All tractors, in their respective classes, pull a set weight in the sledge. When a tractor gets to the end of the 100 metre (300 feet) track this is known as a "full pull". When more than one tractor completes the course, more weight is added to the sledge, and those competitors that went past 300 feet will have a pull-off; the winner is the one who can pull the sledge the farthest.
The sledge is known as a weight transfer sled. This means that as it is pulled down the track, the weight is transferred (linked with gears to the sledge’s wheels) from over the rear axles and towards the front of the sledge. In front of the rear wheels, there is a "pan". This is essentially a metal plate and as the weight moves over this the resistance builds. The further the tractor pulls the sledge, the harder it gets.
The most powerful tractors, such as those in the 4.5 modified class in Europe, can produce over 10,000 bhp.History:
It is said that around the 1860s when farming machines were pulled by horse, farmers would boast about the strength of their horses. They would claim that their horse could tow large loads, such as a fully loaded hay cart or wagon. Farmers would challenge one another to contests to prove who had the strongest horse. A barn door was removed and laid flat on the ground, and the horse was then hitched to it; the farmer would then urge the horse to drag the barn door along the ground. One by one, people jumped on the door until the horse could no longer drag it; the horse pulling the most people the greatest distance was judged the strongest. This event, called horse pulling, is still carried out today with specially bred horses trained to have high strength and low stamina, rather than low strength and high stamina which is normally the case with racing horses. Instead of people, fixed weights on sleds are dragged as far as possible. While it is said that the term horsepower is derived from this event, in reality the term was coined by James Watt.
It wasn't until 1929 that motorized vehicles were put to use in the first events at Vaughansville, Missouri, and Bowling Green, Ohio[citation needed],
the latter being where the current national championships are held.
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 the 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 engine 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 Mr Chevy tractors, and the "Mission Impossible" tractors of Tim Engler, which at one point had up to seven blown alcohol engines on board.
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, jet engines appeared in 1974, with Gardner Stone's "General" Tractor a four-jet-engined 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 450 cubic inches/7.3 liters up to 700/11.5 and probably would have continued, but the NTPA limited it to 650/10.6 naturally aspirated and no blown engine in 1989. Today the 4-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
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