Abstract: A Suspension system is the term given to a system of springs, shock absorbers and linkages that connect a vehicle to its wheels. This report generally gives a review on various types of suspension systems, their layouts at the front and the rear, their role in handling, quality and durability of a vehicle during its operation .Also a brief introduction on role of the suspension systems in interaction with tyres and the dynamics of the complete vehicle is also explained. This report also gives a review of methods used to design, analyze and physically testing of the suspension systems.
Introduction: Suspension systems were developed back perhaps two thousand years or more. He ancient wagons used the elastic wooden poles to reduce the effects of the wheel shock. The first spring to be developed is the leaf spring which is still being used in heavy trucks and goods carrier vehicles for better and smooth ride as it can sustain heavy loads. Now with the availability of new technology and methods the suspension system have become more better and offer better ride handling, quality and durability of the vehicle offering a pleasant and comfortable ride to the occupants in the vehicle. The suspension systems serves dual purposes, one is in the vehicle handling and braking region and secondly it helps in keeping the occupants in the vehicle, comfortable and well associated from road noise, bumps and variations. The better the suspension system the better it will prevent the vehicle from damage and wear contributing in durability of the vehicle. If there were no roads irregularities and roads were perfectly flat we would not have required the suspensions, but that’s not the case we have many road irregularities at present and so to make the ride better we need suspension systems that can make the ride handling, quality and durability of the vehicle more effective and also comfortable to the occupants in the vehicle.
Suspension Systems: The purpose of the suspension system is to isolate the vehicle body from road bumps and vibrations while keeping the wheels in contact with the road.
Principle of Suspension: The suspension system isolates the vehicle body from the road shocks and vibrations which otherwise would be transferred to the passengers and also to the load. It must also keep the tyres in contact with the road. For example when a tyre hits an obstruction, there is a reaction force. The size of the reaction forces depends on the unsprung mass of each vehicle assembly. The sprung mass includes the body, frame and the engine which are supported by the springs. While the unsprung mass include the tyres, wheels, brake assemblies etc. The vehicle ride handling and quality can be improved by keeping the unsprung mass as low as possible, but the suspension systems should be strong enough to withstand the loads imposed on it during cornering, braking and uneven road surfaces.
Suspension Force: Applying a force to an object deforms it. Automotive suspensions generally use the elastic properties of the metals so as to provide the springing medium. The springs are located between the frame and axle assemblies. Leaf springs are semi-elliptical and they absorb the force by flattening under load while coil springs absorb the force by twisting. Torsion bars are held rigid at one end and twist around their center as the suspension arm is deflated. Non metal materials like rubber can provide the main springing action but are commonly used as stops to limit suspension movement. In light vehicle applications, air is normally used for ride height control. For example when the vehicle hits an uneven surface, the spring deforms from their original shape and return to their normal position but tend to over shoot and set up oscillations. This makes the vehicle bounce up and down making the ride uncomfortable due to which the tyres won’t be able to grip the road surface well. The shock absorbers have a marked effect on how well the tyres follow a road surface. They damp the natural bounce over the road and reduce spring oscillations. There are different kinds of shock absorbers but they use a piston sliding inside a cylinder filled with oil.
Unsprung Weight: Parts of the vehicle not supported by the suspensions. Most of the vehicle weight is not supported by its suspensions. It suspends the body and associated parts so that they are insulated from road shocks and vibrations. So the unsprung mass should be kept as low as possible.
Wheel unit Location: When a vehicle is in operation, several forces operate to displace the wheel units- driving thrust, braking torque, cornering forces. These forces must be transferred to the frame of the vehicle but while they act wheel unit must stay aligned with each other and also with the frame. They must be located longitudinally and laterally while still having the freedom to move vertically to allow suspension travel. On vehicle with non independent suspension, leaf springs provide a simple means of performing these functions. On a rear wheel drive vehicle, the axle is located on the spring by a spring center bolt and clamped to spring by a U-bolt.
Dampening: different materials have different levels of elasticity upto certain points they can be deformed. If it returns to its original state very quickly it can produce bouncing effect called an oscillation. Preventing this oscillation is called dampening. In suspension system shock absorbers reduces the oscillations in the spring. Shock absorbers reduce the effect of travelling on a rough ground making the ride quality, handling and increasing the comfort level.
Classification of Suspension System: Suspension systems are an important in the vehicle structure. A suspension system consists of springs, shock absorber, axles, rods and ball joints. Basic types of springs are leaf springs, Coil springs and Torsion bars. They are basically divided into two types of systems one is the independent system and other is the dependent suspension systems. These two are classified further into many types which are discussed below.
Solid Axle: The solid axle provides a simple means of locating and mounting the hub and wheel units. Together with leaf spring it forms an effective, non independent suspension system. Similar with coil spring it is used in the rear suspension of many front engine, rear wheel drive vehicles. It is also called as live axle.
Dead Axle: On front wheel drive vehicles a simple beam axle can be used on the rear, with the coil spring suspension and control arms for location. This is dead axle for the fact that it does not transmit any drive, it only supports the vehicle. It is also another type of non-independent type suspension system, as the deflection of a wheel on one side is transmitted to the other wheel also. Passenger cars do not use this suspension system anymore but they are still being used in the heavy vehicles.
Independent suspensions: In this type of suspension systems the main benefit is that the unsprung mass can be kept low. Also if a wheel on one side hits an irregularity it won’t upset the wheels on the other side on the same axle and it also allows the wheel camber to be adjusted. Double wishbone suspension system also a type of independent suspension system used. It uses two wishbone arms to locate the wheel consisting of two mounting points one on the chassis and one on the knuckle. The shock absorber and coil spring mount to wishbone is used to control the vertical movement. This allows the suspension design engineer to control the parameters like caster angle, camber angle, toe pattern, roll center height, scrub radius and more. The double wishbone suspension can also be referred to as double ‘A’ arms and short long arm suspension if the upper and lower arms are of unequal length. The dvantage of this suspension system is that we can figure out the effect of moving each joint. We can tune the kinematics of suspension very easily and also optimize the wheel motion. It is also very easy to work out the loads that different parts will be subjected to which allows more optimized parts to be designed. They are expensive compared to the Mc Pherson strut suspensions.
Mc Pherson strut: Another type of independent suspension is the Mc Pherson strut which is one of the most common types of independent suspension systems used today. This strut was designed by Earl S Mc Pherson. Its main advantage is that it can be used in the front as well in the rear of the vehicle. It consists of springs and shock absorbers unit called a strut. The lower end of the strut is located by a ball joint fitted to the end of the suspension control arm. Its upper end is located in a moulded rubber mounting. In the front wheel drive suspension, the control arm is a wishbone shape with two widely spaced mounting points. This prevents the forward and backward movement.
In the rear suspension it has to allow the external drive shaft to transmit drive to the wheels. This drive shaft can be used as an upper link of the suspension. The rear suspension must maintain the alignment of the wheels with the front and even the frame.
Rear wheel drive independent suspension: In the rear wheel drive independent suspension the final drive unit must be fixed to the vehicle frame. The Mc Pherson suspension systems consists different types of components into one package making them ideal to be used in the front wheel drive cars. It is simpler and cheaper to manufacture. It requires very less space to engineer into the chassis design, and in the front wheel drive layouts, can allow for more room in the engine bay.
Adaptive Air Suspension: It is an electronically controlled air suspension at all four wheels having an adaptive system. It helps in achieving a sporty handling and a high level of ride quality. This suspension allows the speed dependent lowering of the body giving an increased stability, the handling characteristics are even improved at the same time. There is an adaptive control unit which obtains the data from the sensors thereby controlling the adjustment of individual shock absorbers but depending on driving situations. This kind of suspensions system can maintain a constant height irrespective of load and automatically adjusts the damping and ride height of the vehicle and even enables the driver to influence the suspension characteristics.
Types of springs:
Coil Springs: These types of springs are most commonly used in front suspension systems of modern cars and in many cases have replaced leaf springs in the rear suspension. A coil spring is made of single length spiral wire which is heated and wound on the former to produce the required shape. The load carrying characteristic depends on the diameter of the wire, its springs and spacing of the coils. The comfort level of the springs depends on the deflection rate, as the load increases the ride will become smoother and softer.
Leaf Springs: It is one of the oldest forms of springing. They are used in the rear suspensions for its simplicity. They have more than leaf of different length to form a multi leaf spring. They are held together by a center bolt passing through a hole in center. Rebound clips are provided to prevent excessive flexing of main leaf during rebound keeping them in alignment.
Torsion Bars: It is a long, alloy steel bar fixed rigidly to the chassis at one end and the other to the suspension control arm. Its fitted in unloaded condition, and its control arm raises the bar twisting it at the centre placing it in under a torsional load. When the vehicle is placed on the road with control arm connected to the suspension assembly, the bar supports the vehicle load and twists providing a springing action. Spring rate depends on the length of the bar and its diameter. The shorter and thickness of the bar, stiffer is the spring rate. Torsion bars are used as stabilizers connected between each side of the suspension. During turning the anti-roll bar tries to use its connections to each side of the suspension to resist the roll tendency.
Rubber Springs: They are used in most of the suspension systems as bump and rebound stops. If the suspension reaches the limit of travel, these stops prevent the direct metal to metal contact helping the body of the vehicle and the suspension not to get damaged. They are also used for auxiliary springing for increasing their resistance with suspension contact. The rubber spring work on the principle of elasticity. These springs can be used for many purposes and have many advantages like it can be moulded into any shape and is very silent during use.
Shock Absorber Types:
Hydraulic Type: This is the most widely-used hydraulic shock absorber which is a direct-acting telescopic type. It can be fitted in a self-contained unit, or can be combined with a suspension strut. The strut type uses the same principle of operation but it is considerably larger. The hydraulic shock absorber provides its dampening action by transferring oil, under pressure, through valves which restrict the oil flow. During bumps, or compression, the rod and its piston move into the shock absorber. In rebound, or extension, the rod and piston move out of the shock absorber. For dampening to be effective, resistance is needed in both directions.
Gas Pressurized Type: In the hydraulic type, the oil heats up as the energy of motion of suspension is dampened, due to this rapid movement of the piston takes place it causes the hydraulic fluid to aerate when vehicle moves over road surface. This reduces the dampening effect which causes the performance of the shock absorbers to deteriorate. The gas type uses a mono-tube design where the fluid comes and fills the chambers above and below the piston. As the piston moves in the cylinder, valves control the movement of the oil from one chamber to the other chamber. Pressure on the oil is provided by the nitrogen gas at the base of the cylinder acting on a free-floating separation piston which separates the gas from the oil. On bump the piston moves downward due to which the penetration of the piston rod displaces a quantity equal to its volume. Pressure of oil is even maintained when the piston and the rod are at the top of their stroke.
Load-adjustable type: When the load on the vehicle is more the suspension gets compressed, which causes the rear of the vehicle to be lower than the normal position. Due to this the steering becomes lighter, alignments of the headlight become high, and the compression length of travel is reduced when passing over the bumps, causing discomfort to the occupants. So as to reduce these effects load adjustable type shock absorbers are used. This is a standard hydraulic type, providing dampening action when heavy loads are placed on the rear of the vehicle. We can also adjust the height of the shock absorber by changing the pressure, and also the stiffness of the suspension springs.
Manual adjustable rate type: These types of shock absorbers provide a means of changing the rate of dampening of the springs to suit the varying road conditions. This type has an external adjustable damper rate adjustment, which changes the position of the valves in the piston to vary the number of restriction the oil has to pass and also to vary the force needed to open the valve.
Electronic adjustable rate: This type of shock absorbers consists of electronic controls which are used to alter the dampening rate automatically or as the drivers prefer. It has got a rotary solenoid that alters the dampening rate by changing the restrictions the oil must pass through. The ECU allows different modes of operation by using a selector switch, thus helping to improve the vehicle stability at high speeds.
Automatic load Adjustable Type: These are also called self leveling type shock absorbers. When there is heavy load in the vehicle it causes the compression of the suspension affecting the stability, handling discomfort to the passengers. A lower vehicle performs better on smooth roads but as the road condition vary the roads start getting transmitted causing discomfort. An automatic load adjustable shock absorber automatically adjusts the ride height of the vehicle according to the load placed. This type consists of electrically driven compressor and air dryer assembly, and an ECU. During motion the ECU senses the load and adjusts the ride height accordingly.
Suspension System Layouts:
Driven rear suspension layout:
Live axle rear suspension system. This arrangement consists of shock absorbers, U bolts, fixed shackle and rebound clips.
This arrangement consists of coil springs, flexible bushes, and control arms.
This arrangement consists of coil springs, flexible bushes, panhard rod and control arms.
This type of arrangement consists of coil springs, flexible bushes, watts linkage and control arms.
Non driven rear suspension:
This type consists of coil springs, flexible bushes, panhard rod, forward and rearward control arms.
This type consists of axle beam, torsion bar, trailing arm, panhard rod, coil springs and struts.
This type has coil springs, strut, trailing links and lateral links.
Rear Independent suspension:
This type consists of coil springs, flexible bushes, CV joints, external drive shafts and semi trailing arms.
This type has got coil springs, drive shafts, cross members, universal joints and lower suspension link.
This one consists of coil springs and struts, splined sections, lateral wishbone and external drive shafts.
This one has torsion bar, upper and lower wishbones and torsion bar adjustment. This type can be used in either front or rear suspension.
Front Suspension Layouts:
This is independent driven front suspension consisting of coil spring, lower wishbone and Mc Pherson strut.
This type consists of coil springs, strut, tension rod and lower control arm.
This has a non driven front suspension arrangement having lower and upper wishbone.
This has coil springs, tension rod, lower control arm and upper wishbone.
Arms and Linkages:
The major component of the suspension system must be firmly located to withstand the forces occurring during operation. Control arms are connected to a rear axle assembly to the vehicle body so that it can move freely as it needs to. The arms must be strong enough to withstand the forces but also light in weight so as to keep the unsprung mass low. The front suspension consists of two control arms, while vehicle with strut type have only one control arm. For the rear wheel drive the vehicle has coil springs and trailing arms to position the axis longitudinally. For example the figure below shows that the suspension system uses angled control ride to limit the lateral movement of the rear axle while cornering. They absorb the torque reaction forces.
Ride handling: The suspension system has a very important role in the handling of the vehicle. Ride handling is influenced by suspension system geometry. While designing the factors that are taken into consideration are vibrations from the tyre ground contact, by elastic and damping properties, by vertical properties of the tyres, by oversteer and understeer of the vehicle. The ride handling characteristics of an automobile center depends on the characteristics of the tyres. Tyres have a vertical reaction point with the roadway for the fact that they manage all the input of forces and even the disturbances on the road because they perform on the action of the driver. Therefore tyre characteristics are a key factor where the road has an effect on the vehicle suspension system and in effectiveness of the output forces which control the vehicle stability and cornering characteristics. The basic tyre characteristics are managed by the system of springs, dampers, and linkages that control the way in which tyres move and react to the disturbances. The suspension systems provide steering input for directional control and also compensate for body roll to improve cornering ability and mover vertically in response to road irregularities to smooth out the ride and make sure that the tyres are in contact with ground. Here the wheels are connected to the sprung mass by linkages and are therefore affected by the rolling and pitching movement occurring in the suspensions systems. The handling of the vehicle continuously changes according to the roadway and driving conditions.