A REVIEW OF VEHICLE SUSPENSION SYSTEMS

A REVIEW OF VEHICLE SUSPENSION SYSTEMS FOR RIDE, HANDLING and DURABILITY

ABSTRACT

This project aims to study the Suspension system in a vehicle and its role in influencing the Vehicle Ride Quality, Vehicle Handling and Vehicle Durability. Suspension system in a vehicle includes springs, shock absorbers and linkages which connect a vehicle to its wheel assembly. It isolates the occupants from noise, bumps and other vibrations from the road. Suspension also helps to maintain traction between road and wheel, and to carry the vehicle load. It allows vehicle to corner with minimum roll over tendency. In suspension system springs are used to support load and absorb the road shocks, whereas the shock absorbers dampens the spring action. The suspension of a vehicle plays a very important role in determining the Vehicle ride, Vehicle handling and Vehicle durability. Roads are uneven and they produce pulse like disturbances which affects the vehicle ride. The suspension system absorbs these vibrations from the wheels and dampens its effect and provides a good ride quality. There are a number of suspension systems that are used in automobiles. But each of these suspension systems has their own characters under different working conditions. Every suspension system has its own unique features which make them best suited for some particular working conditions.

1. Types of springs

There are four major types of springs that are used in automobile suspension systems. These are Coils, torsion bars, leaf and air suspensions.

1.1 Coil Spring: The coil spring is made of steel rod, wound into coils. Diameter of the coil at the centre is greater than both the ends. When a coil spring is loaded it gets compressed and when the load is removed it returns to the normal shape due to the resistance of the steel rod.

1.2 Leaf Spring: Leaf springs are one of the most basic forms of suspension which are used in automobiles. The arrangement consists of several flexible steel plates, stacked together and held by clips. When the vehicle is loaded these metal leafs deflects to carry the load.

1.3 Torsion Bar: It consists of a Straight rod of spring steel, rigidly fastened at one end of vehicle body. Other end is attached to an upper or lower control arm. Control arm swings up and down in response to the wheel movement, the torsion bar twist to provide spring action.

1.4 Air Spring: An air spring has a rubber cylinder or an air bag filled of compressed air. A plastic piston moves up and down with lower control arm. This motion causes air to get compressed and provides a spring action. A valve on top regulates the air in cylinder when the load of vehicle varies. [1,2]

2. Types of Shock Absorbers

Shock absorbers are tubular hydraulic devices which are used in a suspension system to dampen the spring oscillations but it does not absorb any shock developed in the system. These devices are assigned near the wheel, spring assembly. One end of the shock absorber is fixed to any moving part of the suspension system such as the lower arm and other end is fixed to the vehicle body. Thus it provides a resistance to the moving or vibrating part in the system, thereby dampening the oscillation. The commonly used shock absorbers are spring assisted shock absorbers, Adjustable shock absorbers, Air shock absorbers and Gas filled shock absorbers.

2.1 Spring assisted shock absorbers: These shock absorbers have an assist spring in between the piston rod and the tube. They combine the spring action with shock absorber action. So these shock absorbers can be used to maintain vehicle height.

2.2 Adjustable shock absorbers: Adjustable shock absorbers are those which can be adjusted to suit a particular road condition. It can be adjusted electronically by a switch or mechanically be screws. This arrangement allows the driver to select between a hard and a soft suspension.

2.3 Air shock absorbers: This type of shock absorber uses compressed air to support the load. It has a rubber boot around the shock absorber to collect the compressed air. As the load on the shock absorber increases the more air is pumped into the system and when this load is removed the excess air is removed from the chamber using air valves.

2.4 Gas filled shock absorbers: In normal shock absorbers the compressed fluid has a layer of air on top of it. When the vehicle undergoes tremendous vibrations the air gets mixed with the fluid to form foam. This air foam resists the piston movement within shock absorber cylinder. In order to prevent this pressurized gases are used instead of air. As the gas is pressurized it reduces the possibility of producing foam. [1, 3]

3. Factors influencing Suspension System Efficiency

The efficiency of any suspension system depends on many other external factors like the vehicle weight distribution, slip angle, tyre used etc. The weight of unsprung mass also influences the suspension performance to a great extend. In order to obtain a high performance suspension system all these factors must be taken into account. To maintain a balance on all these factors designers will have to make compromises between the conflicting targets. [2]

3.1 Slip angle

Slip angle is the term used to denote the tyre slip when the vehicle makes turns or cornering. The slip occurs due to rubber elasticity and this helps to provide tyre a certain traction or grip that helps the vehicle to be steered. During high speed cornering and lane changes, the tyre may be in a slightly different direction from that of the wheel. The angle between tyre and wheel direction is called slip angle.

From the graph the slip angle for a racing tyre can be studied. The lateral force and the slip angle are found to climb up the plot. If an acceleration or additional slip is provided in the lower region, then the slip angle goes on increasing until it reaches an optimum slip angle. At this point the tyre fails to offer any more traction and starts to lose traction which may lead to skid or slippage. [3, 4, 5]

3.2 Camber Angle

Camber angle is the variation of wheels from its vertical position. When the vehicle takes sharp turns or corners the tyre contact patch resist the change in direction of the wheel and tries to retain the tyre in the same direction. This causes the tyre to tilt in the direction in which the vehicle turns. The angle between the wheel vertical and the tyre is called camber angle. There are two types of camber, positive and negative camber. When the tyre tilts towards the body at the contact patch it is said to have a positive camber. When the tyre tilts away from the body at the contact patch, then it's said to have a negative camber. In some independent suspensions the wheel is maintained at a certain camber angle in order to increase stability. But assigning camber angles results in tyre damage. [5, 6]

3.3 Roll over Threshold

A vehicle is said to have rolled over if it has under gone at least a 90 degree rotation about its own horizontal axis. If a vehicle travelling at high speeds makes a sharp corner or hits an obstacle, lateral forces gets generated on the vehicle. When the lateral forces acting on the vehicle creates a large enough roll moment about the centre of gravity of the vehicle for a sufficient time period, it loses the stability and rolls over. The resistance offered by a vehicle to roll over is measured by maximum lateral acceleration produced without causing a roll over. This maximum acceleration is called rollover threshold. [7,8]

3.4 Caster Angle

Caster angle is the angular deflection from the vertical axis of suspension measured in the longitudinal direction. It is the angle between the vertical and the king pin axis and it is measured in degrees. Caster angle is adjusted in some vehicles to serve certain purposes. In heavy vehicles like trucks and trailers caster angle is adjusted to increase the vehicle handling. It is adjusted in such a way that the left wheel opposes left turn and right opposes right turn. If the kingpin leans towards the rear the vehicle is said to have a positive camber. This increases vehicle stability and makes it easy to bring back the vehicle to a straight line after making a turn because the wheels spins back to straight line due to the vehicle load. A high degree of caster angle can result in a hard steering and low speed shimming. Very low caster angle can produce wheel instability. [9,10]

Factors influenced by Suspension system performance

Ride Quality: ride quality is defined as the function of communication or dynamic interaction between the driver and vehicle. The vehicle suspension and tyre characters influence the ride quality to a great extend. To obtain high riding comfort a soft suspension must be installed. A soft suspension doesn't produce any resistance against the wheel movement when the vehicle moves over uneven surfaces. But this suspension does not give the driver a good feed back of the road and may find it difficult to control the vehicle at high speed corners. Whereas a stiff suspension provides better handling and grip on the road. This in turn feels better at a safety point of view. All the suspension systems used in commercial vehicles try to incorporate both these characters by compromising a bit of both. But when it comes to special purpose vehicles the suspension get stiffer. For example a sports vehicle must have a stiff suspension in order to provide the driver with more feedback from the track. As the comfort of a sports vehicle is not the most important feature to be considered a stiff suspension can be installed to enhance the performance on the track. [9, 10]

Vehicle handling: Vehicle handling is the term used to refer the response of the vehicle to a high speed cornering or swerving. It is the communication between a driver and the steering system of a vehicle. There are many factors affecting the vehicle handling. They are weight distribution, suspension, tyres and wheels, unsprung weight etc. The vehicle handling performance can be analysed and optimized using Sequential Quadratic programming (SQP) method and Dynamic Q method. [10, 11]

Vehicle Durability: vehicle durability depends on the built quality of the vehicle. The suspension system can influence the built quality to a certain level. Most of the durability problems arise due to vibrations acting on the vehicle body. If the vehicle has a high performance suspension system, the vibrations acting on the body reduces and thereby reduces the wear and tear. Durability tests are done with and without the system components and the tests included destructive tests and other tests. The present day durability tests are done on prototypes and the results for each test are used to improve the design features. [12]

4. Suspension systems

The suspension systems discussed in this journal are Leaf spring rear suspension, Electronically controlled air suspension, Active suspension, Double Wishbone and Macpherson strut suspension.

4.1 Leaf spring rear suspension

Leaf suspensions are the most basic forms of suspension designs used in automobiles. This suspension was an independent type suspension but had a huge load carrying capacity. It consists of a single or more than one leaf kept on top of the other and held together by metal covers. U bolts are used to attach the leaf spring to the axle. When the vehicle load varies the leaves bend and slides over one another. During the slide and the rebound of the leaves they are held tight by the metal covers to keep the leaves in place. The longest leaf on top which is called the master leaf has got its two ends rolled to form spring eyes. The suspension is attached to the frame by the two spring eye using a spring hanger at the front and a spring shackle at rear.

Working: When vehicle gets loaded the leaves flexes and bends to absorb the load. The front spring eye is attached to the frame by spring hanger using bolt and rubber bushing. This spring eye moves front and back as the spring bends. As the spring shackle is a swinging part it produces a change in distance between the two ends of the leaf as it flexes. The bushings help to absorb the vibrations and thereby reduce the vibrations on the body.

Advantages and disadvantages of Leaf spring

The major advantage in using Leaf spring is that the amount of unsprung mass in the system can be controlled. It can carry more load than most of the conventional suspension systems and this load is placed at a lower height, which increases the stability. Leaf spring also provides good control over wheel base length, when the vehicle is accelerating and decelerating.

The disadvantage in using a Leaf spring is that it consumes more space and thereby a good packaging of the system is difficult. Modification cost involved is high and leaf springs are more vulnerable to damage. [1]

4.2 Electronically controlled Air suspension

This suspension system has air springs that replace coil springs at four wheels. It controls spring rates and provides automatic height and level control. In order to increase its ride and comfort, additional telescopic dampers can also be used. Rear air springs are mount ahead of rear axle on lower suspension arm. The front air springs are part of the front strut assembly. These air struts are mount between steering knuckle and vehicle body. Electronically controlled air suspensions have a driver height control which enables the driver to adjust the vehicle height. All these features enhance the ride quality, handling and control of the vehicle. This system also reduces vehicle wear and tear as it has very low degree of vibration.

Working: An electronically controlled air suspension works on an air compressor which compresses the air and circulates this air within the system. The moisture in the air s removed by an air dryer which is mounted on top of the air compressor. Air which has been compressed and dried is then supplied to each spring through air lines. A solenoid, operated by a control module, is present at each spring opens and closes to control the air pressure at each spring. The system consists of three height sensors. These sensors detect the variations in the vehicle height and adjust the springs by opening and closing the solenoid valve. When the vehicle gets loaded the sensor detects the variation and sends signals to the solenoid which in turn activates the air compressor and pumps in air to the springs. The system pumps in air when the pressure drops below 7.5 bar and turns off when the pressure starts to exceed 10 bar. Springs get pumped up by the air and raise the vehicle body to the optimum height called Trim height. When load is removed from the vehicle, body rises above the trim height. This variation triggers the sensors which in turn opens the solenoid valve and let's air out and maintains the trim height. Spring is provided in the system to ensure that the vehicle body maintains a low natural frequency thereby providing excellent ride comfort.

Advantages of Electronically controlled Suspension System

An electronically controlled air suspension consumes low amount air to operate. The solenoid valve has a greater nominal width and thereby allows quick lift and lowering. All these operations are controlled by a Remote control unit (RCU) which makes it easy to operate. It has high serviceability and concept for diagnosis. One of the major advantages is that the vehicle superstructure can be maintained parallel to the road even if the load distribution is uneven. [1, 13,]

www.meritorwabco.com , www.dunlopsystems.com)

4.3 Active suspension system

Active suspension system is one of the most technologically developed and efficient suspension systems that are used today. In normal suspensions the tyre of a vehicle will not stay in contact with the road and start vibrating when moving thru a hole or hitting a bump. As a result the suspension system in the vehicle extends or compresses to dampen the vibration. This affects the ride quality, handling and sends vibrations into the vehicle body. Apart from such suspension systems, an active suspension system uses hydraulic actuators instead of shock absorbers and springs. A hydraulic actuator can convert hydraulic energy from a pressurised fluid to mechanical motion. In this arrangement the tyres are pushed against the road surface using hydraulic pressure. The pressure exerted by the fluid varies on all four wheels. The system has an oil pump, pump accumulator, pressure control unit and an actuator. The two main active suspension products used today are Bose suspension and Magneto-Rheological Technology.

Working: Active suspension system works similar to an electronically controlled suspension system. When the hydraulic actuator pushes the wheel to the road, the road opposes this with an equal force. But this force exerted by the road varies when wheels moves thru uneven surfaces. These variations are detected by sensors on the actuator and signal the system about the variation. The system analyses the data and actuates servo valves which pumps excess hydraulic fluid into the actuators and keeps the body at trim height. An active suspension system has a number of sensors which could detect the variations in steering position, body stability, acceleration and deceleration. A gyroscope is embedded in the system to detect the Yaw variations. Data from all these sensors are processed and servo valves are operated to increase and decrease the body height to maintain the trim height.

A Bose suspension system uses electromagnetic motors instead of struts and shock assembly. Sensors fixed at various different parts of the vehicle analyses the body and suspension moment. The variations in these are signalled to the electromagnetic motor which complements to the variations. In a Magneto-Rheological system, the damper fluid contains suspended magnetic particles. When the damper bounces a magnetic field is created within and aligns the magnetic particle creating a resistance to movement. This results in dampening of vibrations.

Advantages and disadvantages of Active suspension system

This system operates instantly and thus maintains a high quality in vehicle ride, handling and stability. Active suspension system is very complex and comes only with a hefty price tag.

The disadvantage in using an Active suspension is that the service and manufacture is much more expensive than the conventional suspension systems.

Some of the vehicles using this suspension system are

Audi: A8, Q7, S8, TT

Bentley: Continental flying spur, GT

Jaguar: S-Type, XJ, XK

Land Rover: Range Rover, Range Rover Sport

Rolls Royce: Phantom [1,]

(www.mae.ncsu.edu.com, Automotive Mechanics by Crouse W H and Anglin D L 673 to 674, www.edmunds.com)

BOSE SUSPENSION MAGNETO-RHEOLOGICAL SYSTEM

4.4 Double wishbone suspension

A double wishbone suspension consists of a carrier, upper arm, lower arm, lateral link and a connecting part. It is an independent type suspension system which is also known as Upper and Lower A arm suspension or Short long arm suspension. Each of these arms has two mounting positions fixed to the chassis and one at the knuckle. The wheel is supported by the lower arm, connecting part and the lateral link together. A coil spring and shock absorber assembly is installed at the lower arm to support vertical vibrations. Double wishbone suspension is installed at a low height and takes up little room. The entire suspension system is very light and thus it helps in reducing the unsprung mass, thereby improves the road handling, directional stability and steering control. Double wishbone system is also used as rear suspension system. The pivot locations and control arm design helps in reducing the toe change during tyre vibrations. (Don Knowles - automotive suspension and steering system 3rd edition)

Working: A double wishbone suspension is designed in such a way that the upper arm is shorter than the lower arm. This arrangement helps the system to generate a negative camber when the suspension rises. When the vehicle makes sharp turns or cornering the suspension rises and the outer wheel experiences a positive camber. Both the positive and negative camber gets cancelled due to the suspension rise. As a result the vehicle remains steady on the road and thereby has a good ride quality. This feature helps the vehicle to maintain a precise wheel position under all riding conditions. The vertical motion is absorbed by the coil spring and shock absorber assembly.

Advantages and disadvantages of Double wishbone suspension system

The most important advantage in using this suspension system is that loads acting at various points in the arrangement can be analysed easily and optimised. This helps to analyse and solve various problems within the system and to design light weight components. Apart from any other suspension system, a Double wishbone offers less reaction to a body roll. It helps the vehicle to keep a constant camber angle even when the vertical wheel movement is high. Unlike most of the suspension systems, the double wishbone system provides very good ride quality and handling at corners as the system could generate negative camber to oppose the positive camber generated from the tyres due to corners. It maintains a good tyre and ground contact even when the vehicle moves through rough surfaces.

The disadvantage of this system is that it is more complex than other common suspension systems and requires much more time to service. It also consumes more space which is one of the major draw backs.

(wikipedia, diseno-art.com, free patents online.com, )

4.4 Macpherson-strut Suspension

Macpherson-strut suspension is an independent type system that can be used as a front and rear suspension system. It consumes less space and can be designed using a light weight material. The basic design constitutes of coil springs, shock absorbers and a single control arm at the bottom. It is very similar to the basic spring - shock absorber suspension, but has a strut assembly that is designed as a structural part of the suspension system. The vehicle body is loaded on to the top part of the strut assembly and the steering knuckle is attached using bolts to the lower part of the strut.

Working: Macpherson strut suspension system has a shock absorber and a coil spring assembly to absorb the vibrations generated from the tyres. The oscillations from the spring are controlled by the shock absorber. The shock absorber is installed inside a tubular structure, which is welded above the steering knuckle and has the coil spring on top of it. Top end of this system is fixed to the vehicle body. This entire assembly is supported by the Lower arm. One end of the Lower arm supports the steering knuckle (hub carrier) and the other fixed to the chassis. The disc brake calliper, steering arm and linkage are all installed in the steering knuckle. For a front wheel drive vehicle the drive shaft is connected through the steering knuckle to the wheel spindle.

Macpherson strut (Blundell M, Harty D Multi body systems approach to vehicle dynamics 2004)

Advantages of Macpherson strut Suspension system

Apart from all the commonly used suspension systems, the Macpherson strut design has a rather simple design with very few numbers of components. This makes the system more compact and thereby very well compatible for small cars. It is also easy to design and manufacture. A Macpherson strut suspension weighs very less than other suspension systems. It also effectively distributes the weight over larger area and thus provides better ride quality and stability.

Disadvantages

When lateral forces are generated during cornering and breaking, it induces a radial load on the piston. This reduces the suspension performance. Another disadvantage is that the entire load from the vehicle comes to the strut at the point where the strut is fixed with the body. So this part must be very strong to take all the load acting on it. In a Macpherson strut design, shock absorber is designed as a part of the strut. This makes the shock replacement and servicing rather more complex.

The Macpherson strut design has been modified to improve its performance. This was done by separating the spring from the strut assembly and keeping it in between the lower arm and the vehicle frame. By fixing the spring at the joint, minor vibrations from the wheels were absorbed by the chassis instead of getting spread to the steering. Thus it helped in improving the ride and the stability.

(Total automotive technology by Anthony E Schwaller. Chapter 47, 871 to 873, wikepedia.com, popular science feb 1981, Automotive chassis: brakes, suspension and steering by Tim Gilles. Chapter 14,327 to 329)

REFERENCE

[1] Crouse, W. H., and Anglin, D. L. Automotive Mechanics 10th Edition 2007, pg 658-674 (Tata McGraw-Hill Publishing Company Limited)

[2] Happian- Smith, J. Introduction to modern Vehicle Design 2001, pg 277-333 (Butterworth-Heinemann)

[3] Erjavec, J. Automotive Technology: a systems approach 2004, pg 1050 to 1061(Delmar Publishing)

[4] Bentley, R. Speed Secrets: Professional Racing Driving Techniques 1981, pg 36 (MBI Publications)

[5] Nunney, M. J. Light and Heavy Vehicle Technology 2007, pg 499 (Elsevier Ltd)

[6] Hiller, V. A. H. Fundamentals of Motor Vehicle Technology 4th Edition 1991, pg 318 (Nelson Thomes Ltd)

[7] Harwood, W. G. Highway/Heavy vehicle Interaction 2003, pg 18-19 (Transportation Research Board)

[8] Rating System for Rollover Resistance, The National Highway Traffic Safety Administration 2002, pg 22-23(National Academy of Sciences)

[9] Norman, A.; Scharf, R. and Corrinchock, J. A. Heavy Duty Trucks Systems 2001, pg 661 (Delmar Thomson)

[10]Kulakowski, B. T. Vehicle Road Interaction 1994, pg 150 (American Society for Testing and Materials)

[11]True, H. Dynamics of Vehicle on Road and on Track 2003, pg 24 (Swets and Zietlinger)

[12]Ribbens, J. Simultaneous Engineering for New Product Development 2000, pg 96-97 (John Wiley and Sons)

[13] Heisler, H. Advanced Vehicle Technology, volume 10, pg 447-449 (Butterworth-Heinemann)

(Dynamics of vehicles on road and on track, Hans True, 2003, Swets and Zietlinger, pg 24)

(Simultaneous engineering for new product development Jack Ribbens,2000,pg96)

(William H Crouse, Donald L Anglin 2007 Automotive mechanics, pg 672 to 673,, Advanced vehicle technology volume 10 Heinz Heisler pg 447)

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