Macpherson Suspension System Case Study

In this era of continuous development in the manufacturing sector, design and weight optimization is the prime objective. This objective leads to the research of various metal alloys and composite materials. In automotive industry, the significant increase of the demand for lighter, more fuel efficient vehicles, reduced design-testing iterations, and satisfactory reliability level has promoted this research. The suspension and steering systems are responsible for providing comfort and stability. The performance of both the systems depends on the performance of steering knuckle which is subjected to millions of varying stresses cycles leading to fatigue failure. Due to its critical functionality, design and weight optimization are required.

Components of Macpherson suspension system
1) Steering Knuckle: The knuckle is mounted between the upper and lower ball joints on a Short-Long Arm (SLA) suspension, and between the strut and lower ball joint

Fig.1 Macpherson Suspension System

2) Macpherson Strut: A strut containing shock absorber and the spring carries also the stub axle on which the wheel is mounted. The wishbone is hinged to the cross member and positions the wheel as well as resists accelerating, braking and side forces.
3) Tie Rod or Track Rod: A tie rod is a slender structural unit used as a tie and is capable of carrying tensile & compression loads. It is also referred to as track rod when concerned with automobiles.
4) Wheel Hub Assembly: Wheel Hub Assembly is located between the brake rotors and the axle. It is mounted to the holding bracket from the chassis; on the rotor side.
5) Lower Ball Joint: The lower control arm and ball joint are retained on Macpherson suspensions. The lower ball joint stabilizes the steering and prevents shimmy. Fig. 2 Steering Knuckle
6) Brake Calliper: The brake calliper is the assembly which houses the brake pads and pistons. There are two types of callipers: floating or

Structural Analysis of Knuckle for Multi-Axial Forces
A. Multi-axial force acting on suspension system
Steering Knuckle is subjected to dynamic loads during dynamic conditions like cornering, acceleration and braking. The force acting on the knuckle is as shown: Fig.3 Forces Acting on a Steering Knuckle
B. Cornering
While cornering, a centrifugal force (C.F=mv2/R) acts on the centre of gravity outwards from the centre of turn. This force is directly proportional to the velocity and mass of the vehicle and inversely proportional to the radius of curvature. Lateral load transfer takes place from the inner to outer tires of the vehicle. The weight l is given in eq.1
Fig.4 Load Transfer during Cornering mg×t/2+(CG×m×a)=w_fr×t ---- (1) w_fr=█((1165×9.81×1295/2)+@(553.9×1165×0.8×9.81) )/1295 w_fr=9624.9N The wheel is subjected to vertical, longitudinal and lateral forces which are transmitted to the hub and the wheels. Theoretical calculation of these forces is cumbersome but found easily using sensors. Hence, for this project, the industrial data is used which are shown in Table