The equation of motion for suspension system consists two degrees of freedom, as two independent special coordinates are required to define the complete configuration namely rectilinear motion and rotary motion. The equations of motions are assumed to be simple harmonic with the presence of static coupling and with the absence of dynamic coupling. Finite element technique is applied to solve the equations of motions by choosing BEAM3, MASS21 and COMBINATION 14 with their properties. The reduced option for analysis is used and a comparative study is performed on the automobile suspension system.Functions Of Suspension System
The primary function of suspension system is to isolate the vehicle structure from shock loading and vibration due to irregularities of road surface. Secondly it must do it without impairing the stability, steering or general handling qualities of vehicle. The primary requirement is met by the use of flexible elements and dampers, while the second is achieved by controlling, by the use of mechanical linkages. These linkages may be either as simple as a semi-elliptic spring and shackle or as complex as a double transverse link and anti-rollbar or some other such combination of mechanisms.
The diameter of the tyre, size of contact patch between tyre and road, the rate of tyre acting as a spring, and weight of the wheel and axle assembly affect the magnitude of shock transmitted to the axle, while the amplitude of wheel motion is affected by all these factors and the rate of suspension springs, damping effect of the shock absorbers and the weights of sprung and unsprung masses. The unsprung mass can be defined as that between the road and the main suspension springs .The sprung mass is that supported on suspension springs, though both may also include the weights of the parts of the springs and linkages.
Two types of shocks are applied to the wheels.
1) Shock due to the wheel’s striking on the bump. This is influenced by the geometry of the bump and the speed of the vehicle.
2) Shock caused by the wheels falling into a pothole. This is influenced by the geometry of the hole, the unsprung masses and spring rates, speed being an incidental influencing factor.
A suspension system consists of a spring shock absorber. The hydraulic damping force of the shock absorber can be taken as proportional to the square of the vertical velocity of the sprung mass relative to that of the unsprung mass, the dynamic friction damping force is, in effect, constant regardless of velocity. It follows that while small amplitude, small velocity movements of the suspension are virtually unaffected by the hydraulic damping, the force applied by the friction damping is same for these small movements as it is for large once.
Dampers have two functions. First they reduce the tendency for the carriage unit to continue to bounce up and down on its springs after the disturbance that caused the initial motion has ceased. Secondly, they prevent excessive buildup of amplitude of bounce as a result of period excitation at a frequency identical to the natural frequency of vibration of the sprung - mass system. This natural frequency is a function of the rate of the sprung mass and spring rate.
The suspension system in a automobile requires accurate analysis to find the natural frequencies of the system otherwise which will lead to unbalance of rotary or reciprocating type. The finite element analysis furnishes almost accurate results compared to exact method as tabulated above.