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This paper presents an investigation and research in to various automotive suspension system configurations and their working, methodologies used in designing, analysing and testing them physically. This paper also includes the discussion on the typical values for the characteristics of suspension based on the geometry and compliance in relation to the interaction with the tyres and the dynamics of the complete vehicle.
This paper includes the calculations of suspension system at various conditions applying different loads. The physical laboratory tests such as K&C rig test and shaker rig test are done to find out the suspension characteristics.
This journal paper also includes the relationship between the tyres, suspension system and the vehicle dynamic performance.
Generally all the four stroke internal combustion engines produce high power and acceleration. But all the generated power will not be completely useful if the driver cannot control the automobile. This is where the role of suspension system comes in to play. The main purpose of the suspension system is to increase the frictional force between the road and the automobile tyre. There will be no need of suspension system if the roads are perfectly flat and with no irregularities. As the roads are not perfectly flat, the irregularities on the road surface will interact with the tyres of the automobile. Forces will be applied on the wheels due to these imperfections.
Fig.1 Layout of a Suspension System in a car
When the tyre of the automobile comes in contact with the bump on the road, it will cause the wheel to move up and down perpendicular to the surface of the road. The type of bump with which the tyre comes in contact will decide the magnitude of the force. Due to this force a vertical acceleration is gained by the tyres and is transferred from the wheels to the frame of the automobile in the same vertical direction.
Fig.2 Accelerations acting on a tyre due to a bump
In this case the contact between the tyre and the road can be completely lost as the wheel tends to move in the vertical direction due to the vertical acceleration. After that the wheel can slam back down in to the road due to the gravity effect. This causes more discomfort to the passengers. In order to avoid this problem the suspension system is introduced which will absorb the vertical forces on the wheel and allow the frame and body to ride undisturbed even during the ride on the bumpy roads. So, the suspension supports the vehicle and provides the cushioning of the ride while holding the tyre and wheel correctly positioned in relation to the road.
COMPONENTS OF A SUSPENSION SYSTEM:
The chassis is the part which comprises of all the parts beneath the cars body. The suspension system is also installed on the chassis only. The suspension system of a car consists of various components such as springs, dampers or shock absorbers, control arms, ball joints, steering knuckle and axle.
Springs are the components which will support the weight of the vehicle and absorbs the loads and the forces exerted by the road. It keeps the wheels of the vehicle bouncing up and down while passing a bump on the road. There are various types of springs used for various types of suspension systems depending on the functionality of the spring.
These are the most commonly used springs in a suspension system. These are made by wounding round a heavy duty torsional bar about an axis. The springs compress and expand to in order to absorb the road shocks.
Fig.3 Coil springs
Leaf springs are made by bounding together several layers of metals in form of leaves. These are initial used in horse drawn carriages and later used in many cars. But now these are generally used in heavy load vehicles and trucks.
Fig.4 Leaf springs
Torsion bar works on the twisting property of a steel bar. One end of the torsion bar is attached to the vehicle frame and the other end is attached to a wish bone. This makes the bar to act like a lever which moves perpendicularly. When the vehicle comes across a bump, the vertical action is transferred to the wish bone and then to the torsion bar due to the levering action. This makes the torsion bar to twist about its axis which in turn provides the spring force. These are mostly used in European cars.
Fig.5 Suspension system with Torsion bar
Air springs are the air filled cylindrical chambers which are placed in between the wheel and the body of the car. The wheel vibrations are absorbed by making use of the compressive qualities of air.
Fig.6 Air springs
Damper or Shock Absorber:
A car spring will extend and release the energy it absorbs from a bump at a uncontrolled rate if there is no dampening structure. Until all the energy put in to the spring is used up, it will continue to bounce at its natural frequency. So, the suspension system having only spring will make the ride bumpier which in turn makes the car unsteady.
Fig.7 Shock absorber
The shock absorber is the device which controls the unwanted spring motion through dampening process. The magnitude of vibratory motions is reduced by the Shock absorbers by converting the kinetic energy of suspension movement into heat energy which can be dissipated through hydraulic fluid. The shock absorber is an oil pump which is placed in between the wheels and the car frame. The spring is placed over the shock absorber as shown in the figure. When the car encounters a bump the spring will coil and uncoil and the energy of the spring is transferred to the shock absorber. The motion of the spring is controlled by the to and fro motion of the piston inside the shock absorber.
TYPES OF SUSPENSION SYSTEM:
The suspension system is mainly classified in to two types. They are
Dependent suspension system
Independent suspension system
Dependent suspension system:
In this suspension system the two wheels are interlinked by connecting to the axle. So, when any one of the wheel get over a bump, then the deflection on this wheel will be transmitted to the other wheel on the opposite side of the axle. This will affect the ride quality.
If one of the wheels gets struck, then the opposite wheel does not adjust to the terrain and sits flat on the surface of the road. This will result in the loss traction. The dependent suspension system is used in the rear of many cars and trucks and on the front of four wheel drive vehicles. The major disadvantage of the dependent suspension system is its susceptibility to tramp shimmy steering vibrations and the ability to refine the dynamic response of the vehicle is also limited for the dependent suspension system.
Fig.8 Dependent suspension system
Independent suspension system:
When the vehicle with rigid axle comes across a bump, the axle tilts making the entire vehicle to tilt on one side. This type of phenomenon is not desirable for a proper ride and can be overcome by using the independent suspension system.
In independent suspension system, each wheel can move up and down independently without affecting the wheel on the opposite of the axle. The wheels in this suspension system are connected to a swing axle by using universal joints. Each wheel is mounted on separate suspension system such that the vertical movement of one wheel does not affect the other wheel on the opposite side of the axle.
Fig.9 Independent suspension system
Most of the passenger cars are now using independent suspension system. The compact design of it provides optimum ride quality, vehicle handling and more room for the larger trunk.
Some of the common suspension systems which come under the dependent and the independent suspension systems are:
Short-Long Arm or double- wishbone system
Hotchkiss suspension system
McPherson-Strut Suspension System:
The McPherson strut suspension was invented in the 1940s by Earl S. McPherson of Ford. It has since become one of the dominating suspensions systems of the world because of its compactness and low cost. Unlike other suspension designs, in McPherson strut suspension, the telescopic shock absorber serves as a link to control the position of the wheel.
Therefore it saves the upper control arm. Besides, since the strut is vertically positioned, the whole suspension is very compact. To front-wheel drive cars, whose engine and transmission are all located inside the front compartment, they need front suspensions which engage very little width of the car. Undoubtedly, McPherson strut suspension is the most suitable one. Nevertheless, this simple design does not offer very good handling. Body roll and wheel's movement lead to variation in camber, although not as severe as swing axle suspension.
Fig.10 McPherson Strut Suspension System
Double-wishbone suspension system:
The double-wishbone suspension system is commonly used for the front axle. A coil spring which uses the upper and lower control arms of unequal length is called a short-arm/long-arm or double-wishbone system.
Fig.11 Double wishbone suspension system
The control arms pivot on the vehicle body or frame. The upper end of the coil spring rests in a pocket in the frame. The lower end rests on the lower control arm. As the wheel moves up and down, the control arms pivot and the spring shortens or lengthens. Thus the suspension system absorbs the road shocks and provides vehicle an undisturbed ride.
Hotchkiss suspension system:
The Hotchkiss is a type of dependent suspension system used in rear wheels of many light and heavy trucks. This system has a solid axle which is located by semi elliptical leaf spring. The spring is mounted longitudinally and is connected to the chassis at their end. This type of suspension is cheap and simple in construction.
Fig.12 Hotchkiss suspension system
A Hotchkiss suspension is a live-axle rear suspension in which leaf springs handle both the axle's springing and its location.
This system is mostly used by Ford F-series trucks. Twin I-beam suspension was introduced in 1965. This is a combination of trailing arm suspension and solid beam axle suspension. In this case the beam is split into two and mounted offset from the centre of the chassis, one section for each side of the suspension. The trailing arms are actually leading arms and the steering gear is mounted in front of the suspension setup. The ford claims this makes for a heavy duty independent front suspension setup capable of handling the loads associated with their trucks.
Fig.13 I-Beam Suspension system
In this system the lower control arm gives support from the longitudinal forces and the upper control arm gives support from lateral forces which occur from the braking and driving torques. A four-link suspension system provides an infinite amount of adjustments to compensate for changing weather and road conditions. Because of this reason they are preferred over other types of rear dependent suspension system.
Fig.14 Four-link suspension system
Trailing link suspension system:
In this type of suspension system a coils spring is attached to the trailing link which itself is attached to the shaft carrying the wheel hub. When the wheel moves up and down, it winds and unwinds the spring.
Fig.15 Trailing link suspension system
In this type of suspension the front and the rear suspension system are connected together in order to better level the car when driving. The front and the rear suspension units have the hydrolastic displacers, one per side. These are interconnected by a small bore pipe. Each displacer incorporates a rubber spring and damping of the system is achieved by rubber valves. So when a front wheel is deflected, fluid is displaced to the corresponding suspension unit. That pressurizes the interconnecting pipe which in turn stiffens the rear wheel damping and lowers it.
Hydragas is an evolution of hydrolastic and essentially the design and installation of the system is same. The difference is in the displacer unit itself.
Hydragas suspension was famously used in the 1986 Porsche 959 rally car that entered the Paris- Dakar rally and now we can find it in a MGF roadster.
Fig.16 (a) Hydrolastic suspension system Fig.16 (b) Hydrolastic suspension system
ANALYSIS OF SUSPENSION SYSTEM:
In a suspension system, the wheel is connected to the frame through various links. The vertical motion of the wheels relative to the body is controlled by the spring and the damper of the suspension system. The suspension system of a general road vehicle consists of rubber bushes which reduces the transmission of noise, vibration and harshness in to the passenger's compartment. The compliance in the links, bushes will result in the dependence of slip angle and the camber angle of the wheel on the forces acting. By analysing the suspensions through links, the operation and behaviour of the suspension system can be easily known. In order to analyse the suspension system the following should be considered
The geometry of the idealized suspension system is analysed by investigating the possible arrangements of the links and the wheel motion relative to the chassis which is implicated by the links.
The components of the suspension system which control the wheel motions are checked for any compliance. These components include springs and anti roll bars. The compliances of links and rubber bushes are also checked.
Friction introduced by dampers and the residual friction in the joints
Inertia of the components due to their motion relevant to the suspension action.
CALCULATIONS OF SUSPENSION CHARACTERISTICS
The characteristics of a suspension system is a complete system where it presents the analysis of chassis heave and roll, the roll centre and roll axis, load transfer and the distribution of the vertical forces, and the influence of pitching in response to acceleration and breaking. This can be done by looking at the geometry of steering systems, at the steering effects of wheel bump and chassis roll, and at the steering effects of wheel forces because of system compliance. These factors are of considerable importance in controlling the behaviour and the feel of the vehicle, and every one of them must be carefully controlled if the vehicle is to handle well. Therefore the calculations of suspension characteristics involves various parameters such as
Bump movement, wheel recession and half track change:
The bump is an upward displacement of a wheel relative to the car body, sometimes applied more broadly to mean up or down displacement. It is also known as compression or jounce. When the car is in combined breaking and cornering, the longitudinal and lateral load transfers result in a different combination of heave and bump on each wheel.
Bump movement (BM) is the independent variable and is taken as positive as the wheel moves in the upward z direction relative to the vehicle body. Similarly wheel recession (WR) and halftrack change (HTC) are taken as positive x and y directions respectively.
Fig.17 Calculation of Bump movement, wheel recession and halftrack change
Half track change is a measure of how much the contact patch moves in and out relative to the vehicle body as the vehicle rolls.
Camber and steer angle:
Camber angle is defined as the angle measured in the front elevation between the wheel plane and the vertical. Camber angle is measured in degrees and taken as positive if the top of the wheel leans towards relative to the vehicle body.
The steer or toe angle is defined as the angle measured in the top elevation between the longitudinal axis of the vehicle and the line of intersection of the wheel plane and road surface. Steer angle is taken here as positive if the front of the wheel toes towards the vehicle. Both camber and steer angle can be calculated using two markers located on the wheel spindle axis.
Fig.18 Calculation of camber angle and steer angle
Castor angle and suspension trail:
Castor angle is defined as the angle measured in the side elevation between the steering (kingpin) axis and the vertical. Castor angle is measured in degrees and taken as positive if the top of the steering axis leans towards the rear.
Suspension trail (TR) is the longitudinal distance in the x direction between the wheel base and the intersection between the steering axis and the ground. The suspension trail generates a measure of stability providing a moment arm for lateral tyre forces that will cause the road wheels to 'centre'. The suspension trail combines with tyre pneumatic trail and contributes to the steering 'feel'.
Fig.19 Calculation of castor angle and suspension trail
Steering axis inclination and ground level offset:
The steering axis inclination is defined as the angle measured in the front elevation between the steering (kingpin) axis and the vertical. The angle is measured in degrees and taken as positive if the top of the steering axis leans inwards.
Ground level offset (GO) is the lateral distance in the y direction between the wheel base and the intersection between the steering axis and the ground. The ground level offset is often referred to as the scrub radius as the amount of 'scrub' in the tyre as it steers will depend on the magnitude of the ground level offset. The calculation of the steering axis inclination and ground level offset is shown below
Fig.20 Calculation of steering axis inclination and ground level offset
The point where the lateral forces developed by the wheels are transmitted to the vehicle is known as roll centre. If the roll centre of the front and rear suspension is joined, Roll axis can be produced. It is the axis about which the vehicle rolls over cornering. The distance of the roll centre from ground is known as Roll centre height. It is important to note that the roll axis is present only when the vehicle is following a straight path. As the vehicle rolls, the geometry of the suspension changes causing the roll centre to move. Roll axis helps in determining the roll angle and load transfer on front and rear axle.
The roll centre is found by projecting a line between the wheel base and the instant centre. The point at which this line intersects the centre line of the vehicle is taken to be the roll centre. The roll centre for a double wishbone is shown in construction below
Fig.21 Positions of instant centre and roll centre for a double wishbone suspension
The Roll centre for double Wishbone suspension system is found by extending the upper and lower arms of suspension system. An imaginary line is drawn from the tyre contact patch to that point. The point where the contact patch line cuts the centre line of the vehicle is considered as roll centre and the height from the ground is known as roll centre height.
The roll centre for a McPherson strut type suspension is formed the same way as Double wishbone type suspension system. The point where the lower arm when extended meets the perpendicular line from the strut is the instant centre. The line from the tyre contact patch is joined to that instant centre. The point where this centre line cuts the centre line of the vehicle is called roll centre and the height from the ground is known as roll centre height and the construction is as follows
Fig.22 Positions of instant centre and roll centre for a McPherson strut suspension
PHYSICAL LABORATORY TEST'S FOR A SUSPENSION SYSTEM
Suspension components and tyres have a tremendous influence on ride and handling qualities of motorcars. Therefore the testing for these components is more important. In laboratory many theoretical and practical vehicle dynamics investigations takes place. These test rigs are equipped with highly sophisticated measurement data acquisition systems and they provide a new quality of testing.
Kinematics and Compliance Rig:
Kinematics and compliance properties of vehicle suspensions have a major impact on ride and handling properties. The K & C test rig is designed to investigate both full vehicles and stand-alone suspension systems.
Fig.23Vehicle on the K&C test rig
Design of the K & C Test Rig:
The rig mainly consists of four posts equipped with hydraulic cylinders that allow any desired vertical deflection of the cars wheels. Additionally, there are two more small cylinders in every post to simulate lateral and longitudinal force application. A car can either be tested with its wheels mounted or with special kinematic devices mounted that simulate the kinematics of a rolling wheel. These devices can be adjusted to match the tyre semi diameter and pneumatic trail value. With the devices being mounted, lateral and longitudinal force application is no longer limited to the maximum friction force in the tyre contact patch; this increases the rig's field of operation and it makes testing less complicated.
In both cases there are air cushions mounted between suspension and rig to reduce friction to a minimum value of about 20 N even at maximum wheel load. This allows the suspension to deflect in lateral and longitudinal direction as it does in reality if horizontal forces are applied.
The maximum force range in lateral and longitudinal direction is 10 KN. The rig is adjustable to any wheel base between 2000 mm and 3250 mm; the track width front and rear is independently adjustable between 1180 mm and 1650mm.
Fig.24 Twist beam Axle on the K&C test rig in operation
Test Rig Operation System:
The rig consists of 12 electro-hydraulic control loops that may be operated either in displacement control or in force control mode. The basic functions of the rig such as run-up, run-down or emergency routines are provided by an extremely reliable microcontroller unit. Every control loops are continuously monitored itself. If, for instance, one cylinder exceeds its previously adjusted force limit, all cylinders will be locked up automatically. Thereafter the rig will slowly run down to the neutral position. These safety routines ensure that the suspension will not be damaged by the rig due to overload. Additionally there are two operation modes available for the whole rig. Forces and displacements can either be adjusted manually or automatically. In the automatic operation mode a PC system generates the values of forces and displacements in the course of time. Consequently, full driving manoeuvres such as a steady-state circular run can be simulated on the rig. Apart from the standard investigations there are other investigations which are automatically performed such as
Seven post shaker rig test:
The other type of tests involved in the laboratory is the shaker rig test. These tests are done mainly for the racing cars because the track test are generally expensive and time consuming and in some cases it is impossible to implement. In those cases physical laboratory tests come into a major consideration.
The seven posts are hydraulic cylinders in which four of them have flat pans on which the tyres sit on and support the car. The other three are called the aero loaders and are attached to the sprung mass. The car is placed on the pans and the test is carried as shown in figure.
Fig.25 Racing car undergoing a shaker rig test
RELATION OF SUSPENSION SYSTEM WITH TYRES AND VEHICLE DYNAMICS:
When the tyre of a vehicle passes over the bump then there would be some changes in the geometry of the suspension which in turn affects the various parameters that are related to the suspension. This in turn affects the whole vehicle dynamics. The change in these parameters occurs during cornering of the vehicle and affects the ride quality and tyres as well. Thus the parameters involved in maintaining the relationship between the tyre, suspension system and the vehicle dynamics are given below. The various parameters involved are listed and described in brief above. The change in these parameters is shown below with respect to the bump movement. The various parameters involved are
Front wheel bump steer:
As the front wheels travel through jounce the toe angle becomes more negative and the wheels point away from the centre of the car.
Fig.26 Graph showing the affect of toe angle on front wheel bump steer
Similarly, as the wheels travel through bounce, the toe-angles become more positive and the wheels progressively point towards the centre of the car.
Rear wheel bump steer:
On the rear wheels the bump-steer characteristics are in the opposing direction to that seen at the front. The graph also shows how the compliance washer absorbs a significant amount of the bump-steer toe-angle changes - but it is broadly in the same direction as the standard suspension.
Fig.27 Graph showing the affect of toe angle on rear wheel bump steer
Bump-steer is the least desirable characteristic in suspension geometry as the wheel rides over a bump, the steered wheel is deflected outward - so that there is a small steering movement towards the side that wheel is mounted on the car. Then, as the suspension rebounds, the steering movement is directed in the opposite direction. Thus directional stability is affected.
Steering axis inclination:
Steering axis inclination defines the angle of inward lean upwards or towards the centre of the car of the steering. This angle promotes some steering self-centring and also modifies some of the camber change induced by increasing steering angle in association with caster.
It lessens the negative camber change on the outer wheel, whilst increasing the positive camber on the inner wheel. Wheel offset defines the relationship between the mating face of the hub and wheel and the wheel centre line.
Fig.28 Steering axis inclination offsets
If a wheel has zero offset, then the wheel and hub mating surfaces would be on the same line. A positive offset would see the mounting faces mate towards the outer edge of the wheel whilst a negative offset would be in the opposite direction.
Ideally, the king pin should intersect the ground at the same point as the wheel's centre line. When the King Pin intersect to the wheel's centre line towards the centre of the car when the wheel has too much positive offset, the heavier the steering becomes, the less predictable the steering responses and the less predictable the directional stability. The distance between the wheel centre and the King Pin ground intersect is also known as the scrub radius.
On rear wheel drive with positive scrub radius, the vehicle forward motion and the road resistance on the contact patch produces a turning moment which causes the wheel to toe out and negative scrub radius causes wheel to toe in. But in front wheel drive the above case gets totally opposite. The positive scrub radius causes wheel to toe in and the negative causes toe out.
Half track change:
It is a measure of how much the contact patch moves in and out relative to the vehicle body as the vehicle rolls. When the wheel moves over a bump the half track also changes.
Over bumps the half track angle is positive and negative over rebound.
Fig.29 Graph showing the affect of bump on half track change
The castor angle is the angle measured in degrees formed between the axis of the kingpin and the perpendicular to the ground looking at the vehicle from the side. The stability phenomenon is created on the basis of the distance between the point at which the kingpin axis extension falls in relation to the direction of travel and the point of contact between the tyre and the ground.
Fig.30 Caster angle of a tyre
In the case of positive caster angle where the kingpin extension falls ahead of the point of contact between the tyres and the ground the wheel is pulled, as it is the line of application of the force applied to the axis that passes in front of wheels midpoint without taking the direction of travel into account, and each attempt made by the wheel to deviate from straight line travel will be counteracted by the straightening couple generated by the force and by the rolling resistance of the wheel. With negative castor the wheel is pushed as it is the line of application of the force applied to the axis passes behind the midpoint of the wheel.
Fig.31 Graph showing the affect of bump on castor angle
When the vehicle wheel move over the bump and rebound, the castor angle value changes but remain positive throughout. Consequently the best stability condition for straight line travel is obtained with a positive castor angle. In this case the phenomenon of "wheel wobble" and the consequent effects on steering are avoided.
Camber angle is measured in degrees and taken as positive if the top of the wheel leans towards relative to the vehicle body. It is used in the design of steering and suspension.
Fig.32 Camber angle of a suspension system
Camber angle alters the handling qualities of a particular suspension design and in particular negative camber improves grip when cornering. This is because it places the tyre at a more optimal angle to the road transmitting the forces through the vertical plane of the tyre rather than through a shear force across it. Another reason for negative camber is that a rubber tyre tends to roll on itself while cornering. If the tyre had zero camber, the inside edge of the contact patch would begin to lift off of the ground, thereby reducing the area of the contact patch. By applying negative camber this effect is reduced, thereby maximizes the contact patch area.
Fig.33 Graph showing the affect of bump on camber angle
When the vehicle moves over a bump and rebound camber angle changes. To maintain a proper camber angle of the wheel with respect to the road, the camber angle of the wheel with respect to the vehicle goes negative over the bump and positive over rebound.
The vehicles are used for travelling from one place to another. While doing so safety, comfort and ride quality are also desired. The suspension system plays an important role in safety, comfort and the ride quality. The parameters and characteristics involved in the suspension have their effect on vehicle dynamics. Even the small variation in the characteristics of suspension will affect the ride.
The journal paper explained the different characteristics of the suspension which plays an important role in obtaining the ride comfort. Different types of suspension systems are explained. The physical testing done on the suspension systems are explained. The variation of various suspension characteristics in relation to bump and heave are discussed and are represented in graphs. This helps in finding the flaws in the suspensions which enables the designers to rectify the problems and helps them in developing new suspension systems for the modern cars.