Designing an off-road sport truck

The Tube & Pipe Journal July/August 2003
August 28, 2003
By: Eric Lundin

The main component in any off-road sport vehicle is the frame. Frames for mass-produced vehicles usually are stamped and welded. These are suitable for most drivers' needs, but for intense off-road driving and competitions, a sturdier frame is necessary.

They're big. They're mean. They growl and snarl. Untamed beasts, they're at home anywhere, regardless of the terrain or the climate. They prowl over mountains or deserts or backwoods trails, whether the conditions are warm or cold or wet or dry.

They're custom-made off-road trucks, and they're designed to go anywhere, anytime.Their ability to withstand the rigors of off-road adventures is based on a sturdy design, and one of the primary design challenges lies in the frame. Because they are intended for intense off-road use and substantial abuse, the frames must be much more durable than the frames of mass-produced off-road vehicles.

What's in a Frame?

"Our trucks are based on the Jeep® CJ-7 concept, but we don't use any Jeep parts," said Eric Shurtliff, president of Bulldog Trucks LLC. The company uses a variety of custom-made and off-the-shelf parts to make individually tailored sport vehicles.

"Most mass-produced frames are stamped and welded, but welds can break," Shurtliff explained. To find a better frame for the company's off-road sport truck, Shurtliff turned to Advanced Frame Works, which specializes in manufacturing heavy-duty chassis for off-road vehicles. Rather than using stamped parts, Advanced Frame Works manufactures frames from mandrel-bent rectangular tubing.

"A standard Jeep frame is made from two stamped C channels," explained Jared Prindle, owner of Advanced Frame Works. "The outer channel is 0.125-inch material, and the inner channel is 0.080 in." Like most modern frames on mass-produced vehicles, the standard Jeep frame has a complex geometry that accommodates the vehicle's features. And like most frames on mass-produced vehicles, it's suitable for light-duty driving. The harshest conditions most such vehicles ever encounter are bad weather and slippery roads.

But when mass-produced vehicles are used for intense, off-road recreation and competition, the stresses can be catastrophic. "I have seen frames split from fatigue," Prindle said.

To fabricate a frame that could stand up to extreme off-road abuse, Prindle searched for a material that would be readily available and yet could be formed to replicate, as closely as possible, the dimensions of the Jeep CJ-7. He decided to use 2-in. by 4-in. rectangular tubing with a 3/16-in. wall thickness made from A500, an off-the-shelf mild steel.

Learning to Bend. Finding the material was one thing. Learning to bend it was another.

"Before we manufactured our own raw frame rails, we purchased them from another supplier," Prindle said. The cost of the shipping and the inability to do custom work were drawbacks, though, so the company decided to manufacture its own frames, and it purchased a tube bending machine. Only one obstacle remained.

"We had no experience with mandrel bending," Prindle said.

To build frames for intense off-road use and competitions, Advanced Frame Works manufactures frame rails from 2-in. by 4-in. rectangular tube.

To learn about mandrel bending, Prindle called Tools For Bending Inc. Shane Hartl, sales manager of Tools For Bending, spent two days introducing the Advanced Frame Works staff to the intricacies of benders, tooling, lubricants, materials, and, finally, the fine art of bending. Prindle proved the barter system is alive and well by compensating Hartl with a full-day vehicle test. Not some wimpy test with strictly controlled conditions on a boring test track. A real test. A get-in-sit-down-and-hang-on-tight type of trail-blazing, hill-climbing, river-crossing, gut-wrenching, hair-raising test. Prindle, Hartl, and the vehicle all passed.

The Devil Is in the Details. Prindle found that two important parameters in material selection are the yield strength, or the point at which the material bends, and the tensile strength, or the point at which it fails. The difference between these two characteristics must be at least 10,000 pounds per square in. (PSI) for satisfactory bending. "If the yield strength and the tensile strength are less than 10,000 PSI from each other, the material will crack when it's bent," Prindle said.

Seam location also is important.

"We bend the tube the hard way," Prindle said. "The frame is designed so that the bends alternate directions. Tools For Bending explained that if the seam were on the short side of the tube, the seam would be on the ID on one bend and on the OD of the next bend, and the amount of springback would vary from bend to bend." To keep the amount of springback consistent from bend to bend, the seam is always located on the long side of the tube.

"The mandrel has a seam relief that corresponds to the seam location," Prindle added. "To match the mandrel, the tube seam must be with 1/2 in. of the centerline."

After working out the details of bending 3/16-in.-wall tube, the company added 1/8-in.-wall frames to its product line.

More Tube. Prindle applied what he knew about frames to the skid plates, which protect the vehicle's underside.

"The original plates are stamped steel," Prindle said. A somewhat unscientific experiment at Advanced Frame Works revealed that an average person's weight is enough to cause a stock skid plate to flex. Seeing another opportunity to use tube, Prindle applied his bending knowledge to manufacturing a skid plate with a tubular frame. A follow-up experiment, along the same lines as the first, revealed that Prindle's tubular-framed skid plate didn't flex when a 3/4-ton truck was parked on it.

Above the Frame. There's more to an off-road vehicle than its frame, of course. Like the frame, the rest of the components that go into Bulldog vehicles are sturdier than normal.

"Some aftermarket parts are made from steel that is only half the gauge of the original parts," Shurtliff observed. To keep the weight to a minimum, the company constructs the bodies from marine-grade aluminum. The more intricate parts are stamped, and simpler parts—many of which are flat panels—are manually cut, formed, and welded.

The Beauty in the Beast

Bulldog trucks aren't just sturdy. They look good too. The attractive appearance starts with the undercarriage components (the frame, axles, steering box, tie rods, and other items), which receive three layers of powder coating. The first and second coats are color, and the third coat is a chemical- and UV-resistant clear coat, for a concourse-quality finish, according to Shurtliff.

A standard interior is a mix of luxury and racing accouterments. It is completely boxed in so no sheet metal is visible. A standard interior is fitted out in tucked and rolled leather; the seats are equipped with four-point safety harnesses; and stereos, amplifiers, and speakers are standard equipment.

The standard powerplant is a 5.7-liter V-8 engine that develops 342 horsepower at 5,000 revolutions per minute (RPM) and 375 foot-pounds of torque at 3,000 RPM. The standard transmission is a five-speed manual.

You Want Options? Henry Ford is famous for the options available on the Model T: "The customer can have any color he wants so long as it's black." Shurtliff believes in options. Foremost, the customer can have any color. The company works with a number of paint shops that specialize in custom work. One customer had an American flag painted across the hood and fenders. "It was beautiful," Shurtliff recalled. "If you saw it, you'd swear it was a flag laying on the hood, not a paint job." The same customer purchased two additional hood-and-fender sets with other paint schemes, so he can occasionally change the hood and fenders to alter the appearance of the vehicle.

Nearly every component of the interior and the drivetrain—gauges, seats, upholstery, engine, transmission, differential, wheels, and tires—can be substituted for optional equipment. The company took the optional equipment concept to an extreme when it designed its own urban assault vehicle. "The roll bar had two gun turrets that mounted two huge squirt guns," Shurtliff said. "And it was equipped with a 40-gallon water tank and an air compressor so the guns wouldn't run low on water or pressure."

The only item that isn't optional on Bulldog trucks? That's the 3/16-in.-wall frame made from 4-in. by 2-in. rectangular tube.

Advanced Frame Works, West Highway 87, Lewistown, MT 59457, 406-538-4996, fax 406-538-4998,,

Bulldog Trucks LLC, 1334 E. Chandler Blvd., Suite 5 / D-51, Phoenix, AZ 85048, 800-205-3308, fax 480-283-8174,,

Tools For Bending Inc., 194 W. Dakota Ave., Denver, CO 80223-2195, 800-873-3305, fax 303-777-4749,,

Jeep is a registered trademark of DaimlerChrysler Corp.

Eric Lundin

Eric Lundin

FMA Communications Inc.
2135 Point Blvd
Elgin, IL 60123
Phone: 815-227-8262

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