Strong and sleek

| October 05, 2001

Once upon a time, 40 years ago, diesels weren’t turbocharged, trucks weren’t aerodynamic and the cabover was king. Trucks had trouble starting when the temperature dipped below freezing. There were no seat belt or emission standards, 60 mph fully loaded was hard to reach and the average fuel economy was 4 miles per gallon.

Truck technology and design have changed a lot since then, thanks largely to the brilliant but mostly anonymous work of engineers who, bit by bit, changed trucks from inefficient, noisy, balky devices into the smooth, powerful, comfortable vehicles we have today.

“Back in the old days, drivers carried a lot of parts with them,” says Larry Strawhorn, vice president of engineering for the American Trucking Associations. “They’d often be seen by the side of the road changing parts like fuel pumps. With spoke wheels, the tire/rim assembly was a lot lighter than it is now, and you could heft it up onto the spokes with your back. With disc wheels, you’re less likely to see drivers changing their own wheels. The increased reliability of trucks has made life very different for the driver.”

Early technology

Until the 1970s, most trucks were powered by normally aspirated diesels. These engines, without the pressure boost of turbocharging, capitalized on work done by early diesel pioneers to increase output by increasing engine rotating speed. Nearly all were governed at 2,100 rpm. The most popular Cummins model was the NH-220, says Mark Conover, Cummins market manager for owner-operator/high performance fleets.

These engines ran at their best near the top of the operating range because rpm made the injection and combustion system work better. Drivers were instructed, above all, to keep the engine from lugging – exactly the opposite of the driving technique encouraged today. The minimum rpm under load was 1,500, and cruise occurred at 1,800 rpm and above. The most popular transmissions were very close-ratio 10- and 13-speeds.

Detroit Diesels of the early ’60s were two-strokes known as the 71-Series, with 71 cubic inches per cylinder. In the era before turbos, this design produced more power in a smaller engine envelope. Because of the relatively small cylinders, eight-cylinder models were popular, and 12 cylinders were available.

In this era, all diesels had hydro-mechanical injection systems that metered, timed and injected the fuel without electronics. Macks and Caterpillars used pump-line-nozzle systems, while Detroit used unit injectors and Cummins the PT pump and injector system. Caterpillar engines had pre-combustion chambers: separate, small chambers connected to the main combustion chamber via a narrow passage to more forcibly mix fuel and air.

In this time of much cheaper fuels, gasoline engines survived in many fleets running shorter hauls, Strawhorn says.

Many conventionals had rounded corners and windshields that tilted to the rear, but they looked a lot more aerodynamic than they were. Trucks needed to conform to overall length standards, so the cabover was king in many fleets because it could legally pull a longer trailer. Many cabovers looked like cigar boxes because of the need to preserve cab room in the short space.

Transmission revolution

In 1962, truck transmissions still had a single mainshaft and countershaft and could carry relatively little torque. With so many cabovers on the road, a major challenge was making the gearbox strong, yet short enough to fit into the chassis. The limiting factor was the need to make the gears wide enough for the teeth to carry the torque. In the early ’60s, significant increases in low-end torque made gearboxes just too big, Conover says.

A bright engineer at Fuller hit upon the idea of spreading out the stresses by making the gearbox wider and shorter and putting a countershaft on either side of the mainshaft. The countershaft gears could be half as thick because two of them carried the load. Even the mainshaft gears could be thinner because the torque was now split between two meshes opposite one another, rather than focused in one location. Mainshaft gears would now float on their teeth between the opposing countershaft gears when not engaged, instead of each needing its own bearing. Floating gears, which could move around during engagement, also reduced shift effort. Fuller introduced this twin countershaft design in 1963.

Late 1980s International 9670 has a roof-mounted air deflector, part of the gradual evolution from boxy to aerodynamic.

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