Suspension Unit

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Introduction and insight into general suspensionfundamentals

Suspension Unit


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Why do we need suspension?

Why do we need suspension?

Why not install the spring under drivers seat, if driver

comfort is an answer!!

What are the basic components of vehicle dynamics system? And how do they affect the behavior of the car?

These are the basic questions that you can answer after thissession.


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Weight transfer during cornering

Whenever car is turning, its weight gets transferred to outerwheels.

And hence weight at the outer wheels will be more than theinner wheels.


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Mathematical formulation of weight

transfer

Assume car as a black box, And do not look at the internalstructure of the car.

Now outward centrifugal force on the center ofmass=(MV^2)/R

M= Mass of the car,

V=velocity of the car

R=Turning radius of the car

CG= height of the center of gravity from groundW= Track width of the car.


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Applying moment balance about the contact point of onewheel, we get

dN=(F*CG)/W

Hence while designing the car, our first concern should bedecrease the roll center height as far as possible to avoidtoppling, to facilitate cornering at higher speed.

But they are other constraints such as roll center height

because of which can not decrease it after some point. Why? We will come to that point if time permits.


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Sprung mass: Mass of all the components which aremounted above the spring. Such as, chassis, gearbox, engine,driver weight, batteries and other miscellaneouscomponents.

Unsprung mass: Mass of all the components which arebelow the spring. Such as tires, wheel assembly components,suspension members, brakes components etc.


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Inside a wheel rim


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Basic concept of Suspension geometry

Upright is fixed to the chassis. Hence no movements of upright with respectto the chassis are allowed.

apart from one vertical motion in vertical direction. But this motion is alsoconstrained and hence spring comes into the picture.

We need to have rotational movement to steer the vehicle and hence onemember will be tie-rod and that will be controlled by steering wheel.

Hence we need four members each constraining one degree of freedom,one tie rod and one spring constraining vertical motion.

Instead of having four member constraining each motion we may have lessno. of members and this time some members will restrict two motion.

There are different ways to create this mechanical arrangement, and that mechanical arrangement is

called as suspension unit.


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Types of suspension

Dependant suspension:1. In this type of suspension, two wheels, either front rear are connected through an

axle.

2. Hence if one wheel encounter any bump or for that matter any disturbance, other

wheel will also react.


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Independent suspension:1. In this type of suspension, disturbance or movement in one

wheel does not cause any reaction in other wheel.

2. Typical types of independent suspension include,

a. McPherson Strutb. Double Whisbone/Double A-arm

c. Multilink Type

d. Trailing Arm

e. I-Beam suspension

3. We will look into each type suspension one by one.


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McPherson Strut type suspension


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McPherson struts consist of a wishbone or a substantial compression linkstabilized by a secondary link which provides a bottom mounting point for thehub or axle of the wheel.

This lower arm system provides both lateral and longitudinal location of thewheel.

The upper part of the hub is rigidly fixed to the inner part of the strut proper,

the outer part of which extends upwards directly to a mounting in the bodyshell of the vehicle.

Mounting of spring/strut on the upright allows only rotational motion of strutinside the housing and no other movement is allowed.

This way all the degrees of freedom gets restricted. And the positions as well asfunctioning of tie-rod remains the same.


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Double whisbone Suspension


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Coil Spring type 1 In this type, the lower arm carries most of the load. If you look head-on at this type of system, what you'll find is that it's a very

parallelogram system that allows the spindles to travel vertically up and down.

When they do this, they also have a slight side-to-side motion caused by the arcthat the wishbones describe around their pivot points. This side-to-side motionis known as scrub.

Unless the links are infinitely long the scrub motion is always present. Thereare two other types of motion of the wheel relative to the body when thesuspension articulates. The first and most important is a toe angle (steer angle).The second and least important, but the one which produces most pub talk isthe camber angle, or lean angle. Steer and camber are the ones which weartyres.


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Coil Spring type 2


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Multilink Suspension


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This is the latest development of the double wishbone system described above. The basic principle of it is the same, but instead of solid upper and lower wishbones,

each 'arm' of the wishbone is a separate item.

These are joined at the top and bottom of the spindle thus forming the wishbone shape.

As the spindle turns for steering, it alters the geometry of the suspension They havecomplex pivot systems designed to allow this to happen.

This system gives even better road-holding properties, because all the various jointsmake the suspension almost infinitely adjustable.

There are a lot of variations on this theme appearing at the moment, with hugedifferences in the numbers and complexities of joints, numbers of arms, positioning ofthe parts etc.


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Trailing Arm Suspension


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The trailing arm system a shaped suspension arm is joined at the front tothe chassis, allowing the rear to swing up and down.

Pairs of these become twin-trailing-arm systems and work on exactlythe same principle as the double wishbones in the systems describedabove.

The difference is that instead of the arms sticking out from the side ofthe chassis, they travel back parallel to it. This is an older system notused so much any more because of the space it takes up, but it doesn'tsuffer from the side-to-side scrubbing problem of double wishbonesystems..


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I Beam suspension


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This is a combination of trailing arm suspension and solid beamaxle suspension.

Only in this case the beam is split in two and mounted offset fromthe centre of the chassis, one section for each side of the

suspension. The trailing arms are actually (technically) leading arms and the

steering gear is mounted in front of the suspension setup.

This makes for a heavy-duty independent front suspension setup

capable of handling the loads associated with their trucks. In an empty truck, however, going over a bump with twin I-beam

suspension is like falling down stairs in leg irons.


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Solid Axle Leaf spring


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Solid Axle Coil spring


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This is dependant type of suspension mostly used in the rearsuspension of trucks and heavy vehicles.

This is very sturdy and strong suspension and hence used tocarry highly loaded vehicles.

Solid axle is necessary in some heavy vehicles because ofintegrated differential and drive shaft assembly.


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Revisiting Weight Transfer

We have seen in earlier slides that suspension system is majorlyconsists of spring, and suspension members, i.e. A-arms/whisbones.

As we have discussed earlier, there are two types of masses.

a. Sprung massb. Unsprung mass

When in cornering/braking/accelaration, weight transfer occurs,again weight transfer occurs in both the weighs, i.e. sprung andunsprung.


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Unsprung weight transfer

Unsprung weight transfer will always occur through suspension members andnot through the spring.

That is because spring is used to mount sprung mass on wheels and hence it cannot take unsprung weight transfer.

Small note: From above discussion it is obvious that design of spring in any case

is not at all related to unsprung mass.

The weight transfer for cornering in the front would be equal to the totalunsprung front weight times the G-Force times the front unsprung center ofgravity height divided by the front track width. The same is true for the rear.

So, who will take those imbalanced weight transfers?

Answer is, suspension members.


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Sprung Weight transfer

Sprung weight transfer is the weight transferred by only the weight of the vehicle resting on the springs,not the total vehicle weight.

Calculating this requires knowing the vehicle's sprung weight (total weight less the unsprung weight), thefront and rear roll center heights and the sprung center of gravity height (used to calculate the rollmoment arm length).

The roll axis is the line through the front and rear roll centres that the vehicle rolls around duringcornering.

The distance from this axis to the sprung centre of gravity height is the roll moment arm length. Thetotal sprung weight transfer is equal to the cornering force times the sprung weight times the rollmoment arm length divided by the effective track width.

Calculating the front and rear sprung weight transfer will also require knowing the roll couple

percentage. Some of the sprung weight transfer will be taken by spring and other by suspension member.

This amount depends on the roll moment arm length. More the length, more the weight transfer taken byspring.

Since roll center is required to be below the CG, higher the roll center lower the rolling moment lengthwill be, and hence lower forces will be taken by spring and more by suspension members.


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Jacking forces

Now that we have seen the distribution of the weight transfer forces, we can calculatehow much forces will be there at spring and at suspension members.

Jacking forces are the sum of the vertical force components experienced by thesuspension links.

Generally, the higher the roll centre, the more jacking force is experienced.

As a part of suspension design we should always try to decrease the jacking forces asmuch as possible or to make springs take maximum weight transfer loads.

This is Important because now we can design our suspension members smaller indimensions. And hence weight of the car will be decreased.

Weight of the suspension members is considered as unsprung mass and hence it isdesirable to reduce on their weight.


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Camber angle

Camber angle is the angle made by the wheels of a vehicle; specifically, it is the anglebetween the vertical axis of the wheels used for steering and the vertical axis of thevehicle when viewed from the front or rear.

If the top of the wheel is farther out than the bottom (that is, away from the axle), it iscalled positive camber; if the bottom of the wheel is farther out than the top, it iscalled negative camber.


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Why do we need camber

Camber angle alters the handling qualities of a particular suspension design. in particular, negative camber improves grip when cornering. This is because it places

the tire at a better angle to the road, transmitting the forces properly.

Another reason for negative camber is that a rubber tire tends to roll on itself whilecornering. If the tire had zero camber, the inside edge of the contact patch would begin

to lift off of the ground, thereby reducing the area of the contact patch, ultimatelyreducing the grip.

By applying negative camber, this effect is reduced, thereby maximizing the contactpatch area..

On the other hand, for maximum straight-line acceleration, the greatest traction will beattained when the camber angle is zero and the tread is flat on the road.

Proper management of camber angle is a major factor in suspension design.

And hence a rule in suspension design we have to try and minimize the change in camberin cornering.


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Caster angle

Measured in the longitudinal direction, It is the angle between the pivot line (ina car - an imaginary line that runs through the centre of the upper ball joint tothe centre of the lower ball joint) and vertical.


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Why do we need caster angle


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Steering returnability: Whenever we steer a vehicle/cycle tires experienceslateral forces. Because of which and moment arm about the steering axis, atorque is generated and that will return the vehicle in straight direction.

This wouldnt have happened if the steering axis/caster angle is designed in

other way round. i.e. leaning backward instead of leaning forward.

Try it of you are comfortable with riding a cycle without hands on handle:Leave the handle and slightly turn the handle after you achieving enough speed.Handle will come back to its initial position.

We want similar effect in the car steering for the sake of stability. i.e. smalldisturbances should be taken care of.

And hence it is preferable to have leaning forward or positive camber.


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Anti dive and anti squat

Anti-dive and anti-squat are percentages and refer to the front divingunder braking and the rear squatting under acceleration.

We have seen earlier that if we think car as black box, then weighttransfer will be the same. But how much of that load is taken by springand how much by suspension member is determined by the amount ofroll that chassis undergo.

Same logic applies here, and hence They can be thought of as thecounterparts for braking and acceleration as jacking forces are tocornering.

The main reason for the difference is due to the different design goalsbetween front and rear suspension, whereas suspension is usuallysymmetrical between the left and right of the vehicle.


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Spring rate

Spring rate is nothing but the spring constant. Spring rate is a ratio usedto measure how resistant a spring is to being compressed or expandedduring the spring's deflection. The magnitude of the spring forceincreases as deflection increases according to Hooke's Law. Briefly, thiscan be stated as F=-Kx

The spring rate of a coil spring may be calculated by a simple algebraic

equation or it may be measured in a spring testing machine. The springconstant k can be calculated as follows: K= ((d^4)*G)/(8*N*(D^3))

where d is the wire diameter, G is the spring's shear modulus and N isthe number of wraps and D is the diameter of the coil.


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Travel

Travel is the measure of distance from the bottom of the suspension stroke,such as when the vehicle is on a jack or lifting and the wheel hangs freely, to thetop of the suspension stroke, such as when the vehicle's wheel can no longertravel in an upward direction or bottoming toward the vehicle.

"Bottoming" can be caused by the suspension, tires, fenders, etc. runningout of space to move or the body or other components of the car hitting the

road. Lifting refers to no downward movement is possible due to interference

between suspension/chassis member, damper maxing out etc.

While designing suspension system we have to take care that no members areinterfering with each other before permissible suspension movement.

It is a good idea to install a interfering member to avoid lifting, because lifting

because of suspension member constrain may prove catastrophical in somecases.


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Torsion Bar

Torsion bar is just another way of getting spring effect, other suspension system will beexactly same as earlier.

One end of a long metal bar is attached firmly to the vehicle chassis; the opposite endterminates in a lever, the torsion key, mounted perpendicular to the bar, that is attachedto a suspension arm, a spindle, or the axle. Vertical motion of the wheel causes the bar totwist around its axis and is resisted by the bar's torsion resistance. The effective spring

rate of the bar is determined by its length, cross section, shape and material.


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Progressively wound springs


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Hydra gas Suspension

Hydrolastic is a type of space-efficient automotive suspension system used in many cars. The heart of the system are the displacer units, which are pressurised spheres containing

nitrogen gas.

The system replaces the separate springs and dampers of a conventional suspensionsystem with integrated, space efficient, fluid filled, displacer units, which areinterconnected between the front and rear wheels on each side of the vehicle.

These replace the conventional steel springs of a regular suspension design. The meansfor pressurising the gas in the displacers is done by pre-pressurising a hydraulic fluid.


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Hydro pneumatic Suspension

At the heart of the system, acting as pressure sink as well as suspensionelements, are the so called spheres, five or six in all; one per wheel and onemain accumulator as well as a dedicated brake accumulator on some models.

Spheres consist of a hollow metal ball, open to the bottom, with a flexibledesmopan rubber membrane, fixed at the 'equator' inside, separating top andbottom. The top is filled with nitrogen at high pressure, up to 75 bar, thebottom connects to the car's hydraulic fluid circuit.

Pressure flows from the hydraulic circuit to the suspension cylinders,pressurizing the bottom part of the spheres and suspension cylinders.

Suspension works by means of a piston forcing fluid into the sphere,compacting the nitrogen in the upper part of the sphere.


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damping is provided by a two-way 'leaf valve' in the opening of the sphere. Fluidhas to squeeze back and forth through this valve which causes resistance andcontrols the suspension movements. It is the simplest damper and one of themost efficient.


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THANK YOU

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Suspension Unit