Stability Analysis of Long Combination Vehicles Using Davies Method

Abstract

The cargo transportation in the world is mostly dominated by road transport, using long combination vehicles (LCV’s). These vehicles offer more load capacity, which reduces transport costs and thus increases the efficiency and competitiveness of companies and the country. But the tradeoff of LCV’s is their low lateral stability and propensity to roll over, which has been the focus of many studies. Most vehicle stability models do not consider the longitudinal aspects of the vehicle and the road, such as the stiffness of the chassis, the gravity center location, and the longitudinal slope angle of the road. But, the use of three-dimensional models of vehicles allows a more rigorous analysis of vehicle stability. In this context, this study aims to develop a three-dimensional mechanism model representing the last trailer unit of an LCV under an increasing lateral load until it reaches the rollover threshold. The proposed model considers the gravity center movement of the trailer, which is affected by the suspension, tires, fifth-wheel, and the chassis. Davies method has proved to be an important tool in the kinetostatic analysis of mechanisms, and therefore it is employed for the kinetostatic analysis of the three-dimensional mechanism of the trailer.

1. Vehicle model for lateral stability

According to Rempel [1] and Melo [2], the last unit (semi-trailer) of an LCV is the critical unit, since it is subjected to a high lateral acceleration compared to the tractor unit, which impacts the rollover threshold of the unit and the vehicle. Taking into account this aspect, a simplified trailer model (Figure 1) is modelled and analysed to calculate the SRT factor for LVCs.

Figure 1.

Simplified trailer model.

The tyres, suspension, fifth wheel, and chassis are directly responsible for the CG movements; these movements are dependent on the forces acting on the trailer CG, such as weight (W), disturbance forces imposed by the ground, and lateral inertial force (ma_y ) when the vehicle makes a turn. During cornering or evasive manoeuvres, the weight and the lateral inertial force acting on the vehicle centre of gravity cause its displacement, which can lead to vehicle rollover.

1.1 Tyres system

The tyres system (tyres and rigid suspension) maintains contact with the ground and filters the disturbances imposed by road imperfections [3]. This system allows two motions of the vehicle: displacement in the z-direction and a roll rotation around the x-axis [4], as shown in Figure 2.

Figure 2.

Tyres system.

1.1.1 Kinematic chain for tyres system

Mechanical systems can be represented by kinematic chains composed of links and joints, which facilitate their modelling and analysis [5, 6, 7].

The kinematic chain of the tyres system in Figure 2 has 2-DoF (M = 2), the workspace is planar (λ = 3), and the number of independent loops is one (ν = 1). Based on the mobility equation, the kinematic chain of tyres system should be composed of five links (n = 5) and five joints (j = 5) [7].

To model this system, the following considerations were taken into account:

• There are up to three different components of forces acting on the tyre-ground contact i of the vehicle [8, 9, 10], as shown in Figure 3, where F_xi is the traction or brake force, F_yi is the lateral force, and F_zi is the normal force;

• However, at rollover threshold, tyres 1 and 4 (outer tyres in the turn, Figure 4) receive greater normal force than tyres 2 and 3 (inner tyre in the turn, Figure 4), and thus tyres 1 and 4 are not prone to slide laterally. We consider that tyres 1 and 4 only allow vehicle rotation along the x-axis. Therefore, tyre-ground contact was modelled as a pure revolute joint R along x-axis.

• While tyres 2 and 3 have a lateral deformation and may slide laterally, producing a track width change of their respective axles. As a consequence, tyres 2 and 3 have only a constraint on the z-direction. Therefore, tyre-ground contact was modelled as a prismatic joint P in the y-direction.

• Tyres are assumed as flexible mechanical components and can be represented by prismatic joints P, [11, 12].

• In vehicles with rigid suspension, tyres remain perpendicular to the axle all the time.

Figure 3.

Movement constraints in Tyre-road contact.

Figure 4.

Vehicle on a curved path.

Applying these constraints, Figure 5a shows the proposed kinematic chain model of the tyres system.

Figure 5.

(a) Kinematic chain of the tyres system. (b) Tyres system including actuators.

The kinematic chain is composed of five links identified by letters A (road), B (outer tyre in the turn), C and D (inner tyre in the turn), and E (vehicle axle); and the five joints are identified by numbers as follows: two revolute joints R (tyre-road contact of joints 1 and 4) and three prismatic joints P, two that represent tyres of the system (2 and 5), and one the lateral slide of tyre 2 (3).

The mechanism of Figure 5a has 2-DoF, and it requires two actuators to control its movement. The mechanism has a passive actuator in each prismatic joint of tyres (2 and 5—axial deformation); these actuators control the movement along the x- and z-axes, as shown in Figure 5b.

In this model, the revolute joint (3) and the prismatic joint (4) can be changed by a spherical slider joint (S_d ), with constraint in the z-axis, as shown in Figure 6.

Figure 6.

Tyres system model.

1.1.2 Kinematics of tyre system

The movement of this system is orientated by the forces acting on the mechanism (trailer weight (W) and the inertial force (ma_y )) [13]. These forces affect the passive actuators of the mechanism, as shown in Figure 7.

Figure 7.

Movement of tyres system.

Eqs. (1)–(5) define the kinematics of the tyres system.

$$l_i={\delta }_T+l_r=\frac{3\Delta F}{k_t+a_c}+l_r\approx \frac{-F_{\mathit{Ti}}+F_{\mathit{zi}}^{\mathit{\text{start}}}}{k_T}+l_r$$ E1

$${\beta }_i=90-\mathit{arcsin}\left(\frac{l_{i+1}}{\sqrt{t_{i+1}^2+l_{i+1}^2}}\right)$$ E2

$$t_i=\sqrt{t_{i+1}^2+l_{i+1}^2+l_i^2-2\left(\sqrt{t_{i+1}^2+l_{i+1}^2}\right)l_i\mathit{cos}\left({\beta }_i\right)}$$ E3

$${\delta }_i=\mathit{arcsin}\left(l_i\mathit{sin}\left({\beta }_i\right)/t_i\right)$$ E4

$${\theta }_i={\theta }_j=90-{\delta }_i-{\beta }_i$$ E5

where δ_T is the normal deformation of the tyre [14], ∆F is the algebraic change in the initial load, k_t is the vertical stiffness of the tyre, a_c is the regression coefficient, F_Ti is the instantaneous tyre normal load, l_i is the instantaneous dynamic rolling radius of the tyre i, $F_{\mathit{zi}}^{\mathit{\text{start}}}$ is the initial normal load i, k_T is the equivalent tyre vertical stiffness, l_r is the initial dynamic rolling radius of tyre i, t_i is the track width, t_i+1 is the axle width, and θ_i;j are the rotation angles of the revolute joints i and j respectively.

1.2 Suspension system

This system comprises the linkage between the sprung and unsprung masses of a vehicle, which reduces the movement of the sprung mass, allowing tyres to maintain contact with the ground, and filtering disturbances imposed by the ground [3]. In heavy vehicles, the suspension system most used is the leaf spring suspension or rigid suspension [15], as shown in Figure 8. For developing this model (trailer), it is assumed that the vehicle has this suspension on the front and rear axles.

Figure 8.

Solid axle with leaf spring suspension. Source: Adapted from Rill et al. [15].

The rigid suspension is a mechanism that allows the following movements of the vehicle’s body under the action of lateral forces: displacement in the z- and y-direction and a roll rotation about the x-axis [1, 8], as shown in Figure 9a and b.

Figure 9.

(a) Body motion. (b) Suspension system.

1.2.1 Kinematic chain of the suspension system

The system of Figure 10a has 3-DoF (M = 3), the workspace is planar (λ = 3) and the number of independent loops is one (ν = 1). From the mobility equation, the kinematic chain of suspension system should be composed of six links (n = 6) and six joints (j = 6).

Figure 10.

(a) Movement of suspension system. (b) Kinematic chain of suspension system. (c) Suspension system including actuators.

To model this system the following consideration is considered: leaf springs are assumed as flexible mechanical components with an axial deformation and a small shear deformation, and can be represented by prismatic joints P supported in revolute joints R [16].

To allow the rotation of the body in the z-axis, the link between the body and the leaf spring is made with revolute joint. Applying these concepts to the system, a model with the configuration shown in Figure 10b is proposed.

The system is composed of six links identified by letters D (vehicle axle), E and F (spring 1), G and H (spring 2), and I (the vehicle body), and the six joints identified by the following numbers: four revolute joints R (5, 7, 8, and 10) and two prismatic joints P that represent the leaf springs of the system (6 and 9), as shown in Figure 10b.

The mechanism of Figure 10b has 3-DoF, and it requires three actuators to control its movements, applying the technique developed in Section 1.1, the kinematic chain has a passive actuator in the prismatic joints 6 and 9 (axial deformation of the leaf spring), and a passive actuator in the joints 6 and 9 (torsion spring—shear deformation); but the mechanism with four passive actuators is over-constrained, in this case only one equivalent passive actuator is used in the joint 5 or 8, as shown in Figure 10c.

1.2.2 Kinematics of the suspension system

The movement of the suspension is orientated first by the movement of the tyres system, and second by forces acting on the mechanism (vehicle weight (W) and inertial force (ma_y )). These forces affect the passive actuators of the mechanism, as shown in Figure 11.

Figure 11.

Movement of suspension system.

Eqs. (6)–(12) define the kinematics of the suspension system:

$${\theta }_n=\raisebox{1ex}{$T_{\mathit{xn}}$}\!\left/ \!\raisebox{-1ex}{$k_{\mathit{ts}}$}\right.$$ E6

$$l_n={\delta }_{\mathit{LS}}+l_s=\frac{3∆F{l}^3}{8E_s\mathit{NB}{T}^3}+l_s\approx \frac{-F_{\mathit{LSn}}+F_{\mathit{zi}}^{\mathit{\text{start}}}}{k_{\mathit{Ls}}}+l_s$$ E7

$$r=\sqrt{l_n^2+b^2-2l_{n}b\mathit{cos}\left(90+{\theta }_6\right)}$$ E8

$${\beta }_n=\mathit{arccos}\left(\left(b^2+r^2-l_4^2\right)/\left(2\mathit{br}\right)\right)$$ E9

$${\theta }_{n+1}={\beta }_n+\mathit{arcsin}\left(\frac{b}{r}\mathit{sin}\left(90+{\theta }_n\right)\right)-90$$ E10

$${\theta }_{n+2}={\theta }_n+\mathit{arcsin}\left(\frac{b}{r}\mathit{sin}\left(90+{\theta }_n\right)\right)-\mathit{arcsin}\left(\frac{b}{l_{n+1}}\mathit{sin}\left({\beta }_n\right)\right)$$ E11

$${\theta }_{n+3}=90-{\beta }_n-\mathit{arcsin}\left(\frac{b}{l_{n+1}}\mathit{sin}\left({\beta }_n\right)\right)$$ E12

where T_xn is the moment around the x-axis on the joint n, k_ts is the spring’s torsion coefficient, δ_LS is the leaf spring deformation [17], ∆F is the algebraic change in the initial load, l is the length of the leaf spring, N is the number of leaves, B is the width of the leaf, T is the thickness of the leaf, E_s is the modulus of elasticity of a multiple leaf, l_n is the instantaneous height of the leaf spring n, F_LSn is the spring normal force n, l_s is the initial suspension height, k_Ls is the equivalent stiffness of the suspension, l_n is the instantaneous height of the leaf spring n, b is the lateral separation between the springs, and θ_n is the rotation angle of the revolute joint n.

1.3 The fifth wheel system

This system is a coupling device between the tractive unit and the trailer; but in the case of a multiple trailer train, a fifth wheel also can be located on a lead trailer. The fifth wheel allows articulation between the tractive and the towed units.

This system consists of a wheel-shaped deck plate usually designed to tilt or oscillate on mounting pins. The assembly is bolted to the frame of the tractive unit. A sector is cut away in the fifth wheel plate (sometimes called a throat), allowing a trailer kingpin to engage with locking jaws in the centre of the fifth wheel [18]. The trailer kingpin is mounted in the trailer upper coupler assembly. The upper coupler consists of the kingpin and the bolster plate.

When the vehicle makes different manoeuvres (starting to go uphill or downhill, and during cornering) [18], the fifth wheel allows the free movement of the trailer and more flexibility of the chassis, as shown in Figures 12–14.

Figure 12.

Movement of the fifth wheel—Starting uphill. Source: Adapted from Saf-Holland [18].

Figure 13.

Movement of the fifth wheel—Starting downhill. Source: Adapted from Saf-Holland [18].

Figure 14.

Movement of the fifth wheel—Rotation x-axis. Source: Adapted from Saf-Holland [18].

Rotation about the longitudinal axis of up to 3° of movement between the tractor and trailer is permitted. On a standard fifth wheel, this occurs as a result of clearance between the fifth wheel to bracket fit, compression of the rubber bushes, and also the vertical movement between the kingpin and locks may allow some lift of the trailer to one side [18].

Consider the third movement of the trailer, the mechanism that represents the fifth wheel has similar design and movements to the suspension mechanism (Figure 15), it is located over the front suspension mechanism.

Figure 15.

Kinematic chain of fifth wheel system.

Here l_fw is the fifth wheel system’s instantaneous height, F_FWn is the fifth wheel normal load, and l_fi is the fifth wheel system’s initial height, b₁ is the fifth wheel width.

1.4 The chassis

The chassis is the backbone of the trailer, and it integrates the main truck component systems such as the axles, suspension, power train, and cab. The chassis is also an important part that contributes to the dynamic performance of the whole vehicle. One of the truck’s important dynamic properties is the torsional stiffness, which causes different lateral load transfers (LLT) on the axles of the vehicle [19].

According to Winkler [20] and Rill [4], the chassis has significant torsional compliance, which would allow its front and rear parts to roll independently; this is because the lateral load transfer is different on the axles of the vehicle. Then, applying the torsion theory, the vehicle frame has similar behaviour with a statically indeterminate torsional shaft, as shown in Figure 16.

Figure 16.

Kinematic chain of the chassis.

Here T_CG is the torque applied by the forces acting on the CG, T_f (T₂₈ ) is the torque applied on the vehicle front axle, T_r (T₂₇ ) is the torque applied on the vehicle rear axle, a is the distance from the front axle to the centre of gravity, and L is the wheelbase of the trailer. Applying torsion theory to the statically indeterminate shaft, the next equation is defined:

$$\frac{T_{f}a}{J_{f}G}=\frac{T_r\left(L-a\right)}{J_{r}G}$$ E13

where J_f and J_r are the equivalent polar moments on front and rear sections of the vehicle frame respectively, and G is the modulus of rigidity (or shear modulus).

According to Kamnik et al. [21] when a trailer model makes a spiral manoeuvre, the LLT on the rear axle is greater than the LLT on the front axle; therefore the equivalent polar moment on the rear (J_r ) is greater than the equivalent polar moment on the front (J_f ). These can be expressed as J_r  = x J_f (where x is the constant that allows controlling the torque distribution of the chassis); replacing and simplifying Eq. (13):

$$T_f+T_r\left(\frac{a-L}{\mathit{ax}}\right)=0$$ E14

However, when the trailer model makes a turn, the torque applied on the vehicle front axle has two components, as shown in Figure 17 and Eqs. (15) and (16).

Figure 17.

Torque components.

$$T_{\mathit{fx}}=T_{f}cos\psi$$ E15

$$T_{\mathit{fy}}=T_{f}cos\psi$$ E16

where T_fx (T_x28 ) is the torque applied in the x-axis (this torque acts on the lateral load transfer on front axle), T_fy (T_y28 ) is the torque applied in the y-axis, and $\psi$ is the steering angle of trailer front axle.

1.5 Three-dimensional trailer model

Considering the systems developed, the model of the trailer (Figure 18) is composed of the following mechanisms:

• the front mechanism of the trailer is composed for the tyres, the suspension, and the fifth wheel,

• the rear mechanism is composed for the tyres, and the suspension, and

• the last mechanism is the chassis and links the front and rear mechanism of the model.

Figure 18.

Trailer model.

The kinematic chain of the trailer model (Figure 18) is composed of 28 joints (j = 28; 14 revolute joints ‘R’, 10 prismatic joints ‘P’, 2 spherical joints ‘S’, 2 spherical slider joints ‘S_d’), and 23 links (n = 23).

2. Static analysis of the mechanism

Several methodologies allow us to obtain a complete static analysis of the mechanism. For this purpose, the Davies method was used to analyse the mechanisms statically [11, 22, 23, 24, 25, 26, 27, 28, 29]. This method was selected because it offers a straightforward way to obtain a static model of the mechanism, and this model can be easily adaptable using this approach.

2.1 External forces and load distribution

In the majority of LCVs, the load on the trailers is fixed and nominally centred; for this reason, the initial position lateral of the centre of gravity is centred and symmetric.

Usually, the national regulation boards establish the maximum load capacity of the axles of LCVs; this is based on the design load capacity of the pavement and bridges, so each country has its regulations. In this scope, the designers develop their products considering that the trailer is loaded uniformly, causing the axle’s load distribution to be in accordance with the laws. Figure 19 shows the example of the normal load distribution.

Figure 19.

Normal load distribution.

However, some loading does not properly distribute the load, which ultimately changes the centre of gravity of the trailer forwards or backwards, as shown in Figure 20 respectively.

Figure 20.

Longitudinal CG movement.

In Figures 19 and 20, F_f and F_r are the forces acting on the front and rear axles respectively.

Generally, the CG position is dependent on the type of cargo, and the load distribution on the trailer and it varies in three directions: longitudinal (x-axis), lateral (y-axis), and vertical (z-axis), as shown Figure 21.

Figure 21.

CG displacements.

Here, d₁ denotes the lateral CG displacement, d₂ denotes the longitudinal CG displacement, and d₃ the vertical CG displacement.

Furthermore, Figure 22a and b show that only the weight (W) and the lateral inertial force (ma_y ) act on the trailer CG, but, when the model takes into account the longitudinal slope angle (φ) and the bank angle (ϕ) of the road, these forces have three components, as represented in Eqs. (17)–(19).

Figure 22.

(a) Longitudinal slope of the road. (b) Banked road.

$$P_x=W\mathit{sin}\phi$$ E17

$$P_y=-W\mathit{sin}\varphi \mathit{cos}\phi +m{a}_y\mathit{cos}\varphi$$ E18

$$P_z=W\mathit{cos}\varphi \mathit{cos}\phi +m{a}_y\mathit{sin}\varphi$$ E19

where P_x is the force acting on the x-axis, P_y is the force acting on the y-axis, and P_z is the force acting on the z-axis.

Finally, the load distribution of the trailer on a road with a slope angle is given by the Figure 23 and Eq. (20).

Figure 23.

Load distribution of a trailer on a road with slope angle.

$$P_x{h}_2-P_z\left(a\pm d_2\right)+F_{r}L=0$$ E20

where h₂ is the instantaneous CG height, L is the wheelbase of the trailer, and a is the distance from the front axle to the centre of gravity.

2.2 Screw theory of the mechanism

Screw theory enables the representation of the mechanism’s instantaneous position in a coordinate system (successive screw displacement method) and the representation of the forces and moments (wrench), replacing the traditional vector representation. All these fundamentals applied to the mechanism are briefly presented below.

2.2.1 Method of successive screw displacements of the mechanism

In the kinematic model for a mechanism, the successive screws displacement method is used. Figures 24–28 and Table 1 present the screw parameters of the mechanism.

Figure 24.

Variables of the mechanism position (model of the front of the trailer).

Figure 25.

Variables of the mechanism position (model of the rear of the trailer).

Figure 26.

Vector along the direction of the screws axis (model of the front and rear of the trailer).

Figure 27.

Variables of the mechanism position (side view of the trailer).

Figure 28.

Variables of the mechanism position (three-dimensional model).

Joints and points	s			s0			θ	d
Joint 1	1	0	0	−l13	0	0	θ1	0
Joint 2	0	0	1	−l13	0	0	0	l1
Joint 3a	0	1	0	−l13	0	0	0	t1
Joint 3b	1	0	0	−l13	0	0	θ3	0
Joint 4	0	0	1	−l13	0	0	0	l2
Joint 5	1	0	0	−l13	(t2 − b)/2	0	θ5	0
Joint 6	0	0	1	−l13	(t2 − b)/2	0	0	l3
Joint 7	1	0	0	−l13	(t2 − b)/2	0	θ7	0
Joint 8	1	0	0	−l13	(t2 + b)/2	0	θ8	0
Joint 9	0	0	1	−l13	(t2 + b)/2	0	0	l4
Joint 10	1	0	0	−l13	(t2 + b)/2	0	θ10	0
Joint 11	1	0	0	0	(t2 − b1 )/2	0	θ11	0
Joint 12	0	0	1	0	(t2 − b1 )/2	0	0	l5
Joint 13	1	0	0	0	(t2 − b1 )/2	0	θ13	0
Joint 14	1	0	0	0	(t2 + b1 )/2	0	θ14	0
Joint 15	0	0	1	0	(t2 + b1 )/2	0	0	l6
Joint 16	1	0	0	0	(t2 + b1 )/2	0	θ16	0
Joint 17	1	0	0	−L	0	0	θ17	0
Joint 18	0	0	1	−L	0	0	0	l7
Joint 19a	0	1	0	−L	0	0	0	t3
Joint 19b	1	0	0	−L	0	0	θ19	0
Joint 20	0	0	1	−L	0	0	0	l8
Joint 21	1	0	0	−L	(t4 − b)/2	0	θ21	0
Joint 22	0	0	1	−L	(t4 − b)/2	0	0	l9
Joint 23	1	0	0	−L	(t4 − b)/2	0	θ23	0
Joint 24	1	0	0	−L	(t4 + b)/2	0	θ24	0
Joint 25	0	0	1	−L	(t4 + b)/2	0	0	l10
Joint 26	1	0	0	−L	(t4 + b)/2	0	θ26	0
Joint 27	1	0	0	−L	t4/2	0	θ27	0
Joint 28	1	0	0	0	t2/2	0	θ28	0
Point 29	0	0	1	0	t2/2	0	$\psi$	0
CG (30)	1	0	0	−a ± d2	(t4/2) ± d1	l12 ± d3	0	0

Table 1.

Screw parameters of the mechanism.

In Figures 24–28 and Table 1, l₁₃ is the distance between the fifth wheel and the front axle, l_1;2;7;8 are the dynamic rolling radii of tyres, t_1;3 are the front and rear track widths of the trailer respectively, t_2;4 are the front and rear axle widths respectively, b is the lateral separation between the springs, b₁ is the fifth wheel width, θ_i is the revolution joint angle rotation i, l_3;4;9;10 are the instantaneous heights of the leaf spring, l₁₂ is the height of CG above the chassis, and ψ is the trailer/trailer angle.

This method enables the determination of the displacement of the mechanism and the instantaneous position vector s_0i of the joints, and the centre of gravity (The vector s_0i (Table 2) is obtained from the first three terms of the last column of equations shown in Table 3).

References	s0i
1	$\frac{t_2{s}_{29}-2l_{13}{c}_{29}}{2}$	$-\frac{2l_{13}{s}_{29}+t_2{c}_{29}-t_2}{2}$	0
2	$\frac{\left(2l_1{s}_1+t_2\right)s_{29}-2l_{13}{c}_{29}}{2}$	$-\frac{\left(2l_1{s}_1+t_2\right)c_{29}+2l_{13}{s}_{29}-t_2}{2}$	$l_1{c}_1$
3	$\frac{\left(t_2-2t_1\right)s_{29}-2l_{13}{c}_{29}}{2}$	$-\frac{2l_{13}{s}_{29}+\left(t_2-2t_1\right)c_{29}-t_2}{2}$	$0$
4	$\frac{\left(2l_2{s}_3+t_2-2t_1\right)s_{29}-2l_{13}{c}_{29}}{2}$	$-\frac{2l_{13}{s}_{29}+\left(2l_2{s}_3+t_2-2t_1\right)c_{29}-t_2}{2}$	$l_2{c}_3$
5–16	⁞	⁞	⁞
17	$-L$	0	0
18	$-L$	$-l_7{s}_{17}$	$l_7{c}_{17}$
19	$-L$	$t_3$	$0$
20	$-L$	$t_3-l_8{s}_{19}$	$l_8{c}_{19}$
21	$-L$	$-\frac{2l_{17}{s}_{17}+\left(b_2-t_4\right)c_{17}}{2}$	$\frac{\left(t_4-b_2\right)s_{17}+2l_{17}{c}_{17}}{2}$
22–23	$-L$	$-\frac{2l_{17}{s}_{17}+\left(b_2-t_4\right)c_{17}+2l_9{s}_{21+17}}{2}$	$\frac{\left(t_4-b_2\right)s_{17}+2l_{17}{c}_{17}+2l_9{c}_{21+17}}{2}$
24–28	⁞	⁞	⁞
CG	$-a\pm d_2$	* $h_1$	* $h_2$

Table 2.

Instantaneous position vector s_0i .

$s_i=sin{\theta }_i{c}_i=cos{\theta }_i$

* $h_1=-\frac{\left(2l_{12}\pm 2d_3\right)s_{27+23+21+17}\pm d_1{c}_{27+23+21+17}-b_2{c}_{23+21+17}+2l_9{s}_{21+17}+2l_7{s}_{17}+\left(b_2-t_4\right)c_{17}}{2.}$

* $h_2=-\frac{\pm d_1{s}_{27+23+21+17}+\left(\pm 2d_3-2l_{12}\right)c_{27+23+21+17}-b_2{s}_{23+21+17}-2l_9{c}_{21+17}-2l_7{c}_{17}+\left(b_2-t_4\right)s_{17}}{2}$

Joints and points	Instantaneous position matrix
Joint 1	p’1 = A29 A1 p1
Joint 2	p’2 = A29 A1 A2 p2
Joint 3	p’3 = A29 A 3a A3b p3
Joint 4	p’4 = A29 A 3a A3b A4 p4
Joint 5	p’5 = A29 A1 A2 A5 p5
Joint 6	p’6 = A29 A1 A2 A5 A6 p6
Joint 7	p’7 = A29 A1 A2 A5 A6 A7 p7
Joint 8	p’8 = A29 A1 A2 A8 p8
Joint 9	p’9 = A29 A1 A2 A8 A9 p9
Joint 10	p’10 = A29 A1 A2 A8 A9 A10 p10
Joint 11	p’11 = A29 A1 A2 A5 A6 A7 A11 p11
Joint 12	p’12 = A29 A1 A2 A5 A6 A7 A11 A12 p12
Joint 13	p’13 = A29 A1 A2 A5 A6 A7 A11 A12 A13 p13
Joint 14	p’14 = A29 A1 A2 A5 A6 A7 A14 p14
Joint 15	p’15 = A29 A1 A2 A5 A6 A7 A14 A15 p15
Joint 16	p’16 = A29 A1 A2 A5 A6 A7 A14 A15 A16 p16
Joint 17	p’17 = A17 p17
Joint 18	p’18 = A17 A18 p18
Joint 19	p’19 = A 19a A19b p19
Joint 20	p’20 = A 19a A19b A20 p20
Joint 21	p’21 = A17 A18 A21 p21
Joint 22	p’22 = A17 A18 A21 A22 p22
Joint 23	p’23 = A17 A18 A21 A22 A23 p23
Joint 24	p’24 = A17 A18 A24 p24
Joint 25	p’25 = A17 A18 A24 A25 p25
Joint 26	p’26 = A17 A18 A24 A25 A26 p26
Joint 27	p’27 = A17 A18 A21 A22 A23 A27 p27
Joint 28	p’28 = A29 A1 A2 A5 A6 A7 A11 A12 A13 A28 p28
CG (30)	p’CG = A17 A18 A21 A22 A23 A27 ACG pCG

Table 3.

Instantaneous position matrix.

2.2.2 Wrench—Forces and moments

In the static analysis, all forces and moments of the mechanism are represented by wrenches ($^A ) [13]. The wrenches applied can be represented by the vector $^A  = [M_x M_y M_z F_x F_y F_z ] ^T , where F denotes the forces, and M denotes the moments.

To simplify the model of Figure 28, the following considerations were made:

• for the x-direction a steady-state model was used in the analysis;

• disturbances imposed by the road and the lateral friction forces (F_y ) (tyre-ground contact) in the joints 3 and 19 were neglected; and

• the components of the trailer weight (W) and the inertial force (ma_y ) are the only external forces acting on the trailer CG.

Considering a static analysis in a three-dimensional space [7], the corresponding wrenches of each joint and external forces are defined by the parameters of Table 4, where s_i represents the orientation vector of each wrench i.

Joints and; reference points	Constraints; and forces	si			Inst. position; vector s0i
Revolute joints; 1, 7, 8, 10, 13,; 14, 16, 17, 23, 24,; and 26	Fxi	1	0	0	Revolute joints; 1, 7, 8, 10, 13,; 14, 16, 17, 23, 24,; and 26
	Fyi	0	1	0
	Fzi	0	0	1
	Myi	0	1	0
	Mzi	0	0	1
Spherical slider; joints 3 and 19	Fxi	1	0	0	Spherical slider; joints 3 and 19
	Fzi	0	0	1
Revolute joints; 5, 11, and 21	Fxi	1	0	0	Revolute joints; 5, 11, and 21
	Fyi	0	1	0
	Fzi	0	0	1
	Txi	1	0	0
	Myi	0	1	0
	Mzi	0	0	1
Prismatic joints; 2, 4, 6, 9, 12, 15,; 18, 20, 22 and; 25	Fxi	1	0	0	Prismatic joints; 2, 4, 6, 9, 12, 15,; 18, 20, 22, and; 25
	Fni	0	cos θi−1	sin θi−1
	Mxi	1	0	0
	Myi	0	1	0
	Mzi	0	0	1
Prismatic joints; 2, 4, 18, and 20	FTi	0	−sin θi−1	cos θi−1	Prismatic joints; 2, 4, 18, and 20
Prismatic joints; 6, 9, 22, and 25	FLSi	0	−sin θi−1	cos θi−1	Prismatic joints; 6, 9, 22, and 25
Prismatic joints; 12 and 15	FFWi	0	−sin θi−1	cos θi−1	Prismatic joints; 12 and 15
Spherical joints; 27 and 28	Fxi	1	0	0	Spherical joints; 27 and 28
	Fyi	0	1	0
	Fzi	0	0	1
	Txi	1	0	0
CG (30)	Px	1	0	0	CG (30)
	Py	0	−1	0
	Pz	0	0	−1

Table 4.

Wrench parameters of the mechanism.

All of the wrenches of the mechanism together comprise the action matrix [A_d ] given by Eq. (21) (or the amplified matrix of the Eq. (22)).

$${\left[A_d\right]}_{6\times 148}=\left[{\text{\$}}_{F_{x1}}^A{\text{\$}}_{F_{y1}}^A{\text{\$}}_{F_{z1}}^A\cdots {\text{\$}}_{P_x}^A{\text{\$}}_{P_y}^A{\text{\$}}_{P_z}^A\right]$$ E21

$$\left[A_d\right]=\left[\begin{array}{ccccccc}0& 0& -p_1{F}_{z1}& \cdots & 0& h_2{P}_y& -h_1{P}_z\\ 0& 0& -p_2{F}_{z1}& \cdots & h_2{P}_x& 0& \left(-a\pm d_2\right)P_z\\ p_1{F}_{x1}& p_2{F}_{y1}& 0& \cdots & -h_1{P}_x& -\left(-a\pm d_2\right)P_y& 0\\ F_{x1}& 0& 0& \cdots & P_x& 0& 0\\ 0& F_{y1}& 0& \cdots & 0& -P_y& 0\\ 0& 0& F_{z1}& \cdots & 0& 0& -P_z\end{array}\right]$$ E22

where p_i is a system variable.

The wrench can be represented by a normalised wrench and a magnitude. Therefore, from the Eq. (22) the unit action matrix and the magnitudes action vector are obtained, as represented by Eqs. (23) and (24).

$${\left[{\widehat{A}}_d\right]}_{6\times 148}=\left[\begin{array}{ccccccc}0& 0& -p_1& \cdots & 0& h_2& -h_1\\ 0& 0& -p_2& \cdots & h_2& 0& \left(-a\pm d_2\right)\\ p_1& p_2& 0& \cdots & -h_1& -\left(-a\pm d_2\right)& 0\\ 1& 0& 0& \cdots & 1& 0& 0\\ 0& 1& 0& \cdots & 0& -1& 0\\ 0& 0& 1& \cdots & 0& 0& -1\end{array}\right]$$ E23

$${\left[\Psi \right]}_{148\times 1}=\left[\begin{array}{ccccccc}{F}_{x1}& F_{y1}& F_{z1}& \cdots & P_x& P_y& P_z\end{array}\right]$$ E24

2.2.3 Graph theory

Kinematic chains and mechanisms are comprised of links and joints, which can be represented by graphs, where the vertices correspond to the links, and the edges correspond to the joints and external forces [5, 7].

The mechanism of the Figure 28 is represented by the direct coupling graph of the Figure 29. This graph has 23 vertices (links) and 31 edges (joints and external forces (P_x , P_y , and P_z )).

Figure 29.

Direct coupling graph of the mechanism.

The direct coupling graph (Figure 29) can be represented by the incidence matrix [I] _23X31 [30] (Eq. (25)). The incidence matrix provides the mechanism cut-set matrix [Q] _22X31 [11, 25, 26, 27, 28, 30] (Eq. (26)) for the mechanism, where each line represents a cut graph and the columns represent the mechanism joints. Besides, this matrix is rearranged, allowing 22 branches (edges 1–3, 5–9, 11–15, 17–19, 21–25, and 27—identity matrix) and 9 chords (edges 4, 10, 16, 20, 26, 28, P_x, P_y, and P_z ) to be defined, as shown in Figure 30.

Figure 30.

Cut-set action graph of the mechanism.

E25

E26

All constraints are represented as edges, which allows the amplification of the cut-set graph and the cut-set matrix. Additionally, the tyre normal load (F_Ti ), spring normal load (F_LSi ), fifth wheel normal load (F_FWi ), and the passive torsional moment (T_xi ) are included.

Figure 30 presents the cut-set action graph and the Eq. (27) presents the expanded cut-set matrix ([Q] _22X148 ), where each line represents a cut of the graph, and the columns are the constraints of the joints as well as external forces on the mechanism.

E27

2.3 Equation system solution

Using the cut-set law [24], the algebraic sum of the normalised wrenches given in Eqs. (23) and (24), that belong to the same cut [Q] _22X148 (Figure 30 and Eq. (27)) must be equal to zero. Thus, the statics of the mechanism can be defined, as exemplified in Eq. (28) (or the amplified matrix of the Eq. (29)):

$${\left[{\widehat{A}}_n\right]}_{132\times 148}{\left[\mathrm{\Psi }\right]}_{148\times 1}^T={\left[0\right]}_{132\times 1}$$ E28

$$\begin{array}{c}\\ \\ \mathit{Cut}1\\ \\ \\ \\ ⋮\\ \\ \\ \mathit{Cut}22\\ \\ \end{array}\left[\begin{array}{cccccc}0& 0& \cdots & 0& 0& 0\\ 0& 0& \cdots & 0& 0& 0\\ p_1& p_2& \cdots & 0& 0& 0\\ 1& 0& \cdots & 0& 0& 0\\ 0& 1& \cdots & 0& 0& 0\\ 0& 0& \cdots & 0& 0& 0\\ ⋮& ⋮& ⋮& ⋮& ⋮& ⋮\\ 0& 0& \cdots & 0& h_2& -h_1\\ 0& 0& \cdots & h_2& 0& \left(-a\pm d_2\right)\\ 0& 0& \cdots & -h_1& -\left(-a\pm d_2\right)& 0\\ 0& 0& \cdots & 1& 0& 0\\ 0& 0& \cdots & 0& -1& 0\\ 0& 0& \cdots & 0& 0& -1\end{array}\right].\left[\begin{array}{c}{F}_{x1}\\ F_{y1}\\ ⋮\\ P_x\\ P_y\\ P_z\end{array}\right]={\left[0\right]}_{132\times 1}$$ E29

It is necessary to identify the set of primary variables [Ψ _p ] (known variables), among the variables of Ψ. Once identified, the system of the Eq. (28) is rearranged and divided in two sets, as shown by Eq. (30).

$${\left[{\widehat{A}}_{\mathit{ns}}\right]}_{132\times 145}{\left[{\mathrm{\Psi }}_s\right]}_{145\times 1}^T+{\left[{\widehat{A}}_{\mathit{np}}\right]}_{132\times 3}{\left[{\mathrm{\Psi }}_p\right]}_{3\times 1}^T={\left[0\right]}_{132\times 1}$$ E30

where [Ψ _p ] is the primary variable vector, [Ψ _s ] is the second variable vector (unknown variables), $\left[{\widehat{A}}_{\mathit{np}}\right]$ are the columns corresponding to the primary variables, and $\left[{\widehat{A}}_{\mathit{ns}}\right]$ are the columns corresponding to the secondary variables.

In this case, the primary variable vector is:

$${\left[{\mathrm{\Psi }}_p\right]}_{3\times 1}={\left[\begin{array}{ccc}{P}_x& P_y& P_z\end{array}\right]}^T$$ E31

and the secondary variable vector is:

$${\left[{\Psi }_s\right]}_{145\times 1}={\left[\begin{array}{ccccccccc}{F}_{x1}& F_{y1}& M_{y1}& M_{z1}& \cdots & F_{z1}& F_{z3}& F_{z17}& F_{z19}\end{array}\right]}^T$$ E32

Solving the system Eq. (30) using the Gauss-Jordan elimination method, all secondary variables being function of the primary variables, the last row of the solution system provides the next equation:

$$F_{z3}+\frac{P_1+t_3}{t_1\mathit{cos}\psi }{F}_{z19}+\frac{P_1}{t_1\mathit{cos}\psi }{F}_{z17}$$

$$-\frac{h_1+P_1}{t_1\mathit{cos}\psi }{P}_z+\frac{h_2}{t_1\mathit{cos}\psi }{P}_y=0$$ E33

replacing P_y and P_z :

$$F_{z3}+\frac{P_1+t_3}{t_1\mathit{cos}\psi }{F}_{z19}+\frac{P_1}{t_1\mathit{cos}\psi }{F}_{z17}$$

$$-\frac{h_1+P_1}{t_1\mathit{cos}\psi }\left(\mathit{\text{Wcosϕcos}}\phi +m{a}_y\mathit{sin}\varphi \right)$$

$$+\frac{h_2}{t_1\mathit{cos}\psi }\left(m{a}_y\mathit{cos}\varphi -\mathit{\text{Wsinϕcos}}\phi \right)=0$$ E34

where P₁ is a system variable ( $P_1=\left(2l_{13}sin\psi +t_2\left(cos\psi -1\right)\right)/2$ ), h₁ is the instantaneous lateral distance between the zero-reference frame and the centre of gravity, and h₂ is the instantaneous CG height (Table 2). Simplifying the equation, and making $tan\left(\varphi \right)=e$ , where e is the tangent of the bank angle, we have:

$$\frac{a_y}{g}=\frac{h_1\mathit{cos}\phi +h_2\mathit{\text{ecos}}\phi }{h_2-\left(h_1+P_1\right)e}\times$$

$$\left(1-\frac{t_1{F}_{z3}\mathit{cos}\psi +P_1\left(F_{z17}-\mathit{\text{Wcosϕcos}}\phi \right)+\left(P_1+t_3\right)F_{z19}}{\mathit{\text{Wcos}}\varphi \left(h_1\mathit{cos}\phi +h_2\mathit{\text{ecos}}\phi \right)}\right)$$ E35

According to the static redundancy problem known as the four-legged table [31, 32], a plane is defined by just three points in space and, consequently, a four-legged table has support plane multiplicities. This is why when one leg loses contact with the ground, the table is supported by the other three, as shown in Figure 31.

Figure 31.

Redundancy problem of the four-legged table.

The problem of the four-legged table is observed in dynamic rollover tests when the rear inner tyre loses contact with the ground (F_z19 = 0), and the front inner tyre (F_z3 ) does not, as shown, for example, in Figure 32.

Figure 32.

Dynamic rollover test. Source: Adapted of Cabral [33].

Applying this theory to the vehicle stability, when a vehicle makes a turn, it is subjected to an increasing lateral load until it reaches the rollover threshold [32]. During the turning, the rear inner tyre is usually the one that loses contact with the ground. For this condition (F_z19 = 0), and thus:

$${\mathit{SRT}}_{3D_{\mathit{\psi \varphi \phi }}}=\frac{h_1\mathit{cos}\phi +h_2\mathit{\text{ecos}}\phi }{h_2-\left(h_1+P_1\right)e}\times$$

$$\left(1-\frac{t_1{F}_{z3}\mathit{cos}\psi +P_1\left(F_{z17}-\mathit{\text{Wcosϕcos}}\phi \right)}{\mathit{\text{Wcos}}\varphi \left(h_1\mathit{cos}\phi +h_2\mathit{\text{ecos}}\phi \right)}\right)$$ E36

where ${\mathit{SRT}}_{3D_{\mathit{\psi \varphi \phi }}}$ factor is the three-dimensional static rollover threshold for a trailer model with trailer/trailer angle (ψ), bank angle (e), and slope angle (φ).

The normal forces F_z3 and F_z17 depend on the LLT coefficient in the front and rear axles respectively [4, 21, 34]. Furthermore, this coefficient depends on the vehicle type, speed, suspension, tyres, etc.

This information demonstrates that the ${\mathit{SRT}}_{3D_{\mathit{\psi \varphi \phi }}}$ factor of a vehicle (Eq. (36)) is, in general, inferior to the SRT factor for a two-dimensional model vehicle [35], as shown in Eq. (37).

$${\mathit{SRT}}_{2D}=\frac{a_y}{g}=\frac{t}{2h}$$ E37

where h is the CG height, t is the vehicle track.

With Eq. (36), it is possible to obtain a better vehicle stability representation and the ${\mathit{SRT}}_{3D_{\mathit{\psi \varphi \phi }}}$ factor value attainments closer to reality.

To simplify the solution of the system of equations in Eq. (30), the following hypotheses were considered:

• in the majority of LCVs, the load on the trailers is uniformly distributed (Eq. (20));

• the lateral load transfer of the trailer model is controlled through the torsional moment of the chassis (spherical joints 27 and 28 (Eqs. (15) and (16))).

Eq. (38) shows the final static system for the stability analysis, solving this system using the Gauss-Jordan elimination method, all secondary variables are a function of primary variables, (P_x —force acting on the x-axis, P_y —force acting on the y-axis, and P_z —force acting on the z-axis).

$$\begin{array}{c}\\ \\ \mathit{Cut}1\\ \\ \\ \\ ⋮\\ \\ \\ \mathit{Cut}22\\ \\ \\ \\ \mathit{Eq}.\left(20\right)\\ \mathit{Eq}.\left(15\right)\\ \mathit{Eq}.\left(16\right)\end{array}\left[\begin{array}{cccccccccccc}0& 0& \cdots & 0& 0& -p_1& 0& 0& 0& 0& 0& 0\\ 0& 0& \cdots & 0& 0& -p_2& 0& 0& 0& 0& 0& 0\\ p_1& p_2& \cdots & 0& 0& 0& 0& 0& 0& 0& 0& 0\\ 1& 0& \cdots & 0& 0& 0& 0& 0& 0& 0& 0& 0\\ 0& 1& \cdots & 0& 0& 0& 0& 0& 0& 0& 0& 0\\ 0& 0& \cdots & 0& 0& 1& 0& 0& 0& 0& 0& 0\\ ⋮& ⋮& ⋮& ⋮& ⋮& ⋮& ⋮& ⋮& ⋮& ⋮& ⋮& ⋮\\ 0& 0& \cdots & 0& 0& 0& 0& 0& 0& 0& h_2& -h_1\\ 0& 0& \cdots & 0& 0& 0& 0& 0& 0& h_2& 0& \left(-a\pm d_2\right)\\ 0& 0& \cdots & 0& 0& 0& 0& 0& 0& -h_1& -\left(-a\pm d_2\right)& 0\\ 0& 0& \cdots & 0& 0& 0& 0& 0& 0& 1& 0& 0\\ 0& 0& \cdots & 0& 0& 0& 0& 0& 0& 0& -1& 0\\ 0& 0& \cdots & 0& 0& 0& 0& 0& 0& 0& 0& -1\\ 0& 0& \cdots & 0& 0& 0& 0& L& L& h_2& 0& -\left(-a\pm d_2\right)\\ 0& 0& \cdots & 0& 0& 0& 0& 0& 0& 0& 0& 0\\ 0& 0& \cdots & 0& 0& 0& 0& 0& 0& 0& 0& 0\end{array}\right].\left[\begin{array}{c}{F}_{x1}\\ F_{y1}\\ ⋮\\ T_{x5}\\ T_{x11}\\ T_{x21}\\ T_{x27}\\ T_{x28}\\ F_{T2}\\ F_{T4}\\ F_{T18}\\ F_{T20}\\ F_{\mathit{LS}6}\\ F_{\mathit{LS}9}\\ F_{\mathit{LS}22}\\ F_{\mathit{LS}25}\\ F_{\mathit{FW}12}\\ F_{\mathit{FW}17}\\ F_{z1}\\ F_{z3}\\ F_{z19}\\ F_{z17}\\ P_x\\ P_y\\ P_z\end{array}\right]={\left[0\right]}_{135\times 1}$$ E38

3. Case study

In this study, a B-train trailer with two axles on front and three axles on the rear is analysed. This model has a suspension system with a tandem axle, and its parameters depend on the construction materials. Another important parameter of the model is the dynamic rolling radius or loaded radius l_i . The proposed model considers Michelin XZA® [36] radial tyres. Table 5 shows the parameters of the trailer used in this analysis [32, 38].

Parameters of the trailer	Value	Units
Trailer weight—W	355.22	kN
Front and rear track widths (t1,3 )	1.86	m
Front and rear axles widths (t2,4 )	1.86	m
Stiffness of the suspension per axle (ks ) [37]	1800	kN.m^−1
Number of axles at the front (trailer) (four tyres per axle)	2
Number of axles at the rear (trailer) (four tyres per axle)	3
Vertical stiffness per tyre (kT ) ([37])	840	kN.m^−1
Initial suspension height (l3,4,9,10 ) (ls )	0.205	m
Initial dynamic rolling radius (l1,2,7,8 ) (lr ) (Michelin XZA® [36])	0.499	m
Initial height of the fifth wheel (lfi )	0.1	m
Lateral separation between the springs (b)	0.95	m
Fifth wheel width (b1 )	0.6	m
CG height above the chassis (l12 )	1.346	m
Distance between the fifth wheel and the front axle (l13 )	0.15	m
Wheelbase of the trailer (L)	4.26	m
Distance from the front axle to the centre of gravity (a)	3	m
Offset of the cargo d1	0.1	m
Trailer/trailer angle (ψ)	0	°

Table 5.

Parameters of the trailer model.

To calculate the SRT factor, the inertial force is increased until the lateral load transfer in the rear axle is complete (the entire load is transferred from the rear inner tyre to the rear outer tyre when the model makes a turn). The reduction in the SRT factor (Eq. (36) and the solution of the system of Eq. (38)) results from the combined action of the trailer systems, which allows a body roll angle of the trailer model (Figure 33) [32]. In this figure, it can be seen how the stability factor varies according to the influence of some of the parameters of the developed model.

Figure 33.

(a) Roll angle of the trailer (θ). (b) Change in the SRT factor.

When the model considers all parameters, the LLT coefficient on the front axle is approximately 70% of the LLT coefficient on the rear axle [21]. Applying this concept, the SRT_all factor reduces to 0.3364 g. Finally, the proposed model shows how the lateral offset of the cargo (d₁  = 0.1 m) influences the SRT_off factor: 2 cm of lateral offset corresponds to a loss of stability of around 0.01 g a reduction similar to that reported by Winkler [20, 32].

Additionally, the proposed model shows how a change in the lateral separation between the springs (b) influences the SRT factor. Some LCVs with tanker trailers have a greater lateral separation between the springs, which leads to a decrease in the roll angle and thus an increase in the SRT factor: 1 cm of lateral separation between the springs corresponds to a gain or loss of stability of around 0.001 g, as shown in Figure 33b [32].

This model also allows the determination of the lateral (h₁ ) and vertical (h₂ ) CG location (Figure 34).

Finally, if we consider the recommended maximum lateral load transfer ratio for the rear axle of 0.6 [39, 40], and also include the recommended bank angle and longitudinal slope of the road [41, 42], we can calculate the SRT factor for a trailer model on downhill and uphill corners. Table 6 shows a trailer model with different trailer/trailer angles (ψ) [32].

Figure 34.

(a) Lateral CG location. (b) Vertical CG location.

Slope angle (φ)-(%)		Uphill corners					Downhill corners
Trailer/trailer angle ( $\psi$ )-(°)	Bank angle ( $\varphi$ )-(%)	8	6	4	2	0	2	4	6	8
0	0	0.240	0.223	0.214	0.208	0.202	0.198	0.194	0.190	0.186
0	2	0.261	0.245	0.235	0.228	0.223	0.218	0.214	0.210	0.206
0	4	0.283	0.265	0.256	0.249	0.244	0.239	0.234	0.230	0.226
0	6	0.305	0.286	0.277	0.270	0.265	0.260	0.256	0.252	0.248
0	8	0.327	0.308	0.299	0.292	0.285	0.281	0.276	0.272	0.268
0	10	0.350	0.330	0.320	0.313	0.307	0.303	0.298	0.294	0.290
10	0	0.241	0.223	0.214	0.207	0.202	0.197	0.193	0.189	0.185
10	2	0.262	0.245	0.234	0.227	0.222	0.217	0.213	0.209	0.205
10	4	0.283	0.265	0.256	0.249	0.243	0.239	0.233	0.229	0.225
10	6	0.306	0.286	0.276	0.270	0.264	0.259	0.255	0.251	0.247
10	8	0.328	0.308	0.298	0.291	0.285	0.280	0.276	0.271	0.267
10	10	0.351	0.330	0.319	0.312	0.307	0.302	0.297	0.293	0.288
20	0	0.241	0.222	0.212	0.206	0.200	0.195	0.191	0.187	0.182
20	2	0.262	0.244	0.233	0.226	0.220	0.216	0.211	0.207	0.203
20	4	0.283	0.264	0.254	0.247	0.242	0.236	0.232	0.227	0.224
20	6	0.306	0.285	0.275	0.268	0.262	0.257	0.253	0.249	0.245
20	8	0.328	0.307	0.297	0.290	0.283	0.278	0.274	0.270	0.265
20	10	0.351	0.329	0.318	0.311	0.305	0.300	0.295	0.291	0.286

Table 6.

Static rollover threshold (SRT) of the trailer model with trailer/trailer angle.

In the worst-case scenario, the trailer model, for a downhill corner with a bank angle of 0%, the longitudinal slope of the road of 8%, and a trailer/trailer angle of 20° can reduce the SRT factor of the model by 59.6%, using 0.4511 g as a reference [32].

An analysis of Table 6 leads to the following conclusions for the critical conditions of the trailer:

• a 1% bank angle corresponds to gain in the stability of around 0.01 g;

• when the trailer is in downhill corners, a 1% slope angle corresponds to a loss of stability of around 0.0021 g;

• the trailer/trailer angle is inversely proportional to the SRT factor since when the trailer makes a horizontal curve with a small radio, and the trailer/trailer angle and inertial force are large, the SRT factor is lower.

4. Conclusions

This study demonstrates that the longitudinal characteristics of a trailer model have an essential influence on the SRT factor calculation. In this case, the SRT factor is approximately 38% lower than the previously reported standard value. This value is very close to that reported by Winkler [20] (i.e. 40%), which suggests that the proposed model provides consistent results [32].

This model also shows that the change in the lateral separation between the springs (b) plays an important role, and thus it should be considered in the design and construction of trailers. Greater lateral separation between the springs will increase the trailer model stability [32].

We also found that the parameters of the road, such as the bank angle and the longitudinal slope angle, can affect vehicle stability. This situation is closer to the actual problem: when the road is not planar, the lateral and the longitudinal load transfer play an important role in reducing the stability. On the other hand, this provides a very important warning, because some simplifications carried out when estimating the SRT factor can lead to a considerably higher stability value. This is a point of concern, leading to the perception that our roads are safer than they really are [32].