AUTOMOBILE SUSPESION SYSYTEM
ABSTRACT
Automobile
suspension system is especially created for supporting an Automobile’s body on
its undercarriage including dampers, springs and locating linkages. They
function to control their own oscillation through inter-leaf friction.
TABLE OF CONTENTS
Suspension
system
Layout of Suspension system
Role of suspension system
Principle of suspension system
Components of suspension
system
Types of suspension systems
·
Conventional suspension system
·
Independent suspension system
a) MacPherson
Strut
b) Double
Wishbone
c) Multi link
suspension
·
Air suspension
·
Hydrolastic Suspension
Advantages of suspension
system
Helper spring
Suspension
Geometry
Automobile suspension system
Primary function
Automobile suspension system
Nomenclature
MacPherson strut suspension:
Main Components of the
Automobile Suspension System
·
Spring
rate
·
Calculation
of the spring rate
·
Units of spring rate
·
Wheel rate
·
Roll
couple percentage
·
Weight
transfer
a) Unsprung
weight transfer
b) Sprung
weight transfer
·
Jacking forces
·
Damping
·
Camber
control
·
Flexibility
and vibration modes of the suspension elements
·
Isolation
from high frequency shock
·
Space
occupied
·
Air
resistance (drag)
BREAKDOWN OF STEERING/SUSPENSION COMPONENTS
·
Idler joint
·
Ball joints
·
Control arms and bushes
·
Steering links and the rod ends
·
stabilizer/Sway Bar links
Front wheel automobile
suspension system
Rear wheel automobile
suspension system
Components of the Automobile Suspension System
Key Benefits
Common problems of the suspension system.
Preventive Measures.
Automobile suspension system
Suspension
system
Suspension is the term given to the system of
springs, shock absorbers and linkages that connects a vehicle to its wheels. It
Serve a dual purpose – The suspension
system, part of the undercarriage of an automobile, contains springs that move
up and down to absorb bumps and vibrations. In one type of suspension system, a
long tube, or strut, has a shock absorber built into its center section. Shock
absorbers control, or dampen, the sudden loading and unloading of suspension
springs to reduce wheel bounce and the shock transferred from the road wheels
to the body. One shock absorber is installed at each wheel. Modern shock
absorbers have a telescoping design and use oil, gas, and air, or a combination
to absorb energy.
The job of a car suspension is to maximize the friction
between the tires and the road surface, to provide steering stability with good
handling.
Sir William Brush is the father of suspension system in automobiles.
Layout of Suspension
system
Role of
suspension system
The main role
of suspension system is as follows:
Ø It supports the weight of vehicle.
Ø Provides smoother ride for the
driver and passengers i.e. acts as cushion.
Ø Protects your vehicle from damage
and wear.
Ø It also plays a critical role in
maintaining self driving conditions.
Ø It also keeps the wheels pressed
firmly to the ground for traction.
Ø It isolates the body from road
shocks and vibrations which would otherwise be transferred to the passengers
and load.
— Principle of suspension system
Ø When a tire hits an obstruction, there is a reaction
force. The size of this reaction force depends on the unsprung mass at each
wheel assembly.
Sprung
Mass:
Sprung mass (weight) refers to vehicle parts supported on the
suspension system, such as the body, frame, engine, the internal components,
passengers, and cargo.
Unsprung
Mass:
Unsprung mass refers to the components that follow the road
contours, such as wheels, tires, brake assemblies, and any part of the steering
and suspension not supported by the springs.
Components
of suspension system
There are
three fundamental components of any suspension system.
Ø Springs
Ø Dampers
Ø Anti sway bars
Types
of suspension systems
Ø Conventional suspension system
Ø Independent suspension system
Ø Air suspension system
Ø Hydrolastic suspension system
Conventional suspension system
Two wheels
are mounted on either side of the rigid axle. When one wheel encounters the bump,
both the wheel do not execute parallel up and down motion So it gives rise to gyroscopic effect
and wheel wobble.
Independent suspension system
In this
system one wheel goes down, the other wheel does not have much effect.
ü Basic classification of the independent suspension
system design.
Ø MacPherson
Strut
Ø Double
Wishbone
Ø Multi link
suspension
MacPherson Strut
Ø It is the most widely used front
suspension system in cars.
Ø Comprises of a strut-type spring and
shock absorber combo, which pivots on a ball joint on the single, lower arm.
DOUBLE WISHBONE
SUSPENSION
Ø Type of double-A or double
wishbone suspension.
Ø Wheel spindles are supported by
an upper and lower 'A' shaped arm.
Ø The lower arm carries most of the
load.
Multi-link
suspension
Ø It's currently being used in the
Audi A8 and A4 amongst other cars.
Ø The basic principle of it is the
same, but instead of solid upper and lower wishbones, each 'arm' of the
wishbone is a separate item.
Ø These are joined at the top and
bottom of the spindle thus forming the wishbone shape.
Air suspension
Ø It comprises of compressor, supplying air to air tank.
Ø Pressure maintained – 5.6 to 7 kg/sq.m
Ø Air bags – on each wheel
Ø As load applied , air bags compressed actuating the leveling
valve .
Ø Air from the tank fills the compressrd air bag &
hence raises the level of the frame.
Ø Air from air bag gets released as load on chassis decreases.
Hydrolastic Suspension
Ø The front and rear suspension
units have Hydrolastic displacers, one per side.
Ø These are interconnected by a
small bore pipe. Each displacer incorporates a rubber spring
Advantages of suspension system
Ø Comfort to passengers.
Ø Good handling.
Ø Shields the vehicle from damage.
Ø Increases life of vehicle.
Ø Keeps the tires pressed firmly to
ground.
HELPER SPRING
Ø Directly mounted on main spring.
Ø Take care of large variation spring load.
Ø During light loads, only main spring is active, as
load increase to a particular fixed value, both the springs are active.
Suspension Geometry
A dependent
suspension normally has a beam (driven) live axle that holds wheels
parallel to each other and perpendicular to the axle. When the camber of one
wheel changes, the camber of the opposite wheel changes in the same way (by
convention on one side this is a positive change in camber and on the other side
this a negative change).
An independent
suspension allows wheels to rise and fall on their own without affecting
the opposite wheel. Suspensions with other devices, such as sway bars that link
the wheels in some way are still classed as independent.
A third type
is a semi-dependent suspension. In this case, the motion of one wheel
does affect the position of the other but they are not rigidly attached to each
other. A twist-beam rear suspension is such a system.
Common types seen from behind. From top
to bottom: live axle with Watt bar, suspension like on a bike fork, swing axle,
double wishbone, MacPherson. Some types are missing because trailing arm links
are not presentable in this view and some types use elements which flex to some
movements and are stiff to others and flexible elements are omitted for
clarity.
Suspension systems can be broadly classified into two subgroups.
o
Dependent.
o
Independent.
These terms refer to the ability of
opposite wheels to move independently of each other.
Automobile
suspension system
Primary function
The primary function of a car's
suspension and steering systems is to allow the wheels to move independently of
the car, while keeping it "suspended" and stable. Any play or
uncontrolled motion in these systems results in a deterioration of handling and
accelerated tire wear. Vehicle alignment is closely tied to the condition of
the suspension and steering systems.
The job of a
car suspension is to maximize the friction between the tires and the road
surface, to provide steering stability with good handling and to ensure the
comfort of the passengers.
Worn or loose components
affect the suspension system's ability to control motion and alignment angles,
resulting in a deterioration of vehicle handling and stability, and accelerated
tire wear.
Automobile
suspension system
Automobile suspension system consists of wishbones, the spring,
and the shock absorber to transmit and also filter all forces between the body
and road.
The spring carries the body mass and isolates the body from
road disturbance and thus contribute to drive comfort.
The damper contributes to both driving safety and comfort its
task is the damping of body and wheel oscillations directly refer to drive safety,
as a non bouncing wheel is the condition for transferring road contact forces.
Nomenclature
1.
Steering Gear Box
2. Center Link
3. Pitman Arm
4. Idler Arm
5. Tie Rods
6 Rack and Pinion Assembly
7. Bellows Boots
8. Tie Rods
2. Center Link
3. Pitman Arm
4. Idler Arm
5. Tie Rods
6 Rack and Pinion Assembly
7. Bellows Boots
8. Tie Rods
9.
Control Arms
10. Ball Joints
11. Springs
12. Shock Absorbers
13. Struts
10. Ball Joints
11. Springs
12. Shock Absorbers
13. Struts
MacPherson strut
suspension:
Figures 1 through 3 are drawings of typical
suspension systems found on most vehicles on the road. Figure 1 is the
classical MacPherson strut suspension, which is common on many front drive
vehicles. The strut, which is also a shock damper, moves vertically while the
control arm limits transverse and longitudinal movement. The system is compact,
efficient and adapts easily to front and rear applications. Figure 2 is a view
of an earlier design: the solid axle suspension with king pin. The solid axle
beam is supported by springs and connects to a swiveling axle via the king pin.
This suspension is often used on heavier vehicles such as trucks and on some
older vehicles. Figure 3 depicts a control arm suspension with coil springs.
This independent suspension system is used on many older and rear wheel drive
vehicles. Automobile accident investigation may focus on a vehicle's suspension
system, being guided by evidence of possible malfunction or statements from the
insured driver or witnesses. Automotive suspension failure can be caused by a
design defect, a manufacturing defect, poor maintenance or the accident.
FIGURE
1
FIGURE 2
FIGURE
3
FIGURE 3 elaborates the Control
arm suspension with coil springs
Figure 4 is a view of a MacPherson
front suspension on the right side of a compact car. Evidence suggests that the
lower ball joint (arrow) failed, causing the vehicle to steer uncontrollably,
which resulted in an accident. Figure 5 is a top view of the ball joint showing
wear patterns from the drive shaft rotor just above the ball joint. The ball
joint itself was dry and badly worn with no evidence of lubrication. The
vehicle had over 100,000 miles on the odometer. The wear on the top of the ball
joint suggests that for a period of time, the joint had failed and had moved
vertically and rubbed against the axle rotor. The rotor was acting as a
retainer of the joint, preventing it from separating from the suspension. This
condition would result in excessive play in the steering, plus a loud noise
that should have acted as a warning to the insured driver that a problem
existed. The driver continued to operate the vehicle until the accident
occurred. The failure of the ball joint was determined to be maintenance
related with no evidence of a manufacturing defect.
A car's suspension, with its various components, provides all
of the solutions described as follows to the understanding easily.
|
Principle
|
Definition
|
Goal
|
Solution
|
|
Road Isolation
|
The vehicle's ability to absorb or isolate road shock from
the passenger compartment
|
Allow the vehicle body to ride undisturbed while traveling
over rough roads.
|
Absorb energy from road bumps and dissipate it without
causing undue oscillation in the vehicle.
|
|
Road Holding
|
The degree to which a car maintains contact with the road
surface in various types of directional changes and in a straight line
(Example: The weight of a car will shift from the rear tires to the front
tires during braking. Because the nose of the car dips toward the road, this
type of motion is known as "dive." The opposite effect -- "squat"
-- occurs during acceleration, which shifts the weight of the car from the
front tires to the back.)
|
Keep the tires in contact with the ground, because it is
the friction between the tires and the road that affects a vehicle's ability
to steer, brake and accelerate.
|
Minimize the transfer of vehicle weight from side to side
and front to back, as this transfer of weight reduces the tire's grip on the
road.
|
|
Cornering
|
The ability of a vehicle to travel a curved path
|
Minimize body roll, which occurs as centrifugal force
pushes outward on a car's center of gravity while cornering, raising one side
of the vehicle and lowering the opposite side.
|
Transfer the weight of the car during cornering from the
high side of the vehicle to the low side.
|
Main
Components of the Automobile Suspension System
The
main components of the automobile suspension system are:
Spring rate
The spring
rate (or suspension rate) is a component in setting the vehicle's ride height
or its location in the suspension stroke. Vehicles which carry heavy loads will
often have heavier springs to compensate for the additional weight that would
otherwise collapse a vehicle to the bottom of its travel (stroke). Heavier
springs are also used in performance applications where the loading conditions
experienced are more extreme.
Calculation
of the spring rate
Spring rate is a ratio used to
measure how resistant a spring is to being compressed or expanded during the
spring's deflection. The magnitude of the spring force increases as deflection
increases according to Hooke's Law. Briefly, this can be stated as
F = force the spring exerts
k = spring rate of the spring.
X
= displacement from equilibrium length i.e. the length at which the spring is neither
compressed nor stretched.
Units of spring rate
Spring rates typically have units of N/mm (or lbf/in).
An example of a linear spring
rate is 500 lbf/in. For every inch the spring is compressed, it exerts
500 lbf. A non-linear spring rate is one for which the relation between
the spring's compression and the force exerted cannot be fitted adequately to a
linear model. For example, the first inch exerts 500 lbf forces, the
second inch exerts an additional 550 lbf (for a total of 1050 lbf),
the third inch exerts another 600 lbf (for a total of 1650 lbf). In
contrast a 500 lbf/in linear spring compressed to 3 inches will only
exert 1500 lbf.
Wheel rate
Wheel rate
is the effective spring rate when measured at the wheel. This is as opposed to
simply measuring the spring rate alone.
Wheel rate
is usually equal to or considerably less than the spring rate. Commonly,
springs are mounted on control arms, swing arms or some other pivoting
suspension member.. The wheel rate is calculated by taking the square of the
ratio (0.5625) times the spring rate. Squaring the ratio is because the ratio
has two effects on the wheel rate. The ratio applies to both the force and
distance traveled.
Roll couple percentage
Roll couple
percentage is the effective wheel rate, in roll, of each axle of the vehicle as
a ratio of the vehicle's total roll rate. Roll couple percentage is critical in
accurately balancing the handling of a vehicle. It is commonly adjusted through
the use of anti-roll bars, but can also be changed through 666
Weight transfer
Weight
transfer during cornering, acceleration or braking is usually calculated per
individual wheel and compared with the static weights for the same wheels.
o Unsprung weight transfer
Unsprung
weight transfer is calculated based on the weight of the vehicle's components
that are not supported by the springs. This includes tires, wheels, brakes,
spindles, half the control arm's weight and other components.
o Sprung weight transfer
Sprung
weight transfer is the weight transferred by only the weight of the vehicle
resting on the springs, not the total vehicle weight. Calculating this requires
knowing the vehicle's sprung weight (total weight less the unsprung weight),
the front and rear roll center heights and the sprung center of gravity height
(used to calculate the roll moment arm length).
Jacking
forces
Jacking forces are
the sum of the vertical force components experienced by the suspension links.
The resultant force acts to lift the sprung mass if the roll center is above
ground, or compress it if underground. Generally
Damping
Damping is the control of motion or oscillation, as seen with
the use of hydraulic gates and valves in a vehicles shock absorber. This may
also vary, intentionally or unintentionally. Like spring rate, the optimal
damping for comfort may be less than for control.
Camber control
Camber
changes due to wheel travel, body roll and suspension system deflection or
compliance. In general, a tire wears and brakes best at -1 to -2° of camber
from vertical. Depending on the tire and the road surface, it may hold the road
best at a slightly different angle. Small changes in camber, front and rear,
can be used to tune handling. Some race cars are tuned with -2~-7° camber
depending on the type of handling desired and the tire construction.
Oftentimes, too much camber will result in the decrease of braking performance
due to a reduced contact patch size through excessive camber variation in the
suspension geometry.
Flexibility and vibration modes of the
suspension elements
In modern cars, the flexibility is mainly in the rubber bushings. For
high-stress suspensions, such as off-road vehicles, polyurethane bushings are
available, which offer far more longevity under greater stresses.
Isolation from high frequency shock
For most purposes, the weight of
the suspension components is unimportant, but at high frequencies, caused by road
surface roughness, the parts isolated by rubber bushings act as a multistage
filter to suppress noise and vibration better than can be done with only the
tires and springs. (The springs work mainly in the vertical direction.)
Space occupied
Designs
differ as to how much space they take up and where it is located. It is
generally accepted that MacPherson struts are the most compact arrangement for
front-engine vehicles, where space between the wheels is required to place the
engine.
Air resistance (drag)
Certain modern vehicles have height adjustable suspension in order to
improve aerodynamics and fuel efficiency. And modern formula cars, that have
exposed wheels and suspension, typically use streamlined tubing rather than
simple round tubing for their suspension arms to reduce drag. Also typical is
the use of rocker arm, push rod, or pull rod type suspensions, that among other
things, places the spring/damper unit inboard and out of the air stream to
further reduce air resistance.
BREAKDOWN OF
STEERING/SUSPENSION COMPONENTS
IDLER ARM
On an automobile or truck with conventional
parallelogram steering, the Idler Arm or idler arm assembly is a pivoting
support for the steering linkage. The idler arm consists of a rod which pivots
on a bracket attached to the frame of the vehicle on one end and supports a
ball joint on the other end. Generally, an idler arm is attached between the
opposite side of the center link from the Pitman arm and the vehicle's frame to
hold the center link at the proper height.
PITMAN ARM
The Pitman arm is a steering component in an
automobile or truck. The Pitman arm is a linkage attached to the steering box
sector shaft that converts the angular motion of the sector shaft into the
linear motion needed to steer the wheels. The Pitman arm is supported by the
sector shaft and supports the drag link with a ball joint. It transmits the
motion it receives from the steering box into the drag link, causing it to move
left or right to turn the wheels in the appropriate direction.
BALL JOINTS
The upper and lower ball joints allow the spindle to
rotate when steered, and move vertically to absorb road bumps at the same time.
They are constructed of an inner ball which is bolted to the spindle, and a socket,
which is bolted to the control arm. They are lubricated to prevent wear through
their grease fittings.
CONTROL ARMS AND BUSHINGS
A control arm is a bar with a pivot at each end, used
to attach suspension members to the chassis. When coil springs are used in both
front and rear suspension, three or four control arms are placed between the
rear axle housing and the frame to carry driving and brake torque. The lower
control arms pivot on the frame members and sometimes support the rear coil
springs to provide for up-and-down movement of the axle and wheel assembly.
A-arms are control arms with two inboard pivots, giving strength. Some front
end designs use control arms instead of A-arms, usually to save weight and add
adjustability.
STEERING LINKS AND TIE ROD ENDS
The steering linkage is made of interconnected parts
which move every time the steering wheel is turned. The rotating movement of
the steering column activates mechanisms inside the steering box. Tie rod ends,
which join the key parts, pass on the steering wheel's motion no matter what
the angle of the linkage or the vibration from the road. In a pitman arm
steering setup, the movement inside the steering box causes the Pitman shaft
and arm to rotate, applying leverage to the relay rod, which passes the
movement to the tie rods. The steering arms pick up the motion from the tie
rods and cause the steering knuckles to turn the wheels. The steering linkages
need regular maintenance for safe operation, such as lubrication and
inspection. Faulty steering links can cause tire wear at the least, and
complete loss of control of the vehicle at worst. "Popping" noises
(when turning the wheels) usually indicate worn out steering linkages.
STABILIZER / SWAY BAR LINKS
Some cars require stabilizers to steady the chassis
against front end roll and sway on turns. Stabilizers are designed to control
this centrifugal tendency that forces a rising action on the side toward the
inside of the turn. When the car turns and begins to lean over, the sway bar
uses the upward force on the outer wheel to lift on the inner wheel, thus
keeping the car more level.
Components of the Automobile Suspension System
Control Arm: A movable lever that fastens the steering knuckle to the frame of the vehicle.
Control Arm Busing: This is a sleeve which allows the control arm to move up and down on the frame.
Strut Rod: Prevents the control arm from swinging forward and backwards.
Ball Joints: A joint that allows the control arm and steering knuckle to move up and down and sideways as well
Shock absorbers or Struts: prevents the suspension from bounce after spring compression and extension
Stabilizer Bar: Limits body roll of the vehicle during cornering
Spring: Supports the weight of the vehicle
Control Arm: A movable lever that fastens the steering knuckle to the frame of the vehicle.
Control Arm Busing: This is a sleeve which allows the control arm to move up and down on the frame.
Strut Rod: Prevents the control arm from swinging forward and backwards.
Ball Joints: A joint that allows the control arm and steering knuckle to move up and down and sideways as well
Shock absorbers or Struts: prevents the suspension from bounce after spring compression and extension
Stabilizer Bar: Limits body roll of the vehicle during cornering
Spring: Supports the weight of the vehicle
Key Benefits
|
·
A regenerative damper with extremely
rapid and precise control of coupling between wheels and chassis
|
|
|
·
Almost instantaneous response to
external forces that attempt to modify vehicle ride height and attitude
|
|
|
·
A flexible system for control of ride
height and attitude in response to changing speed and road conditions
|
|
|
·
A multi-layer fail-safe system.
|
Common problems of the suspension system
Shocks and Struts: Shocks and Struts are located behind the wheels of a vehicle. Shocks and Struts are subject to wear and tear just like other vehicle parts. The signs of a shock wear out are if the car bounces excessively, leans hard in corners and jerks at brakes then the shocks and struts are definitely calling for a change.
Ball joints: The wearing out of ball joints can get dangerous because if they separate they cause you to lose control over the vehicle which could also be a life risk.
Preventive Measures
Ø
The shocks and struts should be check frequently for
leakages.
Ø
Ball joints should be checked immediately incase the motion
of the car is not right.
Ø
Make sure to lubricate the ball joints of your car
frequently.
