Spent the last week designing and building the new steering mechanism for the Kalamazoo. After last week's disastrous road test, it was critical that this problem be solved. And while what I came up with isn't pretty, it looks like it will work okay.
In short, I bolted on a lever to the side, which connects to the caster via a connecting rod and ugly universal joint system. The motion is actually pretty nice, even when stationary. (On a nice smooth concrete slab, anyway.) I need to offset the lever above the deck to make it easier for the driver to reach, but other than that it's a much nicer interface than I expected. That was always a driving motivation for the old differential braking system -- how do you pump and steer at the same time? While not perfect, the lever approach is pretty decent. And it feels very awesome. Part rudder, part steam locomotive control. Not bad.
Today's road test did conclusively show that only controlling one caster was not enough. Particularly with any back-and-forward motion it was very easy to get in a bad state where the uncontrolled casters was out of sync, sometimes to the point where the Kalamazoo was completely stalled. It will be easy enough to add a tie rod over to the other caster. I always figured this might be needed, but decided to be lazy and try it this way first. Very little work wasted.
The other big issue coming out of the first road test was the failure of my hacked-up fixie wheels under load. So this week I threw a gob of cash at the problem and bought new ones. Unfortunately, no one makes real 20" fixie wheels. But I got a system the bike store people thought might work. Today's road test showed that, no, it doesn't work. This leaves only one palatable option -- I need to switch to full size bike tires. I only went with 20" because I wanted to keep the deck lower for stability (less of a concern now that I've been on the thing for real) and because I was using 12" pneumatic casters in the front. I might just have to live with an A-Team-van-style raised back end, or shim the front casters by a few inches. We'll see.
Overall, while the broken fixies were a big disappointment, the second road test went pretty well. I was even able to drive (slowly, jerkily) up a slight slope, showing the gear ratio changes are helping. There might be one more improvement there, if an ebay shipment arrives in time. I also have a big hunk of lead strapped to the crankshaft to improve its flywheel effect, though I'm unclear how much that actually helps at these low speeds.
On the way back down to the garage I road it and mostly let gravity do the work while I steered and counter-pumped for braking. This worked great, until it got to the steep section and I had to navigate several parked cars. At one point the rear end lost its grip and I fishtailed most alarmingly towards a well. No control authority, just suddenly riding along on the world's stupidest surf board. Important lesson: don't drive the Kalamazoo obliquely across an incline.