affect the measurements of motion resistance (MR) force. The
values obtained for MR force were corrected by the angle of
movement between the two tractors. Twenty readings were taken
along each test plot using a dynamometer device.
The MR force was measured using a dynamometer device
provided by the Agricultural Machinery class (Agronomy Faculty,
Buenos Aires University, Argentina). A cab-mounted unit collected
the draft force data with a sampling frequency of 200 Hz. The data
were stored in a data logger and downloaded to Excel files for
analysis.
The tyre inflation pressure and tractor ballast were not
modified for the trial, assuming that the tractors are prepared
for tillage with implements that demand large pulling force
(mouldboard plough, chisel plough, heavy discs, etc.). Cross-ply
tyres are usually utilised on commercial farms in Argentina.
Prior to the trial, the two tractors were weighed to obtain their
total and individual axle loads (Fig. 2). The experimental design
consisted of a randomised block (traffic with each tractor) with
three replications. The plot size was 20 m wide and 75 m long
(1500 m2). The tyre/soil contact area was measured by reversing or
driving the tractor into the virgin soil of the experimental field and
spraying the area around the tyre with paint. A hydraulic lift was
then used to raise the tractor so that the tyre track could be
transferred onto a sheet of glass, printed from there onto paper,
and measured with a planimeter. The average ground pressure was
estimated as the total axle load divided by the tyre/soil contact
area for both tyres on the axle. Finally, the tyre widths were
measured in the field under working conditions (Botta et al., 2008).