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SUSPENSION
Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose – contributing to the car's road holding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants comfortable and reasonably well isolated from road noise, bumps, and vibrations,etc. These goals are generally at odds, so the tuning of suspensions involves finding the right compromise. It is important for the suspension to keep the road wheel in contact with the road surface as much as possible, because all the forces acting on the vehicle do so through the contact patches of the tires. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. The design of front and rear suspension of a car may be different.
1.2 ABOUT LEAF SPRINGS
Originally called laminated or carriage spring, a leaf spring is a simple form of spring, commonly used for the suspension in wheeled vehicles. It is also one of the oldest forms of springing, dating back to medieval times.
The advantages of leaf spring over helical spring are that the end of the springs may be guided along a definite path.
Sometimes referred to as a semi-elliptical spring or cart spring, it takes the form of a slender arc-shaped length of spring steel of rectangular cross-section. The center of the arc provides location for the axle, while tie holes are provided at either end for attaching to the vehicle body. For very heavy vehicles, a leaf spring can be made from several leaves stacked on top of each other in several layers, often with progressively shorter leaves. Leaf springs can serve locating and to some extent damping as well as springing functions. While the interleaf friction provides a damping action, it is not well controlled and results in stiction in the motion of the suspension. For this reason manufacturers have experimented with mono-leaf springs.
A leaf spring can either be attached directly to the frame at both ends or attached directly at one end, usually the front, with the other end attached through a shackle, a short swinging arm. The shackle takes up the tendency of the leaf spring to elongate when compressed and thus makes for softer springiness. Some springs terminated in a concave end, called a spoon end (seldom used now), to carry a swiveling member.
There were a variety of leaf springs, usually employing the word "elliptical". "Elliptical" or "full elliptical" leaf springs referred to two circular arcs linked at their tips. This was joined to the frame at the top center of the upper arc, the bottom center was joined to the "live" suspension components, such as a solid front axle. Additional suspension components, such as trailing arms, would be needed for this design, but not for "semi-elliptical" leaf springs as used in the Hotchkiss drive. That employed the lower arc, hence its name. "Quarter-elliptic" springs often had the thickest part of the stack of leaves stuck into the rear end of the side pieces of a short ladder frame, with the free end attached to the differential, as in the Austin Seven of the 1920s. As an example of non-elliptic leaf springs, the Ford Model T had multiple leaf springs over its differential that was curved in the shape of a yoke. As a substitute for dampers (shock absorbers), some manufacturers laid non-metallic sheets in between the metal leaves, such as wood.
Leaf springs were very common on automobiles, right up to the 1970s in Europe and Japan and late 70's in America when the move to front wheel drive, and more sophisticated suspension designs saw automobile manufacturers use coil springs instead. Today leaf springs are still used in heavy commercial vehicles such as vans and trucks, SUVs, and railway carriages. For heavy vehicles, they have the advantage of spreading the load more widely over the vehicle's chassis, whereas coil springs transfer it to a single point. Unlike coil springs, leaf springs also locate the rear axle, eliminating the need for trailing arms and a Pan hard rod, thereby saving cost and weight in a simpe live axle rear suspension.
A more modern implementation is the parabolic leaf spring. This design is characterized by fewer leaves whose thickness varies from centre to ends following a parabolic curve. In this design, inter-leaf friction is unwanted, and therefore there is only contact between the springs at the ends and at the centre where the axle is connected. Spacers prevent contact at other points. Aside from a weight saving, the main advantage of parabolic springs is their greater flexibility, which translates into vehicle ride quality that approaches that of coil springs. There is a trade-off in the form of reduced load carrying capability, however. The characteristic of parabolic springs is better riding comfort and not as "stiff" as conventional "multi-leaf springs". It is widely used on buses for better comfort. A further development by the British GKN company and by Chevrolet with the Corvette amongst others, is the move to composite plastic leaf springs.
Typically when used in automobile suspension the leaf supports an axle and locates/ partially locates the axle. This can lead to handling issues (such as 'axle tramp'), as the flexible nature of the spring makes precise control of the unsprung mass of the axle difficult. Some suspension designs which use leaf springs do not use the leaf to locate the axle and do not have this drawback. The Fiat 128's rear suspension is an example.
TRADITIONAL USE OF LEAF SPRINGS
A leaf spring is a long, flat, thin, and flexible piece of spring steel or composite material that resists bending. The basic principles of leaf spring design and assembly are relatively simple, and leafs have been used in various capacities since medieval times. Most heavy duty vehicles today use two sets of leaf springs per solid axle, mounted perpendicularly to support the weight of the vehicle. This Hotchkiss system requires that each leaf set act as both a spring and a horizontally stable link. Because leaf sets lack rigidity, such a dual-role is only suited for applications where load-bearing capability is more important than precision in suspension response.
1.4 MULTI-LEAF SPRINGS
Multi-leaf springs are widely used for automobile and rail road suspensions. It consists of a series of flat plates, usually of semi- elliptical shape as shown in fig. 4.20. The leaves are held together by means of two U-bolts and a centre clip. Rebound clips are provided to keep the leaves in alignment and prevent lateral shifting of the plates during the operation. The longest leaf, called the master leaf, is bent at both ends to form the spring eye. At the center, the spring is fixed to the axle of the car. Multi- leaf springs are provided with one or two extra full length leaves in addition to the master leaf. These extra full-length leaves are stacked between the master leaf and the graduated-length leaves. The extra full-length are provided to support the transverse shear force.
For the purpose of analysis, the leaves are divided into two groups namely master leaf along with graduated-length leaves forming one group and extra full-length leaves forming the other. The following notations are used in the analysis:
nf = number of extra full-length leaves
ng =number of graduated-length leaves including master leaf
n= total number of leaves
b= width of each leaf (mm)
t= thickness of each leaf (mm)
L=length of the cantilever or half the length of semi- elliptic spring (mm)
F= force applied at the end of the spring (N)
Ff=portion of F taken by the extra full-length leaves (N)
Fg=portion of F taken by the graduated-length leaves (N)
1.5 OVERVIEW OF LEAF SPRING
1.5.1 INTRODUCTION
Semi-elliptic leaf springs are almost universally used for suspension in light and heavy commercial vehicles. For cars also, these are widely used in rear suspension
The spring consists of a number of leaves called blades. The blades are varying in length. The blades are us usually given an initial curvature or cambered so that they will tend to straighten under the load. The leaf spring is based upon the theory of a beam of uniform strength. The lengthiest blade has eyes on its ends. This blade is called main or master leaf, the remaining blades are called graduated leaves. All the blades are bound together by means of steel straps.
The spring is mounted on the axle of the vehicle. The entire vehicle load is rests on the leaf spring. The front end of the spring is connected to the frame with a simple pin joint, while the rear end of the spring is connected with a shackle. Shackle is the flexible link which connects between leaf spring rear eye and frame. When the vehicle comes across a projection on the road surface, the wheel moves up, this leads to deflecting the spring. This changes the length between the spring eyes.
1.5.2 SUSPENSION SYSTEM
The automobile chassis is mounted on the axles, not direct but some form of springs. This is done to isolate the vehicle body from the road shocks, which may be in the form of bounce, pitch, roll or sway. These tendencies give rise to an uncomfortable ride and also cause additional stress in the automobile frame any body. All the part, which performs the function of isolating the automobile from the road shocks, is collectively called a suspension system. It includes the springing device used and various mountings for the same.
Broadly speaking, suspension system consists of a spring and a damper. The energy of road shock causes the spring to oscillate. These oscillations are restricted to a reasonable level by the damper which is more commonly called a shock absorber.
1.5.2 (A) OBJECTIVE OF SUSPENSION
1 To prevent the road shocks from being transmitted to the vehicle components.
2. To safeguard the occupants from road shocks
3. To preserve the stability of the vehicle in pitting or rolling, while in motion
1.5.2 (B) BASIC CONSIDERATIONS FOR VERTICAL LOADING
When the rear wheel comes across a bump or pit on the road, it is subjected to vertical forces, tensile or compressive depending upon the nature of the road irregularity. These are absorbed by the elastic compression, shear, bending or twisting of the spring. The mode of spring resistance depends upon the type and material of the spring used.
Further when the front wheel strikes a bump it starts vibrating. These vibrations die down exponentially due to damping present in the system. The rear wheel however, reaches the same bump after certain time depending on the wheel base and the speed of the vehicle. Of course, when the tear wheel reaches the bump, it experiences similar vibrations as experienced by the front wheel some time ago. It is seen that to reduce pitching tendency of the vehicle, the frequency of the front springing system be less than that of the rear springing system.
From human comfort point also it is seen that it is desirable to have low vibration frequencies. The results of the studies of human beings have shown that the maximum amplitude which may be allowed for a certain level of discomfort decreases with the increase of vibration frequency.
1.5.2 © ROLLING
The centre of gravity of the vehicle is considerably above the ground. Due to this reason, while taking a turn, the centrifugal force acts outwards on the C.G of the vehicle, while the road resistance acts inward at the wheels. This gives rise to a couple turning the vehicle about a longitudinal axis. This is called rolling. The manner in which the vehicle is sprung determines the axis about which the vehicle will roll. The tendency to roll is checked by means of a stabilizer.
1.5.2 (D) BRAKE-DIP
On braking, the noise of the vehicle has a tendency to be lowered or to dip. This depends upon the position of centre of gravity relative to the ground, the wheelbase, and other suspension. In the characteristics the same way, torque loads during acceleration end the front of the vehicle to be lifted. These forces on account of braking and driving are carried directly by deflecting the springs, by wishbone arms or by radius rods.
1.5.2 (E) SIDE THRUST
Centrifugal force during cornering, cross-winds, cambering of the road etc, cause a side-thrust to be applied to the vehicle, such forces are usually absorbed by the rigidity of the leaf springs or by fitting pan hard rods.
1.5.2 (F) UNSPRUNG WEIGHT
Un-sprung weight is the weight of vehicle components between the suspension and then road surface. This includes rear axle assembly, steering knuckle, and front axle in case of rear drive rigid suspension, wheels, tires and brakes. The sprung weight i.e. the weight supported by the vehicle suspension system, includes the frame, body, engine, and the entire transmission system.
When the wheels strike against a bump, they vibrate along with other unsprung parts which store the energy of the vibrations and then further transmit it to the sprung parts via the springs. Thus it is seen that greater the weight of the unsprung parts, greater will be the energy stored due to vibrations and consequently greater shocks.
When a small shock results in the large movements of the wheel, the suspension is said to be soft, such a soft suspension is more comfortable to the occupants. However, excessively soft suspension will result in the loss of braking efforts are decreased. Thus a good suspension system should be an optimum compromise between softness and hardness.
1.5.2 (G) FUNCTION OF SUSPENSION SPRINGS
Springs are placed between the road wheels and the body. When the wheel comes across a bump on the road, it rises and deflects the spring, there by storing energy there in. on releasing due to the elasticity of the spring materials, it rebounds there by expending the stored energy. In this way the spring starts vibrating, with amplitude decreasing gradually on account of internal friction of the spring material and friction of the suspension joints, till vibrations die down.
1.5.3 CLASSIFICATION OF SUSPENSION SPRINGS
The Suspension springs may be classified as follows:
(i) STEEL SPRINGS
(a) Leaf Spring
(b) Coil spring
© Torsion bar
(ii) RUBBER SPRINGS
(a) Compression spring
(b) Compression–shear spring
© Steep reinforced spring
(d) Progressive spring
(e) Face Shear Spring
(f) Torsion shear spring
(a) LEAF SPRINGS
Semi- elliptic leaf springs are almost universally used for suspension of light and heavy commercial vehicles. For cars also there are widely used for rear suspension.
(b) COIL SPRINGS
The coil springs are used mainly with independent suspension, though they have also been used in the conventional rigid axle suspension, as they can be well accommodated in restricted spaces. The energy stored per unit volume is almost double in the case of coil springs than the leaf springs
Coil springs do not have noise problems, nor do they have static friction causing harness of ride as in case of leaf springs. The spring takes the shear as well as bending stresses. The coil springs, however cannot take torque reaction and side thrust, for which alternative arrangements have to be provided.
© TORSION BARS
Torsion bar is simply a rod acting in torsion and taking shear stresses only. These are made of heat-treated alloy spring steel. The amount of energy stored per unit weight of material is nearly the same as for coil springs. Torsion bar is often used with the independent suspension.
The bar is fixed at one end to the frame, while the other end is fixed to the end of the wheel arm and supported in the bearing. The other end of the wheel arm is connected to the wheel hub. When the wheel strikes a bump, it starts vibrating up and down, thus exerting torque on the torsion bar, which acts as spring.
Torsion bar springs are lighter as compared to leaf springs and also it occupies less space. Sometimes the torsion tubes are used instead of the bars; the former one is stiffer than later ones.
There are two main disadvantages of the torsion bar suspension. The first is that it does not take the driving thrust, so that additional linkages have to be provided for that purpose. The second disadvantage is the absence of friction force and hence of damping which is necessity to control the vibration produced due to road shocks.
(ii) RUBBER SPRINGS
The advantages of using rubber as a means of suspension are:
1. It can store greater energy per unit weight than the steel, for this reason rubber springing systems can be made more compact.
2. The rubber has excellent vibration damping properties
3. The absence of squeaking, which is always present in steel springs.
4. The number of bearings is reduced considerably for the rubber suspension system, this means longer life.
5. Rubber is more reliable, a rubber suspension cannot suddenly fail like a metal spring.
The various types of rubber springs used for vehicles are discussed below:
(a) COMPRESSION SPRING
This type of spring is still being used because of the following advantages:
1. It is reliable of simple construction and requires no bonding
2. It provides a rising rate characteristic
3. It can resist occasional overload of large magnitude.
4. It has a large measure of inherent damping than most types of rubber. However, its use is limited because of the fact that some mechanical guide must be provide with this type of spring.
(b) COMPRESSION SHEAR SPRING
In this type, the load is carried partly by shear and partly by compression components in the rubber and hence, although large strains may be allowed in the rubber body, shear stress at the bonding faces is kept small and fatigue properties are excellent.
© STEEL REINFORCED SPRING
Steel rein-forced spring consists of a steel helical spring bonded in a rubber body, the steep springing though carrying only about 20% of load exercises a stabilizing influence on the rubber components there by allowing a greater stroke/diameter ratio to be used without other forms of guiding.
(d) PROGRESSIVE SPRING
It has initially an exceedingly small rate, which rises rapidly as the central cavity closes.
(e) FACE SHEAR SPRING
It consists of a thick disc of rubber having metal plates bonded to its flat surfaces, and axially pre-compressed it operates by a relative rotation of the plates about it thus loading the rubber partly in shear.
(f) TORSION SHEAR SPRING
It consists of an inner metal shaft, tubular or solid, and an outer trough like shell between which rubber body is bounded, the latter being put under pressure by closing the trough with a riveted or spot welded base plate. The spring operates by the rotating of the shaft about its own axis relative to the shell.
1.5.4TYPES OF SUSPENSION SYSTEMS
1.5.4(A) PLASTIC SUSPENSION
Viberitis. P.A of TURINE has developed a new type of suspension based upon the use of resilient plastic rings in compression. The suspension consists of a cylindrical container secured to the chassis, a shaft attached to the axle and free to slide within the plastic rings contained in the cylinder, there are two centering rings, the bottom one fixed to the lower end of the cylinder and the upper one is arranged as high as possible keeping in consideration that in the rebound position shaft must remain supported by it by the plastic rings and absorb the vertical dynamic load.
When the suspension is in the rebound position the disc seats on centering ring radial location of rings is affected by circular pressed steel plates. These plates ate interposed between the plastic rings.
Whenever the plastic rings and of the spring are compressed or extended, the sleeve assembly, surrounding the plastic ring, moves up or down in the cylinder centering ring is carried in a flange sleeve, which is fixed to lower end of cylinder. A flexible gaiter seals the unit and prevents foreign matter from entering the system.
1.5.4(B) INDEPENDENT SUSPENSION
When a vehicle with rigid axle suspension encounters road irregularities the axle tilts and the wheels no longer remain vertical. This causes the whole of the vehicle to tilt on one side. Such a state of affairs is not desirable. Apart from causing rough ride, it causes ‘wheel wobble’. The road adhesion is also decreased. To avoid this, the wheels are sprung independent of each other, so that tilting of one does not affect the other. Besides the independent suspension also have the following advantages over rigid able type suspension.
1. The elastic strain energy per unit spring weight stored in a coil or torsion bar is greater than in case of a semi-elliptical leaf spring, which means lighter springs can be used in case of independent suspension.
2. In case of independent suspension, unsprung weight is reduced, which ultimately reduced the tyre scrub and hence increase tyre lift
3. Compared to the rigid axle, type, softer springs can be used without increasing rolling effect. Soft springs improve ride comfort.
4. When anti-roll bar is used in independent suspension, springs employed may be even softer, in the event of vertical cornering, the anti-roll bar will provide the forces necessary to resist body roll.
5. In case of independent suspension it is possible to locate the springs apart enough obtain under-seer condition.
6. With independent suspension, steering geometry is not altered with spring deflection as in case of conventional rigid axle suspension where effect is especially noticeable during breaking or acceleration.
7. In this case the engine and the chassis frame can be placed relatively lower which means engine position can be moved forward resulting in more space for passengers.
1.5.4© FRONT WHEEL INDEPENDENT SUSPENSION
Independent suspension has become almost universal in the case of front axle, due to the simplicity of such a suspension system.
1.5.4(D) REAR WHEEL INDEPENDENT SUSPENSION
Though the rear wheels are not to be steered, yet there is a considerable difficulty in the rear wheel springing if the power has to be transmitted to the rear wheel. But even the rear wheel independent springing is coming into prominence because of its distinct advantages over the rigid axle type.
Universal couplings keep the wheel vertical, while the sliding coupling is required to maintain the wheel track constant, thereby avoiding scrubbing of the tyres: this method has been used in the DEDION type of axle.
Another method of rear wheel independent suspension is the trailing link type. In this the trailing links are pivoted at right angles to the longitudinal axis of the car and carry the rear wheels at their ends. The trailing links hold the wheels firmly and also sustain accelerating the braking force.
It is claimed that the combined metal – rubber mountings respond softly on straight roads, increasing ride comfort. When cornering, they resist lateral force with a reliable stabilizing effect, even when the car is fully loaded.
Apart from the distinct advantages, which the independent suspension possesses, it has its own drawbacks also:
1. The initial cost is high
2. Greater maintenance required because of larger number of bearings.
3. Misalignment of steering geometry with the wear of components. Thus requires more attention.
4. In the event of body roll, the wheels camber (tilt outwards in case of wishbone type), due to which cornering power is reduced.
5. More rigid sub-frame or chassis frame required
6. Forces due to unbalanced wheels are more pronounced and transmitted easily to the steering wheel.
1.6.4(E) WISHBONE TYPE SUSPENSION
The use of coil springs in the front axle suspension of car is now almost universal. It consists of upper and the lower wishbone arms pivoted to the frame member. The spring is placed in between the lower wishbone and the underside of the cross member. The vehicle weight is transmitted from the body and the cross member to the coil spring through which it goes to the lower wishbone member. A shock absorber is placed inside the coil spring and is attached to the cross member and the lower wishbone member.
The wishbone arms are like the chicken wishbone or letter V in shape, because of which the system is so called, because of this V-shape. The wishbones not only position the wheels and transmit the vehicle load to the springs. But these also resist acceleration, breaking and cornering (side) forces. The upper arms are shorter in length than the lower ones. This helps to keep the wheel track constant, thereby avoiding tyre scrub thus minimizing tyre wear. However a small change in the camber angle does occur with such an arrangement.
The wishbone type is the most popular independent suspension system
1.6.4(f) Mac Pherson Strut Type of Suspension
In this layout only lower wishbone are used. A strut containing shock absorbing and the spring carriers also the stub axle on which the wheel is mounted. The wishbone is hinged to the cross member and positions the wheel as well as resists accelerating, braking and side forces. This system is simpler than double wishbone type described above and is also lighter, keeping the unsprung weight lower. This type of suspension gives the maximum room in the engine compartment and is, therefore commonly used on front wheel drive cars. In India this system has been used in Maruti (Suzuki) 800 cars. This type of suspension with anti-roll bar as employed in Volkswagen Jetta and Passat cars. This is claimed to provide increased road safety, improve ride comfort and light and self-stabilizing steering which means that car continues along its chosen line of travel when the brakes are applied even though the road surface may vary.
1.6.4(G) VERTICAL GUIDE SUSPENSION
The king pin is attached directly to the cross member of the frame. It can slide up and down, corresponding to the up and down motions of the wheel, thus compressing or elongating the springs. In this the track, wheel base and wheel attitude remain unchanged, but the system is having disadvantages of decreased stability.
1.6.4(H) TRAILING LINK SUSPENSION
In this type of suspension, a coil spring is attached to the trailing link which itself is attached to the carrying the wheel hub. When the wheel moves up and down, it winds and unwinds the spring. A torsion bar has also been used in certain designs in place of the coil spring. The system does maintain the camber and the wheel track constant. However, the distance between the front and the rear wheels does change. Difficulty to remedy this defect is the main reason for its very limited use in actual practice.
1.6.4(I) SWINGING HALF AXLE SUSPENSION
In this wheels are mounted rigidly on the half axles, which are pivoted on their ends to the chassis member at the middle of car. The main disadvantage of this system is that up and down movement of the wheel causes the camber angle to vary.
1.6.4(J) INTERCONNECTED SUSPENSION SYSTEMS
In these systems, the front and rear suspension units or else the units on the two sides of the automobile are connected together. These are also called ‘linked system’. Te major advantage of such a system is that tendency of the vehicle to bounce, pitch or roll is reduced and a constant desirable attitude of suspension. The other systems in current use are the Hydroelastic suspension, the Daimler – Benz suspension and the Hydragas suspension system.
1.6.4(K) AIR SUSPENSION
Air suspension systems are coming into prominence because of certain advantages they possess over the conventional metal springs. The advantages are:
1. A vehicle space fro wheel deflection is put to optimum use by virtue if the automatic control devices.
2. Because of the vehicle is also constant, changes in headlamp alignment due to varying loads are avoided.
3. The spring rare varies much less between the laden and unladen conditions, as compared with that of conventional steel springs. This reduces the dynamic loading.
4. The improved standard for ride comfort and noise reduction with air springs reduces both driver and passenger fatigue.
The four air springs, which may be either the bellows type or the piston type, are mounted in the same position where generally the coil springs are mounted. An air compressor takes the atmospheric air through a filter and compresses it to a pressure of 240 Mpa, at which pressure of air in the accumulator tank is maintained, which is also provided with a safety relief. The high pressure air goes through lift control valve and the leveling valves, to the air springs. The control valve is operated manually by means of a handle on the control panel, through a cable running from the valve to the handle.
1.6.4(L) HYDRO ELASTIC SUSPENSION
In this system a displacer unit is fitted at each of the four wheels. The displacer units are all interconnected by means of fluid. In the displacer unit, rubber (under compression and shear) is used as a spring where as fluid rubber pressure acts as damping medium. The stem is connected to the wheel through suitable linkage so that its movement is proportional to the up and down movement of the wheel. A two way valve assembly controls the up and down flow of the fluid. The upper valve opens when the fluid pressure below rises sufficiently. Similarly the lower valve allows the fluid to pass in the downward direction under pressure. The two valves are assembled at right angles to each other. When the piston moves up due to the movement of the wheel, the diaphragm pushes the fluid up through the opening, by pushing the damper valve. The fluid under pressure above the valves, then compress the rubber which acts as spring. The wheel bounce, the body roll, the pitching are controlled.