The suspension system of the automobile is very important device for stabilising the motion as well as reducing the effect of external disturbance. this is very important to give high comfortable for passengers. the suspension system consist of spring and damper elements. What is the design value of the damping ratio for the automobile?
and, How affect this value on the dynamic behavior of the vehicle?
The damping constant is usually in the order of D=0.3
The higher this constant, the better the driving dynamics.
The lower this constant, the better the comfort.
Typically damping of a vehicle comprises of four damping rates: Low Speed Compression, low Speed Rebound, High Speed Compression and High Speed Rebound. Lower Speed Ratios control the rate of weight transfer during maneuvering and motions of sprung mass whereas High Speed ratios takes charge of suspension over bumps or speed breakers.
Damping Constant varies according to the application of vehicle, such as:
1) For Commercial vehicles, Damping Constant is around 0.2 to 0.3
2) For Racing vehicles, Damping Constant ranges from 0.5 to 0.7
When we consider compression and rebounding of suspension reacting to road shocks, this damping ratio value plays a major rule on dynamic motion of vehicle, as in terms of vertical loading of the vehicle (say passenger and cargo). In case of car shock absorber, once the car is said to hit a speed breaker, the suspension system compresses and rebounds with minimum oscillation. this condition is critically damped ( damping ratio, ζ = 1 ), say if more oscillations are there in between compression and rebounding, it is then over damped ( ζ > 1 ), if there is zero oscillation as the system immediately rebounds after compression, it is then under-damped. (ζ < 1). For a good automobile suspension system critically damped system is considered most, as over-damped system would vibrates lot on road shocks (so not advisable) and under-damped system said to absorbs no shocks, which the road shock will be directly transferred to other parts of vehicle, thus if mounting torque of that certain part is too high, it then leads to breakage or crack, and if it is low, it then leads to fatigue losses or else the road shocks can directly transferred to passengers inside the vehicle too (say for example, rigid suspension in a bicycle).
The damping ratio affects the suppression of spring oscillations, beyond a certain limit damping ration has the negligible effect.
Dear Ahmad,
Damping ratio is the significant parameters to design the suspension system. The damping ratio affects the suppression of spring oscillations, beyond a certain limit damping ration has the negligible effect. Generally, the effect of damping can be seen in automobile of sprung mass system, the spring oscillations are damped and the resulting time history response shape is very close to the induced noise profile. It can be concluded that damping control the oscillations in spring. For vehicle control these oscillations should be damped as soon as possible so suspension is ready to counter next road bump in minimum possible time and improve the handling of vehicle. Large damping ratios have usually a small shift in resonance frequency. The peak amplitudes in the frequency response are controlled by the damping ratios, the larger a damping ratio smaller is the peak.
The maximum effective damping coefficient leading to an optimal modal damping ratio can be obtained by finding the maxima of effective damping ratio with respect to the damping coefficient. Negligible errors occur as vehicle suspension systems typically operate with damping ratios around 0.3. For this range of damping ratios, the difference between the natural and damped natural frequencies is much smaller compared with the natural frequency.
Ashish
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