The Typical Shorttracker
the typical shorttracker
What It Takes To Win, Or Lose, Gracefully
JUST ABOUT any 250-cc engine can be made to work well in professional short track racing. Jimmy Odom used a Bultaco Pursang two-stroke Single to win the Astrodome, and Al Kenyon used a similar rig to win the Garden. Mark Brelsford qualified fastest at the Dome on a Harley-Davidson four-stroke Single. Bob Bailey uses an old 250-cc Honda Twin with some success. Kenny Roberts won the Astrodome’s Junior class with what began life as a Yamaha DTI Enduro engine.
All successful short track machines have several things in common. Sheer horsepower is not necessarily one of them; the power band has to be usably broad. The machines are extremely light, with relatively short wheelbases, and moderate rake and trail for quick handling.
Neil Keen’s (55) rig is powered by a
mildly reworked Yamaha 250. The power peak has been raised, but the usable power band is still 4000 rpm wide. The cylinder finning has been cut down to reduce weight. As cooling is not an important factor in short races, the extra finning is not missed.
The engine is housed in a special Trackmaster frame, designed specifically for flat track racing. The wheelbase is variable from 54.5 to 57 in. The shorter settings are used when the track is smooth, while the longer ones are used for bumpy tracks. The goal in arriving at a proper setting involves a compromise: you need the quick handling of a short wheelbase on a tightly turning track, but must sacrifice that for the stability and slow reaction to deflection offered by the longer wheelbase on bumpy tracks.
Keen’s forks are Ceriani, similar in
design to the road racing forks, but with 250 motocross tubes, which are 2 in. longer. The rake is steep, about 25 degrees, although that is variable, too, and changes about 1 degree for each inch the fork tubes are moved upward into the steering crown. Trail—which determines resistance to front wheel deflection, as well as the ease with which the bike may be pointed to the inside of the turn—varies between 3.5 and 4.5 in. The less trail, the easier the bike turns.
It’s worth noting that all the bits and pieces used in Keen’s bike, which carried him to 2nd place in the fourth heat at the Dome, may be bought by anyone.
H-D factory rider Mert Lawwill’s machine (6) is based on the pushrod Sprint CR250 Single, peculiarly adaptable to short track because of its low eg.
His long-time friend and “wrench” Jim Belland built the single toptube/double sling frame to Mert’s specs and is making copies for other riders. Lawwill also uses a Ceriani road racing fork. “Ballpark” estimate of rake is from 25 to 26.5 degrees; trail is about 3 in. He, like many other professional riders, is reluctant to give the exact figures, as it takes a great deal of effort to arrive at a workable combination.
Jim Rice’s pretty BSA 250 is essentially a homebuilt rig with a doublecradle frame carrying the oil supply. The production pushrod engine, not exactly a powerhouse off the shell, has been pushed to an output of roughly 30 bhp. Rice elected to mount a rear brake, which is optional under AMA rules. Like Keen and Lawwill. he mounted Pirelli Universal 4.00-18 tires for the Astrodome event.
