SUSPENSION SYSTEM · 2017. 5. 4. · HEIßING, B. & ERSOY, M. 2010. Chassis Handbook: Fundamentals,...

SUSPENSION SYSTEM

Suspension - set of elements connecting wheel axles with vehicles’ body

• A vehicle suspension is required toperform effectively under a range ofoperating conditions including highlevels of braking and accelerating,cornering at speed and traversingrough terrain – manoeuvres whichare required to be done in comfortand with safety.

• Suspension system connects vehiclesbody with wheel and its tire andallow vertical movement of the wheelin relation to body.

• The wheels, through the suspensionlinkage, must propel, steer, and stopthe vehicle, and support theassociated forces.

Suspension system

1. To provide good ride and handling performance – this requires the suspension to have

vertical compliance providing chassis isolation and ensuring that the wheels follow the

road profile with very little tire load fluctuation;

2. To ensure that steering control is maintained during maneuvering – this requires the

wheels to be maintained in the proper positional attitude with respect to the road

surface;

3. To ensure that the vehicle responds favorably to control forces produced by the tires as

a result of longitudinal braking and accelerating forces, lateral cornering forces and

braking and accelerating torques – this requires the suspension geometry to be designed

to resist squat, dive and roll of the vehicle body;

4. To provide isolation from high frequency vibration arising from tire excitation – this

requires appropriate isolation in the suspension joints to prevent the transmission of

‘road noise’ to the vehicle body.

Types of suspension system

• Each spatial body with six degreesof freedom can be constrainedwith suitable elements – like rodlinks – to reduce the number ofDoF

• A suspension system shouldprovide one degree of freedomfor the wheel. This can be done indifferent ways for example byadding 5 rod link – each of whichwould “fix” one degree offreedom.

• In real suspension systems linksconstrain wheel carriers – whichcan be “carry” one or morewheels.

Types of suspension system

Factors which primarily affect the choice of suspension typeat the front or rear of a vehicle are engine location andwhether the wheels are driven/undriven and /or steered/unsteered. In general, suspensions can be broadlyclassified as dependent or independent types.

Dependant suspension system (rigid). Both wheelsare mounted to jointed, rigid axle which is fixedwith frame or body by means of spring elements.

Independant suspension system. Each wheel isfixed to the body (or frame) individually.

Dependant suspension system is used in majority of trucks, and in some passenger cars as a suspension of rear driving axle

The advantage of a dependant suspension system is its simplicity, which is especially important in vehicles with significant load on axles

Independant suspension system has more advantages comparing with dependent suspension systems, and therefore their more frequent usage, especially in trucks, can be observed in recent years.

Most significant advantages are:

•Reduction of unsprung mass of the vehicle.

•Favourable suspension kinematics in terms of driving stability.

•Possibility of very soft spring elements application

Dependent suspensionSolid-axle, leaf-spring

• A solid axle is one in which wheels are mounted at eitherend of a rigid beam so that any movement of one wheel istransmitted to the opposite wheel causing them to steerand camber together

• Solid beam axles are commonly used on the front ofheavy trucks where high load-carrying capacity is required

• wheel alignment is readily maintained, minimizing tirewear

• The major disadvantage of solid steerable axles is theirsusceptibility to tramp-shimmy steering vibrations

• When a live solid axle is connected to the body withnothing but two leaf springs, it is called the Hotchkissdrive,

• Leaf springs are not suited for taking up the driving andbraking traction forces.

• These forces tend to push the springs into an S-shapedprofile

• The driving and braking flexibility of leaf springs,generates a negative caster and increases instability

JAZAR, R. N. 2008. Vehicle Dynamics: Theory and Application, Springer US.

Dependent suspensionSolid-axle, leaf-spring

• To reduce the effect of a horizontal force and S-shaped profile appearance in a solid axle with leafsprings, the axle may be attached to the chassis bya longitudinal bar - anti-tramp

• Although an anti-tramp bar may control the shapeof the leaf spring, it introduces a twisting angleproblem when the axle is moving up and down,

• Twisting the axle and the wheel about the axle iscalled caster

• If the right wheel goes over a bump, the axle is raised at its right end, and that tilts the left wheel hub, putting the left wheel at a camber angle for the duration of deflection.


• The elements which control the motion of a rigid rear axle must allow translation in the vertical directionas well as rotation about the vehicle’s longitudinal axis. To enable these motions, the axle must beconnected to the vehicle’s body with at least one ball joint and one link element.

• The lightweight, frictionless coil springs which provide the springing function for these designs do notplay a role in controlling the axle’s motion in the lateral or longitudinal directions.

• Lateral forces are transmitted between the axle and the vehicle’s body by a Panhard rod or one of theother types of linkages. The motion of a Panhard rod causes the vehicle’s body to shift laterally duringcompression and rebound. This lateral motion can be eliminated by using a Watt’s linkage for lateralcontrol.

Rigid Axles with Longitudinal and Lateral Links

HEIßING, B. & ERSOY, M. 2010. Chassis Handbook: Fundamentals,

Driving Dynamics, Components, Mechatronics, Perspectives,

Vieweg+Teubner Verlag.

JAZAR, R. N. 2008. Vehicle Dynamics: Theory and

Application, Springer US.

Dependent suspensionSolid-axle

• In other cases, there have been problems, such as axletramp, particularly when high tractive force is used. Tolocate the axle more precisely, or more firmly,sometimes additional links are used, such as thelongitudinal traction bars above the axle, opposing pitchrotation

• In other cases, the leaf springs have been retained asthe sole locating members but with the springing actionassisted by coils, giving good load spreading into thebody.

• with the readier availability of coil springs, in due coursethe rear leaf-spring axle finally disappeared frompassenger cars, typically being replaced by the commonwith four locating links, this system being used byseveral manufacturers.

• In response to the shortcomings of leaf springsuspensions, the four-link rear suspension

• The lower control arms provide longitudinal control ofthe axle while the upper arms absorb braking/drivingtorques and lateral forces.

• Occasionally, the two upper arms will be replaced by asingle, triangular arm, but it remains functionally similarto the four-link.

• The ability to use coil springs (or air springs) in lieu ofleaf springs provides better ride and NVH by theelimination of the coulomb friction characteristic of leafsprings.

DIXON, J. 2009. Suspension Analysis and Computational

Geometry, Wiley.

Dependent suspensionSolid-axle

• The basic geometry of the four-link system isretained in the T-bar system with the cross-arm of the T located between longitudinalribs on the body, allowing pivoting with thetail of the T, connected to the axle, able tomove up and down in an arc in side view.

• The rigid axle is sometimes fitted with a rigidtube going forward to a cross member inwhich it can rotate as in a ball joint. This,perhaps confusingly, is called a ‘torque tube’,presumably because it reacts to the pitcheffect of torque in the driveshafts acting onthe wheels. It does give very good location ofthe axle in pitch. Additional lateral location isrequired at the rear, such as by a Panhardrod

DIXON, J. 2009. Suspension Analysis and Computational

Geometry, Wiley.

Undriven Rigid Axles

• Undriven rigid axles, used at the rear of

front-drive vehicles, have the same

geometric location requirements as live

rigid axles, but are not subject to the

additional forces and moments of the

driveaction, and can be made lower in

mass.

• lateral location can be optained by the

long diagonal member. This form

eliminates the lateral displacements in

bumpof the Panhard rod. If the

longitudinal links are fixed rigidly to the

axle then the axle acts in torsion as an

anti-roll bar, the system then being a

limiting case of a trailing-twist axle.

DIXON, J. 2009. Suspension Analysis and Computational Geometry, Wiley.

Semi-dependent systems

• In this form of suspension, the rigid connection between pairs of wheels isreplaced by a compliant link. This usually takes the form of a beam whichcan bend and flex providing both positional control of the wheels as wellas compliance. Such systems tend to be very simple in construction butlack scope for design flexibility.

Independant suspension:

A broad term for any automobile suspension system that allows each wheel onthe same axle to move vertically (i.e. reacting to a bump in the road)independently of each other. Note that “independent” refers to the motion orpath of movement of the wheels. It is common for the left and right sides ofthe suspension to be connected with anti-roll bars or other such mechanisms.The anti-roll bar ties the left and right suspension spring rates together butdoes not tie their motion together.

Independent suspension Disadvantages:

• Occurrence of the moment increasing side roll

• Inclination of wheels along with the body while cornering

• Different values of side forces on right and left wheels

- angle of body inclination while cornering; -

angle of wheel lean; FY,W,f,i lub O – side forces; mBo,f -the part of mass applied on front axle; Fc,Bo,f -centrifugal force

Independent Suspension

Independent suspensions is introduced to let a wheel to move up and down without affecting the opposite wheelTheir main advantages are:little space requirement;• a kinematic and/or elastokinematic toe-in

change, tending towards understeering• is possible• easier steerability with existing drive;• low weight;• no mutual wheel influence.

REIMPELL, J., STOLL, H. & BETZLER, J. 2001. The

Automotive Chassis: Engineering Principles, Elsevier Science.


The coil spring may be between the lower armand the chassis,. It is also possible to install thespring between the upper arm and the chassis,or between the upper and lower arms. Ineither case, the lower or the upper arm, whichsupports the spring, is made stronger and theother arm acts as a connecting arm.

The McPherson strut is a further development ofdouble wishbone suspension. The upper transverselink is replaced by a pivot point on the wheel housepanel, which takes the end of the piston rod and thecoil spring. Forces from all directions areconcentrated at this point and these cause bendingstress in the piston rod.


Independent SuspensionWhen the two side bars of an A-arm are attached toeach other with a joint, then the double A-arm is called amulti-link mechanism. A multi-link mechanism is a six-barmechanism that may have a better coupler motion than adouble A-arm mechanism. However, multi linksuspensions are more expensive, less reliable, and moreComplicated compare to a double A-arm four-bar linkage.There are vehicles with more than six-link suspensionwith possibly better kinematic performance.

The main advantages of the double wishbone suspension are its kinematics

JAZAR, R. N. 2008. Vehicle Dynamics: Theory and Application,

Springer US.

REIMPELL, J., STOLL, H. & BETZLER, J. 2001. The

Automotive Chassis: Engineering Principles, Elsevier Science.

HEIßING, B. & ERSOY, M. 2010. Chassis Handbook: Fundamentals, Driving

Dynamics, Components, Mechatronics, Perspectives, Vieweg+Teubner Verlag.



Springer US.

HEIßING, B. & ERSOY, M. 2010. Chassis Handbook: Fundamentals, Driving

Dynamics, Components, Mechatronics, Perspectives, Vieweg+Teubner Verlag.

Modern McPherson-type suspensionsystems feature a coil spring inclined inthree dimensions located above thewheel. The spring is oriented such that itscentral axis is offset from the axis of thedamper’s piston rod . The forward end ofthe crescent-shaped lower three-pointcontrol arm is connected to the wheelcarrier via a ball joint. This allows thewheel carrier to rotate freely with respectto the control arm

Independent Rear Driven

• The most common early independent driven suspension was thesimple swing axle, which has the advantage of constant driveshaftlengths, and low unsprung mass


Independent DrivenChapman strut

The Chapman strut is a strut suspension in which the lower lateral location is provided by a fixed-length driveshaft.


relatively recent extension of the wishboneconcept is to separate each wishbone into twoseparate simple links. There are then five links intotal, two for each wishbone and one steer anglelink. This system has been used at the front andthe rear, and, with careful design, makes possiblebetter control of the geometric and complianceproperties. The advantages seem real for drivenrear axles, but undriven ones have not adoptedthis scheme. The concept has also been used atthe front for steered wheels

Independent Drivenmultilink


Multi-arm suspension

• Advantages : • Obtaining arbitrary offset radius and arm of the

disturbing force, • compensation of longitudinal, pitching while braking

and accelerating , • Favourable wheel inclination in terms of toe-in, tire

wear etc.,• Appropriate longitudinal susceptibility obtained without

loss of vehicle steering precision,

• Disadvantages:• More complicated design,• Higher cost of production,• Possible occurrence of kinetic incompatibility, • Sensitive on joint wear,• Greater requirements for of defamations and durability

maintenance,

Independent Undriven


Trailing-Twist Axles

• This design is a logical development of the fully-independent trailing-arm system. Beginning witha simple pair of trailing arms, it is often desired toadd an anti-roll bar. Originally, this was done by astandard U-shaped bar with two mountings onthe body locating the bar, but allowing it to twist.Drop links connected the bar to the trailing arms.

• A disadvantage of this basic system was that theanti-roll bar transmitted extra noise into thepassenger compartment, despite being fittedwith rubber bushes. This problem was reducedby deleting the connections to the body, insteadusing two rubber-bushed connections on eachtrailing arm, so that the bar was still constrainedin torsion

DIXON, J. 2009. Suspension Analysis and

Computational Geometry, Wiley.

REIMPELL, J., STOLL, H. & BETZLER, J. 2001. The Automotive Chassis: Engineering Principles, Elsevier Science.

• Some manufactures describes rear suspension systems by their own definition,which only sometimes correctly defines the original designs, nevertheless it iscommon that typical systems are only different by its name.

• Double wishbone (Honda Accord, Jaguar S Type) independent suspension withdouble lateral arm; this solution has sport origin

• Delta Link (Volvo), Multilink (Mercedes) both are names of multi-arm suspensionsystem where the only difference is various number and configuration of thecontrol arms.

• Scott-Russell rod (Nissan Primera) - a kind of sectional lateral arm with twointernal joints, that cooperates with torsion axle which connects the trailingarms; this system is similar to historical suspension of rigid axle laterally fixed bymeans of Watt rod.

Roll Center and Roll Axis

HAPPIAN-SMITH, J. 2001. An Introduction to Modern Vehicle

Design, Butterworth-Heinemann.

Roll centre and roll axis concepts are importantaids in studying vehicle handling, enablingsimplifications to be made in load transfercalculations for cornering operations.There are two definitions of roll center, onebased on forces and the other on kinematics.1. point in the transverse plane through

any pair of wheels at which a

transverse force may be applied to the

sprung mass without causing it to roll.

2. the roll center is the point about which

the body can roll without any lateral

movement at either of the wheel

contact areas

• In general each roll center lies on the line produced by the intersection of the longitudinalcenter plane of the vehicle and the vertical transverse plane through a pair of wheel centers.

• The line joining the centers is called the roll axis, with the implication that a transverseforce applied to the sprung mass at any point on this axis will not cause body roll

Roll Center and Roll Axis• The position of the roll centers at the front and back and the course of the direction line joining these –

the roll axis C is of decisive importance for the handling properties: the height of the roll centersdetermines both the wheel load differences of an axle and hence the self-steering properties of thevehicle through the tire properties, as well as the necessary roll suspension, which is again crucial tocomfort in the case of unilateral deflection where a high level of roll rigidity is required and a stabilizer isused.

• The position of the roll center also depends on the instantaneous position of the wheel links• The height of the roll center and the change in the roll center with wheel travel is consequently a

compromise between the following requirements• defined changes in wheel load during cornering to achieve the required (understeering) self-

steering properties;• track changes with wheel travel which are not critical for the dynamics of vehicle movement;• roll spring stiffness which is not crucial to comfort;• desired – or permissible – camber change;• as small as possible reaction forces acting on the body;• the position of the roll axis.


The roll axis should rise slightly towards the

rear in order to make use of fractions of the

body damping to damp the yawing

movements of the vehicle.

Roll Center and Roll Axis

• The design of a chassis firstly requires the determination of the height hRo,f of the frontbody roll center (dependent on the track alteration) so that, in a second step, anappropriate rear axle can be provided; in the case of independent wheel suspensionswith a slightly higher

• If the vehicle is fitted with a rigid axle, the body enjoys less anti-roll support on bendsas a result of the shorter effective distance of the springs relative to the track

• To balance this out, it is recommended that the body roll center be designed slightlyhigher at the rear


JAZAR, R. N. 2008. Vehicle Dynamics: Theory and Application, Springer

US.

Roll Center and Roll Axis• The roll axis is the instantaneous line about which the

body of a vehicle rolls. Roll axis is found by connecting theroll center of the front and rear suspensions of thevehicle. Assume we cut a vehicle laterally to disconnectthe front and rear half of the vehicle. Then, the roll centerof the front or rear suspension is the instantaneouscenter of rotation of the body with respect to the ground.

• To find the roll center of the body with respect to theground, we analyze the two-dimensional kinematicallyequivalent mechanism

• The center of tireprint is the instant center of rotation ofthe wheel with respect to the ground, so the wheels arejointed links to the ground at their center of tireprints.

• The instant center 𝐼18 is the roll center of the body withrespect to the ground.

• To find 𝑰𝟏𝟖, we apply the Kennedy theorem and find theintersection of the line 𝐼12𝐼28 and 𝐼13𝐼38

• The point 𝐼28 and 𝐼38 are the instant center of rotationfor the wheels with respect to the body. The instantcenter of rotation of a wheel with respect to the body iscalled suspension roll center

• So, to find the roll center of the front or rear half of a car,we should determine the suspension roll centers, and findthe intersection of the lines connecting the suspensionroll centers to the center of their associated tireprints.

The Kennedy theorem states that the instant center of every three relatively moving objects are colinear.


Roll Centerinternal or external

• Roll center of an independent suspension can be internal or external.• An internal suspension roll center is toward the vehicle body, while an external

suspension roll center goes away from the vehicle body.• A suspension roll center may be on, above, or below the road surface,

Roll Center and Roll AxisMcPherson

• A McPherson suspension is an inverted slider crankmechanism.

• The point 𝐼12 is the suspension roll center, which is theinstant center of rotation for the wheel link number 2

with respect to the chassis link number 1.



The more vertical the McPherson struts and dampers andthe more horizontal the lower control arm GD1, the closerthe body roll center Ro is to the ground. This results in anadverse camber alteration when the wheels are in bumptravel. Lengthening the lower suspension control arm (pointD1 to D2) improves the kinematic properties.

Roll Center


• The height of the (instantaneous center of rotation) P determines the position of the body roll center Ro

• If P is above ground level, Ro will also be above ground.• The virtual center of rotation distance q from virtual

center of rotation to tire contact-patch center can be measured or calculated simply

Suspension relative anglesVehicle geometry

HAPPIAN-SMITH, J. 2001. An Introduction to Modern Vehicle Design, Butterworth-Heinemann.

Suspension relative anglesCamber


• Camber is the angle of the wheel relative to vertical line to the road, as viewed fromthe front or the rear of the car. If the wheel leans in toward the chassis, it is callednegative camber and if it leans away from the car, it is called positive camber.

• A tire develops its maximum lateral force at a small camber angle. This fact is due tothe contribution of camber thrust, which is an additional lateral force generated byelastic deformation as the tread rubber pulls through the tire/road interface.

• To optimize a tire’s performance in a turn, the suspension should provide a slightcamber angle in the direction of rotation.

• The more the wheel must deflect from its static position, the more difficult it is tomaintain an ideal camber angle.

• When a vehicle is loaded with two or three persons a slightly positive camberwould be useful on passenger cars to make the tires roll as upright as possible onthe slightly transverse-curved road surface and give more even wear and lowerrolling resistance.

Suspension relative anglesSwivel pin offset

HEISLER, H. 2002. Advanced Vehicle Technology, Butterworth-

Heinemann

• Swivel pin or kingpin inclination is the lateral inward tilt(inclination) from the top between the upper and lower swivelball joints or the kingpin to the vertical

• If the swivel ball or pin axis is vertical (perpendicular) to theground, its contact center on the ground would be offset to thecenter of the tire contact patch

• If the swivel ball or pin axis is vertical (perpendicular) to theground, its contact center on the ground would be offset to thecenter of the tire contact patch. The offset between the pivotcenter and contact patch center is equal to the radius (known asthe scrub radius) of a semicircular path followed by the rollingwheels when being turned about their pivots.

• No offset (zero offset radius) prevents the tread rolling andinstead causes it to scrub as the wheel is steered so that at lowspeed the steering also has a heavy response

• A compromise is usually made by offsetting the pivot and contactwheel centers to roughly 10-25% of the tread width for astandard sized tire.

Pivot inclination produces a self-centring action which is independent of vehicle speed or traction but is dependent upon the weight

concentration on the swivel joints and their inclin-ation. A typical and popular value would be 8 or 12°.


Suspension relative anglesCaster

• Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, asviewed from the side.

• If the steering axis is turned about the wheel yw-axis then the wheel has positive caster• If the steering axis is turned about the wheel −yw-axis, then the wheel has negative caster.• Negative caster aids in centering the steering wheel after a turn and makes the front tires straighten

quicker thus is used to enhance straight-line stability.• Most street cars are made with 4−6deg negative caster• Zero castor provides: easy steering into the corner, low steering out of the corner, low straight-line

stability.• Negative caster provides: low steering into the corner, easy steering out of the corner, more straight-

line stability, high tireprint area during turn, good turn-in response, good directional stability, goodsteering feel.

• When a castered wheel rotates about the steering axis, the wheel gains camber. This camber isgenerally favorable for cornering

HEISLER, H. 2002. Advanced Vehicle Technology, Butterworth-

Heinemann

Suspension relative anglesToe


• The amount of toe can be expressed indegrees of the angle to which the wheelsare not parallel.

• Toe settings affect three majorperformances: tire wear, straight-linestability, and corner entry handling

• For minimum tire wear and power loss, thewheels on a given axle of a car should pointdirectly ahead when the car is running in astraight line

• Excessive toe-in causes accelerated wear atthe outboard edges of the tires,

• too much toe-out causes wear at theinboard edges

• Toe-in increases the directional stability ofthe vehicle (makes the steering functionlazy)

• toe-out increases the steering response(makes the vehicle unstable)

When driving torque is applied to the wheels, they pull themselves forward and try to

create toe-in. Furthermore, when pushed down the road, a non-driven wheel or a

braking wheel will tend to toe-out.

Suspension relative anglesToe


• Front toe-in: slower steering response, more straight-line stability, greater wear at the outboard edges of the tires.

• Front toe-zero: medium steering response, minimum power loss, minimum tire wear.

• Front toe-out: quicker steering response, less straight-line stability, greater wear at the inboard edges of the tires.

• Rear toe-in: straight-line stability, traction out of the corner, more steerability, higher top speed.


Springer US.

Suspension relative anglesTrust angle

• The trust angle υ is the angle betweenvehicle’s centerline and perpendicular tothe rear axle. It compares the direction thatthe rear axle is aimed with the centerline ofthe vehicle.

• Zero angle confirms that the rear axle isparallel to the front axle, and thewheelbase on both sides of the vehicle arethe same.

Literature

1. CROLLA, D. 2009. Automotive Engineering e-Mega Reference, Elsevier Science.

2. GARRETT, T. K., NEWTON, K. & STEEDS, W. 2000. Motor Vehicle, Elsevier Science.

3. GILLESPIE, T. D. 1992. Fundamentals of Vehicle Dynamics, Society of AutomotiveEngineers.

4. HAPPIAN-SMITH, J. 2001. An Introduction to Modern Vehicle Design, Butterworth-Heinemann.

5. HEISLER, H. 2002. Advanced Vehicle Technology, Butterworth-Heinemann.

6. HEIßING, B. & ERSOY, M. 2010. Chassis Handbook: Fundamentals, DrivingDynamics, Components, Mechatronics, Perspectives, Vieweg+Teubner Verlag.

7. JAZAR, R. N. 2008. Vehicle Dynamics: Theory and Application, Springer US.

8. KARNOPP, D. 2004. Vehicle Stability, CRC Press.

9. LEON, P. 2008. Mechanika ruchu, Warszawa, WKŁ.

10. NUNNEY, M. 2015. Light and Heavy Vehicle Technology, Routledge.

11. RAJAMANI, R. 2011. Vehicle Dynamics and Control, Springer US.

12. REIMPELL, J., STOLL, H. & BETZLER, J. 2001. The Automotive Chassis: EngineeringPrinciples, Elsevier Science.

13. WONG, J. Y. 2001. Theory of Ground Vehicles, Wiley.

SUSPENSION SYSTEM · 2017. 5. 4. · HEIßING, B. & ERSOY, M. 2010. Chassis Handbook: Fundamentals,...

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