Wheel slip occurs when the tyres are turning faster than the ground speed of the tractor. As a result, less than 60% to 70% of the power that a tractor engine develops is used to pull an implement through the soil. It could even drop to 50% on soft and sandy soils.
Observing wheel slip
Wheel slip is a good indicator of whether your tractor set-up is fuel-efficient. Modern tractors typically include wheel slip monitors or monitors can be added aftermarket. If this is not an option, an approximate method for observing wheel slip is the tyre tread pattern produced when the tractor is pulling under load, using the following guidelines. With correct ballast and tyre pressure, the tread pattern will show that the soil between the cleats in the tyres has shifted but the tread pattern is still visible, as shown below:
With too little weight or tyre pressures that are too high, excessive slippage wipes out the tread pattern. If the tractor has high slip levels, tyres will wear excessively and fuel efficiency will be poor. At the other extreme, with too much weight and/or too little pressure, the tread pattern will be sharp and distinct in the soil. The ideal tyre print is one that shows some slippage and some tread pattern.
Measuring Wheel Slip:
Tractors and tyres should be maintained to optimize wheel slippage at 10% to 15%. Less slippage than this results in the expenditure of too much fuel energy to move the wheels, whereas too much slippage (greater than 15%) can result in excessive tyre spin and energy loss through the tyre, which is nonproductive.
To determine percent wheel slip:
1. Place a mark on the inside of the tyre that you can observe from the tractor seat.
2. Flag or mark off a distance of 100 feet part way into the field.
3. Determine the circumference of the tyre by placing a string (or cloth/plastic measuring tape) around the center part of the ribs.
4. Finally, drive the 100 feet with the field operation in progress, counting the number of revolutions of the tyre.
5. Multiply the number of revolutions by the circumference (in feet) of the tyre and divide by 100.
For example: Your tractor wheel circumference measures 14 feet. In plowing 100 feet, you count 7¾ rotations of the wheel. That means your tyre turned 108.5 feet while traveling the field distance of 100 feet, thus 8.5% more, which represents your percent field wheel slip.
(7.75 rotations x 14-foot circumference) / 100 feet = 8.5% wheel slip
(Calculating Wheel Slip)
Adjusting Wheel Slip
If excessive slippage occurs (greater than 15%), you may need to:
· Add weights (iron and/or fluid in tyres),
· Change the air pressure,
· Add duals, or
· Purchase new tyres, if they are heavily worn.
If slippage is less than 10%, you should remove weights and check the tyre pressure.