2. Plain Geometry 101: Custom frame


The street-tracker thing I do, which is a dirt-track-looking bike you ride on the street, well, dirt-trackers don’t work on the street. More trail means more stability, more offset on the triple clamps is less trail. The first street-trackers I built, I used dirt-track frames and dirt-track clamps, and you couldn’t keep your hands on the bars at 70 mph, they wanted to toss you off. The reason being that when you lay the bike over, trail increases. And when it goes past the sweet spot and becomes too stable, when you hit the throttle the rear end wants to step out and throw you off. Having a ton of offset and less trail keeps the front end loose so you can steer with the throttle, you can do a big controlled slide.

Hauling Ass: Mule knows whereof he speaks.

Hauling Ass: Mule knows whereof he speaks.

When guys slide on the road, it’s usually right before a high-side. The forgiveness range is very narrow. If you get on a dirt-tracker with a lot of offset, you can control it really good. I know this because the first short-track bikes I built, I was using streetbike clamps and streetbike forks, and they were real twitchy because the front end was so planted.

I found out over a dozen builds that we had to shorten the triple clamps, but the wheels had to stay out here, clear of the motor. So we had to start moving the steering head forward, a gooseneck thing, instead of bringing the triple clamps back; now they’re stable.

A stock Triumph has too much trail; they steer really, really slow. The sweet spot is like 95 to 100mm trail; the Bonneville’s got like 120mm. When you get into the 75-80 range, you’re coming off. What I generally do is shorten the forks and jack the back and it has the effect of pulling the front end in slightly. I make them with eccentrics, too, so you can adjust the offset, but not many guys do. I had one at (Wayne) Rainey’s house, and Roberts was going on about, “Oh, you have to have an adjustable steering head, you’ve gotta be able to adjust the angle…,” and this other guy, Nakajima, who’s the guy who designs all the MotoGP bikes for Yamaha, he’s looking at it too. And he says to Kenny, “What are you talking about? You don’t know how to ride.” Mr. Nakajima really liked that bike.