1978 Scorpion RD-4

Independent Front Suspension (IFS) in its infancy
1978 was a winter to remember! Not only for the glorious amounts of snow, but for snowmobile innovations as well.

In April 1978 the Scorpion brand was sold to Arctic Cat. At the time, Scorpion was still using leaf spring front suspensions, but one of Scorpion’s engineers, Gerald “Buzz” Irvine, had a concept to develop more travel in front and rear suspensions. He made some great advances… with a little help from his friends.

Jersey Boys
Buzz’s help came from two racers from New Jersey, Dan Oostydk and Dave Echardt. They came to Crosby, Minn. to race cross country for Scorpion and were hired as test riders working with Buzz on his new concept of independent front suspension. Their first test sled was named RD-1 and was quickly worn out in testing on dirt in fall of 1978. They ran this sled until the tunnel buckled! Then came RD-2 and RD-3 with more refinements, but they still weren’t up to snuff. Finally, the RD-4 came along! I happen to own the RD-4, preserved in development stage.

Dan and Dave started racing cross country with the RD-4 concept sled in the late ’70s. At first they had trouble with the steering pulling them off the trail, so more work was done to prevent “bump steer” which meant better steering in rough conditions. Dan said it worked great in the rough trails. With his sled having almost 14 inches of front travel, but they had more problems with the rear suspension.

RD-4 had about 7 inches front and rear suspension travel on the test mule. Also adjustable coil-over spring shocks were used back then. They used handmade skis with the ability to choose five different holes to adjust the ski forward or back for different steering performance in varying conditions. Skis came with handmade ski loops made of rubber hose. With this fade-away front suspension, use of a torsion bar was a must.

The sled was driven with a Power Thrust drive clutch and Cat’s D-Drive 13-inch driven clutch. The idea behind this direct drive system was to cut weight and cost by not using a chaincase and jackshaft. They had to use 4.5-in. smaller diameter drive sprockets to get the right ratio. The 15x118x.75-in. rubber track came clipped for the slide rail suspension to ride on. The drive shaft extended through the tunnel on the right side so a 10-in. disc brake could be used.

The RD-4 had a rubberized front belly pan and side panels for durability. It looked like they used a Brut fuel tank on this sled too. It had a nice 8-in. thick seat with a good back rest to sit back and enjoy the ride. The RD-4 had a Scorpion handle bar pad to finish things off. The sled used Arctic Cat brake and throttle controls with high and low headlight adjustments, and an on-off emergency shutoff. They finished the look of the sled off with a nice Scorpion red hood.

The engine was a Cuyuna 438cc 6-bolt head using special adaptor blocks for Mikuni round slide carburetors. It came with CDI ignition for better starting and reliability.
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