## 3 Results and Discussion
### Simulation Results and Their Discussion
In b7 variant (braking on a surface with reduced adhesion--wet surface) for the CE case the horizontal force in the coupling is lower than the nominal variant (b1) value. This may be due to the fact that get less deceleration (see Table 4 and Figure 18). This is also justified by the tests with the same deceleration (CD), where the horizontal forces in the coupling are approximately similar for both variants b1 and b7. Comparing the results when braking on a road with very low adhesion (b8), the obtained results are qualitatively in line with the expected ones. The braking performance indicators have obviously deteriorated. The operation of the anti-lock braking system improves efficiency. An important effect is the reduction of the horizontal coupling force. However, it should be remembered that due to the lower ability to transfer forces in wheel-road contact also in the vertical direction, it is difficult to conclude on this basis about a lower risk of e.g., of jack-knifing phenomena. It follows from this that the type of surface does not directly affect the horizontal force in the coupling, but it does affect the maximum deceleration obtained. Thus, this is a different effect than, for example, in variants b3f or b4, where the increase in the horizontal force in the coupling was directly caused by the change in mass and the position of the centre of gravity. This can be seen by comparing the results for tests with the same deceleration (CD simulations)--see Table 5 and Figure 17.  
Figure 18: Mean deceleration and braking distance force for constant excitation series (CE) for 9 braking variants: (**a**) Mean deceleration; (**b**) Braking distance. A recurring feature in most of the presented simulations, both for the maximum position of the main control valve positioner (CE) and for the same deceleration in the phase of fully developed braking (CD), is the moment when the horizontal force in the coupling reaches the highest value. It occurs in the final phase of actuating the brakes before the phase of fully developed and stabilized braking. This is related to the reaction time of individual axles of the braking system or the order of axle blocking resulting from the load on axles. The braking process in the phase of actuating the brakes is unstable, and large changes of tangential forces and the angular decelerations of the wheels occur then (e.g., Figure 11a). This is the moment when the risk of braking safety is most significant. The wheel locking order and the status of ABS activation affect the horizontal force in the coupling. Blocking the wheels of vehicle A first (e.g., variant b3r) reduces the braking ratio of this vehicle and thus reduces the force in the coupling. In the b3r variant, the horizontal force in the coupling decreased (compared to the nominal variant), and it increased in the opposite case--b3f. Also, the activity of the ABS significantly affects the values of the horizontal forces in the coupling. In mentioned the case of b3r, after switching on the ABS, the force in the coupling in the phase of fully developed braking increased from 71 \(\div\) 77 kN to about 89 kN. A similar effect can be seen in the b7 variant, where the higher (by approx. 20 kN) value of the force in the coupling was observed for braking with the ABS on in the phase of fully developed braking. From the other hand, in the case of b3f, active ABS reduced slightly the horizontal force in the coupling to 108 kN-111 kN. It is worth emphasizing, however, that this value is still higher than the nominal 100 kN. In general, ABS reduces the risk associated with braking safety, but does not eliminate it completely, and in some cases, as described above, even increases it. For tests with constant excitation (CE), an incorrect axle locking order occurs for the simulation in the conditions of incorrectly distributed load (b3r, b3f) and failure of the braking system (b5). In the b3r and b5 variants, the tractor's rear axle wheels are locked first--there is a risk of jack-knifing.