Vehicle Suspension System For Ride Engineering Essay

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This paper deals with the different types of suspension systems, their working, design considerations, advantages and disadvantages, and role of suspension in contrast to the ride quality, handling and durability. Automobiles travel at high speeds; vibrations are transmitted to the passengers. During turning, vehicle is subjected with side forces. Main agenda behind the design of the suspension is to provide comfort to the occupants and better stability to the vehicle. There is a greater importance on durability testing of vehicle aggregates. Durability of suspension can be test virtually by software "ADAMS" so that designers can perform force, velocity and acceleration analysis of it without building a prototype and on road test which is a time and cost saving approach. Other ways of validation are rig testing, general highway durability, and an accelerated highway durability testing and testing on maneuvers which are explained.

Key words: Suspensions systems, vehicle ride and handling, vehicle durability.

1. Introduction

Suspension systems have been in the usage two centuries or more. The general layout of a suspension system consists of spring connected to the chassis, axle and road wheels. There has been a lot improvement seen in the suspension systems in terms of the usage of damper, linkages and spring type selected to suit the vehicle for different kinds of terrain so that it performs effectively under severe load conditions, cornering at speed, braking and accelerating with safety. Functions of suspension system are given below:

To isolate higher frequency vibrations produce due to tyre excitation.

To provide traction to the wheels in all driving conditions.

To resist rolling of the chassis.

To maintain wheelbase of a vehicle if leaf springs are used (Hotchkiss drive).

To support complete mass of the vehicle.

To provide comfortable ride and handling performance.

General layout of suspension system:

Suspension system consists of spring, damper and suspension linkage. Springs in suspension are used to absorb the vertical movement due to bump & shocks transmitted from the road surface. Different types of springs are metal springs, rubber springs, air springs, plastic springs, hydraulic springs. Spring deflects and oscillates when vehicle passes over the bump. Damper reduces spring oscillations and maintain road grip. During operation the kinetic energy dissipates between the sprung and unsprung masses. It is also known as shock absorber but both spring and damper absorb a shock. Use of suspension linkage is to form a connection between vehicle body and wheel. It also allows movement between them. Linkages control wheel forces and torques depending on the kinematics, geometry and force application of axle. As the name suggest, the function of anti- roll bar is to resist rolling. Anti-roll bar is mounted in transverse direction in such a way that their ends are connected to the locating linkages of wheels. Anti-roll bar provides a torsional resistance when it twists along its length due upward travel of one side of the wheel.

2. Types of suspension systems

Vehicle suspension systems are classified as,

2.1 Dependent suspension 2.3 Active Suspension

2.2 Independent suspension

2.1 Dependent suspension:

In case of a dependent suspension, both wheels are mounted on either side of the axle so that movement of one wheel is transmitted to the opposite wheel. These types of suspension are also known as solid axle suspensions. Solid axle suspensions are widely used in commercial vehicles. Hotchkiss drive is an example of dependent suspension in which semi elliptic leaf springs are attached to the body or chassis with the help of a bracket and shackle links and solid axle is mounted below the spring with metal plate and U bolts. Figure 5 shows typical arrangement of solid axle suspension. Most of the passenger car consists of four link suspension at rear as shown in figure 6. Longitudinal control of axle is provided by lower control arms while the upper arms absorb braking/driving torques and lateral forces. Use of Coil spring in a four link suspension gives better ride and vibration control. It's costlier than Hotchkiss type arrangement but it offers better Anti squat & anti dive performance, roll center location.

Fig. 5 Solid axle suspension Fig 6. Four link rear suspension

Advantages of dependent suspensions:

Constant ground clearance can be maintained in different loading conditions.

Less tyre wear because both the wheels are maintained perpendicular to road in case of body roll and track width remains constant with suspension movements. Tyre life is an important factor while consideration of an operating economics of commercial vehicles.

2.2 Independent Suspension:

Independent suspensions allow each wheel to move vertically independent of other wheel. Most of the passenger cars and light duty truck consist of independent suspensions. Advantages of independent suspensions are:

Provides more room for engine.

Softer springs can be used which provides ride comfort.

Each front wheel follows a prescribed path relative to the body structure which is difficult to achieve with solid axle beam suspension.

Provides high roll stiffness.

A kinematic toe-in change, tending towards understeering is possible.

Front Independent suspension systems can be divided into following types.

2.2. A Wishbone suspension:

A wishbone type suspension consists of two control arms, mounted laterally on either side of the vehicle. Control arms are unequal in length which holds the wheel. This helps to keep wheel track constant which in turn avoids wheel scrub. The inner ends of the control arms are pivoted from car structure and outer ends are ball jointed to the knuckle. Spring and damper are connected to the wishbone, used to control vertical movement. The arms are also called as A-arm because of its shape. This type of suspension is also known as short and long arm (SLA) suspension. Operation of wishbone suspension is shown in figure 8. Only affected wheel tilts when vehicle passes over an obstacle. Also both wheels tilt when body rolls. Most of the front engine, rear wheel drive vehicle includes this type of suspension as it provides a packaging space for engine.

Fig. 7 Wishbone suspension Fig. 8 Wishbone suspension in action

2.2. B McPherson strut type:

If the upper control arm in the wishbone system is replaced by damper and coil spring form as a strut a McPherson suspension is obtained. The lower end of the strut is connected to the wheel house panel. Forces from all direction are acting at a pivot point on a wheel house panel thus producing a bending stress in a piston rod. The upper end of the strut is connected to the vehicle body. The wheel is guided, during its vertical motion, by means of a lower arm and a sliding guide, integral to the shock absorber. Track control is obtained by a single transverse link also known as wishbone. Inner end of wishbone is mounted to the lower the body by rubber bushes and outer end is connected to the stub axle. McPherson is lighter in weight as compared with wishbone suspension. Also it maintains a camber in up and down motion. Its only disadvantage is high installation height which limits the designer's skill to lower hood height.

Fig 9. McPherson suspension used in Audi 6

2.4 Active Suspensions:

It is a type of controllable suspension in which spring and damper are replaced by actuator. As shown in figure 12, an actuator (pneumatic or hydraulic) is driven by a pump controlled by transducer through which signals are given to the actuator. Active suspension reduces sprung mass acceleration more efficiently because it is monitored and actuator produces a force to reduce it. The other components involved in this type of system are an electronic control system, sensors measuring different parameters when vehicle is in motion. It not only maintains desirable ride height by means of electronic control system but also control the vehicle tendency to bounce, pitch and roll thus achieving better ride quality and higher levels of grip and control. Figure 13 shows two vehicles of the same manufacturer. Vehicle on the top has passive suspension system and the vehicle on the bottom has active suspension system. Both the vehicles are driven at same speed over a bump. Effectiveness of active suspension system is clear from the figure.

Fig.10 Active suspension Fig. 11 Comparison between passive and

active suspension

3. Factors to be considered in the designing of a suspension:

3.1 Vertical Loading:

A road wheel is subjected to a vertical force when passes over a bump. The vibrations generated from the road surfaces will eventually get transferred to the occupants. It is important to reduce these vibrations for occupant comfort. The main criterion is to select spring stiffness and damping factor which will reduce vibration levels.

3.2 Side Thrust:

Centrifugal force during cornering, road camber, cross wind creates side thrust to the vehicle. The pan rod is designed considering these factors.

3.3 Rolling:

While taking a turn, a centrifugal force and side thrust can form couple causing turning a vehicle around longitudinal axis. If the vehicle has high centre of gravity then effect of rolling is severe. The roll control in cornering can be controlled by lowering vehicle centre of gravity and using anti roll bar.

3.4 Road holding:

The ability of a tyre to remain in contact with the road can be achieved by increasing the ratio of sprung to unsprung mass that is by decreasing unsprung weight. It is also important to reduce the dynamic variations in the wheel load caused from road roughness to control a vehicle during maneuvers. [1]

3.5 Brake Dive:

Brake dive is the amount by which the front end of the vehicle dips or tail raises on the application of brakes. Anti dive forces can be introduced by designing proper suspension linkage. [5]

3.6 Squat:

Squat is the amount by which the front of the vehicle raises during acceleration. As the load transfer occurs through the suspension, it is possible to reduce the effect of squat. [5]

4. Vehicle Ride

Vehicles are subjected to vibrations when travels at high speeds. These are transferred to the occupants by tactile, visual, or aural paths. The term ride is commonly concerned with tactile and visual vibrations while aural vibrations are categorized as "noise". The vibrations may be divided according to frequency and classified as ride (0-25Hz) and noise (25-20000 Hz). [1] A greater level of concentration can be maintained during driving if the disturbances are less. That results into safer travel. The requirements of a suspension design changes as per different vehicle categories like Passenger cars, Commercial vehicles including trailers. In case of passenger car importance is given to the occupants comfort, safety and control of the vehicle but in case of commercial vehicles importance is given to operating economics where primary function is to transport cargo from one place to other. Suspension design requirements are also changes according to the market conditions, segments and vehicle certification requirements. As an example where the roads are narrow, steep and curved, handling of the vehicle is more important in contrast where the roads are straighter and wide it is important for the vehicle to have good ride quality. As shown in figure 12, the sources from which vehicle is subjected to ride vibrations are road roughness and on-board sources. On board sources are due to rotating components like tyre wheel assembly, driveline and the engine.

Road roughness:

Road roughness includes potholes resulting from the localized road imperfections to random deviations in road surface. It can be described as an elevation profile along the wheel tracks over which the vehicle passes. It can be used as either profile itself or its statistical values; it can represent by Power spectral density (PSD). General amplitude level of the graph is indicative of roughness level, higher amplitudes implies rougher roads. Acceleration is the measure for understanding vehicle ride. Figure 13 shows a plot of power spectral densities versus wave number for bituminous and Portland cement concrete roads. Road inputs to a vehicle are modeled with amplitude that diminishes with frequency to the second and fourth power approximating the two linear segments of the curve. Roughness can be considered as acceleration input to wheels. Considering this road roughness has largest input to the vehicle at high frequency & so greatest source of ride vibrations. As road roughness is considered as vertical input to the vehicle it would have tendency to have bounce & roll motions. [1, 2]

Fig. 12 Vehicle Ride System Fig. 13 Spectral densities for different roads

On board excitation sources:

Tire & wheel assembly is supposed to absorb the road bumps & isolate the vibrations. But due to manufacturing limitations, parts like brake, tyres, wheels, hub may result in nonuniformities like mass imbalance, dimensional and stiffness variations. It causes the force variations in vertical, longitudinal &lateral direction. Driveline excitation is one of the major sources of vibration excitation in vehicle. It arises from the rotating components in the driveline assembly. In the whole assembly propeller shaft has got highest potential of generating vibrations, its due to mass imbalance of the drive shaft & due to secondary couple formed due to angle formed by universal joints.[1] Propeller shafts are subjected to whirling and it increases as the length of the propeller shaft increases. Most of the long wheelbase vehicles consist of centre bearing support with two piece propeller shafts to reduce excitations. The third major source of excitation arises from engine. Due to Compliance of engine mounts, Engine unit can vibrate in all six degrees of freedom (Three translational directions and three rotations around translational axis). Proper design of driveline can be done to absorb vertical vibrations arising from the wheels, for which mounts should be designed near the wheel hop frequency (12- 15 Hz). [1]

Suspension Isolation:

Dynamic behavior of a vehicle can be understood properly by considering the effect of road excitations on the sprung mass. Transmissibility of the system is needed to be considered to understand the isolation characteristics of the suspension. Transmissibility is the ratio of amplitude of body vibration to the amplitude of excitation.

Fig 14. Quarter Car Model Fig 15. Transmissibility-Frequency Ratio Response

Fig 14 shows quarter car model in which tyre is represented as a single spring. The sprung mass resting on the suspension and tyre springs is capable of motion in vertical direction. The effective stiffness of the suspension and tyre springs in series is called the ride rate and determined by equation 1.

RR ……… (1) ……….. (2)

……….. (3)

When damping is present, as it is in the suspension, the resonance occurs at the "damped natural frequency" and is calculated by equation 2. For good ride suspension damping ratio is kept in a range of 0.2 to 0.4 and is calculated by equation 3. The effect of v/ω on transmissibility is shown in fig 15. For good isolation, with damping factor of 0.2 to 0.4 so that transmissibility remains below 1.

It is important to isolate vibration sources because the vibration of the structure can be easily transmitted to the parts and serious noise problems can occur. Low stiffness isolators are desirable to give low natural frequency but stability problem may arise because of isolators which are too soft. Air springs can be used for very low frequency suspensions: resonance frequencies as low as 1 Hz can be achieved whereas metal springs can only be used for resonance frequencies greater than about 1.3 Hz. Metal springs can transmit high frequencies, so rubber or felt pads are often used to avoid metal-to-metal contact between the spring and the structure.

Fig 16. On road acceleration spectra with different Fig 17. Effect of damping on suspension isolation

sprung mass natural frequencies behavior

As shown in figure above lowest acceleration occurs at frequency of 1Hz.At higher values of natural frequency(stiffer springs) acceleration peak in the 1 to 5 Hz increase reflecting a greater transmission of road acceleration inputs & mean square acceleration increases by several hundred percent. In addition, stiffer springs elevate the natural frequency of wheel hop mode near 10 Hz which allows more acceleration transmission in high frequency range. So it's evident from above that keeping suspension soft is recommended for better ride. Normally natural frequency for most cars is between 1 to 1.5 Hz ranges. In case of performance cars where handling is important than ride normally stiffer springs are used which gives the natural frequencies of 2 or 2.5 Hz. Figure below shows nominal effect of damping is illustrated for the quarter car model. For very light damping, say 10% response is dominated by a very high response at the 1 Hz. This type of response causes the sprung mass to amplify long undulations in the roadway which is undesirable. 40% damping curve is close representation of most of cars, which is evident from the graph that amplification of resonant frequency is in range of 1.5 to 2 Hz. If damping is pushed beyond the critical limits say 200%, damper becomes so stiff that the suspension no longer moves & vehicle bounces on its tires resonating in range of 3-4Hz.

5. Tuning of Suspension for Better Ride:

5.1 Wheel Rate:

The wheel rate is related with the force displacement relationship between vehicle body, or sprung mass and ground. The stiffness of the spring can be set so as to be softer during initial bump and stiffer during increased bump travel for better ride comfort and roll control.

5.2 Anti Dive and Anti Squat:

During deceleration the load on the front axle increases due to longitudinal load transfer. As the load transfer is through the suspension system, it needs to be designed to counteract weight transfer and minimise dive and squat.

The figure represents the free body diagram of a vehicle during braking. The dynamic load distribution on front and rear axle under braking condition is given by,

…….. (4) ………. (5)

Under static load condition, spring load is given by ……….. (6)

After taking moment about and substituting values of, and following relationship is obtained==If lies anywhere on the locus of points defined by these ratios then 100% anti-dive is obtained on front suspension but in practice 100% anti-dive is not used because it can cause harshness of the front suspension on rougher roads. Brake hop may be produced if the trailing arm is too short.

5.3 Unsprung mass reduction:

Lighter unsprung mass will produce better ride performance by pushing wheel hop resonant frequency higher.

5.4 Front and rear natural frequencies:

The rear suspensions should have higher spring rate than front suspension so that corresponding periodic times and are such that. When the vehicle passes over a bump, input excitations from road affect the front wheels first. The time lag between the front and rear wheel road inputs is the ratio of wheelbase over vehicle speed. When the rear wheels have passed over the bump, vehicle goes under pitching mode, indicated by point A and B in the figure. Hence the condition is not desirable. With higher rear frequencies, both ends of the car are moving in phase tend to induce bounce and the body moves up and down until the motion is fully damped.

6. Vehicle Handling:

Handling is the behavior of a vehicle, the way it performs during cornering and swivering. It also implies the easiness to control vehicle at longitudinal and transverse accelerations. The common handling problems are understeer, oversteer, bump steer and body roll. At low speed turning, tyre does not produce any lateral forces as they run with zero slip angle. Slip angle is the angle between the direction of wheel towards which it is heading and the actual direction of travel when viewed from top. But at high speeds, under the presence of lateral accelerations tyres develop lateral forces to counteract these accelerations, and slip angles will be present at each wheel. At higher speeds because of the presence of slip angles, centre of turn will move forward. In case of understeer condition the lateral acceleration at the CG causes the front wheels to slip sideways to a greater extent than rear wheels. Therefore on a constant radius of turn when the slip angles of the front wheel are greater than those on rear wheels, more steering force is needed to keep the vehicle on a right path.

In case of oversteer condition the lateral acceleration at CG causes rear wheels to slip greater extent than front wheels. Thus on a constant radius of turn when the slip angles of the rear wheels are greater than those of front wheel, the radius of turn decreased. It means that vehicle will turn more sharply than it should be for the given rotation of steering. When both the front and rear wheels have same slip angles then vehicle is said to be neutral steer condition. Thus the lateral acceleration at the CG causes the identical increase in the slip angle of front and rear wheels. The equation of steering angle is given by

………. (7) ……….. (8)

It is always desirable to make understeer vehicle because in case of oversteer the slip angles at rear are greater as compared with front the term in the equation 8, and hence the stability factor is less than zero which means negative stability factor is undesirable but in understeer the stability factor is greater than zero. Hence most of the average drivers find car more stable if they slightly understeer. There are different ways by which vehicles can be designed to have understeer effect. One way is to increase front roll stiffness by using stabilizer bar at the front or to increase front roll centre height relative to rear by making modifications in the suspension linkages and the other is to move centre of mass forward. Roll steer is another handling problem. The suspension geometry must be such that wheel steers when a vehicle rolls in turning. Roll steer is the steering action of a front wheels with respect to body mass when body rolls. Roll steer coefficient must be always positive on front axle so during right hand roll it causes front wheels to steer to the right. Right hand roll occurs when vehicle is turning to the left and positive roll steer on front axle steers out of the turn and the vehicle goes under understeer mode.

The other factors that affect handling properties are camber, caster angle and aligning torque. The camber is the angle between the vertical plane of the road and wheel centre plane when viewed from front. Camber on the wheel produc camber thrust. Though it produces much less lateral force than slip angle but adds to the cornering force from slip angle thus affecting understeer. While designing of suspension and camber, effect of body roll must be considered. As an example in case of independent suspension where wheels incline with the body when the vehicle is turning. Hence to provide better roadholding and to improve handling a suspension geomety to be designed in such a way that wheels in a bump travel should go into negative camber and into the positive camber during rebound stroke.Figure illustrates this explation. Caster is the angle between the king pin axis and the wheel centre axis when viewed fron side. The distance between a point where king pin axis intersects with ground and wheel center is called pneumatic trail. The lateral force developed by tyre produces a self aligning torque because of trail and adds to the self centering which provides stability to the vehicle.

7. Vehicle Durability:

Durability of a component is the ability to perform its function repetitively. Vehicle operates on different road conditions and terrain, subjected to road loads, excitation frequencies, aerodynamic forces etc. Vehicle has to be design considering these factors otherwise it may fail while performing. It is important to test the vehicle before handing over to the customer otherwise customer may complain in case of field failure and it can affect brand value of vehicle. Vehicle durability test gives an idea about the functioning of a part as per the design requirement and help to produce defect free product before handing over to the customer. Durability of vehicle can be tested by using computer simulations, on road testing and laboratory testing. Computer simulation consists of different CAD and FEA software packages to simulate mathematical models. Vehicle is tested on track and roads in case of on road testing. Laboratory testing uses hydraulic test rig through which wheel forces and moments measured during turning are reproduced. Detailed explanation of each testing is given below.

Computer Simulation:

Design of suspension components begins like any other parts with some realistic assumptions of most severe conditions of service. These systems then go under performance & durability tests to validate the performance of system. A performance criterion for components is often based experience & actual collected road data. Road conditions on various roads are reproduced in computer data which then used to reproduce for lab test cycles & life of the components is validated by stress levels in components normally done by computer aided analysis software's like ABAQUS®. FEA is normally coupled with Multibody system (MBS) analysis of whole system is done to analyse the dynamic behaviour of suspension system under different loading conditions. Software's like ADAMS® & SIMPACK® are used for MBS simulations, this allows designers to optimise the suspension system for different working conditions & also predetermine the vehicle performance. Computer analysis gives faster & reliable results, also reduces costly prototyping.

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