Thursday 16 February 2012

AUTOMOBILE SUSPESION SYSYTEM







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
9. Control Arms
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)
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
Where
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.

The spring rate of a coil spring may be calculated by a simple algebraic equation or it may be measured in a spring testing machine. The spring constant k can be calculated as follows:


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


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.