When is a motorbike not a motorbike?
This is one of the trickiest motorcycles you’ll see in these pages. It definitely looks like a motorbike and rides like a motorbike, but the trick is that it’s not made with a single motorcycle part. That’s the challenge Kim accepted when he set out to build it two years ago. He admits to spending a lot of time on internet forums and this annual competition offered the kind of challenge that fired up his engineering creativity.
He’s already building a Harley-Davidson bobber, which he’s nearly finished, and while that kind of customization might satisfy some builders’ need for expression, Kim couldn’t resist the freedom of having just one rule: you can build anything you like (apart from a death-trap) as long as you don’t use any motorcycle parts.
As soon as Kim found an interesting motor and frame from someone else’s stalled attempt, he was on his way. “The previous owner had done a great job but I had a different vision to his. I changed it a fair bit. I lowered it quite a bit, changed the angle of the downtube. In fact only the backbone is the same. I like the line to the back axle.”
The bike, especially with that vertical downtube, has a real vintage-racer vibe and Kim says that that was the look he was going for. Hence the leaf spring front end. The bike uses the rear tyre from a fat-tyred mountain bike but he didn’t want to use the push bike’s modern headset and fork, so that became a stylish front end on one of his trikes.
Kim welded a box under the steering head to locate half a leaf spring. He cut down the leaves and reduced the overlap to make it softer. He also built the forks, and the rockers and rods that transfer axle movement to the spring. “The geometry is amazing,” says Kim. “It’s brutally low, so you have to be careful, but you can lean it right over and it responds beautifully. It’s a real thrill.” That’s not entirely down to luck: “I calculated the rake and trail and thought it should work.”
Kim loves two-stroke engines and the unusual engine in this one hooked him immediately.
“It’s a Wajax. They were commonly used on fire pumps,” he explains. “In the US they were called ‘Disston’ and were used on two-man chainsaws with about a 2m bar. It’s a twin-cylinder 180cc motor and it puts out about 9hp [6.7kW]. It’s a really interesting motor.” Restoring the magneto would have been a big job so Kim fitted two old car-type ignition coils, one feeding each plug. They are powered from a battery that lives in what looks like an oil tank. It’s a total-loss system but Kim says that he gets about two hours of riding out of it. Perhaps the most impressive part is not obvious from the side view. When he bought the bike it had a gearbox with linkages fashioned from Ford Escort handbrakes. Ingenious, but it was inclined to throw belts.
Kim had another idea. He installed a continuously variable transmission repurposed from a wakeboarding winch. The engine drives a cone on a shaft which is linked via a rubber belt to a cone facing the other way linked to the rear axle. The belt finds its own ideal position between engine speed, torque, and the load on the rear wheel.
Best of all, all the entrants in the competition will get together at a meeting in November to compare machines and to race.
“It’s not slow,” says Kim. “It’s by no means the fastest, but it’s got quite a look about it and it’s a pleasure to ride.”
“It’s a twin-cylinder 180cc motor and it puts out about 9hp [6.7kW]. It’s a really interesting motor”