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Home > LATEST STORIES > BEHIND BARS The Development of the World’s Fastest Doorslammers—Part 1: Pro Mod chassis

BEHIND BARS The Development of the World’s Fastest Doorslammers—Part 1: Pro Mod chassis


Written by John F. Katz
Photography Courtesy of Chassis Engineering and the FSC Staff

The cackling, record-blasting, fin-backed carbon-fiber phenomenon that is Pro Modified racing emerged some 27 years ago from the IHRA Top Sportsman class, and from the nitrous-snorting, mountain-motor outlaws it attracted. Pro Mod has since spawned its own brood of super-lightweight sub-classes that spurn the major sanctions to scorch eighth-mile tracks on their own terms. Our purpose here is not to recount that history, but to outline the significant chassis developments shared by the broader Pro Mod family—development that has included “tremendous changes,” according to Jerry Bickel, of Jerry Bickel Race Cars in Moscow Mills, Missouri.

“The cars keep running faster,” added Rickie Jones, co-owner and vice president of Quarter-Max Chassis & Racing Components in Galesburg, Illinois. “The engines keep getting bigger, hitting the cars harder. Jason Scruggs, in his RJ-built Pro Extreme Camaro, has run 3.492 and 223.54 mph in the eighth-mile. He passed the 60-foot mark in just .885 seconds. That’s on the verge of Funny Car performance. And with that much power, everything gets worked harder.

It’s not surprising, then, that the car constructors we contacted count the double-rail chassis to be the most significant development of the past 25 years. About 10 to 14 inches above the main frame rail, a secondary rail connects the four-bar bracket in the rear to the crossmember at the front of the car. Appearing first in the early 90s, the double-rail chassis is now considered one of the defining features separating a Pro Mod from a Pro Stock.

“Back in the day, we built the chassis kind of loose,” recalled Clayton Murphy, owner of Chassis Engineering in Riviera Beach, Florida. “But we didn’t have the track preparation that we do now. We didn’t have the shock technology, or the tire technology. All of those things have come a long way in the past 10-15 years, even more so in the past five years. So now we’re making cars as stiff as we can make them.”

“You see more X-braces in the back of the chassis,” added Jones. “Across the board, you see more tubes.”

More tubing, yes, but some of it is now smaller in diameter, as builders balance stiffness against weight.

“The main structure of the car is still 1-5/8,” said Murphy, “but the front frame rails might be only 1-1/2, and bars that hold the front strut towers can be 1-3/8.” This trend is reversed in the suspension itself. “We were finding flex in the lower four-link bars, so instead of an inch or 1-1/4, they are now 1-5/8.”

RJ Race Cars, the car-building arm of Quarter-Max, has also increased the size of its four-link bars. The company’s anti-roll bars are thicker, too. “And our four-link brackets are heat-treated now,” said Jones. “Even our rod ends are heat-treated, military-spec, Teflon-lined units from Aurora Bearing, which are all manufactured in the United States.”

About five or six years ago, Bickel began developing specific chassis for supercharged, turbocharged, and nitrous cars, respectively, relying on input from some of his fastest customers: Rickie Smith and Pat Musi on nitrous; Frank Manzo and Howard Boone on blowers, and Mike Moran on turbos.

Steve Summers’ Jerry Bickel-built Pro Mod

“The turbo chassis is the strongest,” Bickel explained, “because the turbocharged engines are the most violent. So they are considerably more braced in the four-link area than the other two chassis. And we change the suspension as well, with different shock valving, different strut travel, even a different engine location.” Additional alterations accommodate service accessibility: “We run the tubing on a turbo chassis so it’s easier to mount the turbos up front.”

Chassis construction is also influenced by differing minimum weight mandates.

“In an NHRA-legal car,” said Jones, “which weighs 2,500 pounds or more, everything is built stronger, so you know it can do its job. It’s a little more challenging when you build a super-lightweight, like a PDRA Pro Extreme or Pro Nitrous car, because it is difficult to save weight and not compromise strength. My Dad and I competed in Pro Nitrous in 2011-12, and applied what we learned to product development. Back then there was no weight rule.” (Now those cars must meet minimums, too: 2,350 pounds for Pro Extreme (2,125 with a small-block); 2,375 for Pro Nitrous.) “You could really build a very lightweight car,” Jones continued, “more like a Super Gas car. The lighter it is, the easier it is to get it moving. But there’s a point where compromising strength compromises performance as well, because your engine power is going into wasted motion in the chassis.” Building a lightweight car, Jones emphasized, “is always a tradeoff. We look at every part of the car, every piece of the chassis, and ask ‘What’s the minimum we need here?’ And if we can take one pound out of each of 100 parts, then we’ve built a car that’s 100 pounds lighter.”

For all of these internal changes, the outside dimensions of Pro Mod and related cars have changed very little. One of the defining rules of the class remains a maximum wheelbase of 115 inches, with the engine set back a maximum of 10 percent from its stock location. Builders can, and often do, choose a slightly shorter wheelbase. Bickel prefers 112. RJ Race Cars has built cars as short as 108. “The longer you make the car, the heavier it is,” Jones noted. “If the wheelbase is five inches longer, that’s five inches more body, five inches more tubing. But as you make more horsepower, a longer wheelbase makes the car more forgiving, with better handling at high speeds.”

In addition to stiffer, more sophisticated, and more specialized chassis, Bickel cited “tremendous” improvements in safety as well.

“That’s been another big jump,” Murphy agreed. “We’ve seen how the head and neck restraints save lives. The pour-in seats (required by most sanctions for at least the past five years) definitely save people’s backs.” Lighter helmets have also reduced neck injuries without any loss of crucial head protection.

Jones pointed to the “high-density foam pads that fit around the driver’s cage—and the new seven-point belts keep the driver’s hips firmly planted in the seat, so the top belts can flex and absorb impact.”

“We’ve also learned a lot about the cage,” Murphy added, “about where the bars need to be placed, and the size of the material. Now we put two bars behind the seat, instead of just one, so even if a tire explodes, nothing can get into the driver’s compartment. And all the cars now have fender headers. We used to raise the rocker bar four to six inches where it passes under the A-post, so we could fit the headers under the car. Now we don’t have to do that, and we can give the drivers a lot more room for their legs and feet.

More recently, chassis builders have added interior panels to help keep the driver’s arms and legs inside the ride.

“We build a semi-containment unit,” Murphy explained, “with smooth carbon fiber panels covering the door X’s, to keep the driver’s feet inside the car, and another panel in the roof, below the top hoop, that doesn’t allow the driver’s hands to get outside of the car if it’s upside-down. Jerry and Rick are doing the same thing. We’ve seen several Pro Mod crashes in the past two or three years that have been extremely scary, and yet the drivers walked away.”

Jones pointed to the significant performance potential of drag-radial tires. In the RJ-built Shadow 2.0 Camaro, Stevie “Fast” Jackson has set the radial-tire record twice, scoring a 3.73 at almost 200 mph at South Georgia Motorsports Park in February; then upping the ante in April at Darlington at 3.71.

“Ultimately, a radial tire is faster,” Murphy confirmed, “because it stays round from start to finish. It rolls smoother. A slick is basically a giant sponge. If you look at a slow-motion video of a slick-tired car leaving the starting line, first the rim starts to rotate, and then it starts to rotate the tire, and then the tire compresses. It’s flat on the bottom and all bunched up. And at that point that tire is not moving forward, it’s trying to pick the nose up. And unless you have enough power to pick the nose up and keep it up, it’s going to drop down and take the weight off of the tire. So your goal is to get the car up on the tire without stopping the tire from moving forward. A lot of tire shake comes from not having enough power to get up on the tire and stay up on it. In a sense, you’re actually fighting the size of the tire.” Or, looked at the other way ‘round, there’s no point in having more tire than your engine can dominate.

“And I’ve been saying for 20 years that a lot of cars are over-tired.” On the other hand, to hook up a radial, “you have to make the car separate a lot more,” Murphy continued, “so the weight of the car physically pushes the tire into the ground.” It is possible to run both radials and slicks on the same car, but switching from one to the other requires significant changes in weight bias and suspension setup.

“The four-link is different, the shock package is different. It’s a totally different application of power.”
“And about a two-day renovation,” added Keith Engling, whose Skinny Kid Race Cars has built Pro Mod-style cars for about 20 years in Commerce, Michigan.

In the near future, in fact, Murphy expects to see “the biggest strides in tire technology.” And that, of course, will affect shock packages, suspension setups, and maybe even the cage itself. “We’re already working on some things at the back of the car.” But that’s all he’d say about that.

One thing not likely to change is the variety of body styles. Engling fondly recalled the late ‘90s and 2000s, when “a body had to be ten years removed from Pro Stock-legal before you could run it in Pro Mod. So Pro Mod was all the old classic cars, the Willys and Studebakers and ’63 ‘Vettes—such a variety of cars! It made a really cool show. Now everything is a Corvette or a Camaro. You have a bunch of eggs running down the track, and they all look exactly the same. That’s why I have $50,000 invested in a carbon-fiber ’72 Cutlass. I couldn’t have a Camaro or Corvette.”

“That’s one of the things that makes them more exciting than Pro Stock,” Bickel agreed. “The most aerodynamic car we have is Ricky Smith’s ZL-1 Camaro. But you see a lot of older cars running pretty fast, too. In drag racing, aerodynamics is not as important as in Cup car. A Cup car runs 170-200 mph all day long. A drag car is accelerating the whole time, and is only running at top speed very close to the finish. So aero is still important, but not as important.”

Perhaps even more importantly, said Murphy, “The fans like the different body styles, and the different power adders. I’m not knocking Top Fuel and Funny Car, but Top Fuel cars all look the same; and in Funny Car you couldn’t tell a Camaro from a Mustang except for the badging on the front. And that’ s true in Pro Stock, too. So from the fan’s standpoint, it’s Pro Mod that’s really cool.”

Nor will we see (as we’ve heard suggested) Pro Mod cars with solid suspension—four sprung wheels, and the attendant suspension tuning, are too closely linked with the identity of the class.

“With a solid-suspended car,” Jones allowed, “every ounce of downforce is going into the tire. But I’d rather have a four-link and shocks that I can tune and adjust. And down-track, at really high speed, a sprung chassis can handle the bumps a lot better.”

“One thing is guaranteed,” Jones concluded. “Engines will change, chassis will change, but our mindset for racing will not. Racers always want to go faster, and so we will continue to evolve and push the envelope to stay ahead of the competition. Ten years ago, even five years ago, we would never have believed that we would be as fast as we are now.”