Today's sleds use a complex system of gears to move weights up to 65 000 pounds/29 000 kilograms. Upon starting, all the weights are over the sled's rear axles, to give an effective weight of the sled plus zero.
As the tractor travels the course, the weights are pushed forward of the sled's axles, pushing the front of the sled into the ground, synthetically creating a gain in weight until the tractor is no longer able to overcome the force of friction.
The effectiveness for a sled to successfully stop a high-speed, fully modified pulling tractor depends greatly on where the two "leg supports" (the support posts that goes from the frame down to the skid pan, also known as the "fulcrum point") are positioned on the frame.
How does the sled work?
A To help understand how a sled works a little history of the device is sometimes necessary. In the early days of pulling people used to stand beside a skid plate in which they hitched to a tractor, horse, or whatever competitive vehicle they were competing with. People stood along the side of the track at regular marked distances from the beginning of the track to nearly the end. As the skid plate drew closer they prepared to jump on it making the skid plate harder for the vehicle to pull as it progressed down the track. Today, a sled is a weight transfer machine that works similar to the original design. It still uses a skid plate (which I will now refer to as a pan) and the machine has been built over the top the pan. A large ramp is placed over the top of the pan with most of it's length existing behind the pan. The sled has regular semi truck wheels in the back that turn as the sled is drug down the track. As the wheels turn a series of transmissions, gears and large chain pull a box full of weight up the ramp as the sled progresses down the track, thus transferring more weight off of the wheels and onto the pan, thus making the entire sled much more difficult to pull as it progresses down the track.
A Tractor pull sled weighs the same from start to finish. It doesn't GAIN weight. The weight moving forward pushes the sled into the ground. The tractor isn't pulling more weight. the weight is being transfered from a rolling force to a dragging force. The force that slows and/or stops the tractor is friction. Not weight.