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.
