14042030 suspension paper

8/3/2019 14042030 Suspension Paper

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AN INTRODUCTION TO AUTOMOTIVE SUSPENSIONS SYSTEMS

AN INTRODUCTION TO

AUTOMOTIVE SUSPENSION

SYSTEMS

Piyush Gaur, MSc Automotive

Engineering

Faculty of Engineering and Computing

Coventry University, UK

Abstract: Suspension system is a term

which is given to a system of springs, Shockabsorbers and linkages that connects a

vehicle to its wheels. A suspension system

serves the two dual purposes. It helps in

contributing of the cars handling andbraking for good active safety and driving

pleasure. Any suspension system of an Automotive is classifies into rigid,

Independent and combination of the above

two. In this paper, a brief introduction to the

suspension and is function is explained. Abrief introduction to its designing procedure

is also explained along with the factors

affecting suspension designing. Doublewishbone, McPherson Strut, Torsion bar,

Quardalink, Twist beam &Leaf Springs hasbeen discussed in detail along with the carson which they are used. The relationship

between the suspension system, the tyre and

the full vehicle dynamics performance has

also been discussed in this paper.

Keywords: Suspensions system, Springs,

Double wishbone, Hotchkiss, Adams,

Vehicle Dymamics, Multibody system

Analysis.

1. INTRODUCTION

Suspension systems date back perhaps two

thousand years or more. Early wagons wereknown to have used elastic wooden poles to

reduce the affects of wheel shock. Leaf

springs in one form or another have been

used since the Romans suspended a two-

wheeled vehicle called a Pilentum on elastic

wooden poles. Later, some innovativecarriage designs included rudimentary leaf

suspension systems. Throughout history, leaf

springs would dominate as the primarysuspension design until fairly recently. Leaf

springs offered the benefit of simplicity of

design and relatively inexpensive cost. By

simply adding leaves or changing the shapeof the spring, it could be made to support

varying weights. As a result, major changes

primarily tended to revolved around the useof superior materials and making

incremental design modifications.

Suspension is a term which is given tosystem of springs, shock absorbers and

linkages that connects a vehicle to its

wheels. Suspension systems serve the twodual purposes. It helps in contributing of the

cars handling and braking for good active

safety and driving pleasure. Secondly, ithelps in keeping vehicle occupants

comfortable and reasonably well associated

from road noise, bumps and vibrations. Thesuspension system also protects the vehicle

and any cargo or luggage from damage andwear. The design of front and rearsuspensions of an automotive may be

different. If a road were perfectly flat, with

no irregularities, suspensions wouldn't be

necessary. But roads are far from flat. Evenfreshly paved highways have subtle

imperfections that can interact with the

wheels of a car. It's these imperfections thatapply forces to the wheels. According to

Newton's laws of motion, all forces have

both magnitude and direction. A bump inthe road causes the wheel to move up and

down perpendicular to the road surface. The

magnitude, of course, depends on whether

the wheel is striking a giant bump or a tinyspeck. Either way, the car wheel experiences

a vertical acceleration as it passes over an

imperfection.

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2. CLASSIFICATION OF SUSPENSION

SYSTEM

Vehicle suspensions can be divided intorigid axles (with a rigid connection of thewheels to an axle), independent wheel

suspensions in which the wheels are

suspended Independently of each other, andsemi-rigid axles, a form of axle that

combines the characteristics of rigid axles

and independent wheel suspensions. On allrigid axles , the axle beam casing also

moves over the entire spring travel.

Consequently, the space that has to be

provided above this reduces the boot at therear and makes it more difficult to house the

spare wheel. At the front, the axle casing

would be located under the engine, and toachieve sufficient jounce travel the engine

would have to be raised or moved further

back. For this reason, rigid front axles arefound only on commercial vehicles and four

wheel drive, general-purpose passenger cars

.With regard to independent wheel

suspensions, it should be noted that the

design possibilities with regard to thesatisfaction of the above requirements and

the need to find a design which is suitablefor the load paths, increase with the

number of wheel control elements (links)

with a corresponding increase in their planesof articulation. In particular, independent

wheel suspensions include:

Longitudinal link and semi-trailing arm

axles, which require hardly any overheadroom and consequently permit a wide

luggage space with a level floor, but whichcan have considerable diagonal springing.

Wheel controlling suspension and shock-absorber struts , which certainly occupy

much space in terms of height, but which

require little space at the side and in themiddle of the vehicle (can be used for the

engine or axle drive) and determine the

steering angle (then also called McPherson

suspension struts).

Double wishbone suspensions or SLA

(Short Length Arm)

Multi-link suspensions, which can have

up to five guide per links and which offerthe greatest design scope with regard to the

geometric definition of guide links per

wheel and which offer the greatest design

scope with regard to kingpin offset, pneumatic offset, kinematic behavior with

regard to toe-in, camber and track changes,

brake/starting torque behavior andelastokinematic property. Broadly speaking

these are the main type of automotive

suspensions systems which are commonly

used in different automotives today. Theseare

Double wish bone suspension system

Mc person Strut suspension system.

Torsion Bar

Quadra Link

Twist Beam

Leaf Springs

A. Double wishbone suspension system

It is the independent suspension designwhich uses a two wish bones arms to locate

the wheel. Each wishbone or arm has two

mounting points on the chassis and onepoint at the knucle .The shock absorber and

coil spring mount to the wishbone is used to

control the vertical movement. This allowsthe suspension designer engineer to control

the following parameters:

Camber angle

Caster angle

Toe pattern Roll center height

Scrub radius, scuff and more.

The double wishbone suspension can also be

referred to as double 'A' arms and short longarm (SLA) suspension if the upper and

lower arms are of unequal length. SLAs are



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very common on front suspensions for

medium to large cars such as the Honda

Accord, Volkswagen Passat, Chrysler 300,orMazda 6/Atenza, pickups, SUVs, and are

very common on sports cars and racing cars.

A single wishbone or A-arm can also beused in various other suspension types, such

as Macpherson strut and Chapman strut. The

suspension consists of a pair of upper and

lowers lateral arms. The upper arm isusually shorter to induce negative camber as

the suspension jounces (rises). When the

vehicle is in a turn, body roll results inpositive camber gain on the outside wheel.

The outside wheel also jounces and gains

negative camber due to the shorter upper

arm. The suspension designer attempts tobalance these two effects to cancel out and

keep the tire perpendicular to the ground.

This is especially important for the outer tirebecause of the weight transfer to this tire

during a turn.

The advantage of a double wishbone

suspension is that it is fairly easy to work

out the effect of moving each joint, so youcan tune the kinematics of the suspension

easily and optimize wheel motion. It is also

easy to work out the loads that differentparts will be subjected to which allows more

optimized lightweight parts to be designed.

They also provide increasing negative

camber gain all the way to full jounce travelunlike the MacPherson strut which provides

negative camber gain only at the beginning

of jounce travel and then reverses into positive camber gain at high jounce

amounts.

The disadvantage is that it is slightly morecomplex than other systems like a

MacPherson strut. Prior to the dominance of

front wheel drive in the 1980s, manyeveryday cars used double wishbone front

suspension systems or a variation on it.

Since that time, the Macpherson strut hasbecome almost ubiquitous, as it is simpler

and cheaper to manufacture. In most cases, a

Macpherson strut requires less space to

engineer into a chassis design, and in frontwheel drive layouts, can allow for more

room in the engine bay. A good example of

this is observed in the Honda Civic, whichchanged its front suspension design from a

double wishbone design, to a Macpherson

strut design after the year 2000 model. The

changes was made to lower costs, as well asallow more engine bay room for the newly

introduced Honda K-series engine.

Fig 1: Double Wishbone suspension

system

B. MAcPherson Strut

McPherson struts are popular struts that are

used mainly in the front suspensions on

vehicles especially cars. This strut containsdifferent types of components into one

package making them ideal for front-wheel-

drive cars. The McPherson struts are used indifferent types and models of cars. The strut

is used for both rear and front suspension

but mainly used in the front suspension

because it provides a steering pivot. Thesubframe of the strut is capable of providing

the lateral and longitudinal location of the

wheel. The strut was designed by Earl S.McPherson. This strut was used in Ford

Vedette in 1949. The strut consists of a

wishbone or a compression link which is

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stabilized by a secondary link. The

secondary link is important for providing a

bottom mounting point for the hub or axle ofthe wheel. The lower arm of the strut is

helpful in providing both lateral and

longitudinal location of the wheel.

Fig 2: Mc Pherson Strut

The body is suspended on the coil springwhereas the shock absorber, which is usually

in the form of a cartridge mounted within

the strut. The assembly is simple and can be preassembled into a unit. Moreover, it

allows for more width in the engine bay by

eliminating the upper control arm. This is

useful for smaller cars particularly withengine having transverse orientation just like

most front wheel drive vehicles have.

C. Torsion Bar System- A torsion bar

suspension, also known as a torsion spring

suspension or incorrectly torsion beam, is ageneral term for any vehiclesuspension that

uses a torsion bar as its main weight bearing

spring. One end of a long metal bar isattached firmly to the vehicle chassis; the

opposite end terminates in a lever, mounted

perpendicular to the bar, that is attached to asuspension arm, spindle or the axle. Vertical

motion of the wheel causes the bar to twist

around its axis and is resisted by the bar's

torsion resistance. The effective spring rateof the bar is determined by its length,

diameter and material.

Torsion bar suspensions are currently used

on trucks and SUV from Ford, Dodge, GM,

Mitsubishi and Toyota. Manufacturers

change the torsion bar or key to adjust the

ride height, usually to compensate for

heavier or lighter engine packages. While

the ride height may be adjusted by turning

the adjuster bolts on the stock torsion key,

rotating the stock keys too far can bend the

adjusting bolt and (more importantly) place

the shock piston outside the standard travel.

Over-rotating the torsion bars can also cause

the suspension to hit the bump stop

prematurely, causing a harsh ride.

Aftermarket forged torsion key kits userelocked adjuster keys to prevent over-

rotation, as well as shock brackets that keep

the piston travel in the stock position.

Fig 3: Torsion bar action

D. Twist Beam Suspension- The Twist-

beam rear suspension is a type of

automobile suspension based on a large H

shaped member. The front of the H attaches

to the body via rubberbushings, and the rearof the H carries each wheel, on each side of

the car. The cross beam of the H holds the

two trailing arms together, and provides

the roll stiffness of the suspension, by

twisting as the two trailing arms move

vertically, relative to each other. The coil

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springs usually bear on a pad alongside, or

behind, the wheels. Often the shock is

colinear with the spring, to form a coilover.

This location gives them a very high motion

ratio compared with most suspensions,

which improves their performance, and

reduces their weight. The longitudinal

location of the cross beam controls

important parameters of the suspension's

behavior, such as the roll steercurve and toe

and cambercompliance. The closer the cross

beam to the axle stubs the more the camber

and toe changes under deflection. A key

difference between the camber and toe

changes of a twist beam vs independentsuspension is the change in camber and toe

is dependent on the position of the other

wheel, not the car's chassis. In a traditional

independent suspension the camber and toe

are based on the position of the wheel

relative to the body. If both wheels compress

together their camber and toe will not

change. Thus if both wheels started

perpendicular to the road and car

compressed together they will stay

perpendicular to the road. The camber and

toe changes are the result of one wheel being

compressed relative to the other. This

suspension is used on a wide variety of front

wheel drive cars, and was almost ubiquitous

on European superminis. It was probably

introduced on the Audi 50, which was

rebadged as the Volkswagen Polo. This

suspension is usually described as semi-

independent, meaning that the two wheels

can move relative to each other, but their

motion is still somewhat inter-linked, to a

greater extent than in a true IRS. This limits

the handling of the vehicle, and VW have

dropped it in favor of a true IRS for the Golf

Mk V in response to the Ford Focus' Control

Blade rear suspension.

Fig 4- Twist Beam Rear Axle

E.Leaf spring

Originally called laminated or carriagespring, a leaf spring is a simple form

ofspring, commonly used for

the suspension in wheeledvehicles. It is also

one of the oldest forms of springing, dating

back to medieval times. Sometimes referred

to as asemi-elliptical spring orcart spring,

it takes the form of a slenderarc-shaped

length ofspring steel ofrectangularcross-

section. The center of the arc provides

location for the axle, while tie holes are

provided at either end for attaching to the

vehicle body. For very heavy vehicles, a leaf

spring can be made from several leaves

stacked on top of each other in several

layers, often with progressively shorter

leaves. Leaf springs can serve locating and

to some extent damping as well as springing

functions. While the interleaf friction

provides a damping action, it is not wellcontrolled and results in stiction in the

motion of the suspension. For this reason

manufactures have experimented with

mono-leaf springs. A leaf spring can either

be attached directly to the frame at both ends

or attached directly at one end, usually the

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front, with the other end attached through a

shackle, a short swinging arm. The shackle

takes up the tendency of the leaf spring to

elongate when compressed and thus makes

for softer springiness. Some springs

terminated in a concave end, called aspoon

end(seldom used now), to carry a swiveling

member.

Fig 5- Leaf Spring

3.1 Suspensions Geometry& Tyres role

for Effective vehicle Handling

The stability and effective handling of the

vehicle depends upon the designers

selection of optimum steering and

suspension geometry which particularly

includes the wheel Camber, Castor and King

Pin inclination. It is essential for the

suspension members to maintain these

factors throughout the whole life of a car.

Unfortunately, the pivoting and the

swiveling joints are both subjected to the

wear and damage and must be periodically

checked. With the understanding of the

principles of the suspensions geometry and

their measurements it is possible to diagnose

and rectify the steering and suspension

faults. Following are the parameters which

are kept in mind for designing of suspension

systems

3.1 Wheel Camber Angle- Wheel camber is

the lateral tilt or sideway inclination of the

wheel relative to the vertical. When the top

of the wheel lean inwards towards the body

the camber is said to be negative, conversely

an outward leaning wheel has positive

camber. Road wheels were originally

positively cambered to maintain the wheels

perpendicular to the early cambered roads.

Practically for most of the suspensions

system wheel cambered has been reduces to

0.5 degrees to 1.5 degrees. if one wheel is

slightly more cambered then the other, due

may be to body roll with independent

suspension or because of misalignment ,the

steering wheel will tend to wander or pull to

one side as the vehicle is steered in the

straight ahead position. To provide a small

amount of understeer, the front wheels are

normally made to generate a greater slip

angle then the rear wheels by introducing

positive camber on the front wheel and

maintain the rear wheels virtuallyperpendicular to the ground.



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3.2 King pin inclination king pin

inclination is the lateral or inward tilt from

the top between the upper and lower swivel

ball joint or king pin to the vertical. If the

kingpin is perpendicular to the ground ,its

contact centre on the ground would be offset

to the centre of the tyre contact patch, the

offset between the pivot centre and contact

patch centre is known as the scrub radius.

When turning the steering the offset scrub

produce a torque T created by the product of

the reactionary force f and offset radius .A

large pivot to wheel contact centre offset

requires a large input torque to overcome the

opposing ground reaction, therefore the

steering tends to be very heavy. A positiveScrub Radius or Kingpin Offset is when the

Kingpin Angle hits the road surface on the

inside of the centre line of the tyre contact

point (see the diagram below), a negative

Scrub Radius is when the Kingpin Angle

hits the road on the outside of the centre line

of the tyre contact point. The Kingpin

Angle, along with the Castor, dictates the

self-centering action of the steering and the

affect the steering will have under braking.

Fitting larger wheels can alter the Scrub

Radius if the correct offset is not chosen

which in turn can affect the handling.

3.3 Castor Angle- Castor Angle is the angle

to the vertical plane on which the steering

axis sits as viewed from the side. In other

words if we imagine looking at the side of

the front wheel, the Castor Angle is theangle an imaginary line makes that is drawn

through the centre of top ball joint (or top

mount of a suspension) and down through

the lower suspension arm ball joint. Looking

on the diagram, if we follow the Castor

Angle line down we can see it hits the

ground in front of where the tyres contact

with the ground, this is Positive Castor. This

means the tyres will always follow the

steering input or in other words act just like

a normal furniture castor wheel. Castor

Angle determines the amount of self-



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centring the steering will have, influence the

straight-line running and with the Kingpin

Angle it will influence the camber change

when cornering as a function of the steering

input. Castor Angle traditionally used to be

very small as large amounts of Castor Angle

created heavy steering,. Large Castor Angles

mean greater, dynamic camber changes can

be created and that means better negative

camber when cornering and smaller camber

on the straight, ideal for both performance

and wear of the tyres unfortunately too large

a castor angle can lead to poor turn-in.

Fig 8- Negative & positive Camber

3.4 Toe Pattern - Toe describes the angle at

which a wheel sits on a horizontal plane

relative to the longitudinal axis of the car. Inother words if we imagine looking vertically

down on top of a wheel mounted on a car, if

the front of the wheel is angled inwards

more than the rear of the wheel then it issaid to have toe-in, if its the other way

around then the wheel is said to have toe-out. If the wheel is parallel with the

longitudinal axis of the car then it has zero

toe. Toe can be measured in degrees but

more commonly, its measured as thedistance difference between the front of the

wheel rim and the rear of the wheel rim.

Total Toe is the overall distance for a pair of

wheels whereas Individual Toe is half theTotal Toe and relates to individual wheels.

Toe-in increases lateral stability but can lead

to wear on the inside shoulder of the tyre.Front end toe-in dampens turn in response

but improves the self centring action of the

steering while rear toe-in helps to reduce

oversteer due to the improvement in lateralstability. Toe-out reduces lateral stability and

can lead to wear on the outside shoulder of

the tyre. Front end toe-out can improve turn-in response while rear end toe-out

encourages oversteer due to the reduction in

lateral stability. Toe can be altered on the

front by adjusting the track-rod ends and onthe rear by adjusting the toe control arms.

Fig 9- Toe pattern on suspension

geometry

3.5 Roll center Analysis One important

property of the suspension relates to the

location at which lateral forces developed by

the wheels are transmitted to the sprungmass. This point, which has been reffered to

as the roll center, affects the behavior of

both the sprung and unsprung mass, and thus

directly influences the cornering. Eachsuspension has a Roll center, defined as a

point in the transverse vertical plane throughthe wheel centers at which the lateral forces

may be applied to the sprung masses without

producing the suspension roll. It derives

from the fact that all suspensions have a rollaxis, which is the instantaneous axis about



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which the unsprung masses rotate with

respect to the sprung mass when a pure

couple is applied to the unsprung mass. Theroll center is the intersection of the

suspension roll axis with the vertical plane

through the center of two wheels .The rollcenter height is the distance from the ground

to the roll center .The suspension roll axis

and roll center can be determined from the

layouts of the suspensions geometry in theplan and elevation views. From the analysis,

we draw the concept of Virtual Reaction

point. It is another word of Instanteouscentre.

Fig 10a- Roll Center of McPhersonStrut

Fig 10 b- Roll centres of other automotive

suspensions

3.6- Tire behavior in Vehicle handling - A

tire is a simple visco-elastic toroid whichserves the three basic functions -1. It

supports the vertical load, while cushioning

road shocks 2. It develops longitudinalforces for acceleration and braking and also

develops lateral forces for cornering. Tofacilitate precise description of the operating

conditions, forces and moments experienced by the tire, a SAE has defined the axis

system shown in fig below-



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Fig 11- SAE axis systems

Wheel plane Central plane of the

tire normal to the axis of rotation.

Wheel center- Intersection of the

spin axis and wheel plane.

Center of tire Contact- intersection

of the wheel plane and projection

Loaded Radius- Distance from

center of the tire contact to the wheelcenter in the wheel plane.

Longitudinal force(Fx)- Componentof the force acting on the tire by theroad in the plane of the load and

parallel to the intersection of the

wheel plane with the road plane .Theforce component in the direction of

the wheel travel is called Tractive

force.

Lateral Force (Fy) - Component of

the force acting on the tire by theroad in the plane of the road and

normal to the intersection of thewheel plane with the road plane.

Normal Force (Fz) - Component of

the force acting on the tire by the

road which is normal to the plane ofthe road .the Normal force is

negative in magnitude.

Over turning Moment (Mx)-

Moment acting on the tire by the

road in the plane of the road and

parallel to the intersection of the

wheel plane with the road plane.

Rolling Resistance Moment (My)-Moment acting on the tire by the tireby the road in the plane of the road

and normal to the intersection of the

wheel plane with the road plane.

Aligning Moment (Mz)- Moment

acting on the tire by the road whichis normal to the plane of the road.

Slip angle () - Angle between thedirection of the wheel heading and

the direction of the travel.

Camber angle () - angle betweenthe wheel plane and the vertical.

3.7 Mechanics of forces Generation in

tires- The forces on a tire are not applied ata point, but are the resultant from normal

and shear stresses distributed on a contact

patch. The pressure distribution under a tire

is not uniform but vary in X and Y direction.When rolling, it is generally not symmetrical

about the Y-Axis but tends to be higher inthe forward region of the contact patch.Because of the tires visco elasticity,

deformation in the leading portion of the

contact patch causes the vertical pressure to be shifted forward. the centroid of the

vertical force does not pass through the spin

axis and therefore generates rolling

resistance. With a tire rolling on the roadboth tractive and lateral forces are developed

by shear mechanism. Each element of the

tire tread passing through the tire contact patch exerts a shear stress which, if

integrated over the whole area is equal to the

lateral /tractive forces developed by thetires.



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Fig 12- Tire Deformation in the contact

patch

3.8 Forces Developed on the tires and

their effect on vehicle handling

3.8 a- Tractive properties- Under

acceleration and braking ,additional slip is

observed as a result of the deformation of

rubber elements in the tire tread caused asthey deflect to develop and sustain he

frictional force. As the tread elements first

enter the contact patch they cannot developthe frictional force because of their

compliance-they must have bend to sustain a

force. This can happen only if the tire isdmoving faster than the circumference of the

tread. As the tread element proceeds back

through the contact patch its deflectionbuilds up currently with vertical load and it

develops much more friction force.However, ,approaching the rear of thecontact patch the load diminishes and there

comes a point where the tread element

began to slip noticeably on the surface such

that the friction force drops off, reachingzero as it leaves the road. Thus acceleration

and braking forces are generated by

producing a differential between the tire

rolling speed and its speed of travel. The

consequence is production of slip in thecontact patch. Slip is given by

S= (1- r) 100 where,V

R= Tire effective rolling radius

= Wheel angular velocity

V= Forward velocity

3.8b.Effect of tractive properties on

Vehicle Handling- Longitudinal traction properties are the properties of the tires

system that determine braking performance

and stopping distance.Beacuse of the weight

transfer during deceleration, all wheelcannot be brought to the peak traction

condition except by careful design of the

braking system so as to proportion of thefront and rear braking forces in accordance

with the prevailing loads under these

dynamic conditions. Since it is practicallyimpossible to design a conventional braking

system that can achieve exact proportioning

under all conditions of load, center ofgravity location, and load condition, it is

inevitable that the driver will experiencelock up problem. Therefore, the slidingcoefficient of friction is an important tire

performance property. With the use of

antilock braking system thr brake system

maintains the wheel near the peak of thetraction curve and does not allow lock up .

3.8c. Cornering properties and Its effect

on the Vehicle Performance- One of the

very important fuctions of the tire is to

develop the lateral forces necessary tocontrol the direction of the vehicle, generate

lateral acceleration in corners or for lane

change, and resists external forces such as

wind gusts and road cross slope. Theseforces are generated either by lateral slip of

the tire, by lateral inclination, or a

combination of the two. The integration of



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all the forces acting on a contact patch yields

the net lateral force with a point of action on

the centroid.The asymmetry of the forcesbuild up in the contact patch causes the force

resultant to be positioned toward the rear of

the contact patch by a distance known asPneumatic Trail. By SAE convention the

lateral force is taken to act at the center of

the tire contact. At this position net resultant

is a lateral force,Fy and aligning moment,Mz.The magnitude of aligning moment is

equal to the lateral force times the

pneumatic trail.

Vehicle stiffness is one of the primary

variables affecting steady state and transient

cornering properties of vehicles in thenormal driving range.Understeer gradient,

the characteristic commonly used to qualify

turning behavior, is directly influenced bythe balance of the cornering stiffness on

front and rear tires, as normalized by their

loads.A higher relative cornering stiffness onthe rear wheels is necessary to achieve under

steer.

Fig 13- Lateral force vs slip angle

graph

3.8d- Camber thrust and its effect onvehicle handling- A second means of lateral

force generation in a tire derives from

rolling at a non-vertical orientation, the

inclination angle being known as camber

angle.With Camber, a lateral force known asCamber Thrust is produced. The

inclination angle is defines with respect to

the perpendicular from the ground plane,positive corresponding to an orientation with

the top of the wheel tipped to the right when

looking forward along its direction of

travel. It is the primary cornering force bywhich motorcycles and the other two

wheeled vehicles are controlled. On

passengers and trucks, camber thrustcontributes to understeer behavior, but

normally as a secondary source. On vehicles

with independent suspensions where

significant camber angles may be achieved,this mechanism may contribute up to about

25 percent of the under steer gradient. On

vehicles with Solid axles, little camber canoccur such that its contribution to turning

performance is very less.

Fig 14- Lateral Angle vs Camber Angle

3.8e- Aligning Torque And its effect on

Vehicle performance- Aligning torque as a

torque acting on the vehicle contributes a

small component to the understeer of avehicle. The fact that positive aligning

moments attempt to steer the vehicle out of

the turn means that they are understeer in

direction. Overall, the direct action of themoments contributes only a few percent to

the understeer gradient of a vehicle. The

aligning moment has a more direct influence



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on understeer by its action on the steered

wheels. The moment is normally in the

direction to turn the steered wheels out ofthe turn. the steered wheels out of the turn.

Even though the steer deflection angles in

response to aligning moments may be small,this is normally an important contribution to

under steer gradient.

4. Simulation and Analysis of Suspensions

Systems- Computer aided simulation of

vehicle handling characteristics is nowadays

universally acknowledged as an efficientmethod in the process of developing new

vehicles. Simulation software tools are used

both by automobile manufacturers and

suppliers to an increasing extent. Theoutstanding quality of simulation results for

chassis development is acknowledged

without exception. ADAMS as amultybody-simulation-tool is in service in

automotive engineering all over the world.

The dynamics of rigid bodies can hereby beanalyzed mathematically very exactly.

Fig 15- ADAMS model of a car

The main use of ADAMS within the

automotive industry is to simulate the

performance of suspension systems and full

vehicle models. The analyst will often wishto validate the performance of a suspension

model over a range of displacements

between full bump to rebound before the

assembly of a full vehicle model. The final

model may be used for ride and handling,

durability or crash studies. A detailed model

may include representations of the body, sub

frames, suspension arms, struts, roll bars,

steering system, engine, drivetrain and tyres.

The main analysis code consists of a number

of integrated programs that perform three-

dimensional kinematic, static, quasi-static or

dynamic analysis of mechanical systems. In

addition there are a number of auxiliary

programs, which can be supplied to link

with ADAMS. These programs can be used

to perform modal analysis, model vehicle

tyre characteristics, pre-process using a

library of macros, automatically generate

vehicle suspensions and full vehicle models,

or model the human body. Once a model has been defined ADAMS will assemble the

equations of motion and solve them

automatically. It is also possible to include

differential equations directly in the

solution, which allows the modeling of

active suspensions or steering, braking and

speed controllers. Programs such as

ADAMS have developed to such an

advanced stage that they form an integral

part of a modern computer aided

engineering installation. The program will,

for example, link or interface with CAD

systems, finite element programs, software

used for advanced visualization or additional

software modules such as those used for tyre

modelling. The combined use of these

systems can lead to the development of what

may be referred to as virtual prototypes,

which is computer models that can simulate

the tests and conditions that a real prototype

would be subject to during the development

of a new engineering product.



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Fig 16- Integration of Adams with CAE

The types of analyses that can be performed

and the use in design will be addressed in

three main areas:

(i) The use of kinematic or quasi-static

analysis to simulate the motion of the

road wheel relative to the vehicle

body passing through the full rangeof vertical movement between the

rebound and the bump positions. Theoutput from these analyses is mainly

geometric and allows results such as

camber angle or roll centre positionto be plotted graphically against

vertical wheel movement.

(ii) The use of static, quasi-static or

dynamic analyses to simulate thediffusion of loads from the contact

patch through the suspension system

and into the body mounts. Thesetypes of analyses are used to

represent typical in service loads that

need to be considered to provide therequired durability. Typical load

cases will include those due to

driving, braking and cornering

leading on to the simulation of themore severe cases to which a

prototype vehicle would be subjected

such as driving through a pothole.The output from these analyses will

be the peak loads produced at

locations such as the suspension armto body mounts and the spring seats.

These results can then be used as

inputs to finite element models in

order to determine the structuralstresses and strains required for the

design of the components and to

perform further fatigue assessments.

(iii) The third type of analysis is the use

of dynamic analyses to determine the

natural frequencies in the suspensionsystem required for the consideration

of the ride performance of the


Geometry

ADAMS

SimulationResults ProcessingModules

CAD/Solid Modelling

IGESTranslation

ADAMS

System Model Definition

HumanFactors Modelling

Hydraulic,Pneumatic

Subsystem Modelling

ControlSystem Modelling

Body properties,geometry,

postures monitors

Bond-graphModels

Controllaws

Mass

properties

F.E.

FlexibleBody Modelling

ActuatorModelling

VehicleModellingSuspensionmodels, tyre

models,drivetrains

DifferentialEquations

Mass,stiffness,damping

models

Interactive Real-Time Kinematics Kinematic Path Optimisation

EquationGeneration

Assembly//InitialConditionAnalysis

Kinematic Analysis

Static/Quasi-Static Analysis

Dynamic Analysis

Linearisation/Model Analysis

ADAMS

System Simulation Modules

ADAMSDataLanguage

PlantModel

LoadsBoundary

Conditions

Signal Processing Data Tabulation

Configuration Display Results Plott ing

SuperimposedDisplay/Animation

High-speedShaded

ImageAnimation

Photo-realisticRendering

Film-recordedAnimation

ADAMSResults Files


15/19


vehicle. An example of this would be

to recreate test procedures carried

out in the laboratory such as theinput of an oscillatory load at the

tyre contact patch where the

frequency is varied with time. This isoften referred to as a frequency

sweep and will identify which

frequencies will excite the

suspension leading in severe cases toproblems such as wheel hop where

the resulting excitation of the road

wheel can lead to violent bouncing.

Modelling of suspension system

consist of the following four types ofmodel which are used with ADAMS.

They are

(i) TheLinkage Modelwhere the suspension

linkages and compliant bush connections are

modelled in detail in order to recreate as

closely as possible the actual assemblies on

the vehicle.

(ii) The Lumped Mass Model where the

suspensions are simplified to act as single

lumped masses which can only slide in the

vertical direction with respect to the vehicle

body.

(iii) The Swing Arm Model where the

suspensions are treated as single swing armsthat rotate about a pivot point located at the

instant centres for each suspension.

(iv) TheRoll Stiffness Modelwhere the body

rotates about a single roll axis that is fixed

and aligned through the front and rear roll

centers.

The four suspension arrangements are

shown schematically in Figure18.

Fig 17.1 Linkage model

Fig 17.2- Swing Arm Model



16/19


5. Physical Testing of Simulation System-

Basically two tests are commonly used to

test the exact geometry of suspensionsystem. These two test are- 1. K and C test

2. Shakers Rig test.

5.1- K & C Testing - For the understanding

of vehicle handling characteristics,

investigations on suspension kinematics andcompliance steer are of major interest.

Kinematics means the movements of the

wheel relative to the body that result fromspring travel. Compliance steer results from

additional forces in the contact area of thetires. These forces caused by lateral or

longitudinal accelerations of the vehicledeform the suspension parts and its bushings

and lead to additional camber and toe

angles. Compliance steer of axles has a greatinfluence on the handling performance of

vehicles. By a specific interpretation of the

suspension elements, the engineer is beingforced to get a compliance steer that

supports a controlled road performance of

the complete vehicle. Some types of axleshave however conceptionally causeddisadvantages regarding compliance steer

such as twist- beem rear axles .These shall

be minimized in the most effective way byconstructive features. Because of the

diminution of vehicle development time it is

necessary to get object measuring resultsfrom new axles very fast and easily. These

results are also necessary to validate the

compliance steer of vehicle models for the

simulations of vehicle dynamics. The qualityof the model compliance steer influences

very clearly the results of the multy-body-

simulations. Pure static mechanical modelsdo not deliver adequate simulation results

for modern vehicles.

The IKA kinematics and compliance Test

Rig can be used for the measurement of the

influences of vertical deflections and bothlateral and longitudinal forces on the axle

geometry of complete vehicles or of axle-

systems. By the help of four hydrauliccylinders that are fixed to the four wheels

arbitrary wheel suspension positions can be

realized. The test bench mainly consists of

12 hydraulic actuators (one for longitudinal,lateral and vertical force generation on each

wheel) that can be operated individually.. In

order to simulate a contact zone betweenwheel and the ground the test bench can be

equipped with aerostatic bearings. Highly

sophisticated sensors, amplifiers and

measurement data acquisition systemsrecord any value in the course of time that

might be of interest. Fig. 17c shows the

optical Autocollimator sensors that are usedto measure the camber and toe angles. A

large number of fastening devices, which

serve to fix the vehicle body to the test rig,eliminate the influence of body stiffness on

the measurement. Moreover, the fastening

systems allow the easy fixing of any car tobe tested without the need to produce costly

adapters. Apart from that it is also possibleto fix and to examine single axles and wheelsuspensions without examining the complete

vehicle.

The complete system is controlled by areliable computer system to reduce the

operator's influence on the results and to

achieve an optimum reliability andrepeatability of the measurements. Extensive

routines shall exclude a malfunction of the

test bench to avoid a damage of the vehicle.This system is presented in Figure 16 .

Typical characteristics which are supposed

to be examined are: roll axis position, roll

stiffness or steering compliance. But alsocomplete driving maneuvers such as 'steady

state cornering' or 'breaking maneuvers' can

be simulated at the test bench. Knowledge



17/19


can be gained about the self steering

properties of the vehicle by using this

method. The technical data of the test rigare:

variable wheelbase: 2000 to 3250 mm variable track width: 1180 to 1650 mm

max. vertical displacement at the wheel:

400 mm

max. wheel load: 14 kN

max. lateral force (per wheel): 10 kN

max. brake force (per wheel): 10 kN

max. Traction force (per wheel): 5.5 Kn



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Fig 18- Measurement of Toe and camber

angle

5.2 Shaker Rig Test- Dynamics and

vibrations are much harder to understandthan static forces. Ever since man has been

building and driving cars, the complex

systems of springs and dampers have createda complicated symphony of noises and

vibrations. The passenger car industry has

been mounting cars on four-post shaker rigsfor years, since it allows for more precise

evaluations of body and suspension

dynamics than running on a road. The inputs

can be simple repetitive vibrations (sinewaves), or they can be representations of

real roads. While undergoing input from the

road, sophisticated dynamic measurementdevices provide insight to how the system is

working. Generally, unwanted noises and

vibrations entering the passenger

compartment are the focus of theseinvestigations. In racing, the only objective

is to go fast. One of the main limiting factors

is how well the tires stay in contact with thetrack surface. The basic use of the shaker rig

is to optimize the springs and shocks to

minimize tire load variations whilemaintaining reasonable body motion control.

We have all seen a car running down the

highway with a bad or missing shock

absorber. The body is bouncing up and downlike a boat on big waves, and the tire may

also be hopping up and down showing

daylight on each up cycle. This is arepresentation of what happens when the

system of springs and masses is very under-

damped. The shock absorbers are the key

element here. They have to do the job ofcontrolling the body motions as well as the

wheel motions. One device (normally a

shock absorber) mounted between thechassis and the suspension is asked to

control a spring connecting two different

masses, each with its own natural frequency.

To further complicate the issue, the fourcorners of the car work independent of each

other but are tied together by the body

structure. The wheels basically movestraight up and down relative to the chassis

while the body has several motions relative

to the ground. Engineers call these bodymotions heave (movement up and down),

pitch (forward and back) and roll (side to

side). Each of these motions is resisted bythe springs at the four corners of the car.

Resistance to these motions causes forcevariations between the tire and the road. Thetrick is to find the balance point. Tie the car

down too tight and the force variation goes

up, but freeing it up too much can do the

same thing in the opposite direction. Therehas to be a compromise for the correct

amount of damping that gives the best load

control. Finally, there is one last, but veryimportant, variable to throw into the mix.

Driver preferences come into play here in a

big way; some drivers like very little bodymotion, while others don't mind a car that

moves around a little more.The seven-post

shaker rig is used in racing work because

aerodynamic downforce and track bankingadd to the wheel loads. The amount of load

added can be more than the static initial

weight, so it must be included in the test



19/19


procedure. The seven posts are hydraulic

cylinders. Four of them have flat pans the

tires sit on and support the car. The otherthree are called the aeroloaders and attach to

the sprung mass. Normally, two are mounted

to the front of the chassis some distanceapart while the third one is mounted at the

rear on centerline. Loading on these

cylinders is done to pull the car down,

opposing the four wheel pans. Theaeroloaders simulate other forces on the car

such as the squashing from inertia loads as

the car rolls through a banked turn ordeflections due to aerodynamic loading. By

adjusting the load on the three downforce

rams we can simulate any combination of

roll, heave or pitch displacement to recreatespecific conditions seen on the track and

repeat that condition. Normally, wheel

travels from actual test-session recordingsare re-created in the lab. By using the

correct deflections indicated by wheel travel

with the same springs and bars as those usedin the track test, the loads will be correct.

Deflections are used because race teams

seldom have vertical loads as ameasurement.

Fig 19- Seven post shaker rig

Conclusion- Suspension systems are one ofthe most important system of an automotive

.In this paper, Various types of suspensions

and their functions has been introduced. The

types of cars on which the suspension

systems are used has also been discussed.

Particular Emphasis has also been given onthe mechanics of tyres, Suspension

Geometry and how it affects the vehicle

performance. Various physical test like K &C Rig test and Shakers Rig test has also

been given and fully explained. More

emphasis is given on the Modelling and

analysis of the Automotive Suspensions.

References-

1. Advance Vehicle Technology byHeisler

2. Vehicle Dynamics by ThomasD.Gillespie

3. www.howstuffwork.com

4. Advance Race Dynamics by

Milliken & Milliken

5. www.sidebrake.net/forums/index.ph

p?topic=841.0

6. http://www.circletrack.com/techarticl

es/seven_post_shaker_rig_suspensio

n_dynamics/index.html

COVENTRY UNIVERSITY UK Page 19
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