Analysis of the chassis design for a high mobility wheel platform

1. Introduction

The suspension system is generally used to connect wheels with the vehicle frame. Its main task is isolation of vibrations from the vehicle body, i.e. damping vibrations generated at the wheels by moving over uneven surfaces, the wheel adhesion to the road surface as well as enabling the wheels to ensure maximum transfer of the driving forces and braking. The technical elements of the suspension system are divided into:

- steering – defining the geometry of movement of wheels relative to the vehicle (e.g. A-arms);

- springs – storing vibration energy, oscillating motion (e.g. leaf spring, coil spring);

- damping – vibration energy dissipation (e.g. shock-absorbers).

These suspension elements transfer the forces generated at the space between the wheels and the surface to the rest of the vehicle. On account of the passenger comfort and minimisation of forces acting on the vehicle load, the suspension system should be characterised by large vertical travel, since it is responsible for good off-road driving properties. Suspension systems should also be characterised to some degree by longitudinal flexibility and some transverse flexibility as well. Strong transverse rigidity allows for precise guidance of the vehicle (stabiliserroll). Consequently, one can be defined that a vehicle suspension system, which consists of damping, springing and steering elements, are responsible to a considerable extent for damping of vibrations generated by road roughness. Parameters of those elements have nonlinear characteristics which determine the system’s response to vehicle vibrations from road roughness [1-3].

Typical spring elements (coil spring, torsion bar, leaf spring and air spring) used in vehicle suspension systems are presented in Fig. 1.

The suspension system with a coil spring, a torsion bar or a leaf spring absorbs additional load through compression. The ride height depends on the loading condition. The air spring is a rubber cylinder filled with compressed air and the vehicle ride height can be corrected by adjusting the air pressure [4-6].

Fig. 1Vehicle spring elements [7]

Because of numerous advantages, special-purpose vehicles often use suspension systems based on leaf springs. The evolution of a conventional laminated leaf spring into a parabolic spring is illustrated in Fig. 2. Application of a small number of leaves in the spring particularly contributes to the reduction of frictional forces in a positive manner. This is possible not by the use of additional spring leaves but by the use of stronger leaves capable of withstanding higher bending forces at the spring centre. This leads to a higher degree of material utilisation. The bending stress along the leaf is constant if the spring halves display a parabolic profile in the longitudinal section. Leaf springs with such leaves are called parabolic springs [7-11].

Fig. 2Comparison of leaf spring elements: a) conventional trapezoidal b) improved trapezoidal parabolic [5] c) parabolic

a) Conventional trapezoidal spring, $m=$ 128 kg

b) Improved trapezoidal spring, $m=$ 94 kg

c) Parabolic spring, $m=$61 kg

The three springs considered here are characterised by the same length, spring rate and nominal load. The advantage of the parabolic spring, apart from the decrease of friction, is the decrease of the spring weight by around 50 % (minimisation of unsprung masses). The main advantage of the leaf spring is the combination of suspension and axle location, which is partially lost with the transition to parabolic springs featuring fewer leaves. In the course of braking, the parabolic springs tend to be affected by the $S$ impact, which must be prevented using additional torque support. Otherwise, relative velocities between the road and tyres can result in brake-hopping [10].

2. Parameters of the high mobility wheel platform

The high mobility wheel platform (KPWM) is presented on Fig. 3. It is one of vehicles developed by the Szcześniak company for military applications.

The technical parameters of the high mobility wheel platform are presented in Table 1.

Fig. 3High mobility wheel platform [12]

The 6×6 power transmission system enables driving in both paved roads and off-road routes. A block diagram of the 6×6 power transmission system has been provided in Fig. 4. Torque generated in the engine is transferred via the clutch and the transmission to the drive shaft, the drive shaft transfers the torque to the transfer case, and then, via drive shafts, 3 driving axles are propelled. All driving wheels are propelled, and such a solution has been applied in order to increase the driving force in wheels. This has made it possible to improve drive properties of the vehicle as well as its off-road mobility.

Fig. 4Block diagram of the 6x6 power transmission: 1 – engine, 2 – clutch, 3 – transmission, 4 – transfer case, 5 – driving axle, 6 – drive shaft

The high mobility wheel platform (KPWM) is, by definition, intended to be a lorry of superior drive properties compared to the heavy vehicles currently available in the market. Unlike in commercial vehicles, the design of the vehicle in question is intended for driving in difficult terrain, carrying heavy load and operating under adverse weather conditions. One of the characteristic features of the design is also that it enables finding an assembly line in the country of the party interested in the purchase and licenced assembly of the chassis. The design in question is not intended for a single customer only, but rather for a target group of parties sharing needs, best represented by armed forces [13-16].

The main frame (Fig. 5) of KPWM is a backbone of the entire vehicle, which supports all elements. It should meet the strength requirements set up to carry the burden of rough roads as well as the load from the body and its parts and the cargo being transported. The material used for making the frame is resistant to static and dynamic loads as well as resistant to the conditions during the vehicle’s operation. The frame is made of steel elements forming a flat grid: stringers and cross beams connecting them in accordance with the design.

Fig. 5Frame of the special-purpose vehicle (KPWM) [15]

The suspension system features build-in elastic elements, such as reinforced parabolic leaf springs in the front suspension (Fig. 6(a)) and semi-elliptic leaf springs in the rear suspension (Fig. 6(b)). Also shock absorbers and a stabilizer were used to reduce lateral body roll during cornering.

Fig. 6Front (a) and rear (b) suspension of the special vehicle (KPWM) [15]

a)

b)

3. Conclusions

The high mobility wheel platform (KPWM) is an example of a vehicle offering improved off-road mobility capabilities. The solutions assumed while designing the front axle suspension system as well as that of the twin rear axles ensure a reasonable compromise between mobility capabilities of a vehicle carrying load in difficult terrain and costs as well as possibilities to repair the suspension system in the field. Solutions based on elastic components, such as parabolic leaf springs, are not optimum solutions when considering the driving comfort criterion, but their legitimacy stems from the fact that it is the off-road mobility which constitutes the most significant criterion applicable to this platform.