The 36 year-old sport of extreme pulling is growing in popularity every year. There are over 4,800 tractors competing nationwide on a circuit that ranges from county fairs to motorways. The purse money is still small at the regional level, so the competitions run on pride and passion. About fifty percent of the tractors come from the farm community, and the rest from motorheads who do this in their spare time – not that they have much of that during the May to October season, when there is a pull just about every weekend somewhere in their region.
Thumper II, owned and driven by Tom Durfee, was the 2003 Regional Champion in the Super Modified Class of the NYTPA and second in 2004. This year the team from Warsaw, New York, consisting of Tom Durfee, his brother Wayne Durfee, and friends George Barnison and Pat Coffey, is aiming not just to recapture the regional title, but to win at the Nationals.
In order to get this far, he’s already put close to $100,000 and countless hours into Thumper II, mostly in four key areas:
- Two 2,500 horsepower Allison aircraft engines
- A custom-built chassis
- Transmission
- Two firestone rear tires
THE ENGINES
Tom Durfee rattles off the specs of the Allison V12 engines “They’re 1710 cubic inches apiece and they put out about 2,500 horsepower each,”. The engines are capable of greater horsepower, but the Super Modified class has restrictions. They have 2 spark plugs for each of the 12 cylinders, making for a very crowded distributor cap. These engines, originally built for World War II aircraft, are relatively unusual on the circuit. Most of the competition is powered by diesel Chevy or Chrystler Hemi engines. These Allisons have been modified by Steve Bollinger of Stonington, Illinois, who runs his own competition tractors on the Midwest circuit. They are adapted to run on ethanol (ethyl alcohol) and they are eye catching. A shiny chrome block at the front of the tractor is the fuel tank. A pair of chrome tank holds water, and a gauge on each engine shows the level. The water is there to thin out the alcohol when necessary. The engine also has a supercharger that adds air to the mix. Keeping all of these elements in balance is a series of control boxes that measure engine pressure. “ When everything is all working, one sends one to the other, the other sends it there, one says give it here, and boom! “ says Tom. Like the supercharger you might have on your car engine, these engines run best in cool days when the air pressure is on the low side. Under ideal conditions, the plenums will end up as frosty as a chilled beer mug, Tom says. Optimum engine performance is the basis of any good pull. But there are many other factors involved, including the basis for it all, the chassis.
THE CHASSIS
The front tires are tiny. Their main function is to support the weight of the engines, which are seated side by side, while the tractor is stationary or being towed. The chassis was built by Tim Engler’s shop in Indiana. The two struts of chrome moly tubing that extend from the front to the rear axles are reinforced vertically with triangular bracing for strength, but they still have some give. There is enormous torque on the tractor when the engines are really revving, and if the body is too rigid, it will impact the balance of the rear end. Another factor influencing chassis design is the weight limit for the division of 7,500 pounds. The chassis has mounts at the front and rear where movable weights can be added. The weight of the tractor needs to be a few hundred pounds under the limit in order to be able to deploy the added weight strategically`. To get the most pull out of the tractor, it is critical to be able to fine-tune the balance. If the front rises too high off the ground as the tractor is pulling, the draw bar (where the sled is attached) is forced below the optimum 18 inches. Adding height to the draw bar increases the tractor’s pulling ability, so this is the maximum height allowable for the tractors in competition. Compliance is carefully tested. The tractor is weighed and then the draw bar height measured on a level concrete platform before it is allowed onto the track. The pit crew hovers, doing final checks on the tire pressure, and redistributing weights if necessary.
TRANSMISSION
To translate the energy of the engines efficiently to the rear, the right transmission is key. Thumper II’s is of the type called a reverser. It has only two gears, forward and reverse, This is common, though some of the tractors use a four-speed truck transmission. Tom explains why a good transmission is critical, “A lot of times in the modified class you want the fastest gear that you can to turn the tires, because a slower gear might not turn the tires hard enough to get it to lock up. ‘Lock up’ means grab traction on the track.” In order to achieve that, the driver has to spin the tires up to speeds 85 or 90 miles an hour. Tom can tell when he’s not geared right, because his front end will rise too high, more than eight to ten inches off the ground.
REAR TIRES
Another critical performance factor is the tread on the rear tires. Thumper II wears 30 point 532 Firestone tires, but they aren’t exactly custom issue. Four to five inches of rubber have been taken off of them with a grinder. The original tread is only barely visible at the edge of the tires. Then, with a hand grinder, a new v-shaped tread pattern has been cut in, barely deeper than half an inch at the center. The shallow tread allows the wheels to spin while building up engine revs, but is still deep enough to grip the track when tires are forced down as the tractor begins to pull. Once again, it’s a balancing act to get the tread just right. Since not all pulling surfaces are alike, this can be a tough call. The ideal surface for Thumper II is a clay track, with enough moisture in it to give the clay body.
THE SLED
The weight that the tractors pull is not just a piece of stone anymore – it’s a complicated mechanism in its own right. It’s not simply dead weight, like the old stone sleds used at the first tractor pulls. The weight is on a sliding bar. This enables the tractors to get some momentum before the weight slides up the bar. The further it goes, the more weight the tractor is pulling. The sled is controlled by an operator, and the operator’s skill level can also impact the tractor’s performance. The sled operator is also there for another reason -- safety.
SAFETY FEATURES
Like any motor sport, there’s an element of danger in pulling. There are many safety features built into the tractors, from the cage of steel tubing that surrounds the driver to what is probably the most important, the automatic kill switch. If either of the engines seize, a switch will shut down the tractor. The driver of the sled also has a manual control. Engine problems can range from a thrown rod to fire or even explosions. Many of the tractor parts, like the clutch, are inside protective housings, so that if they explode the driver and audience won’t be hit by flying parts. Wheelie bars at the back help prevent a tractor from rearing up too far, The extreme power of the tractors mean that loss of traction or an engine problem can send the vehicle into an uncontrolled skid. The serious steering is done not with the wheel but with the brakes, a separate one for each wheel. The brake discs are custom made, of course, and that’s another part the driver and his crew pay close attention to before going out. Losing control of the vehicle at a minimum results in an out-of-bounds disqualification, and at the worst a rolled tractor.
The drivers are competitive risk takers, but they aren’t stupid. Tractor pulls are by no means as hazardous as, say, skydiving, about which Tom Durfee says, “Why would anyone jump out of a perfectly good running plane?”
He emphasizes that pulling is a family sport, both amongst the competitors and the spectators. So go to a tractor pull, pay a little extra to go behind the scenes, and ask as many questions as you like. Maybe you’ll find Tom Durfee and Thumper II there!
