Rallycross: Protect the underside

Image by Josh Sikora
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emailBy Matt Wolfe | May 9, 2020 12:52 PM

Underbody protection is a unique need within SCCA’s motorsports offerings. Most Solo, SCCA Road Racing, or even RoadRally cars won’t ever encounter a hazard during an event that would necessitate armoring the underside of the chassis.

Meanwhile, even a relatively smooth RallyCross course can still pose hazards that can be minimized through basic preventive measures.

The most common form of underbody protection for RallyCross is a skid plate. A skid plate is generally a flat piece of metal mounted underneath the engine that protects the motor and transmission from debris and impacts. Most skid plates are made from aluminum, though there are certainly opportunities to utilize cheaper, or more exotic materials depending on your goals and budget.

When designing or selecting a skid plate, it’s important to remember the intended application. While RallyCrossing is certainly more intense than going for a Sunday drive down Main Street, it’s not exactly clearing minefields either. While a 3/16-inch or thicker plate may be what is recommended for a dedicated stage rally car, you may be able to use a thinner plate.

“One of the most important things about skid plates is what they’re mounted to,” explains Ryan Thompson of Thompson Racing Fabrication, which has been building and protecting rally cars for years. “The plate needs to be attached to something that isn’t going to deform on impact.

“You also need to consider how that mounting point is dissipating the energy of the hit. It’s also important to know how much space is between the plate and what you’re protecting. If the plate is right up against the oil pan, it’s going to transfer the impact right into the pan when the plate deforms.”

In addition to a skid plate, it’s common practice for rallyists to install some form of protection around the floor pan, wheel wells, and other areas of the car in order to guard those items from mud, snow, and gravel spray. Over time, this flying debris can destroy chassis components, or the chassis itself.

So, what type of material is best for underside protection? “We use a lot of HDPE plastic,” explains Thompson. “We buy it by the roll and use it for pretty much everything under the car. We usually attach it with rivet nuts because they’re light and cheap. You can use weld nuts, but they’re heavier and more expensive.

“Whenever we put something on the car, we always try to think about how this item can be lighter (or) better. We also use the thinnest material we can, because we’d rather save the weight and just replace the plastic when we need to.”

A bonus of installing HDPE panels is they simplify post-event cleanup. The material is nonstick by nature, so mud, dirt, ice, and snow that might otherwise stick to the vehicle’s floor pan will often fall right off with a light kick to the rocker panel. Skid plates also help as they keep a lot of dirt and dust out of the engine bay.

So, the next time you’re contemplating upgrading a component or two on your RallyCross car, consider investing a little time and money into protecting the underside of your current investment. It may pay dividends.

This story originally appeared in the May 2020 issue of SportsCar magazine, the official publication of the Sports Car Club of America and just one of the many benefits of membership in the SCCA.

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