A REVIEW OF VEHICLE SUSPENSION SYSTEMS FOR RIDE, HANDLING AND DURABILITY
The main objective of this assignment is to investigate and research into various automotive suspension system configurations and the methodologies used to design, analyse, and physically test them.In this assignment, the suspension system geometry is discussed.After that, different types of suspension systems are explained.Then the role of suspension system in the context of vehicle ride quality, handling, and durability is studied with the detailed analysis of vehicle durability through durability testing. At the end, the testing of the suspension system is discussed with the special focus on Adhesion.
Suspension system is defined as the system of springs,shock absorbers and linkages that connects a vehicle to its wheels.Suspension system generally have dual purpose,firstly it contributes to the car road handling and braking for good active safety and driving pleasure and secondly keeping the occupants of the vehicle comfortable and reasonably aloof from road noise,vibrations and bumps.The steering is also an important part in the suspension system,both have to work together to get an effective suspension system.
The stability and effective handling of a vehicle depends upon the designer's astute selection of the optimum steering and suspension geometry which generally includes the wheel camber,castor and kingpin inclination.
2.1Wheel Camber Angle
It is the lateral tilt or sideway inclination of the wheel relative to the vertical.When the top of the wheel leans inwards towards the body the camber is said tobe negative,alternately an outward leaning wheel has positive camber.
Road wheels were earlier positively cambered to maintain the wheel perpendicular to the early highly cambered roads and likely shaped to help in the drainage of rain water.With nowdays because of underground drainage,road camber has been vastly reduced or even removed and like 0.5 to 1.5 degrees.
Cambered wheels with a positive steer develop a little more slip angle as compared to the uncambered wheels.When subjected to sudden crosswinds or irregularities on roads,the tyres do not instantly deviate from their steered path,as a result more stable steering is achieved.With the use of wider tyres as as a standard on few cars the camber should be kept minimum so as to reduce tyre edge wear alarmingly unless the suspension system is being designed to cope with the new generation of low profiile wide tread width tyre.
2.2Swivel or Kingpin Inclination
Kingpin Inclination is the lateral inward tilt(inclination) from the top between the upper and the lower swivel ball joints or the kingpin to the vertical.The swivel ball'conatct centre on the ground would be offset to the centre of the tyre contact patch,if its axis is vertical or perpendicular to the ground.The offset between the pivot centre and the contact patch centre is equal to the radius(scrub radius) of a Semicircular path followed by the rolling wheels when being turned about their pivots.
The offset scrub produces a torque 'T' created by the product of the offset radius 'r' and the opposing horizontal ground reaction force 'F'(T=Fr(Nm) when turning the steering.The steering will tend to be heavy ,so to solve this problem,a large pivot to the wheel contact centre offset requires a big input torque to overcome the opposite ground reaction.An adjusment is made by offsetting the pivot and contact wheel centres to approx 10-25% of the tread width for a standard sized tyre.This small adjusment permits the pivot axis to remain within the contact patch,so enabling a rolling movement to still take place when the wheels are pivoted to reduce the tyre scruff and creep(slippage).
When we combine the camber and swivel joint inclination it is known as included angle and the intersection of both of these axes at one point at ground level is known as centre point steering.
The inclination of the swivel ball joint axis or kingpin axis in the forward and backward direction,so that the tyre contact centre is either behind or in front of the imaginary pivot centre produced to the ground ,is known as the castor angle.When the wheel contact centre trails behind the pivot point at ground level,it is said to be positive castor,ahe pnd when the centre leads the pivot axis,it is said to be negative castor.when pivot centre and the wheel contact centre meet the castor angle is said to be nil.Because of this,the steered wheels become unstable when the vehicle is travelling on a straight path.
The castor angle effects could be seen in the following figure,as the steering is partially turned on one lock
The trail or lead distance between the pivot centre and the contact patch centre rotates as the steered wheels are turned so that the forward force FD and the equal but opposite ground reaction FR are still parallelbut offset by a distance 'x'.So a twisting moment or a couple M is generated of magnitude M=FX ,where F=FD =FR .As the vehicle is moving ,M continously trying to reduce itself to zero by cancelling the offset' x'.Because of this the offset 'x' produces a self-rightning effect to steered wheels. As a result with wheel traction and the vehicle speed,the self rightning action increases.
2.4Swivel joint positive and negative offset
The positive offset is the offset of the ball joints on the inside of the tyre cotact,where as in negative offset,the ball joint touches the ground on the outside of the contact centre.
When we apply the brakes,the force of inertia and the positive offset distance together produces a torque due to which the wheels pivot about the contact patch centre in an outward direction at the front.When we drive the right wheel on a slippery surface,the vehicle tend to go the left because as we apply brakes vehicle resist moving forward and as a result the left wheel also turn to the left.Therefore the positive offset restricts the natural tendency of the vehicle to move left if the right wheel skids instead of going straight.
In negative offset, the swivel ball joint inclination centre line meet the ground surface on the outside of the contact patch centre.Because of this, the vehicle which is moving forward produces a torque which makes the wheel turn inwards at the front.
This happens as stub axle assembly and ball joints are being forced forwards and around the centre made by the offset distance which is negative.
2.5MacPherson Strut friction and Spring offset
The MacPherson strut has a problem in the sliding movement of the strut as the cylinder rod bearing and the damper piston move closer together,as a result they want to stick to each other.We cannot alter their distance because side loading will increase because of this.We can generally reduce by:-
(a)Using 'Stiction'by reducing the friction between the sliding members.This can be done by applying a layer of impregnated poly-tetra-fluorethylene(PTFE) which provides a very low coefficient of friction
(b)Secondly by removing the bending moment on the strut while driving in a straight line,but the bending moment under cornering conditions will remain.
2.6Suspension Roll Centres
It is the centre of a suspension system relative to the ground about which the body will instantaneously rotate.With the angle of roll and the suspension the position of the roll centre varies.
Roll axis is the line which joins the front and rear suspension roll centres.Roll centre's height is different for the front and the rear suspension.The inclination of roll centre depends upon certain factors such as centre of gravity height and weight distribution between the front and rear axles of the vehicle.
2.7Short Swing Arm Suspension
An overturning moment is generated because of which the body rolls outwards from the centre of turn,while cornering.The instantaneous response of a vehicle is that the inner and outer swing arm move up and down respectively at their ends and forces the inner and outer wheels to tilt on their instantaneous tyre to ground centres,Iwg1 and Iwg2,opposite to the body roll.
The effective body roll will only take place if there is two movements within the suspension geometry.
(a) The instantaneous centres Iwb1 and Iwb2,rotate about their instantaneous centres Iwg1 and Iwg2 in relation to the body roll movement.
(b) Iwb1 and Iwb2,move in a circular path in which the centre made by the intersecting projection lines drawn through the tyre to ground instantaneous centres Iwg1 and Iwg2.
As a result the of tilting and rotation of the swing arms about the tyre to Iwg1 andIwg2 centres,an arc will be produced which is tangential to the circle on which the centres Iwb1 and Iwb2 touch.As a result the meeting point where the two projection lines meet is known as the instantaneous centre of rotation for the body relative to the ground or generally as the' body roll centre'.
The roll centre height for a short swing arm suspension by taking into account similar triangles from the figure above is:-
where h=Roll centre height
l=Swing arm length
2.8Long Swing Arm Suspension
This suspension system is pretty much the same as the short swing arm but the difference is in the arms as it extends to the opposite side with respect to the wheel it supports and because of this both the arms overlap each other.
The roll centre is known through a straight line which joins the tyre contact centre and the swing arm pivot centre for each half suspension.The point where both the lines will meet is the 'body roll centre' and the its height is the distance above or below the ground.
As this suspension system has a longer arm than the short arm suspension system,therefore the lines joining the tyre contact centre and the swing arm pivot has a slope which is not so steep.As a result the crossover point which is helpful in finding the roll centre height is lower for the long swing arm as compared to short swing arm.
The short arm has a disadvantage that the camber changes a little more when body rolls and secondly while cornering the axle arm occasionally jack the body up.
3.0TYPES OF SUSPENSION SYSTEM
3.1Transverse Double Wishbone Suspension
If we draw the lines through the upper and lower wishbone arms and extend them until they meet either inwards or outwards,the point where they meet usually is a virtual instantaneous centre for an imaginary triangular swing arm suspension.The arc made by the arms is similar to the virtual arm,therefore we can derive the body roll centre.
For inwardly converging the roll centre can be derived in two ways:
Firstly,by extending the straight lines through the wishbone arms upto the point when they meet opposite to the body at theie virtual centres Iwb1 and Iwb2.
Secondly by drawing straight lines between Iwg1 and Iwg2 and the virtual centres Ibw1 and Ibw2 for each half.The point where both these lines will meet is called as the body roll centre Ibg.
For the outward coverging,the body roll centre is known by having two set of lines.Firstly we project the straight lines through the arms till they meet each other on the outside of each wheel at their centres Iwb1 and Iwb2.Then we draw sraight lines between Iwg1 and Iwg2 and the virtual centres Iwb1 and Iwb2 for each half and then extend them till they meet near the middle of the vehicle.
3.2 Macpherson Strut Suspension
For this suspension,the centres Ibw1 and Ibw2 and the vertical swing arm are found as we project a line perpendicular to the direction of strut side at the upper pivot.Then we draw the second line which intersects the first one.This is the virtual centre about which virtual swing arm will pivot.
Straight lines are drawn in each half between the contact centre and the virtual centre.The point where they both meet is the body roll centre.
3.3Rigid axle beam suspension
This suspension system has both the axles rigidly supported by a common tranverse axle beam member which can be anything from a axle beam,a live rear axle hollow circular sectional casing or a De Dion beam.
In this system there is no independent movement of the two stub axles.As a result any body roll take place between the axle beam and the body itself.
There are different methods to cotrol and locate the axle movement:
(1)Longitudinally located semi-elliptic springs
When we support the body with these springs,the roll centre will be approximately at spring-eye level it will become lower and lower as the camber keeps on changing from positive when unloaded to negative when loaded.
(2)Transverse located Panhard rod
The panhard rod used in this acts as a lateral body to take the load,which is attached at the ends to the body and the axle.As the side force is applied ,the body tilts to move either end and also lifts or dips.At the same time the body keep rolling to about the mid position of the panhard rod.
(3)Diagonally located tie rods
In this system ,there is a trailing four link suspension which consists a pair of long lower trailing arms damp the braking torque and driving reactions and they support the tie rods to control the lateral movement if any.It also helps in providing the lateral support and the driving thrust for a helical coil spring.
(4)Transverse watt linkage
In this system ,they have a pair of horizontal rods having their outer ends joined to the body and the inner ends tied to a lever which has the pivot movement to the axle beam.The body will roll about the watt linkage lever pivot point when the body is subjected to an overturning moment.This is the body roll centre.
4.0ROLE OF THE SUSPENSION SYSTEM
4.1Ride and Handling
Ride and handling parameters of vehicle are mainly related to the characteristics of the tyres.Tyres are the reaction point of the vehicle with respect to the road.They take the load of forces and disturbances from the road,and they follow the driver's commands.Their characteristics therefore play an important part in the life of the vehicle with respect to the tyre forces and disturbances,and also the forces which controls the vehicle's stability and cornering characteristics.The tyres typically consists of a system of springs,dampers and linkages.
The steering and bounce characteristics of the wheels play an important part in the final output.Wheels improve cornering ability of a vehicle by compensating for the body roll,they provide directional control as a link to the steering wheel,and they move forward vertically with respect to irregular roads and bumps by helping in smoothning the ride maintaining adhesion with the road.They are through linkages are connected to the sprung weight,and because of this get affected by the rolling and pitching movements about the reaction centres of the suspension system.According to the road and driving conditions,the mechanical properties required for the directional control,cornering forces and the ride comfort keeps on changing accordingly.The suspension and steering linkages are generally designed in such a way that wheels can be moved as required by the driver to meet the various dynamic requirements of a vehicle.But the designer is mostly bounded by the other structural members,the engine and the drivetrain and other parts of a vehicle that has to be fit in the vehicle.Because of this the geometrical errors crept in the vehicle and the suspension system is not the ideal one.
4.1.1 Ride Quality
Ride quality of a vehicle is influenced by many key factors which includes high frequency vibrations,body booming,body roll and pitch and vertical spring action which is one of the key factors in the smooth ride of a vehicle.If the body produces a booming resonance,if the vehicle moves abruptly during acceleration and braking,if it's noisy or rolls excessively,the occupants will have an uncomfortable ride.
Ride quality of a vehicle has some important factors,one of them is the vehicle's response to bumps and it is affected by low frequency bounce and the rebound movements of the suspension system.Because of the bump,the part of suspension ,the undamped suspension will have a series of oscillations that will continue according to the natural frequency of the system.When the natural frequency is between 60 to 90 cycles per minute(CPM),or approx. 1 to 1.5Hz,the ride seems to be most comfortable.When the frequency reaches 120CPM or around 2Hz,the ride is said to be harsh.For example,the suspension of a normal family car has a frequency of about 60 to 90 CPM.With a stiffer suspension,a high performance sports car has a frequency of around 120 to 150 CPM(2 to 2.5 Hz).
The natural oscillations of an adult human body during walking is originally perceived as human sensitivity to ride frequency.An adult walks at a rate of 70 to 90 steps per minute(frequency) with up and down movement of the torso 2 inches(amplitude) with each step.As a result the past designers basically made the effort to limit the vehicle oscillations to these limits,and the ride was also comfortable,and therefore this theory is perceived as correct, but today our knowledge and technological advancements in the field human sensitivity to vibrations is more accurate than before.Today we know that the natural frequeny is affected by the amplitude and there are some uncomfortable frequencies also.Taking an example, frequency between 30 to 50 CPM will produce motion sickness.Frequencies between 300 to 400 CPM will affect the visceral region of the body.1000 to 1200 CPM frequencies affect the head and neck regions.These are basically the vibrations that are likely to be produced from tyres and axle hops.Longitudinal vibrations are generally in the torso.Longitudinal vibrations in the range of 60 to 120 CPM(greatest comfort for vertical vibrations) are most uncomfortable for humans.These vibrations happen when the seat lean rearward at a higher than normal angle or when the vehicle pitches.
The types of disturbances is directly related to the human sensitivity.A good ride depends also upon the overall design of the vehicle apart from the suspension system design.The high frequency vibrations of the wind and the drivetrain noise must be reduced and thoroughly damped to have a comfortable ride and the suspension system should be placed on the required rubber mountings to damp high frequency road vibrations.But inspite of this,the natural frequency of the suspension system is still the backbone of the comfortable ride.
The natural frequency is found out by the static deflection rate of the suspension system.It is the rate at which the suspension compresses as we apply weight.There are other factors which affects the natural frequency of the suspension system such as the system friction and the damping effects.Spring rate is quite different from the static deflection rate.Springs are generally located inside the wheels subjected to mechanical forces of the linkages.It is the distance the sprung weight travels downward when weight is applied.Natural frequency of 1 Hz will be produced with a static deflection of 10 inches.The natural frequency of a suspension can be found out by using the formula
Where NF=natural frequency in cycles per minute(divided by 60 Hz)
SD=static deflection in inches
According to the Newton's first law,the body will continue to move in a straight line until acted upon by a disturbing force.Second law is about the balance between the disturbing force and the reaction of the moving body.In an automobile the force causing and the force opposing the turn will always be equal, regardless of the force is in the form of windgust,any pothole on the road or cornering force produced by the tyres.
The vehicle's inertia forces and the cornering forces of the tyres largely influences the vehicle's feel and handling.Vehicle's weight and balance is the main source for finding the magnitude and the direction of the inertia forces.Angular direction acts as a force which acts in the direction opposite to the turn centre.To produce a stable and controlled turn,and to overcome these forces,a combination of tyres and suspension system is required.Suspension system support ,turn, tilt the tyres and and maintain a perfect relationship with the ground and the vehicle so that their capabilities can be increased.
4.1.3Tyres in a Turn
At low speeds(at parkings) the wheels turn according to their geometrical alignment.The wheels move in the direction they are moving and the vehicle turns about the point made by the projection of the front axles meeting the projection of the rear axles.Due to the slip angle of the tyres,as the speed increases the actual turn centre moves in a forward direction.Slip angle is directly related to the cornering force of the tyre.At higher cornering speeds, as lateral load increases,the tyres moves in the opposite direction to which they are heading because of the creep on the outside.The difference between direction of travel and the direction in which tyres are moving is called the slip angle.
The lateral cornering force get influenced by the vertical load on the tyres at a given slip angle,they are directly proportional to each other.As the cornering force is developed by the tyres with respect to the vertical load,it is called 'cornering coefficient'.It reduces as the vertical load increases.Another cornering force is generated from the tyres camber angle.When the tyres rolls at camber it produces a lateral force in the direction in which it's moving.This is known as 'camber thrust'.
4.1.4Oversteer and Understeer
The Oversteer and Understeer characteristics of a vehicle is generally described by the weight of the vehicle.A vehicle which is heavier at the fronts will tend to understeer and if it's heavier at the back,it tends to oversteer.A vehicle is said to be in perfect steer conditions when weight is evenly distributed between the front and rear axles.The design of the suspension as well as the selection of wheels and tyre size also plays an important part in oversteer and understeer .
Understeer results when the slip angle of the rear tyres is less as compared to in the front.Therefore a greater steering angle is needed to maintain the turn.As the turn cannot be managed because of the steering lock,the vehicle moves to the outside.During understeer,as the driver is taking a turn,the vehicle refused to go in the desired direction.Oversteer produces almost the opposite condition.
During oversteer,slip angle of the front tyres is less than in the rear tyres.As a result the rate of turning increases by itself,therefore driver have to reduce the steering angle to comoensate for oversteer.The steering angle reaches complete lock in the opposite direction during severe oversteer.The vehicle is in 'spin out'condition now.A vehicle which is in understeer condition is considered safer for an average driver.
The cars should be designed with a neutral steer to prevent the above conditions.It is the scientifically correct method in which the slip angles of the front and rear tyres increases together through the range of steer angles.But it has a problem that the car with a neutral steer condition tends to go into the oversteer condition.Therefore designers today create a little a bit of understeer intentionally in order to avoid oversteer.
4.2Durability of a Suspension System
The durability of a suspension system is one of the important part in the development of the complete vehicle because the suspension parts are continuously and directly subjected to various dynamic loads as the vehicle is in motion.Efforts are being made to improve the durability of the suspension system of a vehicle by changing the geometries of the suspension components and parts.
We can take the example of a testing of the Ford Ikon car by using the standard adopted by the ford motor company known as PASCAR to prove the structural durability and integrity of these cars as they cover more than 1.4 million km on Ford's state of the art proving ground in Lommel, Belgium.This whole process uses the detailed research into the different driving patterns to establish the data.
The PASCAR standard demands that all major systems must pass through durability test that covers 160,000 km of driving in the hands of the 90th percentile customer which represents the worst drivers so that they can check the vehicle in any condition from best to the worst conditions.It demands 90% of the worst surfaces, highest speeds and the most demanding start and stop conditions.The PASCAR road and driving conditions for Ikon were based on the customer reviews and the most severe conditions in India which equals the 600,000 kms of normal running conditions.The remaining 800,00 kms were driven on normal road surfaces, but it included mud,hill and water baths.Severe acceleration,braking,cornering and gear changing were employed to ensure Ikon performance across the conditions.
The vehicle durability is also tested on special surfaces known as 'Corrugations', with the help of which appraisals can be done on the cars with the data collected includes hub accelerations,hub displacement, body acceleration and strain gauge.It can also be used for ride and durability model verification.
4.2.2Laboratory based Durability testing
This testing is generally for the vehicle structure using a 4 poster Test Rig.It provides only vertical Accelerations and displacements.From this majority of damaging inputs can be found out.It responds well to the track tests achieving nearly 90% accuracy.The non damaging data can be cut off from the acquired signal giving reduced test duration.But this testing has some limitations also:-
(a) No torque or heat inputs can be derived from the powertrain
(b) Longitudinal inputs(pothole braking) are not reproduced
5.0TESTING OF SUSPENSION SYSTEM
An automotive suspension system's main objective is to provide comfort as well as the safety to the passengers.Irregularities on the road surface causes the tyre and suspension components to displace from their original place,as a result energy is stored in the suspension spring. This energy is then converted to damped oscillations.Proper co-ordination between the suspension components will present a vehicle which is smooth and safe in the presence of road surface irregularities.
In 1971,the European Shock Absorber Manufacturer's Association(EuSAMA) was established having a set of guidelines called "Recommendations For a Vehicle Suspension Performance Evaluation".This document standardised the "road adhesion"measurement, which is superb vehicle safety comparison.
Adhesion is known as the minimum percentage of the instantaneous remnant vertical tyre contact force between the tyre and the road surface.This is calculated by taking the ratio of the minimum remnant vertical load to the static weight.
The phase angle is the angular difference between the absolute difference between the absolute sinusoidal position of the SA400 platform(X3) and the sinusoidal vertical contact force between the tyre and the SA400 platform.
Phase Angle is calculated all through the frequencies of the test.The minimum phase angle,also called damping is the lowest phase angle measured between the sprung and unsprung mass resonant frequencies.The measurement of damping is basically the strength of the damper.SA400 calculates the side to side balance value for both adhesion and the damping.Large values of side to side balance for adhesion or damping describes the problems with the components on one side of the vehicle
The SA400 take conclusions from the adhesion,damping and balance measurements by comparing the measurements to the already set specification. Initially the specifications were universal but later it was found that the SA400 performance can be improved by changing the specifications,with respect to the vehicle being tested.
During this assignment,we had studied the various aspects of the suspension system and research into the the various suspension characteristics and the methods used to analyse and physically test them.We have studied the different types of suspension system,their geometrical configurations and the role of suspension system with respect to vehicle ride quality, handling and the vehicle's durability. We had studied the various types of testing such as the durability testing of two types and the testing of the suspension system as a whole. Basically we have achieved our objective of going through the suspension system, studying the physical test of a suspension system and studying the different roles of the suspension system. We found out that the suspension system plays a very substantial role in an automobile and for the well being of the passengers sitting in it. Without it, a vehicle will not be able to work .And finally we can improve and sustain the performance of a vehicle by carefully maintaining the suspension system.
1. Advanced Vehicle Technology,2nd Edition by Heinz Heisler.
2. SAE Technical Paper Series(2000-01-1329),Experimental Evaluation of a Non-Intrusive Suspension Testing Apparatus.
6. Lecture Notes(Ride and Durability) by Mr. Mike Dickison