The values of k were calculated by developing a schematic
model of the main section of a tractor as three rectangles (one
for the cabin, twofor the left and right rear tyres,whose outlines
act over the front wheels). The value of the drag coefficientwas
taken from the literature. Mohamed-Kassim and Filippone
(2010) indicate a value of 0.80 for a European heavy road-truck
with a trailer and no spoiler to reduce turbulence. Without any
precise information, 0.70was, hence, considered reasonable for
agricultural tractors. Although the ratio Fa/Ft is proportional to
the speedcubed, at 11.1ms1 Fa/Ft is close to 0.015 (¼1.5%) for all
three vehicles, as themagnitude of k is 105 s3m3.
Therefore, the equation of the tractor dynamics takes the
following form: