Suspension Reasoning

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Introduction= For automotive applications, a vehicle suspension allows the controlled movement of the wheels relative to the vehicle body. In order to control it, passive, semi-active and/or active components are introduced. These enable the suspension system to adapt to various driving conditions. In most vehicles, this is achieved with a passive compliant structure (suspension). The vehicle suspension system consists of wishbones, springs, and shock absorbers to transmit and filter all forces between the body and road. If a vehicle has a passive suspension it will tend to oscillate under given inputs. To reduce the amount and duration of oscillation, some form of damping is required.

Purpose of a suspension

The automotive industry uses different kinds of suspension systems. Important criteria are cost, weight, performance, packaging, manufacturing, kinematic properties and compliance attributes. A suspension and damping system can serve multiple purposes. Some of the reasons for having one are described in table 1.

Purpose	Description
Improving driver and passenger comfort	A suspension acts as an interface between the the vehicle body and the road, reducing the passenger accelerations and movements. The accelerations of the vehicle body are also dependent on the amount of damping in the system.
Reduce impulse forces	A compliant structure helps to reduce the peak forces in the vehicle structure due to external impacts. This helps to improve vehicle reliability and allows lighter structural designs.
Static load distribution	A suspension is a controlled compliant structure that can be used to distribute the normal load between the wheels.
Handling	The suspension can be used to influence the handling of the vehicle by changing the tire normal load distribution. This is achieved by adjusting the suspension stiffness distribution, the suspension geometry or by controlling the aerodynamic platform. The vehicle damping can also be used to influence the transient vehicle balance by changing the damping coefficients.
Improve mechanical grip	A suspension is compliant over bumps and impacts. This can help to reduce the tire load variation (TLV) seen at the tire-road interface. TLV has a large impact on grip due to the delayed response of the tire to load variations and due to the load sensitivity of the tire.
Aerodynamics	The balance and effect of the aerodynamics can be influenced on ride-height sensitive vehicles with a suspension system. The position and orientation of the vehicle body is also dependent on the amount of damping in the system.
Transient Response	The vehicle heave, pitches and rolls under inputs such as steering, throttle, brake application as well as large bumps and curb strikes, etc. Dampers help to control the response of these inputs.
Forced Vibration	The vehicle can be excited in heave, pitch, roll and single wheel vibrations. This occurs particularly near the system’s resonance points and is due tothe consistent unevenness/bumpiness in the track. Damping can help to reduce the magnitude of this response.