springs instead of the elastomers, which were far too progressive to allow full travel. This would be bad on a mountain bike because the fork is much longer, but on the skibike taller is better. I also replaced the steerer and fork crown with a narrower one to make the fork more compact, increase the lean angle before the axle will hit the snow, and let us use a 1 1/8" stem. To keep the front ski level, we built a strange spring device that attaches to a replacement brake bridge we fabricated.

The rear suspension is based around three things we were able to scrounge from various sources. We got some 1" by 2" aluminum box section from a machine shop scrap bin, some easy to use bearings from the hardware store, and shocks from a Bultaco Frontera. The shocks were the best score, as they should be just about perfect, if a little stiff. We used the pivot point on the frame to attach a swingarm to which the rear ski mounts. The shock mounts about halfway down the swingarm and uses the frame's mounting points for the other end. This shock is so dang stiff we are doing everything we can to exert as much leverage on it as possible. No actual math is involved - we're just eyeballing it.

As for frame geometry, experience with the first skibike showed that a steep head angle is a good thing for keeping the steering from being too powerfull and causing the rear end to come around. Building our own suspension is nice because it lets us adjust the height of the rear end. Higher means more clearance, more potential for suspension travel, and steeper head angle - so we went pretty high. I would say the head angle is about 10 or 15 degrees from vertical. We'll se how it works. The rear end should be roughly adjustable for height so the geometry can be tuned a bit.

Ok, so I now have the rear end built up and guess what? The shock is WAY too soft. I sit on it and squish - 8 inches of downward travel under bodyweight. Luckily, the swingarm also feels much to long. The riding position feels really far forward, especially with the steep head angle. I think I will chop 2 inches off of the swingarm (from 18" to 16", pivot to pivot) which should do alot to stiffen the rear end, but I imagine I will have to move the pivot point of the shock too. This will be a little funky because it is so clean right now and adding little triangular extensions will be less than elegant. Dang. So much for eyeballing it. At this point the pivot was on the bracket in the notch of the "V" on the frame, where it was designed to go, but of course the frame was designed for a high springrate low travel motocross shock. This was a super clean setup and it moved the mass of the rear shock more towards the center of the bike, which was nice.

So I moved the upper pivot of the rear shock to just behind the seat tube. This is about as maximum-compression-force, minimum-travel as I can get and it seems to work just dandy. We should still get 8 or 10 inches of rear travel too :) Now all I need to do is figure out some kind of multi-link swingarm fork to get 10 inches up front!