## 3 Results and Discussion
### Simulation Results and Their Discussion
In the case of b3f, the axle of the semitrailer is blocked--the risk of instability of the semitrailer. The variant b2 is a debatable issue, where the front axle of the tractor and the complex axle of the semitrailer are locked approximately simultaneously, while the rear axle of the tractor is not locked. In the case of tests with constant deceleration (CD), only in one variant (b3f), the wheels were locked with the ABS turned off, qualitatively the same as for the maximum excitation (CE). Analysing the test results for the constant excitation (CE), it can be seen that each deviation from the nominal variant affects the braking efficiency measures. The set with a laden semitrailer (nominal b1) reached a deceleration of about 6.0 m/s\({}^{2}\), and the braking distance was up to 40 m. The shortest braking distance (37 m with ABS off and 34 m with ABS on) and the biggest average deceleration (5.8 \(\div\) 6.5 m/s\({}^{2}\)) were obtained for the unladen semitrailer (b2). Longer braking distances are in the simulations for a shift of the centre of gravity forward b3f (centre of gravity shifted to the front of the semitrailer) and b3r (centre of gravity shifted to the rear of the semitrailer). For variant b3r the average deceleration is 4.9 m/s\({}^{2}\) (ABS off) and 5.2 m/s\({}^{2}\) (ABS on), and the braking distance is 42 \(\div\) 44 m. In the case of b3f average deceleration is 4.3 m/s\({}^{2}\) (ABS off) and 4.6 m/s\({}^{2}\) (ABS on), and the braking distance is 47 \(\div\) 48 m. In the latter case, it is worth recalling that we also have another unfavorable effect--an increased horizontal force in the coupling in relation to the nominal variant b1. The longest braking distance (73 m for ABS off and 66 m for ABS on) on a dry surface was obtained for the variant with failure of the semitrailer braking system (b5). The extended time to reach the maximum braking torque of the semi-trailer in the case of b6 also reduced the average deceleration, which resulted in an extension of the braking distance by 2 m.  
On a wet surface (b7), the braking distance is 53 m with the ABS off and 44 m with the ABS on (and the average deceleration is 4.1 m/s\({}^{2}\) and 4.9 m/s\({}^{2}\), respectively). On ice (b8) the braking distance is 207 m with the ABS off and 191 m with the ABS on (and the average deceleration in both ABS status cases is close to 1.0 m/s\({}^{2}\)--slightly less than 1.0 m/s\({}^{2}\) with the ABS off and slightly greater than 1.0 m/s\({}^{2}\) with the ABS on). The ABS system's significant impact on these braking performance measures is evident here. The presented results were compared in the scope of the main indicators of braking efficiency available in the literature (average deceleration, braking distance) obtained in actual road tests. A truck with a similar mass to the b2 variant shows decelerations in fully developed braking at the level of 6.27-7.67 m/s\({}^{2}\)[14]. The braking distance of the tractor with a partially loaded semitrailer with a set weight of approx. 22 t (i.e., approx. 9 t more than in the variant of b2) in [15] was given at approx. 41m, and its deceleration at approx. 5.7 \(\div\) 6.0 m/s\({}^{2}\). For the unloaded set (about 15 t), the braking distance was 30-32 m, and the deceleration was about 7.25 \(\div\) 7.5 m/s\({}^{2}\). In [16], the average deceleration for a loaded vehicle of similar mass to the variant b1 is 4.4 to 5.4 m/s\({}^{2}\). For a tractor with damaged trailer brakes (a variant close to b5), the deceleration was from 3.43 to 4.22 m/s\({}^{2}\)[16]. In addition to deceleration and braking distance, the horizontal force in the coupling was compared. In the work [79] you can find information on the value of this force for vehicles with parameters similar to cases b1 and b5 with small decelerations of 0.5-0.6 m/s\({}^{2}\) at the level of approx. 11 and 14 kN, respectively. In the simulation under similar conditions, values of approx. 12-15 and 13-16 kN were obtained. In sum, it can be assumed that the simulation tests' results are similar to the real ones.