By Ed Stiles, University Communications

A team of UA engineering students has built a four-engine "tractor" that features twin I-beam suspension, rear swing arms, dual wheels and a homebuilt limited slip setup.

Early in June they loaded up the tractor and headed for Peoria, Ill. and the American Society of Agricultural and Biological Engineers' "1/4-scale Tractor Student Design Competition." The competition runs through this weekend.

To say that UA's entry is unusual is something of an understatement. This is the first year that competition officials have allowed entries with multiple engines. So the UA team decided to use as many engines as possible to pack in the maximum horsepower while staying under the tractor's 900-pound overall weight limit.

This led to linking four 16-horsepower Briggs & Stratton engines to a central drive shaft with a system of pulleys and belts. All 30 teams at the competition are required to use the same type of Briggs & Stratton engine.

Mechanical Engineering student Kurtis Pfeifer works on the gear mechanism in one of the tractor's rear swing arms.

The drive shaft on UA's tractor goes from the engines to a rebuilt 1969 VW transaxle that includes the team's homebuilt limited-slip device, which breaks away at a hefty 250 pounds.

Even with this very stiff rear end, "the steering brakes will still work fine," said the team's design guru and ace TIG welder Travis Wuertz, a graduate student in Mechanical Engineering. "When you think about it, we'll have on the order of 4,000 pounds of torque between the two sides, so what's another 250?"

From the transaxle, power is transferred to the dual rear wheels by swing arms that house motorcycle sprockets and chains.

Power and Speed are Essential

Tractor pulls involve pulling a weighted sled that creates more and more drag the farther it travels. The tractor that can pull the sled the greatest distance wins.

"We found that when you're hurting for traction, when you have more power than you have traction, speed makes a huge difference," Wuertz said. So the UA entry will have about 4,000 pounds of torque at 7 mph in first gear.

However, the pulleys on the engines and the chain-and-sprocket system that drives the rear wheels can be changed to produce a version of the "tractor" that will go 80 mph. The students will use a fiberglass sprint-car body and lights to turn their tractor into a street-legal hot rod.

"Most teams enter with just a tractor," Wuertz said. "I just can't see putting in this amount of work to just have a tractor to pull a sled. So our main goal is to build something that not only has the power, but we can switch the pulleys and change the gearing so we can top out at 80 mph. Our goal is to drive it from the hotel to the competition."

Most teams assemble mostly off-the-shelf parts and link their engine or engines to a hydraulic pump and send power to the wheels via hydraulic hoses. This sidesteps aligning drive shafts, dealing with mechanical transmissions, or fabricating lots of components.

A few teams also use CVTs (Continuously Variable Transmissions), which also make design and shifting easier.

Striving for Maximum Power

UA's team goes the more difficult, fully mechanical route because it offers maximum efficiency and a wide range of speeds.

But this means fabricating most of the parts and, with the weight of four engines, building an aluminum frame.

"After being back at the competition for three prior years, I know that no other team has the ability to make a lightweight tractor like we can because no one uses aluminum like we do," Wuertz said.

There's good reason for this. Aluminum is difficult to weld and frame design has to account for the material's low resistance to bending loads. The UA team has beaten those disadvantages to capitalize on aluminum's strengths — it's about a third the weight of steel and twice as strong per weight in tension and compression. "A lot of aluminum alloys also are as strong or stronger per volume than steel," Wuertz added.

Aluminum allowed the team to run four engines and still make the 900-pound weight limit by keeping the entire frame and its associated components under 70 pounds.

The UA team prides itself on building most of the tractor's components from scratch, except for the engines, transaxle, brakes and tires.

Mechanical Engineering student Chris Cagle works on a front wheel bearing. Part of the custom-made twin I-beam front suspension can be seen at right.

The rear swing arms, for instance were machined from 100-pound solid blocks of aluminum, eventually becoming 17-pound components. This required a huge amount of time. The parts could have been made commercially with a CNC machine in about a week, but the students strive to make as many of their own parts as possible. So they put in the long hours at the Agricultural and Biosystems (ABE) Engineering machine shop out at Campbell Avenue Farms.

The team also designed and scratch built a twin I-beam front suspension similar to the ones used on Ford pickup trucks.

Lots of Custom Fab Work

"We have a lot more custom fab work than I've seen on any of the other tractors," Wuertz said. "Everybody else pretty much orders parts and then they put their tractor together."

During the past four years, UA's team has finished higher in the tractor pull part of the competition each year, going from 12 to 7 to 6 to the number 2 spot last year.

"We missed first place by five feet last year because we did the last pull in three-wheel-drive and ran out of traction," Wuertz said. Last year's tractor had four-wheel drive and like its predecessors was built primarily from donated junkyard parts. A bearing in one of the used parts failed last year.

This year, the team members focused on raising about $20,000 so they could buy new parts or, in the case of the VW transaxle, rebuilt parts.

Team members Diana DeRosa and Stephanie Sara were primarily responsible for raising the money. DeRosa has graduated and is pursuing a medical degree at the University of Texas, but is at the competition in Peoria with the UA team.

During the last weeks of the project, team members were arriving at the shop around 9:30 a.m. and leaving at 3 a.m. In fact, their dedication to the project seemed to be almost limitless. The team's driver, Wes Williamson, went on a running/diet regimen and dropped 15 pounds, down to 135, to be sure the tractor/driver weight would come in under 1050 pounds for the lightest of the four pulling weight classes.

In addition to the tractor pull, the tractors also will be judged on maneuverability, braking, ease of maintenance and other factors, including the team's project report.

A Great Tool for Students

Charlie De Fer, who runs the ABE shop, said, "The cool thing about this project is that the students go from design concept to building everything they can in-house to testing and then competing. It's a great tool for engineering students. This is where they really get a lot of hands-on experience."

In addition to Wuertz, Williamson, DeRosa, De Fer and Sara, the UA tractor team members include John Richards, Kurtis Pfeifer, Doug Pratt, Chris Cagle, Rick Collazo, Marvin Wuertz, Priyanca Sarkar and the team's faculty advisor, Professor Don Slack.