Independent Suspension-Seminar Report

  INTRODUCTION

Independent suspension is a broad term for any automobile suspension system that allows each wheel on the same axle to move vertically (i.e. reacting to a bump in the road) independently of each other. This is contrasted with a beam axle, live axle or de Dion axle system in which the wheels are linked - movement on one side affects the wheel on the other side. It is common for the left and right sides of the suspension to be connected with anti-roll bars. The anti-roll bar ties the left and right suspension spring rates together but does not tie their motion together.

Most modern vehicles have independent front suspension (IFS). Many vehicles also have an independent rear suspension (IRS). IRS, as the name implies, has the rear wheels independently sprung. A fully independent suspension has an independent suspension on all wheels. Some early independent systems used swing axles, but modern systems use Chapman or MacPherson struts, trailing arms, multilink, or wishbones.

Independent suspension typically offers better ride quality and handling characteristics, due to lower unsprung weight and the ability of each wheel to address the road undisturbed by activities of the other wheel on the vehicle. In the case of straight line drag racing though, it can be more of a burden because of the design, IRS may cause the vehicle to experience wheel hop on a hard launch. Independent suspension requires additional engineering effort and expense in development versus a beam or live axle arrangement. A very complex IRS solution can also result in higher manufacturing costs.

The key reason for lower unsprung weight relative to a live axle design is that, for driven wheels, the differential unit does not form part of the unsprung elements of the suspension system. Instead it is either bolted directly to the vehicle's chassis or more commonly to a sub-frame.

The relative movement between the wheels and the differential is achieved through the use of swinging drive shafts connected via universal (U) joints, analogous to the constant-velocity (CV) joints used in front wheel drive vehicles.

   THE NEED FOR SUSPENSION

The study of the forces at work on a moving car is called vehicle dynamics. Some of the concepts are needed to be understood in order to appreciate why a suspension is necessary in the first place. Most automobile engineers consider the dynamics of a moving car from two perspectives:

• Ride - a car's ability to smooth out a bumpy road
• Handling - a car's ability to safely accelerate, brake and corner

These two characteristics can be further described in three important principles - road isolation, road holding and cornering. These principles are explained below and also how engineers attempt to solve the challenges unique to each.

Road Isolation: The vehicle's ability to absorb or isolate road shock from the passenger compartment. Allow the vehicle body to ride undisturbed while traveling over rough roads. Absorb energy from road bumps and dissipate it without causing undue oscillation in the vehicle.

Road Holding: The degree to which a car maintains contact with the road surface in various types of directional changes and in a straight line (Example: The weight of a car will shift from the rear tires to the front tires during braking. Because the nose of the car dips toward the road, this type of motion is known as "dive". The opposite effect "squat"  occurs during acceleration, which shifts the weight of the car from the front tires to the back. Minimize the transfer of vehicle weight from side to side and front to back, as this transfer of weight reduces the tire's grip on the road.

Cornering: It is the ability of a vehicle to travel a curved path. Minimize body roll, which occurs as centrifugal force pushes outward on a car's center of gravity while cornering, raising one side of the vehicle and lowering the opposite side. Transfer the weight of the car during cornering from the high side of the vehicle to the low side.

   TYPES OF SUSPENSION

§  Conventional suspension system

§  Independent suspension system

§  Air suspension system

§  Hydro-Elastic suspension system

Under the independent suspension system, it is further classified into:

§  Swing Axle

§  Sliding Pillar

§  MacPherson Strut

§  Double Wishbone

§  Multi link

§  Trailing and Semi-trailing

  

 SWING AXLE

A swing axle is a simple type of independent suspension first used in early aircraft (1910 or before), such as the Sopwith and Fokker, usually with rubber bungee and no damping. Some later motor-car rear swing axles have universal joints connecting the drive-shafts to the differential, which is attached to the chassis. They do not have universal joints at the wheels: the wheels are always perpendicular to the drive shafts. Swing axle suspensions traditionally used leaf springs and shock absorbers. This type of suspension was considered better than the more typical live axle for two reasons:

• It reduced unsprung weight since the differential is mounted to the chassis
• It eliminates sympathetic camber changes on opposite wheels

However, there are a number of shortcomings to this arrangement:

• A great amount of single-wheel camber change is experienced, since the wheel is always perpendicular to the driveshaft
• "Jacking" on suspension unloading (or rebound) causes positive camber changes on both sides, which (In extreme cases) can overturn the car.
• Reduction in cornering forces due to change in camber can lead to over steer instability and in extreme cases lift-off over steer

Mercedes-Benz addressed the inherent handling issues by producing swing axles with a single-pivot point located under the differential, and thus well below the axle. This configuration markedly reduced the tendency to "jack-up" and the later low pivot swing-axle equipped cars were praised in contemporary publications for their handling.

Swing axles were supplanted by de Dion axles in the late 1960s, though live axles remained the most common. Most rear suspensions have been replaced by more modern independent suspensions in recent years, and both swing and de Dion types are virtually unused today. One exception is the Czech truck manufacturer Tatra, which uses swing axles and a central 'backbone' tube instead of more common solid axles. This system is claimed to give greater rigidity and better performance on poor quality roads and off road.

Another use of the swing axle concept is Ford's "Twin I-Beam" front suspension for trucks. This has solid axles (so they do not transmit power). Though it is touted as an independent suspension system in that each tire rises and falls without affecting the position of the other, the parallelogram action of the A-arm suspension system is not present. Each tire in fact moves with a similar camber change to that of the powered swing axles for the rear wheels listed above. But the pivot point of the axles is located not in the middle of the car but nearly on the other beam of the chassis, so the effect is far less hazardous.

SLIDING PILLAR

Sliding pillar suspension is one in which the stub axle and wheel assembly are attached to a fixed vertical "pillar" or kingpin which slides up and down through a bush or bushes which are attached to the vehicle chassis, usually as part of transverse outrigger assemblies. Sliding pillar independent suspension was the first recorded instance of independent front suspension on a motor vehicle. In this system, the stub axle carrying the wheel was fixed to the bottom of a pillar which slid up and down through a bush in a transverse axle member fixed to the front of the chassis. The top of the pillar was fixed and pivoted on a transverse semi-elliptic spring.

 Macpherson strut

The most widely used front suspension system in cars comprises of a strut-type spring and shock absorber combo, which pivots on a ball joint on the single, lower arm. The steering gear is either connected directly to the lower shock absorber housing, or to an arm from the front or back of the spindle. In this case, when you steer, it physically twists the strut and shock absorber housing and consequently the spring to turn the wheel.

Double wishbone suspension

The double wishbone suspension can also be referred to as double 'A' arms, and short long arm (SLA) suspension if the upper and lower arms are of unequal length. A single wishbone or A-arm can also be used in various other suspension types, such as MacPherson strut and Chapman strut. The upper arm is usually shorter to induce negative camber as the suspension jounces (rises). When the vehicle is in a turn, body roll results in positive camber gain on the inside wheel. The outside wheel also jounces and gains negative camber due to the shorter upper arm. The suspension designer attempts to balance these two effects to cancel out and keep the tire perpendicular to the ground. This is especially important for the outer tire because of the weight transfer to this tire during a turn.

Between the outboard end of the arms is a knuckle with a spindle (the kingpin), hub, or upright which carries the wheel bearing and wheel. Knuckles with an integral spindle usually do not allow the wheel to be driven. A bolt on hub design is commonly used if the wheel is to be driven. In order to resist fore-aft loads such as acceleration and braking, the arms need two bushings or ball joints at the body.

At the knuckle end, single ball joints are typically used, in which case the steering loads have to be taken via a steering arm, and the wishbones look A or L-shaped. An L-shaped arm is generally preferred on passenger vehicles because it allows a better compromise of handling and comfort to be tuned in. The bushing in line with the wheel can be kept relatively stiff to effectively handle cornering loads while the off-line joint can be softer to allow the wheel to recess under fore aft impact loads. For a rear suspension, a pair of joints can be used at both ends of the arm, making them more H-shaped in plan view.

Alternatively, a fixed-length driveshaft can perform the function of a wishbone as long as the shape of the other wishbone provides control of the upright. This arrangement has been successfully used in the Jaguar IRS.

 MUTLI-LINK SUSPENSION SYSTEM

            A multi-link suspension is a type of vehicle suspension design typically used in independent suspensions, using three or more lateral arms, and one or more longitudinal arms. These arms do not have to be of equal length, and may be angled away from their 'obvious' direction.

Typically, each arm has a spherical joint (ball joint) or rubber bushing at each end. Consequently, they react on loads along their own length, in tension and compression, but not in bending. Some multi-links do use a trailing arm or wishbone, which has two bushings at one end. On a front suspension one of the lateral arms is replaced by the tie-rod, which connects the rack or steering box to the wheel hub.

Multi-link suspension allows the auto designer the ability to incorporate both good ride quality and good car handling in the same vehicle. In its simplest form the multi-link suspension is orthogonal - that is, it is possible to alter one parameter in the suspension at a time, without affecting anything else. This is in direct contrast to a double wishbone suspension where moving a hard-point or changing a bushing compliance will affect two or more parameters.

TRAILING AND SEMI-TRAILING

A trailing-arm suspension is an automobile  suspension design in which one or more arms (or "links") are connected between (and perpendicular to and forward of) the axle  and the chassis. It is usually used on rear axles. A 'leading arm' as used on a Citroën 2CV, has an arm connected between (and perpendicular to, and to the rear of) the axle and the chassis. It is used on the front axle.

Trailing-arm designs in live axle setups often use just two or three links and a Panhard rod to locate the wheel laterally. A trailing arm design can also be used in an independent suspension arrangement. Each wheel hub is located only by a large, roughly triangular arm that pivots at one point, ahead of the wheel. Seen from the side, this arm is roughly parallel to the ground, with the angle changing based on road irregularities.

A semi-trailing arm suspension is a supple independent rear suspension system for automobiles where each wheel hub is located only by a large, roughly triangular arm that pivots at two points. Viewed from the top, the line formed by the two pivots is somewhere between parallel and perpendicular to the car's longitudinal axis; it is generally parallel to the ground. Trailing-arm and multilink suspension designs are much more commonly used for the rear wheels of a vehicle where they can allow for a flatter floor and more cargo room.

 SUSPENSION TERMINOLOGY

 Camber: This is the angle of the rim/tire from vertical as viewed from the front or the rear of the car. Be sure the wheels are pointed straight ahead when measuring this angle.

Caster: This is the angle of the steering axis as viewed from the side of the car. The axis may pass through upper and lower ball joints or the upper strut bushing and a lower ball joint. Be sure the wheels are pointed straight ahead when measuring this angle.

Center Of Gravity: This is the imaginary point in a car where it would be exactly balanced if lifted by a hoist.

Ride Height: This is the height above the road that the car sits.

Roll Center: This is an imaginary point about which the cars rotates while in a turn. Each axle has it's own roll center. The higher the roll center, the more tipsy the car will feel.

Sprung Weight: This is the weight of a car that is supported by the suspension. The engine, body, interior, passengers, cargo, etc. are all sprung weight.

Toe-In/Toe-Out: Toe is the dimensional difference of the distances between the front and rear edges of the wheels on an axle. If the front edges are closer than the rear edges, there is toe-in. Toe-out is when the rear edges are closer together.

Unsprung Weight: This is the weight of a car that is not supported by suspension. Wheels, tires, brakes, hubs, etc. are unsprung weight. Suspension components such as control arms, anti-roll bars, shocks, and struts are a percentage sprung weight and a percentage unsprung weight. The actual percentage depends on the application.

Weight Distribution: This is the amount of weight on the front and rear axles expressed as percentages.

Bump Steer: This happens when the suspension compresses, causing the control arms and tie-rods to move vertically. Because they differ in length and location, the result is the rim/tire being steered without any movement of the steering wheel. Cars having control arms and tie-rods parallel to the road will exhibit minimal bumpsteer.

Counter Steer: If a car is torque steering to the left, turning the steering wheel to the right will maintain a straight line of travel. A car that is oversteering to the right can be brought back into line by turning the steering wheel to the left. In both cases the driver is counter steering to correct the car's direction of travel.

Drift: As a car loses traction in a turn it moves toward the outside of the turn. This movement is called drift.

Neutral Steer: This is the theoretic ideal steer characteristic when the front and rear tires lose traction at the same time.

Oversteer: When the rear tires lose traction before the front tires, a car is oversteering. Recovery from an oversteer situation must be quick since directional control can be lost.

Roll Steer: This characteristic is the result of body roll at the rear suspension. Because the mounts (forward and rearward) are typically not at the same level above the road, as the wheels move up and down they also move forward and rearward. As the body rolls one wheel advances in front of the other, steering the car from the rear.

Torque Steer: Front-wheel drive cars have a tendency to pull to one side during acceleration, particularly noticeable during quick starts from a stop. Because the driveshafts are different lengths, the twist in them during acceleration is different causing one wheel to accelerate quicker than the other, making the car turn to one side.

Understeer: When the front tires lose traction before the rear tires, a car is understeering. Instinctively, a driver will compensate for understeer simply by turning the steering wheel further. This makes understeer safer than oversteer for the average driver, so automobile manufacturers design this characteristic into their cars.

   WORKING OF A Suspension:

When people think of automobile performance, they normally think of horsepower, torque and zero-to-60 acceleration. But all of the power generated by a piston engine is useless if the driver can't control the car. That's why automobile engineers turned their attention to the suspension system almost as soon as they had mastered the four-stroke internal combustion engine.

The job of a suspension is to maximize the friction between the tires and the road surface, to provide steering stability with good handling and to ensure the comfort of the passengers. In this article, we'll explore how car suspensions work, how they've evolved over the years and where the design of suspensions is headed in the future.

If a road were perfectly flat, with no irregularities, suspensions wouldn't be necessary. But roads are far from flat. Even freshly paved highways have subtle imperfections that can interact with the wheels­ of a car. It's these imperfections that apply forces to the wheels. According to Newton's laws of motion, all forces have both magnitude and direction. A bump in the 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 tiny speck. Either way, the car wheel experiences a vertical acceleration as it passes over an imperfection.

Without an intervening structure, all of wheel's vertical energy is transferred to the frame, which moves in the same direction. In such a situation, the wheels can lose contact with the road completely. Then, under the downward force of gravity, the wheels can slam back into the road surface. A system that will absorb the energy of the vertically accelerated wheel, allowing the frame and body to ride undisturbed while the wheels follow bumps in the road is needed.

 Suspension Parts:

­            Suspension of a car is actually part of the chassis, which comprises all of the important systems located beneath the car's body. These systems include:

The frame is a structural, load-carrying component that supports the car's engine and body, which are in turn supported by the suspension

The suspension system is a setup that supports weight, absorbs and dampens shock and helps maintain tire contact.

The steering system is a mechanism that enables the driver to guide and direct the vehicle.

The tires and wheels are components that make vehicle motion possible by way of grip and/or friction with the road 

    COMPARISON BETWEEN CONVENTIONAL AND INDEPENDENT SUSPENSION SYSTEM

The four wheels of a car work together in two independent systems - the two wheels connected by the front axle and the two wheels connected by the rear axle. That means that a car can and usually does have a different type of suspension on the front and back. Much is determined by whether a rigid axle binds the wheels or if the wheels are permitted to move independently. The former arrangement is known as a dependent system, while the latter arrangement is known as an independent system. In the following sections, we'll look at some of the common types of front and back suspensions typically used on mainstream cars.

Dependent Front Suspensions

Dependent front suspensions have a rigid front axle that connects the front wheels. Basically, this looks like a solid bar under the front of the car, kept in place by leaf springs and shock absorbers. Common on trucks, dependent front suspensions haven't been used in mainstream cars for years.

Independent Front Suspensions

In this setup, the front wheels are allowed to move independently. While there are several different possible configurations, this design typically uses two wishbone-shaped arms to locate the wheel. Each wishbone, which has two mounting positions to the frame and one at the wheel, bears a shock absorber and a coil spring to absorb vibrations. Double-wishbone suspensions allow for more control over the camber angle of the wheel, which describes the degree to which the wheels tilt in and out. They also help minimize roll or sway and provide for a more consistent steering feel. Because of these characteristics, the double-wishbone suspension is common on the front wheels of larger cars.

Dependent Rear Suspensions

If a solid­ axle connects the rear wheels of a car, then the suspension is usually quite simple based either on a leaf spring or a coil spring. In the former design, the leaf springs clamp d­irectly to the drive axle. The ends of the leaf springs attach directly to the frame, and the shock absorber is attached at the clamp that holds the spring to the axle. For many years, American car manufacturers preferred this design because of its simplicity.

The same basic design can be achieved with coil springs replacing the leaves. In this case, the spring and shock absorber can be mounted as a single unit or as separate components. When they're separate, the springs can be much smaller which reduces the amount of space the suspension takes up.

Independent Rear Suspensions                 

If both the front and back suspensions are independent, then all the wheels are mounted and sprung individually, resulting in what car advertisements tout as "four-wheel independent suspension." Any suspension that can be used on the front of the car can be used on the rear, and versions of the front independent systems described in the previous section can be found on the rear axles. Of course, in the rear of the car, the steering rack , the assembly that includes the pinion gear wheel and enables the wheels to turn from side to side is absent. This means that rear independent suspensions can be simplified versions of front ones, although the basic principles remain the same.

   Advantages and Disadvantages

ADVANTAGES

• Bigger deflection of front wheels, no reaction on steering
• Greater distance for resisting rolling action
• Front axle improves road holding tendency of tyres.
• Minimum vibrations
• It provides smooth and comfort ride
• Rear wheels remain stable

 DISADVANTAGES

• Better shock absorber required.
• Expensive
• Tyre wear increases due to transmission of torque.
• Increased cost
• Complicated design
• Steering action is not proper

  FUTURE OF INDEPENDENT SUSPENSION

 SMART SHOCKS

It is one of the latest developments in the suspension systems. Smart shock absorber can quickly change its cushioning characteristics without input from an electronic controller. The new shock absorber enables automakers to provide a vehicle with greater control during hard cornering, while at the same time supplying a softer ride over bumpy surfaces.

SIX-WHEELED SUSPESION 

The wheel suspension is being researched at the next level. Though it is not available commercially, It has been successful in its test. A 6 wheel suspension system is proposed and designed with two wheels in between the front and rear wheels. These wheels act the support that does the actual suspension work by taking the load of the wheels while they move on roughs and bumps. It has a unique feature of provide the smooth rides and minimizing energy losses and vibrations.