The subject matter considered to fall into areas covering suspension design, computer modeling and simulation, ADAMS program and the design study.
The stability and effective handling of a vehicle is dependent on the selection of the optimum steering and suspension geometry which particularly includes wheel camber, castor and kingpin inclination. The suspension members should maintain these settings throughout out their service life. The pivoting and swiveling joints of the suspension system are affected to both wear and damage, therefore they must checked periodically.
[1]Vehicle dynamics relates tire and aerodynamic forces to overall vehicle accelerations, velocities and motions, using Newton's laws of motion. It encompasses the behavior of the vehicle as affected by drive line, tires, aerodynamic and chassis characteristics and it is a complex subject where large numbers of variables are involved.
SUSPENSION GEOMETRY:
[3]The purpose of suspension system is to reduce the vertical wheel load variations and isolation of road inputs from the body. The suspension system addresses mainly six basic needs for the vehicles having more than four wheels
· Reduction of vertical wheel load variation.
· Isolation of road inputs from the body.
· Control of transmission of handling loads to body.
· Control of wheel plane geometry due to compliance effect.
· Control of wheel plane geometry due to Kinematics effect.
· Comprehension of component load environment.
The above mentioned needs are very crucial and important for setting, investigating and verify the design targets.
The suspension geometry is the broad subject of how the unsprung mass of the vehicle is connected to the sprung mass. The angular relationship between the suspension, steering linkage and the wheels is known as Suspension Geometry. Suspension connections dictate the path of relative motion and control the forces that are transmitted between them. A particular geometry must be designed to meet the needs of particular vehicle for which it is to be applied.
In an independent suspension system, linkage assembly is intended to control the wheel motion relative to the vehicle body in a single prescribed path (fixed motion of path). The knuckle is not allowed to rotate other than as determined by this fixed path. The linkages are expected to position the knuckle (wheel) accurately in all directions while allowing it to move up and down against the spring and shock.
As the independent suspension system allows one path of motion to the knuckle relative to the vehicle body, which also means that the suspension provide five degrees of restraint i.e.it severly limits the motion in five directions.
Suspension design is a compromise between tire capacity handling and ride quality of the vehicle. The isolation from road-induced disturbances is certainly a high desirable design aim. The stability and control of the vehicle totally relies on maintaining the contact between the wheels and road surface.
The automotive industry uses different kinds of wheel/axle suspension systems depending on the need of vehicle. Main criteria are costs, pace requirements, kinematic properties and compliance attributes. The MacPherson suspension, the double wish bone suspension and the multi link suspension are the most common multipurpose suspension systems.
SUSPENSION KINEMATICS:
[2]The kinematic behavior of the suspension linkages is not obvious from its appearance and it is obvious far from it. A suspension system should incorporate a good kinematics design to keep the tire as perpendicular to the pavement as possible, optimal damping effect and spring rates to keep the tire to the road surface at all times. Also strong components which do not get deflected under the loads induced up on them.
The suspension kinematics is the study of motion without reference to mass and force. It describes the controlled orientation of wheels by the suspension links, by making assumptions of the rigid parts and also frictionless joints. By visualizing the attitude of the vehicle in a corner, the suspension design can be made to keep as much as the tire on the ground as possible,
For a race car the kinematics, roll over, handling for the suspension design issues are discussed.
The kinematic and dynamic analysis is performed by using the MBS and roll over, ride and handling are simulated and tuned on geometry, springs and dampers to achieve performance.
The choice of camber gain, roll centre placement and scrub radius should be based on how the vehicle is expected to perform. By visualizing the attitude of the vehicle, the suspension design can be made to keep the tire as much as possible on to the road surface.
This paper has concentrated on the design aspects and the test evaluation is agreed perfect with the simulation models. The computational tools have supplied the necessary support by which the sizing and design details occur in coherent form with engineering principles.
Suggestions made for the suspension kinematics:
Scrub radius is considered positive when the steering axis intersects the ground to the inside of the wheel centre line. The amount of scrub must be kept small since it causes excessive cornering forces and it is desirable as it can provide feedback through the steering wheel for the driver.
To reduce the scrub radius, King Pin Inclination can be incorporated into the suspension design only if the ball joint near the centre line of wheels is not feasible.
With the positive castor the outside wheel in a corner will camber negatively which helps to offset the positive camber associated with KPI and body roll.
Castor causes the wheel to rise or fall as it rotates about the steering axis where it transfers the weight diagonally across the chassis
It is more desirable to have the roll centre close to the round place in order to reduce the amount of chassis vertical movement due to the lateral forces.
The roll centre is the instant centre which moves with the suspension travel, so the migration of the roll centre must be checked to ensure that the jacking forces overturning moments follow a relatively linear path for the predictable handling.
USE OF MULTI BODY DYNAMICS SIMULATION:
[3]Modern Multi Body Dynamics Analysis software allows describing the individual components of the systems and it will automatically calculate the contribution of them.
[6]The MBS is being an established tool for the virtual design of full vehicle behavior. The applications of MBS in an automotive industry are
1. Calculation of suspension characteristics such as camber angle, steer angle and steer angle as a function of vertical movement of the suspension
2. Full vehicle ride and handling simulations
3. Prediction of joint and bush reaction forces for various loadcases at the tire to road surface contact patch.
4. Advanced simulation of features such as Anti Lock Braking system,
The MBS studies are valuable in providing guidance for suspension systems design and reduce product development cost and time. The calculations and assessment of the internal cross sectional forces of suspension components are enabled.
THE ADAMS PROGRAM:
[4]ADAMS (Automatic Dynamic Analysis of Mechanical Systems) is the most widely used multi body dynamics and motion analysis package which helps in understanding the dynamics of moving parts, distribution of loads and forces in the mechanical system, also to improve and optimize the performance of the system.
ADAMS incorporates real physics by simultaneously solving the equations for kinematics, statics, quasi-static and dynamics. It enables to create and test virtual prototypes of mechanical systems in a fraction of time and cost required for physical build and test.
[3]Suspension geometry analysis is one of the earliest applications of the MSC ADAMS program by the Automotive Industry in 1977. The output from this type of analysis is mainly geometric and allows results such as half track change, camber angle, roll centre position to be plotted graphically against the wheel vertical movement.
For Vehicle Suspension systems, ADAMS provides useful spline editing and plotting capabilities which considerably simplifies the modeling and inclusion of non linear elements such as bushes.
Suspension Design In MBD Simulation:
The suspension system development is an iterative process, the parameters such as sprung and unsprung mass assumptions may change during the development process. The MDS models are useful for estimating the component specifications for any changes in the sprung mass assumptions. They also identify and optimize the important suspension component specification using DOE studies and decides the suspension component specifications for different value variants of the base vehicle.
In [5] the author describes studies of suspension component and system level analysis consisting of rigid or flexible parts which are connected by joints by the application of ADAMS Program. In the paper four case studies for suspension system performance optimization using MBD studies are presented.
The important parameters to be finalized in the early stage of suspension design are the parameters that affect the vehicle ride comfort and handling like suspension spring rates, geometry, suspension travel, jounce rebound bumper, bushing compliance and stabilizer bar designs. Vehicle track, sprung mass centre of gravity, wheel base and weight distribution are the vehicle level parameters which affects the ride comfort and handing.
The papers states that the large spring and geometric ratios are generally aimed to improve the suspension NVH. These reduce the loads on bushings, shock absorber damping force requirements by which improve the durability of the components.
KINEMATIC LINKAGES
[2] Presents an approach to suspension linkage design which avoids the geometry iteration process typically required to ensure specific kinematic behavior in the design condition.
In [3] a methodology to calculate the suspension characteristics as the suspension moves between the bump and rebound positions is illustrated. This study based on the front double wishbone suspension of a passenger car where the suspension connections are considered as joints, linear or non linear bushes which establish the effects on suspension geometry changes during the vertical movement.
The simulations results with measured suspension rig test data provided by the vehicle manufacturer are compared. The reaction forces at the bushes leads to distortions which produces the change in suspension geometry. This result confirms that the geometry changes are dependent on the position and orientation of the joints, and there will be little difference between the models using rigid joints, linear bushes and non linear bushes.
REFERNCES:
