1 Investigation of load assumptions
1.1 Full braking
Full braking is the greatest load to which a forward securing arrangement is exposed. Recent developments in the field of truck tires, coupled with modern brake systems and asphalt roads, permit braking deceleration values that are perfectly capable of approaching 0.8 g (1). Other factors, such as the distribution of axle weights, also play a role in this context.
The connection between the loading area of a truck or semitrailer and the tyre footprints is not rigid but resilient, which means that the inertial force of the cargo does not follow directly from the braking deceleration, but instead initially brings about a forward tilting of the loading area. This "pitching angle" is not at a steady state throughout full braking, but has pitching oscillations superimposed on it. The amplitude of the pitching oscillations is very highly dependent on buildup time, i.e. the time taken for the braking force to increase to its full value.
During full braking, the following forces act forwards on the cargo in the coordinate system of the loading area (parallel to the loading area):
The normal force acting from the cargo on the loading area is generally reduced by two causes, namely, as a result of the inclination of the loading area, by the
The upwardly directed vertical component of the inertial force, and the reduced normal force arising from the geodetic inclination of the loading area reduce both the friction relative to the loading area and the moment of stableness of a cargo unit.
Figure 1: Full braking on downward sloping road
Figure 2: Full braking on a level road from 90 km/h with 0.8 g braking deceleration
Figure 2 shows the numerical solution of the equations of motion over a period of 6 seconds. The forces acting on the Cargo have been converted into units of g. The vehicle is stationary after approx. 3.3 seconds.
The truck is loaded in such a way that, at 0.8 g deceleration, a steady-state pitching angle of 4° is obtained. The maximum pitching angle after 0.9 seconds amounts to 5.5° as a result of the superimposed pitching oscillation. This oscillation is strongly damped and largely subsides by the time the vehicle is at a standstill, but is re-excited by the familiar jerk at the end of the braking maneuver.
The maximum longitudinal load on the cargo at 0.9 seconds amounts to 0.98 g, at which point the normal force has simultaneously declined to 0.92 g.
Numerous further simulated full braking maneuvers at other speeds, uphill and downhill road gradients and other vehicle types (e.g. semi trailer with a smaller pitching angle) reveal similar profiles. The following general conclusions may be drawn:
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