You couldn't blame motorists for doing a double take when they drove by a corn field near Slater, Iowa, last November.

In the field were two tractors connected by a cable to a third tractor that was pulling a five-shank, disk ripper through the corn stubble.

The tractors made a quarter-mile run, stopped while the tires were changed and then repeated the runs.

Each time, one of the lead tractors pulled ahead of the other, the first time by 37 feet, 4 inches, and the second time by 36 feet, 8 inches, as their tires bit into the corn stubble in the freshly harvested fields.

You may have guessed by now the tractors were part of a traction test set up, in this case, by Firestone Agricultural Tire Co., to determine the validity of claims made by a competitor. Firestone released a video of the traction test to a group of agricultural editors attending a media event in Des Moines, Iowa.

“Firestone's ‘Tread-to-tread traction demonstration’ is a direct response to (the competitor's) claims that the traction of its Agribib R-1W is equal to or better than that of the Firestone Radial All Traction 23 R-1,” said Ken Allen, vice-president of Firestone Agricultural Tire.

“The Firestone 23 degree R-1 tractor has consistently outperformed the 45 degree R1-W,” he said. “When (the competitor) released a video challenging the superiority of Firestone tires and our 23-degree bar design, we decided to conduct a test to set the record straight.”

Prior to the demonstration, two tractors of the same brand and horsepower were weighed, ballasted to compensate for weight differences and tested on a dynamometer for equal power. The rear tires — Firestone 480/80 R46s and a comparable tire from the competition — were inflated at 11 pounds per square inch each.

“We wanted to eliminate all the variables except the tires,” said Wayne Birkenholz, manager, global field engineering for Firestone, who led the team that conducted the test. “That's one of the reasons we switched the tires from one tractor to the other and repeated the test — to make sure we had a fair comparison.”

Firestone first introduced it 23-degree tractor tires in the 1950s and has done extensive testing of the tread design at its research facility in Columbiana, Ohio, through the years, said Tom Rodgers, marketing manager for Firestone Agricultural Tire. (The location is where Harvey Firestone tested his first rubber tractor tire more than 100 years ago.)

“We had tested the tires under field conditions many times, but not in this particular manner,” he said. “This demonstration was designed to eliminate any questions about the performance of Firestone's Radial All Traction 23 R-1.”

The demonstration involved two tractors and an anchor tractor. A cable was attached to the drawbar of one drive tractor, run through a pulley on the anchor tractor and attached to the second drive tractor. Starting side by side, running in the same gear and at the same RPM, the two drive tractors pulled the anchor tractor along.

“In this configuration, each drive tractor pulls an equal load, yet can float in relation to each other, depending on the grip or slip of the tires they run on,” said Firestone's Allen.

“It's really a tug of war — the tractor with the most traction steadily gains more cable and pulls ahead of the other tractor. The difference in traction is measured by the distance between the two tractors at the end of the run.” (When the technicians averaged the two runs, the Firestone tires out-pulled the competition by 37 feet.)

Birkenholz and Ken Brodbeck, manager of O.E./Export Sales Engineering, said the difference was in the increased number of bars on the Firestone tire owing to the 23-degree design of the tires.

“The two variables in this test were the tread design and the number of lugs on each tire,” said Brodbeck. “You get more bite on the ground with more lugs from the 23-degree tire than from a 45 degree. A lot of people have the impression a deeper bar should be better, but actually the 23-degree bar angle is the better design.”

“Farm operators have significant amounts of time, fuel and money at stake when they make their tire choices,” said Allen. “That's why we decided to conduct a controlled demonstration that compared performance. We believe our Tread-to-Tread Traction Demonstration does just that.”