What's That Behind You?

by Bryan Lively
photos by Bryan Lively

To most of us, the name Jay Brownlee Davidson does not ring a bell, but we all know his work in some form or other. For those in the know, Davidson was a pioneer in Agricultural Engineering and was the founder of the ASAE, or American Society of Agricultural Engineers. It seems that Davidson's passion was measuring power, be it a team of mules or a traction engine (tractor), and he successfully patented three inventions to used in that capacity. Little did Mr. Davidson know that two of the three devices he created during his tenure at Iowa State University would spark the course for the development of the modern weight transfer machine, or pulling sled, as we all know it.

It is understood that in the history of tractor and truck pulling that it did not begin with some contraption like that of Davidson's, it began with the stone boat/deadweight sled. Deadweight pulling is still alive and well today, with mostly horses and mules tugging on this type of sled and a few local antique clubs keeping the past alive as well. From the deadweight sled, we moved into the step-on sled era, making pulling a participatory sport for fans. This form of sled was highly inconsistent as people could step on the sled at varying points and moments, or a small person may replace a larger individual along the track. By the late sixties and early seventies, the earliest versions of a weight transfer machine surfaced, beginning the era of repeatable, consistent passes for pullers.

So, what is a weight transfer machine? There are countless definitions, from those that are imbedded in a half-dozen or so patents to those found on the Internet. A weight transfer machine for our purposes is a device that progressively transfers a portion of its' own weight from a starting position of little or no resistance to an ending position of resistance, enough to slow or completely stop the vehicle supplying the force to overcome the resistance. The sled designs from that early day that epitomize that definition were basic yet remain a cornerstone of our modern motorsport. A weight box, perched over the rear tires, moved from the rear to the front of the sled by means of a chain driven by a ground drive off the rear tires, moving the weight off the rear wheels and onto the sled pan and producing the resistance needed to stop a tractor.

Pulling sleds remained largely unchanged in design for a number of years, until the mid eighties when a means to produce more passes in a given amount of time was created, the self-propelled sled. This improvement allowed for quicker returns to the starting line and more accurate placement of the sled by the sled operator. John Ellenbaum is credited with the first self-propelled sled in 1985. Beyond this advancement, sleds did not change much at all, until the development of the first of the “race” sleds. While truck and tractor pulling was an exciting sport with the smoke and the noise that it produced, it was also much harder on equipment due to the length of time runs down a 300-foot track would take. More time under load meant more fuel burned and in turn created more heat, leading to many melted engine parts. Passes that once took in the 15-second range now take only 10-12 seconds on a 300-foot track, a significant difference in load. This major change also further shortened the length of events, turning pulls that once lasted well beyond midnight into events ending at a decent hour, with no loss of action or number of hooks.

What makes a “race” sled a race sled? A number of factors come into play to make a tractor or truck blast down a track, only to be stopped safely and abruptly. On the surface, it would seem that sled weight would be a huge determining factor; however, sleds weigh much the same as they did 20 years ago, in the neighborhood of 50,000 pounds. If the discussion had to be narrowed down, it lies in one area of the sled more than any other: the pan. The pan is much lighter now, weighing 2000 lbs versus 6000 lbs in years past. In addition, the pan is off the ground for the early portion of the run because of the amount of weight that has been shifted to the rear of the sled to above and behind the rear axles, and the angle of pan in relation to the ground. This change sounds like a recipe for a runaway tractor or truck, but it is the ability of the pan to drop into the track and transfer 95% of the sled's weight (versus only 50% of an older sled) onto it through the box moving well beyond the pivot point (fulcrum) to further induce a quick stop.

When fully describing what makes these sleds tick and provide for fast and action-packed runs, it all begins at the starting line. The two rear axles have unique duties and work independently of one another. The front axle is engaged for a pass down the track while the rear axle goes along for the ride until it switches roles and propels the sled back to the starting line. With the front axle engaged, the sled is typically set in a “240” gear, meaning that the gear will allow the box to “top out” at 240 feet. Weight has a more profound effect upon the distance a vehicle travels, with each block in the box typically being the equivalent of 20 feet in change; gearing fine-tunes distance, as it provides only 5 to 10 feet of incremental difference. With the sled moving and the box rolling forward, the box has two sets of switches it engages. The first is highly adjustable and can be moved to start the drop of the pan either early in the run or nearly at the time the box is topped out. The second set, found in the top third of the frame rails, act in two fashions: the first disengages the air clutch that connects the box final drive to the box transmission, while the second acts to drop the pan further, literally vaulting the rear axle into the air.

With the change in sleds came subtle changes in the vehicles pulling them. Gone are the days of heavy lug tires required to motivate a sled off the line and replaced by the double cut tires that are more akin to a drag slick than they are their farm-based counterparts. Shorter lugs also meant the ability to turn a more aggressive gear, bringing tire speeds up to produce quicker starts. This is ultimately the most important factor in producing a winning run, as the truck or tractor with the most ground speed at the beginning of the pan drop will be more likely to win than a slower counterpart, because the rate of the pan drop is constant and the more ground covered before it's down, the better.

Even with these advances, one aspect that has not been forgotten is safety. According to Larry Richwine, NTPA Director of Technical Services, each sled approved to operate at a NTPA event has undergone an annual inspection prior to the beginning of the pulling season and they receive a pre-event inspection of brakes, the engine kill switch tether, as well as the driveline. Tech Officials also work in conjunction with sled operators at NTPA events to determine sled settings that allow tractors and trucks to run to their potential without sending all of them past the 310 foot mark.

Perhaps the greatest determining factor of successful sled operation is what lies between the ears of the sled operator. Careful attention to track conditions and making sled adjustments based on those observations go a long way towards an effective sled setting. Operators like Brian Neal of the Hager Arctic Cat Sled maintain notes on track conditions, number of weights carried, switch settings, and the results of classes for future reference, others like Dave Hager rely on thirty-plus years of experience to make adjustments to set a class correctly. Good sled settings work two-fold, as they allow pullers to enjoy the rush of the pass and most importantly put on a good show for pulling fans, be it a class of Pro Farm tractors or Unlimited Mods. Next to sled settings, maintenance on a sled is possibly the most crucial job of the sled operator, and it is a monumental job indeed; a truck or tractor may make four or five passes a weekend, while a sled might in some instances cover a track 150 or more times in that same weekend, totaling upwards of 16 miles of travel and never leaving the same 300 foot piece of real estate. Essentially, a well-operated, durable sled will almost “disappear” in the midst of the action and will keep an event running smoothly with a minimum of downtime.

So what does the future hold for sled design? It is honestly hard to say. With the rise of fuel prices, the future might bring lighter sleds that require less fuel to transport and operate, without losing any ability to stop the best in pulling. Until then, these unsung heroes of the pulling track will continue to work tirelessly to produce shows that keep old fans coming back and draw new fans to the sport, one hook at a time.