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How engineers invented the rubber-belted ag tractor

Part two: We finish the story by revealing how two research engineers at Caterpillar teamed up to merge ag crawlers with rubber belts

In the late 1970s, Caterpillar had been selling a limited number of SA (Special Application) crawlers to farmers. But the high-horsepower, articulated, four-wheel drive ag tractors that were pouring onto the market were eating into demand for those machines. For that and other reasons Caterpillar executives eventually decided to develop and market their own four-wheel drive ag tractor.

At the same time R&D work progressed on the Cat wheeled ag tractor program, Dave Janzen, an engineer working at the company’s Peoria Proving Ground in Illinois, was redesigning the SA crawlers to make them more appealing to farmers. At the Proving Ground he modified a D4SA specifically for ag applications, and greatly improved its field performance.

Janzen put the prototype D4SA on a diet and it now weighed roughly 100 pounds per engine horsepower, giving it a power to weight ratio similar to ag tractors. That significantly increased its working speeds compared to other crawlers. And moving the chassis forward relative to the tracks gave it a forward weight bias that improved its ability to pull.

But the D4SA still suffered from the limitations of steel tracks and having the same “jerk” steering system common to most crawlers of the day. Those were big drawbacks compared to what dedicated ag tractors offered.

Removable tread

As Janzen continued working on his D4SA, Ron Satzler, another Cat engineer working at the same Peoria Proving Ground, happened to be experimenting with alternative uses for the removable, cable-reinforced rubber tread for beadless tires the company’s Rubber Products Division had developed. The tread was about the same size as a steel crawler track. Seeing the similarities wasn’t difficult.

Satzler had been using a modified road grader as a test bed to evaluate the performance and durability of the rubber tread when used as a track. The tread-come-track had proven its durability in that application after a host of tests and evaluations. So Satzler and his team knew they were onto something. And while the rubber belts did not get approval for use on the construction equipment being produced at the time, Janzen’s D4SA ag tractor seemed an ideal use for it.

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“I think it was the two teams that got together and proposed putting the rubber team’s track on the ag team’s D4SA,” said Janzen. “It was obvious to everyone that the rubber track should be installed on the D4SA. That resulted in the creation of the red D4 BAT (BAT stood for Belted Ag Tractor) as it was known, which was ready for testing in 1980.”

After being fitted with rubber tracks, Janzen’s original D4SA was taken to a local farm for field trials. The farm owner used Versatile tractors. So the belted prototype was painted the same colours as a Versatile four-wheel drive tractor, in case anyone saw it working in the distance they would hopefully mistake it for one of the farmer’s Versatiles. That earned the prototype the name red D4 BAT.

Satzler and Janzen were now working together full time on the belted ag tractor’s development.

Higher working speeds

To give it a wider range of working speeds, a 13-speed truck transmission was installed in the red D4 BAT. But those higher working speeds exaggerated the steering problem and showed a need for track suspension. To eliminate the steering problem, Satzler modified a differential steering system that was in limited use on other Cat crawlers at the time. Using that updated system gave the ag tractor stable steering characteristics at the much higher working and road speeds; and it kept power flowing to both tracks, even during turns. Satzler’s design remains the basis for steering systems on the Challengers to this day.

After being fitted with all the design updates, the red D4 BAT had now overcome the limitations that hampered Janzen’s original steel-tracked prototype, and it now showed a lot of promise. But as the project progressed, Cat found itself in a difficult financial position; it was bleeding red ink. Money for R&D was scarce and executives no longer wanted to fund both the wheeled tractor and belted tractor development programs.

Even though the wheeled tractor was virtually ready for production, executives cancelled it in favour of the belted tractor after evaluating its potential, much to the shock of many engineers that had been working on the wheeled tractor.

Cat’s industrial design department had developed a body style for the wheeled tractor that was transferred to the first production version of the belted tractor, the Challenger 65. The designer who created the Challenger’s body style had previously worked for General Motors and brought some automotive styling cues to the tractor’s design, including the raked-back windshield. Many ag tractors of the day used an opposite slope on windshields.

Initially, senior management considered keeping a steel-tracked version as an economy alternative to the rubber-belted tractor. The steel-tracked AG6 with Challenger body styling was built in small numbers but was soon discontinued.

Shortly before the 1987 market launch of the Challenger 65, the company had settled on using the Challenger name. But the space shuttle Challenger disaster occurred shortly before the tractor was to be released, and executives worried that using the same name might be a marketing problem. But they decided to stick with it, and most prospective customers didn’t seem to mind or notice.

In 2001 Caterpillar management decided ag tractors were no longer a good fit for the company and the Challenger tractor brand was sold to AGCO. The original red D4 BAT and the remaining wheeled prototype tractor were donated to Iowa State University when ownership of the brand changed.

However, both original prototypes are now in the hands of a private collector, although neither tractor is entirely complete, having had parts pirated at various times during ongoing development projects at the Proving Grounds. But they were saved from the scrap yard and remain symbols of an important milestone in ag machinery technology.

About the author


Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.



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