From cabover to conventional

| November 01, 2007

A classic cabover design with this International Transtar, made from 1968-78.

In the 1970s, it seemed as if everybody wanted to drive a truck, wear a cowboy hat and gab in a new language on the CB. But wanting to be a truck driver in the ’70s didn’t make much sense if you wanted a comfortable life. The cab-over-engine (or COE) design was still king, and the driver was a pawn in an uncomfortable chess game he had little influence on.

But in 1980 things began to change. Rules that had kept freight rates artificially high were loosened substantially by the Motor Carrier Act. Along with the reduction in rules, the federal overall length laws were changed. The COE design had resulted from the need to shorten the cab to increase trailers’ cubic capacity and payload when overall length was regulated. Now only the trailer length had to meet a standard, and it had increased, too. This paved the way for a future in which dozens of other changes would make the truck driver feel like a king of the road.

Larry Strawhorn started his career at Mack Trucks, Inc., in 1965 and was, up until 2002, the vice president of engineering at the American Trucking Associations. He notes a prime force in the evolution of truck design – the carriers that were regulated up until 1980 are now often staffed by Teamsters Union drivers, while most newer carriers are not. “The formerly regulated carriers have to pay the drivers quite well,” says Strawhorn, “while [many of today's carriers] attract drivers by buying fancy trucks.”

There was a time, Strawhorn says, when the engine air intake was put inside the cab to reduce dust. Drivers had to listen to the intake noise as well as put up with the drafts and dust this brought into the cab.

But the ride revolution was soon under way. Air suspensions would give way to air cabs and seats. Rob Swim, director of dealer marketing at International Truck and Engine, recalls, “When I first started selling trucks, there would be a simple, vinyl-covered bench seat or two seats attached via tubing right onto the floor.”

Swim recalls driving one COE with a seven-speed transmission and an old 6-71 two-stroke Detroit Diesel, which produced a high-pitched roar. The truck’s sound insulation was so poor, his ears were actually hurting by the time he got out of the truck. Because of the old COE’s engine doghouse between the seats, he felt like his right foot was on fire.

Drivers were also irritated by the need to tilt the cab to work on the truck. “Since they lived in the truck, they felt like you were tilting their house 90 degrees,” Swim says. In those days, he told himself, “I’m sure glad I’m a salesman and not a driver.”

Fuel was constantly increasing in price in those days, and many segments of the interstate highway system were reaching completion, raising average cruise speed. The potential for aerodynamics to save fuel became paramount for many truck designers. One such person was Larry Orr, who became chief engineer of PACCAR in the early 1980s. According to Kenworth Trucks: the First 75 Years, by Doug Siefkes, Orr had begun to do wind-tunnel research into aero design with Tim Kangas, PACCAR’s assistant director of R&D, before 1980. They used truck models made of wood and wax. Initial tests were conducted on a COE model. “It proved the whole idea of aerodynamics and fuel economy,” Orr says.

By the time this design was ready, however, Orr had decided that “the rules had changed and conventionals were the future.” The next result, after another year, was the T600, which had a sloped hood, lowered bumpers, sleek fenders, side skirts, roof fairings – the “most important single feature” in terms of aerodynamics, says Ed Saxman, Volvo’s drivetrain product manager – and an air cleaner that had been moved under the hood. A set-back front axle that carried more weight allowed for longer, smoother riding springs.

The longer springs put on many trucks have produced more comfortable, lower frequency ride vibrations, says Saxman. The springs change thickness throughout their lengths, which kills other forms of vibration, too. And their slower movements also dampen out the natural vibration of imperfectly balanced tires, which had been transmitted to the cab by earlier spring designs.

A sharper turning radius was another design innovation that saw full use in the T600. Dubbed “The Anteater” by many drivers, the T600′s fuel economy proved out, and the design has survived with continuous refinement through the present T660.

At the same time, in 1989 an International Trucks aerodynamics expert named Gene Olson reported to Owner Operator magazine that cabovers could be just as aerodynamic as conventionals. International made COEs like the 9700 for some time after that, and they all had the rounded corners and other features Olson described as necessary to keep the flow of air “attached,” running smoothly against the surfaces rather than being bounced outward.

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