Suspension System

Suspension systems have been widely applied to vehicles, from the horse-drawn carriage with flexible leaf springs fixed in the four corners, to the modern automobile with complex control algorithms. The suspension of a road vehicle is usually designed with two objectives; to isolate the vehicle body from road irregularities and to maintain contact of the wheels with the roadway. Isolation is achieved by the use of springs and dampers and by rubber mountings at the connections of the individual suspension components.

From a system design point of view, there are two main categories of disturbances on a vehicle, namely road and load disturbances. Road disturbances have the characteristics of large magnitude in low frequency (such as hills) and small magnitude in high frequency (such as road roughness). Load disturbances include the variation of loads induced by accelerating, braking and cornering. Therefore, a good suspension design is concerned with disturbance rejection from these disturbances to the outputs. Roughly speaking, a conventional suspension needs to be “soft” to insulate against road disturbances and “hard” to insulate against load disturbances. Therefore, suspension design is an art of compromise between these two goals.

Today, nearly all passenger cars and light trucks use independent front suspensions, because of the better resistance to vibrations. One of the commonly used independent front suspension system is referred as double wishbone suspension.

Basic Suspension Parts

Spring: The spring is the core of nearly all suspension systems. It’s the component that absorbs shock forces while maintaining correct riding height. The increased effect of shock impairs the vehicle's handling the amount of deflection exhibited under a specific load. A mounting plate welded to the lower arm serve as a lower spring seat. The upper seat is bolted to the strut piston rod. A bearing or rubber bushing in the upper mount permits the spring and strut to turn with the motion of the wheel as it steered.

Shock Absorber: Shock absorber damp or control motion in a vehicle. If unrestrained, spring continue expanding and contracting after a blow until all energy is absorbed. Shock absorber can be mounted vertically or at an angle. Angle mounting of shock absorbers improves vehicle stability and dampens accelerating and breaking torque.

Lower Control Arm: The suspension lower mounting position continues to be the frame, as on the traditional suspension, because the lower control arm and ball joint are retained. The control arm serves as the lower locator of the suspension.

Ball Joint: A ball joint connects the steering knuckle to the control arm, allowing it to pivot on the control arm during steering. Ball joint also permit up and down movement of the control arm as the suspension reacts to road conditions. This ball joint are load caring and supports the car weight it also called tension loaded or compression loaded ball joint.

Bump Stop: Bump stop are located on lower control arm and it avoid direct contact of arm with chassis/body while car movement upward (jounce) and downward (rebound).

Types of Suspension Systems

Suspensions generally fall into either of two groups-solid axles and independent suspensions. Each group can be functionally quite different, and so will be itemized accordingly for discussion.

1) Solid Axle Suspension Systems

In solid axle suspension systems, wheels are mounted at the ends of a rigid beam so that any movement of one wheel is transmitted to the opposite wheel causing them to steer and camber together.

Solid drive axles are used on the rear of many cars and most trucks and on the front of many four-wheel-drive trucks. Solid beam (non-driven) axles are commonly used on the front of heavy trucks where high load-carrying capacity is required.

Solid axles have the advantage that wheel camber is not affected by body roll.

Thus there is little wheel camber in cornering, except for that which arises from slightly greater compression of the tires on the outside of the turn. In addition, wheel alignment is readily maintained, minimizing tire wear. The major disadvantage of solid steerable axles is their susceptibility to tramp-shimmy steering vibrations. The most common solid axles are Hotchkiss, Four link and De Dion.

2) Independent Suspension Systems

In contrast to solid axles, independent suspensions allow each wheel to move vertically without affecting the opposite wheel. Nearly all passenger cars and light trucks use independent front suspensions, because of the advantages in providing room for the engine and the better resistance to steering vibrations. The independent suspension also has the advantage that it provides inherently higher roll stiffness relative to the vertical spring rate. Further advantages include easy control of the roll centre by choice of the geometry of the control arms, larger suspension deflections, and greater roll stiffness for a given suspension vertical rate.

Over the years, many types of independent front suspension have been tried such as MacPherson, Trailing arm, Swing axle, Multi link and Double wishbone suspension.

Many of them have been discarded for a variety of reasons, with only two basic concepts, the double wishbone and the MacPherson strut, finding widespread success in many varied forms.

Double wishbone Suspension (SLA, A-arms)

The most common design for the front suspension used two lateral control arms to hold the wheel. The upper and lower

control arms are usually of unequal length from which the acronym SLA (short-long

arm) gets its name.

These are often called “A-arms” in the United States and “wishbones” in Britain.

This layout sometimes appears with the upper. A-arm replaced by a simple link, or the

lower arm replaced by a lateral link, the suspensions are functionally similar. The SLA

is well adapted to front-engine, rear-wheel-drive cars because of the package space it

provides for the engine oriented in the longitudinal direction.

Design of the geometry for a SLA requires careful refinement to give good

performance. The camber geometry of an unequal-arm system can improve camber at

the outside wheel by counteracting camber due to body roll, but usually carries with it

less-favourable camber at the inside wheel (equal-length parallel arms eliminate the

unfavourable condition on the inside wheel but at the loss of camber compensation on

the outside wheel). At the same time, the geometry must be selected to minimize tread change to avoid excessive tire wear.

The compact design of a coil spring makes it ideal for use in front suspension

systems. Two types of coil spring mountings are used. In the first type the spring is positioned between the frame and the lower control arm as shown in Figure.