What do they pull?
The
tractors pull a machine that literally becomes heavier the further it is pulled
down a 100 meter track. This
machine is referred to as a ‘Weight Transfer Machine’.
The weight transfer, or sled as it is commonly known, looks like a standard flat bed semi-trailer with a weight box instead of a standard load, this weight box weighs about 14 tonne. At the front of the sled, positioned where you would usually find the coupling point between the semi-trailer and prime mover, a steel skid pan is mounted. The total weight of the sled is approximately 22 tonne. The tractor is hitched to the sled by a chain. The weight box begins the pull located over the rear axles of the sled and as the tractor pulls the sled along the track a series of gears winch the weight box toward the front of the sled thus transferring the weight contained in the box onto the skid pan.
 |
The ‘Mini’ tractors utilise a different type of sled. This is in the form of a tandem axle trailer which begins the pull rolling on its wheels. The Mini Sled weighs between 1 and 4 tonne. As the sled is dragged along, a jockey wheel turns a hydraulic pump which operates two hydraulic rams at the rear that lift the sled off its wheels transferring the weight onto a skid pan.
 |
 |
This
transfer of weight and the subsequent friction between the skid pan and the
ground eventually overcomes the grip and power of the tractor. When the tractor
is unable to pull the sled any further the distance pulled is measured using
laser surveying equipment. This provides an accurate reading to within a
centimeter
(and sometimes it is only a centimeter that separates 1st from 2nd
place). The sled is then returned to the starting position, the track is
repaired by a grader and roller and the sled is then ready for the next tractor
to try their luck.