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Mechanical lead

In order to fully exploit the advantages of four-wheel drive and to avoid premature wear of transmission and tires, the peripheral speed of the front axle must be higher than the rear, on unequal wheel tractors.

The "MECHANICAL LEAD" (L) is the percentage relative ratio between the peripheral speeds of the front and rear tires. Commonly, the correct running of the tractor in 4wd foresees a value of L included between +1% and +5% (peripheral speed of front tires > peripheral speed rear tires).

Values of L out of this range could cause problems with handling and instability (L < 1, the rear axle "pushes" the tractor in respect to the front axle) or cause excessive lead between the front and rear tires (L > 5, excessive mechanic effort of all the transmission chain).
In both the cases, this will cause difficulties with the management / insertion of the "gears" and the tires are subject to rapid and uneven wear.

Calculation of the front lead
Rolling circumference on the field
Mechanical ratio on the field


The measurements must be implemented with the following conditions:
- Tractor equipped with standard front ballast.
- Nominal tires pressure.
- On a of smooth and level surface at least 50 m.

          Rc (front) * RM - Rc (rear)
A = ------------------------------------ * 100 = (+1 +5)
                   Rc (rear)


The measure must be calculated in the following way:
- Remove the 4wd (the rolling of the front and rear wheels has to be independent)
- Mark the front and rear tires sidewall, on the middle of the tire-foot print area
- Mark the rolling surface in correspondence of the middle of the tire-foot print area
- Move the tractor forward so that the front and rear tires complete exactly 5 revolutions
- Mark on the ground the point at the end of the 5th revolution, for both the front and rear tires
- Measure the distance rolled respectively by the front (f) and rear (r) tires

- CALCULATION OF Cr (front) = F[m] / N° of revolutions = a / 5

- CALCULATION OF Cr (rear ) = R[m] / N° of revolutions = p / 5


The measure must be implemented in the following way:

- Insert the double traction.
- Mark the front and rear tires sidewall, on the middle of the tire footprint area.
- Move the tractor forward so that the rear tire complete exactly 5 revolutions.
- Count the complete number of revolutions of the front tire.
- Calculate the fraction of revolution of the front tire in this way: count the number of lugs included between the one in the middle of the foot print area and the previously taken as reference point. Divide this counted number by the total lugs number.

CALCULATION OF RM = (FRONT TIRE revolutions. + rev. fraction) / COMPLETE REAR TIRE revolutions.

Last updated 2010-02-04 |  Print this page Print this page |  E-mail this page E-mail this page   Facebook   Twitter
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