In the late 1990s, Lance Smail was a lone wolf competing in the vaunted AMA Supercross championship series on this Tom Moen-prepared, four-stroke powered KTM. By virtue of being the sole four-stroke in a series populated exclusively by two-stroke machines, Smail and the KTM were instant outsiders. However, at every stadium the bike appeared, the fans went crazy for it. In a field of buzzing, shrieking two-strokes that sounded like agitated bees, the distinctly aggressive thumping of the KTM, with its signature backfire, made it a crowd favorite. (Click image to enlarge);%20function%20voidFunc()%20{%20}%20voidFunc();)
At that time, the use of four-stroke technology in off-road motorcycling, due to the relatively complicated and heavy workings of valves, camshafts and timing chains, had pretty much been relegated to recreational use. Naturally, when the one-off KTM lined up next to the all-conquering two-strokes in their sacrosanct domain of Supercross, it bordered on being little more than a sideshow.
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To help put it in perspective, internal combustion engines have four basic phases—intake, compression, power and exhaust. Four-stroke engines require four strokes of the piston in order to accomplish the process (hence the name), with a power stroke being generated every two revolutions of the crankshaft. Two-strokes, through an ingeniously simple design of integrated transfer ports in the cylinder walls, combined with the science of back-pressure generated by the exhaust system, are able to achieve the essential intake/compression/power/exhaust phases in just two strokes of the piston, thus producing a power stroke with each revolution of the crank. The design eliminates the need for intake and exhaust valves, camshafts, and timing chains. As a result the engines possess very high power-to-weight ratios.
Unfortunately, the very heart of the two-stroke’s engineering necessitates the lubricating oil for the piston and top-end be mixed with the gasoline. This in itself presents obvious problems with regard to air pollution; as a two-stroke spews burnt oil directly into the atmosphere. More damning still is the fact that two-stroke engines are not very efficient. Even the best-tuned units will lose a percentage of this gas/oil mixture, completely unburned, due to an unavoidable degree of seepage from the exhaust port. The fallout? Two-stroke engines are going the way of the dinosaur. In their place a new breed of high performance, cleaner burning four-strokes have taken over the racing world. However, the motorcycle pictured here earned its place in the annals of Supercross as being the first.
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The somewhat antiquated motor, borrowed from one of KTM’s more sedate enduro models, was bored out to 538cc to make full use of the AMA’s displacement allowance for four-strokes. Every aspect of the engine was modified to achieve the kind of performance necessary to go up against the exotic machines in the competition’s arsenal. A lighter clutch was installed and a full pound-and-a-half shaven off the bulky crankshaft. In the end, the motor produced enough power to eliminate the need for the low-end pull of first gear, as well as the seemingly endless top-end available in fifth, so they were pulled out to save weight, making the bike a three-speed. With prodigious use of lightweight parts they were able to get the bike’s overall weight down to 243 lbs (compared to an average of 220 for the competition).Moen then concentrated on suspension and handling to augment the four-stroke’s controllable, smooth power delivery. The first tests of the machine as a viable Supercross tool proved that the comparatively smooth power delivery had some advantages over a two-stroke’s notoriously instant throttle response. The big KTM was more forgiving and easier to ride.
Less than a decade since that watershed Daytona event, manufacturers have turned their collective engineering acumen toward developing a new generation of highly competitive, lightweight four-strokes. In fact, with the proliferation of thumpers seeding the starting gates of motocross and Supercross events today, it is hard to imagine just how novel this machine was in those early appearances. Smail, Moen and the four-stroke KTM were true pioneers that cleared the way for the rest of the pack.
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Specs
ENGINE
538cc, liquid-cooled,
SOHC 4-valve single
MAX HP
52 @ 8,500 rpm
FUEL
VP Racing C12
EXHAUST
FMF Racing
DRIVE SYSTEM
3-speed transmission
with wet clutch
WEIGHT
243 lbs
FRONT SUSPENSION
Fully adjustable
50mm Marzocchi forks;
11.0 inches of travel
REAR SUSPENSION
Fully adjustable Ohlins shock
with piggyback reservoir and
progressive rate linkage;
12.2 inches of wheel travel
