It takes loads of science, and a little bit of art, to make a WRX STI ready for the rough-and-tumble world of rallycross.
From the roll cage to the fenders to the dashboard, it takes truly impressive tech to achieve the world-class custom fabrication necessary for rallycross. That’s the job of Vermont SportsCar (VSC) fabricators, who work with Subaru Tecnica International engineers to create body panels, structural parts and interior components for the Subaru Rally Team USA (SRTUSA) rallycross car. The goal is to create rallycross vehicles that are strong, light and custom-tailored to the kind of extreme duty that your WRX STI will likely never see.
Strip It Down!
The process starts with a bare WRX STI shell, stripped of all drivetrain and interior pieces. The first component to build is the roll cage.
A stock WRX STI provides a very rigid platform for daily use, but the Global Rallycross Championship (GRC) competition is much tougher. The roll cage not only strengthens the chassis, but also helps protect the driver during a shunt, allowing the suspension to work more consistently even after those hard jumps (and landings!). Since the SUBARU BOXER engine in the rallycross car puts out almost twice the horsepower of a stock WRX STI, there’s also additional stress on every component.
Nick Carter, head of fabrication at Vermont SportsCar. Photo: © Lars Gange, subaru.com/rally 2017
Nick Carter, head of fabrication at VSC, explains the process. “It all starts in the design office with the engineers,” he says. “They use a full 3-D CAD model of the WRX STI shell to design the roll cage.” Once the design is finalized, VSC orders the roll cage tubing to their exact specification. “Then we get to work installing the cage, which involves quite a bit of fitment checks, precise measurements and lots of welding,” says Carter. Besides the roll cage and body-shell reinforcements, VSC also fabricates subframes, gearbox cradles and other components – again, making sure they’re as light and durable as possible and able to handle rallycross conditions.
The sexiest components VSC fabricates are made of composites. “Let’s face it, carbon fiber is bling for [drivers],” says Robert “WeeGee” Smith of VSC. “We all like the way it looks on a hood or another panel. But for us, it’s more about function than fashion.”
Smith is in charge of composite fabrication at VSC, and he comes by his expertise through both training and experience. About 10 years ago, after spending several years as a rally technician, he started looking at composite parts from different suppliers and decided he could do better. After attending Abaris Training in Reno, Nevada, where many of his classmates were in the aerospace business, he returned to rally with the goal of creating custom-tailored composites. “I really like the challenge of it all,” he says.
A mold for a front bumper at Vermont SportsCar. Photo: © Lars Gange, subaru.com/rally 2017
So Long, Heavy Metal
Nearly every body panel on the rallycross car is a composite – the bumpers, fenders, rear quarters, deck lid, hood. Only the exterior driver’s door is sheet metal, per GRC rules. Composites are also used inside for the dashboard, switch panels and the ducting for the rear-mounted radiator.“We use composites throughout the car because, in general, they’re light and strong, and you can tailor them for specific uses,” says Smith. “We can engineer and design things that can’t be built out of steel. And we can make it strong or stiff in one direction or in multiple directions. We can also make it compliant so it can be re-formed.”
Carbon fiber is light and strong, but shatters upon impact. Kevlar® is strong but heavy, and designed to “give” upon impact – think bulletproof vests. More basic thermoplastics can be re-formed if they’re hit, so that material is used for the bumpers.
Mix It Up
That being said, it’s not an either/or situation. “We can also tailor the part by mixing in different materials, either carbon fiber or Kevlar or both, to stiffen or lighten or change the characteristics of that part,” says Smith. “This can help us come up with a very specialized, high-performance component.”
The process of fabricating each piece is long and complex. First, VSC modifies, for example, a factory steel fender, cutting it and adding body filler and foam to make the wheel arch bigger, or adding more clearance for suspension components. The mocked-up fender then goes to a body shop, where it’s finished and painted. “Then we mount it on the car to see what it looks like,” says Smith. If the team is pleased with the look and the fit, that piece becomes a pattern to make a mold, which is then used to create the composite piece.
“This works well because we have all the factory flanges and mounting points for the new part in place,” says Smith. “It bolts on the same way a factory piece would. Overall, we want the car to look like what the factory would build if they were [creating] the car out of composites.”
The Bottom Line Strength and weight reduction are always the objectives. “If we can shave off some weight, the engineers will be able to use it to modify the weight distribution or the center of gravity,” says Smith.
All the long hours and painstaking fabrication work at VSC pay off when the team gets to an event. “The harder we work in the shop, the less work we’ll have to do at the race,” says Smith.
Q&A with WeeGee Smith
Q: How do you initially decide what composites to use for each piece?
A: We decide what type of material to use based on what the function of the part is designed to do, such as if it is to be flexible or do we need stiffness, or will it have a short life span or do we want longevity.
Q: Do you change composition of any piece after race data?
A: Yes, if we find a better material, in the initial build of the part, such as if it is too heavy or too light or if it fails before its designed time.
Q: How long does each composite take to create?
A: A front bumper will take a full day [eight hours] and a front fender will take [five hours]. It will depend on the complexity of the part design and the number of plies or thickness of materials used in the layup.
Q: How many extra parts are needed for a race?
A: Depends on the race and the part. A one-day race we will bring two sets of all bodywork and four front and rear bumpers.
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Subaru Rally Team USA driver Chris Atkinson in Red Bull Global Rallycross action. Photo: © Lars Gange, subaru.com/rally 2017