Fundamentals• PURPOSE OF THE SUSPENSION SYSTEM

– Maintain correct vehicle ride height – Reduce the effect of shock forces – Maintain correct wheel alignment – Support vehicle weight – Keep the tires in contact with the road – Control the vehicle’s direction of travel

Main Components of a Suspension

Springs

Shock Absorbers

Strut

Tires

SpringsSupport the weight of the vehicle, maintain ride height, and absorb road shock

bounce : vertical movement of suspension

jounce : compression, upward movement

rebounce : extension, downward movement

The spring bounces at its natural frequency.

Sprung Mass : Mass carried by the spring, like the vehicle body, transmission, frame

Unsprung Mass : Mass that is not carried by the springs, like the tyre, wheels.

TYPES OF SPRINGS

COIL SPRINGS

LEAF SPRINGS

TORSION BAR

AIR SPRINGS

Coil springs

Diameter and length determine whether the spring is strong or flexible

No inter-leaf friction therefore smoother ride

Variable rate achieved by varyingMaterials of different thickness

Progressive winding of the spring

Variable rate providesLower spring rate under non-load conditions

Higher spring rate under load conditions

Leaf springs

Multi leafStacked steel plates of different lengthsSpring compress to absorb road shocksLeaf springs bend and slide for suspension movement

Mono leafTapered leaf spring- thick in middle and tapers toward sidesComposite material

Mounting configuration Longitudinally in pairsSingle traverse mounted leaf spring

Torsion Bar

Torsion bar straight or L-shaped bar of spring steel

Torsion bar twists to provide suspension

Mounting configuration Longitudinally – mounted solidly to the frame at one end and other end to the moving part of the suspension

Traverse mounting possible.

Air springs

Air spring is a rubber cylinder filled with compressed air

Movement of the piston attached to lower control arm affects the air compression to provide the spring action

On board compressor provides air through a valve at the top of air bag

Valve opens to add or release air according to the load requirements

Most popular on passenger cars, buses, and heavy trucks

Shock absorbers/ DampersControl movement

SpringSuspension

Turns K.E thermal/ heat and dissipates though hydraulic fluidLocated in pressure tube and at the end of suspension which forces the fluid in to the piston of shock absorberHas a piston with orifices to control flow of hydraulic fluidResistance to the flow depends on:

Number and size of orificesSpeed of movement of the suspension

Velocity sensitive faster the movement of suspension more resistance

Types of damping

Viscous

Coulomb

Structural

Fd

x

• Damping force linearly dependent on velocity

• Pure viscous damping force is an ellipse w.r.t displacement

• With the spring force added, the Damping force is inclined forming a hysteresis.

• Fd = πcwx

Viscous damping

Fd

x

• Damping force dependent on direction of velocity

• Pure coulomb damping force is a rectangle w.r.t displacement

Coulomb Damping

StrutsA common type of damper unitFirst main job:

Damping functionVelocity sensitiveSimilar structure of shock absorberMajor structural component of suspension – replace the upper control arm and ball joint

Second job:Structural support support the spring

hold tire in aligned positionBear side load placed on suspension Affect riding comfort, steering, braking, wheel alignment and wear on suspension

Suspension typesAbility of opposite wheels to move independent of

each other – Dependent – Independent– Semi dependent

Dependent: A live axle holding both wheels parallel to each other and perpendicular to the axle

Change in Camber of one will affect the same in otherIndependent: Not connected at all or connected through

universal joints with a swing axleAllows wheels to rise and fall without affecting opposite

wheelsAnti roll bars are classified as independentSemi-dependent: Swing axle is used, but the wheels are also

connected with a solid tube deDion axle

Ride Control

Objectives of a suspension steering stability good handling Maximize passenger comfort Limits of vibration Exposure Limit Fatigue Reduced comfort boundaries Design aspects Vibration isolation Suspension travel Road Holding

Ride Control

Vibration isolationResponse of sprung mass to the excitation from the ground

Judicious selection of damping required for control over the natural frequency of sprung mass and ride comfort

Suspension travel The deflection of the suspension spring or the relative motion between the sprung and the unsprung mass

Road Holding Vibration effects normal load acting b/w the tire & road

In turn effects the cornering force, tractive effort and the braking force developed by the tire

Suspension for AFVRequirement:

High loadGreater suspension travelProtection form land-mines & anti-tank weaponsWheel pairs of six/eight for ride over rough groundWide range of ambient temperature: -10 ˚C to 50 ˚C

EvolutionFixed suspensions Leaf spring suspensionsChristie suspension: coil springs inside armor hull and bell crankHorstmann suspension –combination of bell crank and exterior coil springsTorsion bar suspensionHydro-pneumatic suspension

Hydro –pneumatic suspension• Working principle:

As the road wheel rises the axle arm is lifted and this rotates the crank which moves the piston via the con rod; the piston displaces oil through the damper valve and moves the separator piston; this causes the gas to compress which produces the

spring force

Hydro-pneumatic suspensionAdvantages:

Progressive spring ratesofter rate around the static position and a stiffer rate near full deflection

Totally independent System is external more space inside the vehicle Better cross-country performanceSensitive equipment subjected to lower acceleration levels Reduced shock loadsvary the gas pressures of each unit

Variants:Hydro strutHydrogasTandem hydro strut for

torsion suspensionVariable damper & spring

Static Test and Dynamic Test

Static Test Load at Rebound,

Static, Bump Spring Characteristic Curve

Dynamic Test• 300 ± 200 mm, 0.1

Hz

• 250 ± 100 mm, 0.8

Hz

0

2

4

6

8

10

12

14

16

18

20

0 100 200 300 400 500 600

Estimated Value

Before Dynamic

After Dynamic

Lo

ad, T

on

s

Displacement, mm

Spring Characteristics

Endurance Test Results

0

20

40

60

80

100

120

140

160

0 60 120 180 240 300

FPC

SPC

DAMPER

Time, min

Tem

per

atu

re,

°C

Variation of Temperature with Time

Status of suspension technology

France

Leclerc/ EPC MBT

50t 6x6 Twin-cylinder hydro pneumatic– 425 total travel

AMX-40 MBT

43t 6x6 Torsion bar

AMX-32MBT

39t 6x6 Torsion bar

AMX-30 and AMX-30-S

36 5x5 Torsion bar

GermanyLeopard 2 55.15 t 7x7 Torsion bar

suspension and advanced friction dampers

Leopard1A1

40t 7x7 Torsion bar& hydraulic shock absorbers

Leopard1A4

42.2t 7x7 Torsion bar & hydraulic shock absorbers

TAM 30.5 6x6 Torsion bar

Leopard2 MBT

Leopard 1A1 MBT

IndiaVijayanta:40,4tonnes and 6x 6 configuration with torsion bar suspensionIndian production of Vickers MBT, UK

CI Ajeya:43.5 tonnes and 6x 6 configuration with torsion bar suspensionIndian production and upgraded T-72M1

IndiaArjun: Combat weight of 58.5 tonnesHydro- mechanical Suspension . 7 x7 road wheel configuration

Ex-tank: Combat Weight 47tonnes has Suspension type Torsion bar with Hydrogas struts. Indian production of T-72M1 series tank chassis fitted with the complete turret of the Arjun MBT

Israel

Mk 1 Merkava

60 t 6x6 Horstmann suspension with helical spring

MK3 Merkava

63t 6x6 concentric coil springs and have a hydraulic rotary damper

MK4 Merkava

65t 6x6 Horstmann suspension with rotary shock-absorbers

United Kingdom

Challenger 1 MBT weighs around 62 t and has hydro pneumatic suspension

The In-arm suspension is for high-performance system with minimal space claim and weight

Vickers MBT MK1weighs around 38.6tonnes and MK3 weighs around 38.7 tonnes have six road wheels on each side with torsion bar suspension

Tandem Hydro strut suspension

United KingdomHydro strut are used in Centurion, M109, and Cheiftian

Khalid MBT weighs 58 tonnes has hydro pneumatic suspension with 6 road wheels on each side

Cheiftian has 12 MK types and have Horstmann suspension systems

United States

M1A2 Abrams with combat weight of 63,086 kg has advanced torsion bar suspension

Suspension System for M1 MBT

The suspension systems of M48, M60 and centurion and T-series are being retrofitted with in-arm suspensions developed by Textron Marine called 14k, 10k and 6k systems (In-arm suspension)

Russian federation

T-72 S MBT

46.5 t 6x6 torsion bar suspensions with hydraulic rotary shock absorbers

T-80 BT-80 U

42.5 t46 t

6x6 torsion bar suspension

T-90 MBT

46.5 t 6x6 torsion bar suspension &hydraulic shock absorbers

Semi active rotary damper

BAE Systems Land Systems OMC (previously Reumech Ermetek) of South Africa and Horstmann Defense Systems of the UK teamed to develop, as a private venture, the Adaptive Damping System (ADS).

In-arm suspension

A compact light weight design to meet the requirements of air portable tracked vehicles

Options for in-arm suspension include variable height , Adaptive damping system, and suspension lockout as well as the development of super light weight with extensive use of titanium.

In arm suspension is used on surrogate vehicle chassis

Concept of active and semi active suspension

Acceleration of the sprung mass is sensed

Correction signal, as per control strategy, is fed to the

variable damper/ actuator.

Variable damping achieved by variable orifice, MR, ER

fluids, proportional or servo control valves.

Active suspensions consume very high power and are

more complex.

Un-sprung mass

Sprung mass

Control & power supply

Variable Damper

sensor

sensor

Semi-Active suspension system concept

Un-sprung mass

Sprung mass

Control & power supply

Force actuator

sensor

sensor

Active suspension system concept

Requirements of spring and damper

Frequency Requirement Spring Requirement Damper

For ride For handling

For ride For handling

Near Natural frequency of unsprung mass

soft stiff low

Near Natural frequency of sprung mass

Little influence;

soft

high

High (> natural freq) Not influential

low high

Low (< natural freq) stiff Not influential

High

Variable damping

Electro-rheological fluid mixture of dielectric base oil and fine semi

conducting particles Its resistance to flow is related to the electric

voltage across it

Magneto-rheological fluidsmart material --mixing fine particles in a fluid of

low viscosity The particles form into fibrous chain like

structures in the presence of a magnetic field

MR Damper

The fluid must pass through the MR valve for flow

MR valve is a fixed size orifice with the ability to apply a magnetic field

results in an apparent change in viscosity of the MR fluid

Imparts pressure differential for the flow of fluid proportional to the flow required to move the damper rod.

The accumulator is used to avoid cavitation in the low pressure side of the MR valve

Accumulator

MR valveMR Fluid

Control strategies

The Conventional PID Optimum Control And Ricatti EquationPredictive ControlRobust Control / L1-Optimal ControlSkyhook Damping Control

The Conventional PID

Damping set to two values:• Maximum • Minimum

Setting parameter: Relative velocity (Rv) b/w sprung mass and unsprung mass

Rv is in the direction of absolute velocity of sprung mass damping is set to maximum

Robust and simple

Optimum Control And Ricatti Equation

Penalty function J = ∫(XT Q X + UT R U) dt Objective function is linear quadratic Ricatti solution X = AX +B An optimum control strategy is one that

minimizes the following:• rms value of the sprung mass acceleration• the rms value of the suspension travel• the rms value of the dynamic tyre deflection

Predictive Control

Input obtained by solving an open-loop optimal control problem over a fixed prediction horizon into the future

Applied input is determined on-line at the recalculation instant

Implemented until new measurements become available

As applied input is based on an optimal control problem, it is possible to take specifications into account

Robust Control / L1-Optimal Control

for plants with parametric or dynamic uncertainties

time-invariant and time-varying controllers Norm-based optimization is used to address

robustness and performance issues of the control system

Criteria: H∞, H2 and in particular the L1

norms, as well as multi-objective synthesis problems

Skyhook Damping Control

An optimum control strategy, followed for suspension control problems

The damper is considered to be positioned such as to connect its one end to the sprung mass and the other end to an inertial reference in the sky,

instead of the unsprung mass.

Csky

M

Base wheel

Testing of AFV suspensions

Control Room

Controllers

Oil Reservoir

Heat Exchanger

AccumulatorFilter

Pump

Suspension Test RigPower Pack

Actuator

Pit Mounted Hydraulic System

Specimen

Instron test rig• Specification

– Hydraulics• Powerpack:4 nos• Max pressure: 250 bar• Flow: 80 lpm/ power pack

– Vertical actuator• ±300mm stroke• Load: 25T dynamic, 50 t static• Frequency: 0.1 Hz/ ±250mm,

0.8 Hz/ ±100mm– Measurements:

• Load cell• Pressure: 0-1000 bar• Surface temperatures: 0-200˚C

One-axial dynamic test rig

Two-axial Quasi-static test rig

Hydraulic Power packHydraulic Power Unity of 550 KW able to power 1000 l/min @ 280 barIt consists of pumps for both motion and fluid cooling, drive motors, controls, fluid reservoir, and associated equipment and valves.

Acceptance Test rig Minimum low end specifications for production testing of

the proven design of MBT Arjun suspensions

It has a hydraulic actuator controlled by a electro-proportional valve and powered by a hydraulic power pack of 400 lpm at 250 bar

The actuator is configured in an inclined mounting, which does not need a special structural strong floor

The motion of the actuator is transmitted to the test specimen via bell crank mechanism

If proven, it can be used by the production agencies for qualification of the production line of HSUs.

Test specifications

Test Frequency (Hz) Amplitude (mm)

Cycle 1 0.1 200

Cycle 2 0.8 100

Sample Stroke Pattern

Acceptance Test rig

ActuatorLH & RH Mounting for test units Electric motor -Pump

Conclusion

State of the art is the passive type, hydro-pneumatic suspension, for the AFVCurrent research actively pursued on semi-active and active suspensionKorea, USA. etc., are ready to field AFV with semi-active Hybrid (passive suspension for some stations and other with active suspension/semi-active) is the next optionCVRDE fully prepared to take up development of semi-active suspensions for AFV’s