RADIO-CONTROLLED RZRs?

No fear of big jumps

— When I moved to California to work for what was then 3&4 Wheel Action magazine, my new friends and roommates were into radio-controlled cars and trucks. We built tracks in our backyards, raced our R/Cs, and learned how to work on them and make them better. Long before GoPro helmet cameras, I often wished we had a small video camera so I could see the track from the R/C’s view, like I was actually inside the cheapest form of motor racing.

Looking back on those early days of radio-controlled racing, it’s amazing how the technology for scale models influenced Trophy Truck design (and vice versa) and now our UTVs. When I got into it, gas conversions for 1/10th-scale electric trucks were just becoming available.

My first R/C was a Losi 1/10th-scale truck with a “five-link” trailing-arm rear suspension, much like today’s RZR XPs and Mavericks. This electric R/C was well used, and the electrics were pretty much used up, so I got fed up with messing with it and bought a used Kyosho USA-1 1/8th-scale nitro-burning monster truck. Kyosho had introduced its Stadium Truck, which used the same basic chassis and suspension as the USA-1, and it started a new class—1/8th-scale gas 4WD trucks. I bought Stadium Truck wheels and tires, which were scale versions of the BF Goodrich KR2 A/T UTV tires we use now.

They also came with tire foam, precursors to today’s ATV and UTV Tire Blocks. I glued up a set, tested them and then grooved fronts for more side bite and rears for more forward bite, like Nic Granlund does on his WORCS YXZ1000R.

I scrapped the USA-1’s eight plastic shocks, removed the shock towers and fabricated new towers out of the Losi’s carbon fiber chassis. While the Kyosho Stadium Truck had long rear and shorter front shocks, I bought four rear aluminum upgrade shocks and matched them to the taller front and rear carbon fiber shock towers. I also installed front and rear adjustable torsion bars and set preload for a very low ride height for turning. In the air, longer shocks extended to soak up the landing, just like Micky Thompson Grand Prix, CORR, TORC and Lucas Oil Trophy Trucks.

Standard 1/8th-scale 4WD design has a 3.5cc nitro-burning engine driving a center gear via a centrifugal clutch, and the gear is attached to a center differential with front/rear brake rotors, so the center diff drives the front and rear diffs. The USA-1 didn’t have a center diff at all, so it pushed the front end in turns badly. I bought a one-way bearing main gear kit, so the front wheels were only driven when the gas was applied, and letting off allowed the front wheels to freewheel and turn in better.

This was almost 20 years before Polaris’s On-Demand 4WD system, but it was a more simplistic version that vastly improved my Stadium Truck’s handling. Front and rear diffs were filled with medium- and high-viscosity fluids, respectively, while the center diff had lighter fluid. I filled the front diff with heavier fluid and the rear with grease for maximum drive. Between that and the grooved tires and longer-travel suspension, my homemade Stadium Truck ripped like a full-scale version.

I also adjusted brake bias so that when I let off of the throttle completely, the rear brakes would drag slightly. This would set the front end for better turn-in traction and drift out the rear for turns. In the air, the rear tires would lock up to drop the front end for down-slope landings. Once I got it set like I wanted, I wouldn’t change anything except shock fluid weight for each track’s conditions.

It could charge whoops and skim the tops like a full-scale Trophy Truck, but it was hard on diff gears, so I upgraded to high-grade steel bevels like those in today’s UTVs.

With an air-cooled engine and a boxy Stadium Truck body, one had to be careful not to lean out the jetting too much for fear of damaging the engine. We cut out the “windshield” for cooling air to the engine and also the other side for quicker pit stops via a zip-tie on the fuel cap. I ended up with basically a half windshield like we use on today’s UTVs. I also used scrap body plastic to duct cooling air around the head fins. This let me lean out the mix more for better power.

I also made front and rear lower-shock and CV guards out of scrap plastic, like Assault and Fox do for our UTVs today.

At my first Gas National in Hemet, California, I won the B Main, which seeded me into the A Main, but I didn’t do so well against a deep field of pros. I was stoked, though, and flew to the next round near Andy Griffith’s Mayberry neighbor, Mount Pilot. The track had a huge jump only my Stadium Truck would clear, and I got as high as third in the A Main but had to settle for fourth.

A GoPro would’ve been great to use in North Carolina, because I’m never going to go that big in a UTV!

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