What Is Dual-Purpose?

Today's sand machines might range from 1,800 to 3,000 pounds, depending on what a customer wants. At right is an actual SCORE race car, powered by a VW motor. A nearly complete dual-purpose car, center, will be powered by a V-8. At left is a powder-coated chassis for a prerunner that will have a longer wheelbase and heavier chassis bracing.

Every dual-purpose machine is custom-built just like a race car. Steel tube is measured, cut, and assembled on a chassis jig. Shock and engine mount positions are precisely calculated. It takes about a week to produce a chassis. Workers then pre-fit each component in the chassis to confirm that the particular set of components specified for each car is exactly positioned. From there, the long and complex process of installing and plumbing the engine, transmission, steering, and other components begins.

Just like a race car, the dual-purpose chassis is heavily braced using triangular structures. For safety and structural integrity, the chassis is built from 4130 chromoly steel tubing. Needless to say, the welds have to be perfect.

As with a desert racer, the dual-purpose car's radiator is mounted high and to the rear for maximum protection from flying debris. A larger V-8 demands dual fans, and a heat exchanger may be used to cool the oil. Plumbing needs to be carefully routed away from passengers.

One difference between a dual-purpose machine and the classic dune buggy frame is this side intrusion bar. The bar adds weight, but since the dual-purpose machine will have a powerful V-8 engine, the added structural integrity is a safety feature that can more easily be built in without sacrificing performance.

A crucial area is the front nose where the chassis comes to point. This is where the power steering ram is mounted. Cooley: "Notice the heavy bracing. If the chassis is not braced correctly, you can actually see the chassis kind of twist and turn. If it's not reinforced, it's going to break out of the car."

The power steering racks for dual-purpose cars have to be heavier than pure sand machine racks. The idea is to have steering that is strong enough to turn larger, heavier tires with very low effort. Power steering first started appearing in off-road racing around 1990.

With hydraulic ram steering in use, combined with tall tires, the front spindles and wheel mounting hardware has to be extraordinarily light and strong. The knuckle is machined from billet. Note the heavy Heim joints.

Bodywork is done by hand, according to the customer's preference. This car will have a hand-shaped fiberglass body and weigh in the 3,500-4,000-pound range. The car will take about four months to build and cost roughly $140,000.

On the heaviest cars, which are very close to Class 1 race cars, the structural bracing is extreme. This rear shock mount is triangulated, braced, and gusseted heavily. The lower bumpstop (not shown) is equally well engineered.

Off-road racing has pushed the boundaries of shock technology in recent years, with the result that highly advanced dampers are now routinely adopted for recreational use. Long-travel coilover shocks with remote reservoirs are commonly used on dual-purpose machines. Twenty-one inches of travel is not uncommon. Rear damping, depending on the weight of the vehicle, may be handled by 3-inch bypass shocks that allow for very precise tuning of both the jounce and rebound cycles.

Prerunners and dual-purpose machines typically seat four, multiplying the need for sound restraint systems. The rear seats, particularly, need harnesses that prevent smaller passengers from sliding out. These rear seats have alternate belt mounting slots placed lower in the seat back so harnesses can be properly adjusted for youngsters.