December 9, 2014
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Do you remember Gyro Gearloose? He was an anthropomorphic chicken created by the Walt Disney Company who appeared in the Scrooge McDuck comic-book series. Gyro was the town of Duckburg’s most famous inventor. Because his inventions were rarely successful, the name Gyro Gearloose has become synonymous with any inventor with a crazy or crackpot idea. Motocross has had more than its fair share of Gyro Gearlooses — some amazingly successful and others forgotten. MXA presents the latest installment in its series of weird and wacky inventions.


ATK 406.

Look closely at the early model ATK 406 two-stroke above. Although it was built long after water-cooling had become the norm, a large number of offroad, cross-country and enduro riders didn’t trust radiators and wanted an air-cooled bike. Horst, with the help of AMA National Champion John Martin, gave them what they wanted. Although the Rotax engine was long in the tooth, Horst redesigned the intake to produce more power (and the air filter was placed in the black box in the fuel tank). If you look at the rear wheel you won’t see a rear brake. Why not? Because Horst wanted to decrease unsprung weight, so he mounted the rear disc on the countershaft sprocket.

1993 ATK 406 two-stroke. Note that the rear brake pedal is mounted backwards so that it can’t be bent backwards in a berm.

In this photo you can see several unique features of this 1993 ATK 406. The chain is routed over Horst’s patented A-Trak system that removes chain torque from the suspension when accelerating. The rear disc is behind the countershaft guard. Horst even put the gas cap high up and angled it to the side to make it easier to put fuel in. The chromoly frame was nickel-plated. The swingarm is steel (and lighter than the aluminum swingarm).

ATK 604 four-stroke.

At one time, and for many years, the ATK 604 was the largest selling four-stroke motocross bike made. It sold for twice the price of a Japanese 500c two-stroke, but people lined up to buy them through the 1980-90s. The ATK 604 used a Rotax engine, single no-link shock and many of Horst’s others innovations. Later, Horst would replace all of the aluminum swingarms on his bikes with rectangular steel swingarms. Contrary to what people believed, the chromoly steel swingarm was two pounds lighter and 30 percent stiffer than the aluminum one. Horst would later add an electric start version and claims that no one ever ordered the kickstart version after that.

AMP Research KTM 125SX Prototype.

Horst sold ATK in 1990 and started AMP Research, which would later become a big player in suspension mountain bikes (his own bikes plus prototypes for Mongoose and Specialized), Bed Extenders and truck accessories. KTM commissioned Horst Leitner to build them a prototype 125cc two-stroke (above) in 1990. Definitely not a man to play it safe, Horst designed a rectangular chromoly steel frame that only used three tubes. The engine hung from the frame and there was no frame cradle (just a skid plate). A long single radiator was mounted on the right side of the frame, while the exhaust pipe went up the left side (where the other radiator would have been). The muffler exited directly behind the engine through the hole where the shock would normally go. The single shock is visible behind the aluminum air boot and is mounted on a strut and angle radically forward. The bike was incredibly light. KTM paid Horst for the prototype, but didn’t produce it. Late in 2013 Horst sold AMP Research to an automobile accessory company and retired.


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