ON TWO WHEELS
yield right-of-way to a full-rigged ship on wheels that claimed their ability to maneuver was constrained by draft.
F
Lee's version, based on an old three-speed bike, rigged with the high-wind sail. The rig is made entirely from discarded windsurfer parts. The pedal-powered drive train is left in place, handy for getting out of irons.
I was off. Lee borrowed another bike from one of her friends, now engaged in a discussion with the creator of a walking robot. There was a little bit of upwind work to get back to the RV anchorage, and it was a scary ride while I struggled to learn to handle this machine. I nearly collided with a car made entirely out of marshmallows, almost knocked over some naked people on stilts, and had to
inally back at the RV, I invited Lee and her friends inside the big land yacht to cool off, an invitation they accepted gratefully. "We have everything we need to be comfortable here," boasted the RV owner as he passed out cold drinks. "We even h a v e a n U L PA filter on the AC, which captures particles down to a tenth of a micron. That's onethird the size of your usual threetenths micron HEPA filter. But we neglected to bring bicycles, and this place is far too big and far too hot to see it all on foot." "And too hot to pedal around in a conventional bike," added Lee. "But there's usually a good breeze blowing. We just have to be careful not to get too far downwind, because not all the streets line up for good upwind angles, and short-tacking makes heavy weather of it." "Next year we bring sailing bikes for the whole crew!" asserted the land-yacht skipper. "Some hints for building them," Lee advised, "so you don't repeat my early mistakes." I took careful mental notes. "I first had the mast step attached to the front forks and handlebars," she explained. "Big mistake: The mast step
has to be attached to the frame. Otherwise there's too much coupling between the sail force and the steering. You need to use the bike's fork rake and trail distance for stability, and that works, like, independently of the balance between heeling force and righting moment. And no mainsheet, you hand-hold the boom so you can change heeling moment very quickly as you sail through turbulence. Hand-holding the boom also brings most of the righting moment through your body instead of the mast step, to keep the load on the mast step structure relatively light." "I like the fact that you can sail on two wheels," said the RV owner. "Combining gyroscopic stability with sail control. It seems like a much more elegant solution than those three- and four-wheeled land sailers I see." "Actually," Lee corrected, "gyroscopic stability has, like, very little to do with it. It's all in the trail distance between the ground contact point and the axis of rotation of the front forks."
The mast has to be attached to the frame, otherwise there's too much coupling between the sail force and the steering. "Really?" I questioned. "What about those experiments with the bicycle wheel and the swivel chair we all did in school?" "The gyroscopic force is there, but it's small compared to the centrifugal forces that keep the bike upright. I mean, haven't you ever wondered why a little two-wheel scooter, with a tiny front wheel that has almost no rotational inertia, is still easy to balance?" I confessed that I had never wondered about that, even though I had logged a lot of helm time on one of those scooters in the distant past. Then Lee went on to describe an experiment whereby a counter-rotating rim was installed on an otherwise normal bike, to negate gyroscopic effect, and the bike was still perfectly rideable. "So that's why bikers chop their bikes," added one of Lee's friends. "The increased rake angle increases the trail distance for better high-speed stability." October, 2018 •
Latitude 38
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