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Jun. 3 – With the retuned engine in the frame and the reworked suspension in place, it still takes an experienced eye to see that the ongoing project bike being built by tuner Ralph DiSanto for Mark Burkhart is going to be a Supermoto racebike, not a motocrosser. But that's about to change.
More than any other components, wheels, tires and brakes make a Supermoto bike look unique. And the difference goes far beyond looks.
The
stock YZ450F comes with the usual 21-inch front and 19-inch rear wheels,
wearing motocross knobbies. In their place, DiSanto will bolt on two
Excel 17-inch rims laced to the stock hubs with Buchanan spokes. Inside
the hubs are low-friction ceramic bearings from All Balls Racing.
The
front Excel rim is 3.5 inches wide and the rear is 5 inches wide, a big
change from the stock wheels.
While DiSanto tinkers with other details on the bike, racer Mark Burkhart goes to work with the tire irons (right), installing the Maxxis Goldspeed racing slicks, specially made for Supermoto racing.
The front is a 120/76-17 and the rear is a 165/56-17. On race days, Burkhart will choose tire pressures ranging from as low as 19 psi in the front to as high as 24 psi in the rear. The Maxxis tires also come in various compounds. Right now, for the shakedown run to come after the bike is bolted together, Burkhart is putting on a medium compound.
Choosing a tire is just the first of many race-day decisions a racer and his tuner have to make, however. They must also decide how, when and how much to groove the slicks — if at all.
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Many of the Supermoto racers hand-cut grooves in their tires, "tuning" the rubber to the track just like flat-track racers have done for years. Like everything else in Supermoto, the art of grooving tires is so new, every try is an experiment. The best approach depends on the nature of the track—how much asphalt versus dirt, whether the dirt is packed or loose, and what kinds of surfaces the rider is on when he needs to hook up for acceleration.
And, as Burkhart has learned from some of the masters, sometimes the best grooving is none at all. That's hard for a motocrosser like Burkhart to believe, but he was persuaded by none other than former World Champion Grand Prix racer Kevin Schwantz, who has also taken up Supermoto racing.
"Schwantz actually talked me into not grooving it," Burkhart says. "At first I thought, 'He's nuts.' But then I thought, 'He probably knows something, though.' "
With
the slicks mounted on the Excel rims, DiSanto bolts on the new brake
components. The stock front 250mm floating disc provides plenty of
stopping power on a loamy motocross track, but it won't do for Supermoto
duty. DiSanto replaces it with a platter-sized 320mm Moto-Master disc
from White Brothers. Laid side by side, the difference in the two discs
is striking.
The
front disc (right) is gripped by a four-piston Moto-Master caliper,
linked by a custom-made Spiegler braided-steel brake line to the Brembo
master cylinder. DP Brakes road-racing pads slot inside the caliper.
At the rear, the Moto-Master disc is the same size as the stock piece, but lighter.
The tricky part is squeezing that 165-section rear tire into a swingarm that's built to carry a 110-section motocross tire. It takes the precise use of spacers, a lot of patience and some carefully applied force, but eventually it all goes together. The fit inside the aluminum swingarm is snug, to say the least.
The
major parts are all in place, now, and Burkhart's racer is undeniably a
Supermoto bike, not a motocrosser. But there are still loads of details
to finish.
Cycra, one of Burkhart's top sponsors, provides the handguards and the neat front fender, specially made for Supermoto and less ungainly than the big motocross fender.
Works
Connection provides a skid plate (right) and radiator and frame guards.
One slick piece from Yamaha is the carbon-fiber guard over the rear
brake disc. The stock plastic piece tended to melt from the heat
generated by the hard braking of Supermoto action.
With
sponsor graphics in place, the bike is ready for a shakedown run at the
nearby kart track where Burkhart practices.
After a few laps around the tight and constant turns of the track,
the tires are smoking hot, but Burkhart has barely broken a sweat under
the early-summer sun. After years of enduring the rigors of motocross,
Burkhart finds Supermoto to be far less exertion.
It's kind of like trading the life of an NFL wide receiver for the job of an Olympic hurdler — still lots of speed, but with less of a pounding. And more tire smoke.
With a full season of experience behind him, and an even better bike beneath him, Burkhart plans to fight for the AMA Supermoto title this year.
"I
know I could have won last year," Burkhart said. "We're trying out stuff
that works better and better, all the time. We'll definitely get faster
this year."
If you think what goes into building the bike is interesting, check out the nearest round of the AMA Supermoto Championship. You'll find that what comes after the building process — great racing — is even better. Catch all the details on www.amaproracing.com
Related story:
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Project Supermoto, Part 1
From stock to box: The bike is torn down -
Project Supermoto, Part 2
From frame to game: The build begins -
Project Supermoto, Part 3
Wheels, tires—and laps
© 2004, American Motorcyclist Association