A GUIDE TO

A GUIDE TO

TRACTOR PULLING

History of Tractor pulling

It all started many years ago in the USA when neighbouring farmers argued about which had the most powerful tractor, (originally horses). These debates were finally decided by contest in the farm yard by chaining the tractors to a large weight and seeing which tractor could pull it the furthest.

It was soon found that the amount of weight required to provide a good test of strength was too much to allow the tractors to get moving in the first place.

A system of increasing the weight progressively was needed.

These contests soon became something of a local spectacle, the spectators were put to good use. Each tractor was hitched to a large barn door laid flat in the field, ten or so spectators were asked to stand on it and the tractor pulled away, as it progressed along a marked-out line a certain number of people “hopped on” at each 10 yard mark. In this way the “sled” became progressively heavier the further it was pulled.

The only remaining problem was trying to maintain consistency, as people left and were replaced by others of differing weight more arguments and controversy reigned.

The birth of tractor pulling and the sled followed and grew to become events in their own right with crowds now often exceeding those for major regional football and baseball matches.

Tractor Pulling Today

Tractor Pulling crossed the Atlantic in the 1970’s and is now seen all over Europe at agricultural shows, motorsport events and stand-alone pulling events as well as on the Eurosport satellite TV channel.

The PPA (The Power Pulling Association) are the body for Mini (950kg) Pulling in the U.K. and stage events throughout the summer months at public events and agricultural shows. Membership is available to competitors and spectators alike and is a source for all Pulling details and events lists etc.

The sport consists of different classes of “Tractors” of 950kg & 1050kg weight categories attempting to pull a weighted sled along a 90m earth track.

The objective of all competitors is the same- to pull the weighted “sled” as far as possible. Those that reach the full 90m distance achieve a “Full Pull” and return, with any other tractor that got a full pull to try again, but this time with a greater weight on the sled. The tractor that pulls the furthest is the class winner.

The ability of a competitor to win depends not only on the power available from his machine but his ability to “read” the track, balance his tractor and keep control of it whilst under full load.

For the tractors and indeed the spectators the overall objective of the sport may seem quite straight forward - if it were not for one thing - THE SLED!

THE SLED.....Simple description

The sled, or "Weight Transfer Machine" to give it its correct name, is the very sophisticated machine necessary to provide a fair and effective load for a wide range of competing tractors.

Use of a straight forward weighted "trailer” would not work.

If it were sufficiently heavy to provide enough drag to eventually stop the tractor then that same drag would probably be too much to allow the tractor to ever get moving in the first place.

What is needed is a form of trailer that offers little drag at the beginning of the pull (thus allowing the tractors to get moving) but offers an increasing amount of drag as the tractor pulls it along the track - hence the complication involved within the Sled mechanics.

At the start of the tractors pull the weight box is positioned at the back of the sled over the twin axles. As the tractor pulls the sled down the track the weight box is automatically driven up the rails towards the front of the sled.

As the weight box moves up the length of the rails the effect of its weight is lessened on the sled wheels but begins to increase its effect on the large skid pan below the front of the sled.

The weight of the box progressively pressing down through the skid pan increases the friction with the track and hence exerts increased drag.

The weight has been "transferred" from a rolling weight to a dead weight. This weight transfer effect allows a well set-up sled to provide little drag at the start line, allowing the tractor to quickly accelerate away from the start, but sufficient transfer to stop most of the tractors in the class within a few metres of each other. The diagrams overleaf show the Sled in its starting condition - (rolling weight), and then in its fully transferred condition - (dead weight).

· All hydraulic derived from a hydraulic pump driven from the flywheel and feeding a split pressure valve to two hydraulic systems. Also a compressor to charge and maintain the air system and an alternator to charge and run the electrical system.

· Ground drive:- to self propel the sled so that it can be driven about and back up the track avoiding the need for a tow back tractor. This is the high volume side of the hydraulics and uses a hydraulic motor through an air operated clutch to the rear axle.

· Auxiliary hydraulics:- This is the low volume side and operates the steering, front wheel raise and lower, and box return and movement when under the operators control.

· Air system:- this operates the air over hydraulic brakes for the two axles, the box and box drive brakes, the emergency kill switch system, the back pan push down, the clutch for the box and the ground drive engage, disengage. Oh and you can blow up the tyres as well!

· Electrical system:- This operates all the electrical controls, the lighting, the engine fuel system, the kill switch motor and most everything else in a roundabout way.

· The control station:- This enables the sled operator to totally control the sled from one point, the drivers seat, however once the sled green light is on the driver has no influence over the sled other than the ability to use the kill switch safety system.

· The sled also incorporates:- an on board diagnostics system to quickly find faults or wrong settings and also has many fall back arrangements enabling it to continue operating (albeit in a less efficient manner) even if a major component such as the engine should fail.

· Statistics:- Weight around 4 tons, stopping efficiency about 400% or equivalent to a dead weight of about 13 tons. Box gears, 4 with 3 start points making 12 combinations. Box capacity 1 ton of steel weights.

Once the sled is on its way down the track the sled operator has no control over the advance of the weight box, its advance is a function of the forward movement imparted to the sled by the pulling tractor. So why does he sit there going up and down the track all day getting covered with dust, stones and/or mud?

The sled operator has final control of the pulling tractor by means of the "Kill System" and can terminate any pull should he feel that the tractor driver is out of control or becoming so.

The system will also activate automatically in the very unlikely event of the pulling chain breaking.

Every machine that hooks to the sled is not only attached by a very strong (certified) chain but also by a kill wire.

The kill wire is spooled out from an electric winch at the front of the sled and hooked onto a fitting on the rear of the tractor.

On diesel engined tractors the fitting is attached to flaps that can cover the air inlets to the turbochargers and starve the engines of air and result in a rapid halt. On ignition engines the fitting is an electrical multi socket which when withdrawn will kill all the electrics to the ignition systems and fuel systems of all the engines and stop them.

Should the kill system operate, either by command of the operator or by the tractor "breaking away" the following specific actions occur;

Finally the kill wire winch is energised to rapidly withdraw the kill fitting from the tractor.

These actions, sequenced automatically and very quickly, ensure the whole rig comes to a rapid and controlled halt.

The sled, whilst on the start line is fully braked, the brakes cannot be released until the operator is aboard and has inserted a "key" attached to his person.

The pulling area of the track is 90m in length and 10m wide, with a 30m long “run off” area at the top end. It may be a purpose built track of clay and earth mixture as installed at permanent venues or a plain earth track at “one off” shows. There are distance markers along its length. At the 20m marker is usually a white flag. Should the driver realise that his machine is not working well, or he has selected the wrong gear, he can make an attempt to back-off the throttle and stop. If he does so before the 20m marker then he will be allowed a second attempt, at which he must get it right as this is now to be his measured pull.

after his pull the sled operator will decide if the sled is “set”, i.e. correctly weighted for the class and track conditions.

If the sled is deemed not set then the first tractors pull is discounted and he drops six places in the running order to try again. Alterations to the sled weights are made and the second tractor becomes the new test puller. If the sled is deemed to be set after this pull, then the competitor may accept his distance or elect to discount it and pull again six places lower in the running order. The rest of the class pull the sled as set.

The rear tyres of the tractors have most of their tread shaved off. Too much tread would give too much grip, the tyres would dig in, destroy the track, very low gearing would be needed to get the engines going, the pull would be very slow and the tractors drive line would very soon be destroyed.

The tractors move forwards by using friction with the track rather than traction. A shallow tread reduces the grip but maintains friction, this allows high engine speed, high wheel speed, (up to 140mph for the Mods), rapid acceleration and is less damaging on the drive line. An initial rapid acceleration of the sled helps keep it moving later when the weight box effect starts to increase the drag.

READY FOR THE OFF

The track is being rolled and graded after the previous competitors pull.

The Sled is taken back to the start line and positioned visually at the zero meter position. Each driver may “spot” the sled at any position across the track at the start position, (he may feel that the right hand side of the track is in better condition and will benefit his pull).

The sled will be showing a red flashing beacon. The measuring tower operator aims an infra-red beam at the reflector on the sled roof. The measuring unit will calculate the distance from the tower to the sled. The tower operator then signals his readiness by changing his “traffic lights” from red to green.

Whilst the measuring is taking place the next competitor is called to the track. Each driver has three minutes, from the moment his is called, to hitch to the sled and be ready to launch. Exceeding this three minute period may result in disqualification.

The first and second track marshals are showing red flags. When the first track marshal and hook man are satisfied that the tractor is secured to the sled the sled operator will check that the measuring tower is at green and release the sled braking systems and switch from his red to green flashing beacon.

The only remaining “reds” on the track are the flags of the two marshals. The second marshal (at the finish end of the track) will see the sled go green and if his end of the track is clear will raise his green flag. The first marshal (at the start line) re-checks all the previous signals and if all are at green and he is happy that the driver and tractor are secure will raise his green flag. The tractor may now leave the line.

The driver has not only to control his tractor but must always be watching the marshals. If a marshal raises a red flag the driver must stop immediately. The driver may be “red flagged” for various reasons - perhaps a marshal has spotted a dangerous fault on the tractor, or he has gone out of bounds, or achieved a full pull.

Once the tractor has stopped the measuring system will take another measurement to the sled and calculate the distance pulled, (to the nearest centimetre).

Power Pulling in the U.K. consists of two main classes of competing Mini tractors; 950Kg & 1050Kg.

The Club is full of friendly fans and helpful competitors, always willing to explain the details of their machines, the sport and their tactics and to help the novice.

As the Tractor progresses down the track, at full revs and under full load, the fuel consumption may be anything from 2 to 12 gallons for each 100m pull. How does this compare to the more normal 30 mpg we expect from our family car!

The laser measuring system records only the straight line distance achieved, ie. 100m.