Prior to the invention of the tractor, when
farm implements were pulled by
horses, farmers would boast about the strength of their teams and seek to compare and contest in teams with one another to see who had the most powerful animals. In some cases, they compared horse teams pulling large loads over distance, such as a fully loaded hay
cart or
wagon. In other situations, a flat board or skid would have a horse or team of horses then hitched to it; weight would be added, usually in the form of rocks, and the driver would urge his horses to pull the load, with more weight added as competitors were eliminated; the animals pulling the most weight or for the greatest distance were judged the strongest. These events became the formalized sport of
horse pulling, which is still carried out today with
draft horses, specially bred to have high strength for pulling heavy loads. Today, fixed weights on drags are dragged for a set distance and additional weight is added in successive rounds. While it is said that the term
horsepower is derived from this event, the concept was developed earlier, in experiments and measurements performed by
James Watt and
Mason Worrell. It wasn't until 1929 that motorized vehicles were put to use in the first events at
Bowling Green, Missouri, and
Vaughansville, Ohio. Although the sport was recognized then, it did not really become popular until the '50s and '60s. It was also realized, at that time, there were no uniform set of rules. The rules varied from
state to state,
county to county, and competitors never knew what standards to follow. This made the sport difficult for new entrants. In 1969, representatives from eight states congregated to create a uniform book of rules to give the sport much-needed structure, and created the
National Tractor Pullers Association (NTPA). The NTPA's early years were events that used standard farm vehicles, with the motto "Pull on Sunday, plow on Monday". Pulling remained basically the same through the '70s, with only stock and modified tractors. Stock tractors were commercially available tractors produced by manufacturers, and modified tractors were the basic tractor chassis with another non-tractor engine mounted on it. Tractors remained single-engined until two Ohio brothers, Carl and Paul Bosse, introduced the
crossbox, which could allow multiple engines to be attached to a single
driveshaft. Other innovators during this period included Bruce Hutcherson, with his triple-Rodeck-engine–powered "Makin Bacon Special", Dave and Ralph Banter and their Chevrolet-powered tractors, and the "Mission Impossible" tractors of Tim Engler, which at one point had up to seven
supercharged methanol-fueled engines. Subsequently, modified tractors with four engines were common, while stock tractors tried to catch up by adding multiple large
turbochargers, along with
intercoolers, but both retained the appearance of a
tractor. Soon tractors became single-use machines that were not used on the farm, making the "Pull on Sunday, plow on Monday" motto obsolete. Throughout the '70s and '80s the modified division continued to thrill crowds by adding more engines, and soon the tractors lost their tractor appearance and turned into high 'spec'
dragsters. The limit was reached in 1988 when a tractor with seven engines was built. As well as
piston engines,
turbine engines (frequently mistakenly called "
jet engines") appeared in 1974, with Gardner Stone's "General" Tractor, a four-
turboshaft unit hitting the hook in 1989. The growing popularity of the sport caused the creation of a new
four-wheel–drive division in 1976, which captured a large fan base. The engine sizes in these vehicles continued to increase, from up to and probably would have continued, but in 1989 the NTPA limited displacement to and required
natural aspiration, banning supercharging. Today the four-wheel–drive division is one of the most popular with the success of trucks like the Holman Brothers "4-Play" Chevy and Bob Boden's "Studley Studebaker".
Two-wheel-drive (2WD) truck class The
two-wheel–drive (2WD) division was introduced in 1983. The division imposes a weight limit of on each competing truck, a maximum width of , and a maximum distance of from the centerline of the rear axle to the front of the vehicle (including weight racks and tow hook). (The length restriction allows for up to of cosmetic fiberglass, however.) Alcohol methane engines with up to eight cylinders are permitted, but diesel engines are not. Any
wheelbase is permitted. The National Tractor Pullers Association restricts engines to and two valves per cylinder. They permit tubular steel frames. The maximum tire size for the 2WD class is , with a maximum circumference of when mounted on an rim and inflated to . The ground patch is not to exceed on original tread.
Super stock, pro-stock, and mini-modified Super Stock tractor Open class uses primarily methanol fuel (some are diesel versions). The Super Stock Open machines can generate over and of torque, with billet or re-cast engine blocks. Super Stock Open and Super Stock Diesel tractors may use up to four turbochargers in three stages. The Diesel super stock tractors generate close to and of torque and are allowed to compete in the Open class, which very rarely occurs anymore. However, a true Open (methanol fuel) tractor is not allowed to compete in the Diesel class. There is light Super Stock class which is and the Heavy Super Stock Classes that are . Diesel Pro Stock Tractors are limited to one turbocharger and diesel fuel is the only allowable source for power, in keeping with the 'spirit' of the original tractors. The maximum engine displacement is . These engines can achieve around and of torque. In recent years, new classes have been created to appeal to different groups of pullers. There is now a class called Limited Pro Stock that is limited to engine and turbocharger. This class typically pulls at and is slightly restricted, as opposed to the Pro Stock class, which can run up to engine and an unrestricted size
turbocharger, along with intercoolers. The latest addition to Pro Stocks is the Light Pro Stock Class that typically pulls depending on location. These tractors are limited to engines but can run any size turbo. They are not allowed to run intercoolers, however these tractors are making an average of . The light overall weight makes this a driver's class as significant skill is required to keep the tractor on the track. The mini-modified class is a highly specialized and custom built tractor to be fitted with a naturally aspirated engine, at minimum. NTPA Minis are limited to (always an aftermarket V-8 engine block) and uses up to a 14-71 hi helix supercharger. With the driver, they weigh only . Today's engine is capable of a minimum of on methanol or ethanol. Their reputation is known as the wildest ride in pulling, as naturally it is a very high horsepower to weight ratio. Whereas, their larger counterparts, the Modifieds, will weigh , utilizing the same engine that a Mini has, but, with multiple powerplants per custom built tractor chassis. Usually, a maximum of five engines is all that will make the weight limit.
Nitromethane and oxidizers were outlawed in 1976. == Antique tractor pulling ==