Dirt Modified Chassis - Fast By Design

A saying in thoroughbred racing, "horses for courses," applies to the ever more popular PMC dirt Modified chassis developed by DIRT star Billy Decker, car owner Randy Ross, crew chief Scott Jeffery, and fabricator Pete Chuckta. The team's ancient but skillfully tweaked "big track" Olsen chassis has won the Eckerd 200 on the treacherous Syracuse, New York, mile three times. But while they were always in the hunt, Decker and Jeffery decided after the 2000 season that they needed something more to have a chance at dominance. Something besides the "cookie cutter" cars seen at the 100 or so short-track events they run on the far-flung DIRT circuit.

Refinements
"We talked to Pete Chuckta and he was willing to get involved. We knew he was our man when he said that he didn't want to do something that everybody else had," Jeffery says. "He started by building the 'cage in halves, like a sprint car. Pete made a jig so we could lay out the whole side of a car. He welds the pieces together, then stands the two sides up and connects them.

"Pete also added bracing to help support the 'cage. DIRT requires 111/42-inch tubing in the 'cage and 0.120-inch wall thickness, so that's pretty standard, but he added side bars near where the radius rods mount and an extra connector where the sides and the top of the 'cage come together. We also wanted a double thickness around Billy's head so we narrowed up the 'V' in the roof. The driver can still get out through it, but a bumper won't fit in there if you're laying on your side and get hit."

"You wouldn't know it unless you measured it, but the 'cage is also taller than the other brands," Chuckta says. "That lets me mount the seat higher, which makes it easier to use the more efficient, non-quickchange rearends that guys use at Syracuse and more and more at their home tracks. The input shaft is higher on them so you need more driveline clearance. Plus, I also like having the body mass of the driver a bit higher to promote roll."

Versatile Rear Axle Locators
The resulting 106-inch-wheelbase chassis was "narrower than our Olsen but wider than a TEO (a predominant DIRT chassis) because we needed more room for the driver," Jeffery says. "We were looking for adjustability, so Pete incorporated brackets for both a panhard bar and a W-link to locate the rear axle laterally. The panhard bar works on dry, slick tracks, but we go to the W-link when the track is rough with a big cushion. It's a lot more forgiving. The panhard bar keeps the car bouncing around on a rough surface and makes it really hard to hold your line in traffic.

"The W-link plants the right rear while the panhard helps keep the left wheel on the track. Brewerton (New York) Speedway, for example, has a lot of bite and the TEOs with a panhard can't swap, so when they slide up into the cushion, it plants the rear of the car too well. When they bury the gas pedal, it lifts the front of the car and they can't steer."

The other unusual aspect of the PMC car's rear suspension is the use of a "Z" link to locate the axle fore and aft, making it resemble a dirt Late Model.

"We were trying to eliminate roll steer," Jeffery says. "We wanted to keep the car tighter off the corner. With the standard birdcage hooked on the bottom to the torsion arm and on top to the rollcage with a radius rod, we didn't have the drive we needed off the corner. It was always loose and wanted to light the tires up. Now we can put more drive on one wheel than the other to help the car come off straight with forward drive."

"There's not a huge difference from one setup to the other, but the W-link does make the car feel different on certain parts of the track," Decker says. "It also gives me peace of mind on a rough track because I know it's a lot stronger, and I don't worry the whole race about it failing."