When front-wheel assist (FWA) and mechanical front-wheel drive (MFWD) tractors are working in tilled soil the front wheels operate more efficiently at a higher slip rate (lead) than the rear wheels, which are tracking in the soil consolidated by the front wheels.

Lead occurs when the front tyres are rotating slightly faster than the rear tyres. Lag occurs when the front tyres are rotating at a slower rate than the rear tyres.

Lead makes MFWD and FWA tractors easier to steer, and also allows them to operate more efficiently. Always check what the tractor manufacturer’s recommended lead percentage is.

Steps recommended by many manufacturers for checking

• Always check with the manufacturer for the specific method they recommend for checking the lead or lag.

• Before performing the lead or lag exercise, you may choose to have the spray tank half full.

• Check the required cold tyre pressure is matched to the potential load using websites such as https://www.tirepressurecalculator.com/ and

• Always check and record the current tyre pressures before starting.

Step 1: Set up the tractor as it would be used with an implement and/or loader. Use the normal tyre pressure and optimum ballast distribution.

Step 2: Locate a suitable smooth, hard, flat, level surface about 75 to 100 metres long. Park the tractor at a start point and mark the front tyre and rear tyres at the bottom of the tyre (at the 6 o’clock position). Mark the ground adjacent to the same position on the tyres (front and rear).

Step 3: Disengage the MFWD/FWA and drive the tractor forward at walking speed in a straight line until the front tyre has made exactly 10 revolutions and the position of the mark on the tyre at ‘6 o’clock’ has returned to the ground.

Step 4: Engage the MFWD/FWA and reverse the tractor along the same path to the starting point. Count the revolutions of the front tyre, and stop the tractor when the 6 o’clock position on the rear tyre returns the original position (mark on the ground from the start of the test).

Interpreting the results of the lead or lag test

If the mark on the front tyre has gone one-quarter of a turn past the 6 o’clock position, the lead is 2.5 per cent.

Note: The tractor would have lag if the mark on the front tyre has not returned to the 6 o’clock position.

Refer to the tractor manufacturer’s technical information for the recommended lead, e.g. ideally it will be around 2 to 3 per cent, although anywhere from 1 to 5 per cent is generally acceptable.

If the lead is too low, consider increasing the tyre pressure in the front tyres, or decreasing the pressure in the rear tyres (or a combination of both), within tyre manufacturer’s limits. Generally this can make a 1 to 1.5 per cent change to the lead (and vice versa).

If changing tyre pressures alone is not sufficient to increase the lead, changing the size of the front or rear tyres (or both) may be an option.

If changing the tyre size, you may need to purchase different brands of front and rear tyres to increase or decrease the rolling circumference to improve the lead.

Tips

• Generally, tyre pressures are quoted as cold tyre pressure.

• It is important to check the MFWD/FWA lead as there could have been an incorrect gear installed in the transmission, or a replacement transmission may have been installed into the tractor.

• As a quick guide only: with one tyre lug on the ground ‘at 6 o’clock’ the tyre pressure is about correct if there are 3 lugs touching the ground.