This paper covers the basic concepts of suspension system and its frame design. The suspension section addresses the basic design parameters of the present specific suspension systems like Hydro-electric, Electro - magnetic suspension systems. The design section gives a brief overview of the design and methodology used in these suspensions. In this case study, demonstrated an understanding significance of the Suspension system types, design requirements and its materials. The typical values for suspension characteristics based on geometry and compliance to be discussed in relation to the interaction with the tyres and the dynamics of the complete vehicle.
In today's world, now understood by everyone the importance of Automobile. An automobile plays a vital role in our daily life. One of the first things we are bound to notice is how the vehicle differs from other vehicle in this competitive automobile industry. If the vehicle rides smoothly that can credit a good suspension system. Next to the engine, the most important part of a car is the suspension system. The suspension system functions to limit the impact of a particular road condition to the passengers. The ease by which we drive the car, the rapid acceleration to sudden stops and tight cornering are all handled by a very efficient suspension system. (Ref.1.R. Kayne)
A Suspension system consists of springs, shock absorbers and linkages that connect a vehicle to its wheels. Suspension systems serve the two dual purposes. It helps in keeping vehicle occupants comfortable and reasonably well associated from road noise, bumps and vibrations. The design of front and rear suspensions of an automotive may be different for a vehicle. A typical Front wheel suspension system of the car shown in Fig.1
Fig. 1 front wheel suspension system of the car
The suspension of a car is actually part of the chassis, which comprises all of the important systems located beneath the car's body.
These systems include:
The frame - structural, load-carrying component that supports the car's engine and body, which are in turn supported by the suspension
The suspension system - setup that supports weight, absorbs and dampens shock and helps maintain tire contact
The steering system - mechanism that enables the driver to guide and direct the vehicle
The tires and wheels - components that make vehicle motion possible by way of grip and/or friction with the road
So the suspension is just one of the major systems in any vehicle. The three fundamental components of any suspension: springs, dampers and anti-sway bars. Shown in fig2
Fig.2 anti-sway bars and springs
SUSPENSION SYSTEM FUNCTION
Suspension system functions are to provide vertical compliance so the wheels can follow an uneven road/terrain, thus isolating the chassis from induced forces and vibration. Suspension maintains the wheels in the proper steer and camber attitudes to the road surface. React to the control forces produced by the tires-longitudinal (acceleration and braking) forces, lateral (cornering) forces, and braking and driving torques. Resist roll of the chassis. Keep the tires in contact with the road with minimal load variations. Achieving these functional objectives may be attempted through geometric suspension Design and/or through active suspension methods. (Ref.6.R.G. Longoria)
TYPES OF SUSPENSIONS
There are various types of suspension systems used in automotive industry. Generally all the suspension system fall in to either of two groups dependent and independent suspensions. Both groups are functionally different and has classification based on front and rear suspension. The design of suspension various according to the nature of vehicle drive (front and rear).
Types: Conventional, Independent, Air suspension, Hydro elastic, Electromagnetic suspension.
Here suspension parts are discussed and noted as springs and dampers parts plays a vital role in functioning, so given detailed description follows
SPRING AND DAMPER THEORY
The suspension of a vehicle can be considered as a basic two mass, two spring systems the primary mass is the body shell with all its mechanical components. It is supported by the primary spring's medium which today is most often a set of helical, constant rate coil spring. The secondary mass is all made of individual masses of the unsprung components of the suspension, drive, breaking and steering systems. They are isolated from the road surface by the tyres which act as secondary spring.
Spring always considered with energy so given some typical values of parts
Typical energy capacities (Nm/kg) of spring parts
Leaf spring 50, Torsion bar 200, Coil spring 200, Rubber cone 375, Hydrolastic rubber 750,
Gas spring 1500(Bibliography Ref.1 Geoffrey P.Howard)
In practice damping exists in three forms: friction viscous and that due to the pressure of air. Ignoring the generally slight difference between static and dynamic friction. Friction in a suspension is harmful because until a disturbing forces exceeds the static friction of the damper and no suspension movement occur and the forces is passed on to the sprung part of the vehicle. This force is a result of an obstruction which deflects the tyre and for a given bump the tyre rate determines the magnitude of the force.(Bib 1.page 55.donold bestow&GH)
Apart from hydro-pneumatic and hydro elastic suspension, in which diaphragms move the fluid, hydraulic dampers depend upon piston working in cylinders for developing the pressures necessary to provide the damping forces.
Research and Analysis on ACTIVE SUSPENSIONS like Hydro and electromagnetic suspension system
ANALYSIS ON SUSPENSION SYSTEM
In my analysis, various suspension systems of the automotive have different features and purposes. So i have researched two major suspensions are HYDRAULIC and ELECTRO-MAGNETIC .my analysis follows brief description of both suspensions and difference between them. Electro-magnetic suspension is new one and it has unique features and hydraulic suspensions using various vehicles in cars are 2002 Nissan altima, 2006 Bmw Z4Si(Ref.7.web)
HYDRAULIC SUSPENSION SYSTEM
Hydraulic Suspension or "Hydraulics" for short is another form of suspension setup for modern day road vehicles. This suspension uses four independent dampers filled with Hydraulic fluid which are controlled by a main control unit, normally found inside the vehicle.(Shown Fig 3). The four dampers in the Hydraulic System are individually controlled by a main control unit which allows each damper to be pressurized and de-pressurised to allow a sudden up and down movement at the flick of a switch.
Advantage of this suspension system is that when back left strut is fully down the back right could be fully up and the front two half up or half down. And
â€¢ Very high force density,
â€¢ ease of control,
â€¢ ease of design,
â€¢ Commercial availability of the various parts,
Fig 3. Hydraulic suspension system
The main disadvantages of the hydraulic system are:
â€¢ Considered inefficient due to the required continuously pressurized system,
â€¢ Relatively high system time constant (pressure loss and flexible hoses),
â€¢ Environmental pollution due to hose leaks and ruptures, where hydraulic fluids are toxic,
â€¢ Mass and intractable space requirements of the total system including supply system albeit
That it mainly contributes to the sprung mass.
Due to the high force density, ease of design, maturity of technology and commercial availability of the various parts in suspension, hydraulic systems are used in body control systems.
An electromagnetic suspension system could encounter the disadvantages of a hydraulic system due to the relatively high bandwidth (tens of Hz), no need for continuous power, ease of control and absence of fluids. Linear motion can be achieved by an electric rotary motor
with a ball-screw or other transducer to transform the rotary motion to linear translation. However, the mechanism required to make this conversion introduces significant complications to the system. These complications include backlash and increased mass of the
moving part due to connecting transducers or gears that convert rotary motion to linear motion (enabling active suspension). More important that they also introduce an infinite inertia and therefore, a series suspension, e.g. where the electro-magnetic actuation is represented by a rotary motor connected to a ball-screw bearing is preferable. These direct-drive electro-magnetic systems are more suited to a parallel suspension, where the inertia of the actuator is minimized. FIGURE.4
(ELECTROMAGNETIC SUSPENSION SYSTEM)
Compared to hydraulic actuators, the main advantages of electro-magnetic actuators are:
â€¢ Improved dynamic behaviour,
â€¢ Stability improvement,
â€¢ Accurate force control and
â€¢ Dual operation of the actuator.
And the disadvantage is:
â€¢ Increased volume of the suspension, since the force density of the active part of hydraulics is higher than for electro-magnetic actuation, i.e. system mass and volume could be less.
(Journal 2. A S S)
Toe setting: This is an angle measures between longitudinal axis and car. It can be measured at certain level where both left and right wheel rims at centreline level blow shown figure of model. Figure.5
(TOE-IN AND TOE OUT)
Figure.5 Ref .http://www.rockcrawler.com/techreports/glossary/index.asp
Caster angle and mechanical trail: A steered wheel is arranged to trail by a small angle and forward movement gives a stabilising effect. Figure.6
Figure.6 Ref .http://www.rockcrawler.com/techreports/glossary/index.asp
Camber angle: this is an angle between vertical and a plane of wheel. The sign positive when top of wheel learns out. Opposite for negative sign that is top leans in Figure.7
Wheel offset: the lateral distance between the points where the swivel axis of a steered wheel meets the ground and centre of the tyre footprint is known as the offset. The axis passes outside it is negative. If it coincides the systems is termed centre point. Figure.8
(Figure .8 Ref.http://www.bargaincarrims.com/wheel-offset.php)
Ackerman effect: this is the degree by which the inner wheel in a turn is arranged to cover a tighter radius than the outer wheel. Figure .9
(Figure.8 Ref. http://www.smithees-racetech.com.au/ackerman.html)
Roll centres and roll axis: under lateral cornering forces, the body will roll on its springs about centres at both ends of car. The line joining these centres is the roll axis and its inclination to the horizontal plane plays an important part in the handling characteristics.
Anti-dive and anti-squat: to resist front end dip under heavy braking, or rear end squat under acceleration, suspension pivots are usually angled to provide upward reaction automatically in response to high wheel torque inputs. Figure .10
(ANTI SQUAT OF REAR SUSPENSION)
(Figure.10 Ref. http://www.moddedmustangs.com/forums/autocross/184197-torque-arms-amazing.html)
Torque steer: with powerful front drive cars, there can be an interaction of the drive system on the suspension geometry which causes direction influences on the steering.
Ct=Cr-Cl=T(tan Î±/2-tan Î²/2)
Roll steer: there are several ways in which the effects of roll can cause either front or rear wheel to steer the car slightly, and this characteristic may be used for fine tuning of a car's handling.(Bib.2 page 16)
This is a distance between the front and rear wheel centrelines which is illustrated in below Figure. It is very important for considering resists the overturning moment due to the inertia force at the centre of gravity (CG) and the lateral force at the tires. When designing, track width is important since it is one component that affects the amount of lateral weight transfer. Understanding kinematic analysis of the suspension geometry is important.In case of selecting the track width, the front and rear track widths do not necessarily have to be the same. This design concept is used to increase rear traction during corner exit by reducing the amount of body roll resisted by the rear tires relative to the front tires. Based on the corner speeds and horsepower-to-weight ratio of cars, this concept should be considered by the designer. Figure .11
The wheelbase also needs to be determined. Wheelbase is defined as the distance between the front and rear axle centrelines. It also influences weight transfer, but in the longitudinal direction. Except for anti-dive and anti-squat characteristics, the wheelbase relative to the CG location does not have a large effect on the kinematics of the suspension system. However, the wheelbase should be determined early in the design process since the wheelbase has a large influence on the packaging of components. For track width and wheelbase starting points, the designers should research the dimensions of the opposition.
TIRE AND WHEEL
After track width and wheelbase considerations have been addressed, the next step in the design process is tire and wheel selection. Since the tire is important to the handling of the vehicle, the design team should thoroughly investigate the tire sizes and compounds available. The tire size is important at this stage of the design since the height of the tire must be known before the suspension geometry can be determined. For example, the tire height for a given wheel diameter determines how close the lower ball joint can be to the ground if packaged inside the wheel. Tire Size - The designers should be aware that the number of tire sizes offered for a given wheel diameter is limited. Therefore, considering the importance of the tire to handling, the tire selection process should be methodical. Since the amount of tire on the ground has a large influence on grip, it is sometimes desirable to use wide tires for increased traction.
The designer can now set some desired parameters for the suspension system. These usually include camber gain, roll centre placement, and scrub radius. The choice of these parameters should be based on how the vehicle is expected to perform. By visualizing the attitude of the car in a corner, the suspension can be designed to keep as much tire on the ground as possible. For example, the body roll and suspension travel on the skid pad determines, to a certain extent, how much camber gain is required for optimum cornering. The amount of chassis roll can be determined from roll stiffness while the amount of suspension travel is a function of weight transfer and wheel rates. Once a decision has been made about these basic parameters, the suspension must be modelled to obtain the desired effects. Before the modelling can begin, the ball joint locations, inner control arm pivot points, and track width must be known. The easiest way to model the geometry is with a kinematics computer program since the point locations can be easily modified for immediate inspection of their influence on the geometry. Should a dedicated kinematics computer program not be available, then a CAD program can be used simply by redrawing the suspension as the points are
The steering system transfers the sreering action from the gear box to the steering arms of the wheels which provides overall directional control of the vehicle. The steering action is achieved by translation of displacement of the relay linkage in presence of arbitrary suspension motions. There is obvious potential for steering actions arise from suspension motions, which are known as steering geomerty errors. Figure .12
È¢ is the equivalent steer angle
l is the wheel base
R is the radius of rotation
The steer angle is given by
cot È¢ = cot È¢o + cot È¢/2
Radius of rotation is given by
BUMP (ROLL) STEER
As the suspension moves between bump/rebound, small amount of steer change may be introduced due to suspension geometry. When the car is streed left it is understeered and usually it ill be scaring for normal people when they drive but young people like top drive like that.
It is desirable to add an understeer characterstics as follows.
+ve toe -ve toe
Inside renound outside bump
-ve toe +ve toe
Dotted lines are roll steer exaggerated. The above schematic representation is showing that under steer charcterstics which are desirable to have in vehicle. And the suspension curves are as follows.
Steer or Toe - - - - Front
Rebound 0 Bump
There are various factors affecting the vehicle handling. The suspension has major effects on cornering this happens due to the various steering positions like neutral steer, under steer and over steer. Vehicle handling is used to refer the response of the vehicle to a high speed cornering or swerving. Apart from that it is also influenced by weight distribution, suspension, tires and wheels, unsprung weight etc. The vehicle handling performance can be analysed and optimized using Sequential Quadratic programming (SQP) method and Dynamic Q method.
Neutral steer is a constant-radius turn where no change in steer angle will be there. Physically the neutral steer case corresponds to the balance on the vehicle such that the force of the lateral acceleration at CG causes an identical increase in slip angle at both front and rear suspension.
Under steer on a constant radius turn, the steer angle will have to increase with the speed proportional to the lateral acceleration. Thus it increases linearly with the lateral acceleration and with the square of speed. It is necessary to maintain the radius of turn, so that front wheel must be steered to great angle. (Bib.2)
Over steer on a constant turn, the steer angle has to decrease with the increase in speed. In this case the lateral acceleration will increase the slip angle on the rear wheel more than the front. The rear wheels drifts more in this case unless the steer angle is reduced to maintain the turn.
VEHICLE DURABILITY TEST
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. Figure.13
(Vehicle Durability Test Rig)
VIRTUAL TEST RIG
This is a modern physical test method which provides an optimized route through the design and development process. Virtual test rigs are proving ground techniques used to obtain load case information and the utilization of fatigue analysis method to predict component lives. Though these techniques were developed initially for durability purpose, their benefits are boundless. The virtual test rigs are used practically to play through high frequency data and to assess noise and ride characteristics.
It also helps in doing vehicle dynamic simulations like steering kick back, as tire modelling becomes more accurate, handling operation may also be conducted. Engineers used one of the in house virtual durability test rigs in ADAMS/Car to generate component load time histories and to perform a fatigue life prediction of the rear lower wishbone and an equivalent physical lest carried on the hydraulic suspension rig for correlation purposes. This is shown in figure.14
(Multi-Axis Rig Test and Test Suspension)
KINEMATIC AND COMPLIANCE RIG TEST
K and C test rig machines used to measure the quasi-static suspension characteristics which are essential to understand the vehicle ride and handling. Kinematics has defined as study of motion without reference to the mass and force. Where, Compliance is defection resulting from the application of force. K & C rig has ability to apply the lateral and longitudinal forces at the tire contact patch. Suspension parameters such as toe and camber are measured at the wheel hub, while vertical, longitudinal and lateral tire loads are measured at the tire contact patch.
ANALYSIS AND TESTING OF SUSPENSION SYSTEM
Ultimately the quality of design will be judged on the performance of vehicle, for this an early analysis is required for suspension design as it is essential as an individual unit. In order to quantify the performance of the suspension system a range of characteristics are identified through simulation of single suspension system or quarter vehicle modelling. There are several types of analysis are used during the design and development of a vehicle. Kinematic, static, mobility, roll centre and dynamic force of analysis are used in simulation of single suspension system. Computational analysis are used for quarter vehicle modeling.The model geometry has also been analysed using an equivalent MBS model in ADAMS software allowing a comparison of results from theory and MBS and also providing readers with an insight in to the computational process involved. ADAMS/car programs provide templates with pre-programmed configurations of suspension system widely used by automotive manufacturers. This software used to analyse the suspension geometry is now well established. The output from this type of analysis is mainly geometric and allows results such as camber angle or roll centre position to be plotted graphically against vertical wheel movement. Figure 15
In this case study, it will be helpful to understand the Suspension systems and the importance for vehicle dynamics. Here attempted to give a clear view on suspension systems. Concentrated on two major types of suspensions (Hydraulic and Electro-mag).both compared with each other and believes that best suitable suspension for a vehicle is electro-magnetic and the design stage itself and helps us to get an excellent ride comfort, road control, safety and vehicle durability. The design section gives a brief overview of the design and methodology used in these suspensions .Further studies need to be carried out in this field so that will help to develop much more efficient suspensions systems in the future.
1. Research on Automotive Suspension Systems for Ride, Handling and Durability
Introduction http://www.wisegeek.com/what-does-a-cars-suspension-system-do.htm written by R.Kayne
2. Image of typical Front wheel suspension system
3. Suspension system parts
4. Hydraulic suspension system
6. Vehicle Dynamics - Part 6Vehicle Suspension and Ride,R.G. Longoria, Department of Mechanical Engineering. The University of Texas at Austin.
7. Hydraulic cars in market
1. Journal on "Introduction to Formula SAEâ Suspension and Frame Design" Edmund F. Gaffney III and Anthony R. Salinas
University of Missouri - Rolla
2. Active Electromagnetic Suspension System for Improved Vehicle Dynamics
Bart L.J. Gysen, Johannes J.H. Paulides, Jeroen L.G. Janssen, and Elena A. Lomonova
Eindhoven University of Technology, The Netherlands.