Auto chassis


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Auto chassis

  1. 1. CONSTRUCTIONAL DETAILS OF FRAMES The frame is usually a high strength metal structure used to support the body, engine, transmission,suspension and to keep a definite relative position between them. The frame must be capable of carrying thevehicle weight, withstand the large vibrations and shocks created by the unevenness of the road surfaces andat the same time must be light and rugged. To attain strength the frames are made of C-shape or U-shape,box shape, hat section, double channel or I- section, pipe section etc. The frame can be separate from thebody or the body can be welded together to form the frame. The longitudinal members of the frame arebrought closer together at the front to give an adequate steering lock and at the rear are upswept to clear theaxle movement. The thickness of the longitudinal members is frequently increased at the middle where thegreatest bending loads occur. On the sides of the frame some brackets and dumb irons are provided tosupport the body, springs, bumpers and bearings for spring shackles. The engine is mounted at one end ofthe frame, usually on the front end and supported on the frame with the help of rubber blocks to avoid theeffects of road shocks on the engine and the engine vibrations on the body.Sub frames:Generally different units are directly bolted to the main frame of the vehicles. But sometimes these units aremounted on the separate frame, which is known as the sub-frame. The sub-frame carrying components arefurther supported on the main frame at some points usually by riveting. By the use of the sub-frame the unitsare isolated and safeguarded from the effects of flexing and twisting of the main frame. The advantages ofthe sub-frame are 1. The assembly lines as well as overhauling and repair of the units are simplified by the use of sub- frame. 2. The vibrations of the engine or other units and shocks of the suspension springs transmitted to the main frame are absorbed and damped out by the sub-frame. 3. Any alterations in the unit systems can easily be performed by making changes in the sub-frame instead of main frame.Functions of the frame: 1. To support the body, power plant (Engine), transmission system, suspension system and other chassis components. 2. To maintain correct spacing and relationship between the other units. 3. To withstand all the static and dynamic loads without undue deflection or distortion. 4. To get the normal functions of the supported units and freedom from stress, strain and wear caused by operation in misaligned condition. 1
  2. 2. Loads on the frame 1. Weight of the vehicle body and the passengers or pay load which causes vertical bending of the side members. 2. Vertical loads when the vehicle come across a hump or ditch, which results in longitudinal torsion due to one wheel lifted (or lowered) with other wheels at the usual road level. 3. Loads due to road camber, side wind, cornering force while taking a turn, which results in lateral bending of side members. 4. Engine torque or braking torque (inertia force) tending to bend the side members in the vertical plane. 5. Impact loads due to the sudden collision of the vehicle. 6. Loads for long periods while taking turn and applying brakes at a time.Materials for the frames:Frames should be constructed with materials of good quality so that these bear all the uneven twists, shocksand other loads in different conditions. Generally steels used for pressed frames are mild steel, carbon steel,and nickel alloy steel sheets. An aluminium alloy called Alpax has also been used as a frame material. Thecomposition of a nickel alloy steel as specified is given below.Carbon – 0.25% to 0.35% Manganese – 0.35% - 0.75% Silicon – 0.30% maxNickel – up to 3% Phosphorous – 0.05% max Sulphur – 0.5% maxTypes of framesThe main types of the chassis frames are:a) Ladder frame. b) X-frame (backbone). c) Perimeter frame or space frame Fig. Ladder framesH-shaped frame (ladder frame) is the most basic shape of chassis and consists of two long side membersriveted together with the cross members. This type of construction is commonly used for the commercialvehicles like truck, passenger heavy vehicles, dozers etc. This is not very common in passenger cars due toits hard ride. 2
  3. 3. Fig: X-frameThe X – frame narrows in the center giving the vehicle a rigid structure that is designed to withstand a highdegree to twist. A heavy front cross member is used to support the upper and the lower suspension controlarms and the coil springs. A variation of the X-frame is the back bone frame structure which has a singlethick beam in the middle section. It can be found in some sports cars. Fig: Perimeter frameThe perimeter frame is a similar in construction to the ladder frame. The full length side rails support thebody at its greatest width which provides more protection to the passengers in case of a side impact to thebody. The areas behind the front wheels and in front of the rear wheels are stepped to form a torque boxstructure. In a head on collision, the stepped areas absorb much of the energy. In a side impact collision thepassenger compartment is protected from collapse since the center frame rail is near the front floor sidemember. In case of a rear end collision the rear end cross member and the kick-up absorbs the shock. Mostof the conventional frames used today are the perimeter designs, which include the following body sections.a) Conventional front body. b) Conventional main body.The front body section is made up of the radiator support, front fender and front fender apron (mud guardand wheel arch). These components are installed with bolts and form an easily disassemble structure. Themain body is made up of the dash panel, under body, roof and so on to form the passenger and luggage 3
  4. 4. compartments and similar in structure that of the uni-body. The dash panel, sometimes termed as the firewall or front bulk head is the panel dividing the front section and the center, passenger compartment section.It normally welds in place. The front of the under body has a propeller shaft tunnel built into it forming achannel cross section through the center of the floor pan and cross members are welded to it where it joinsthe frame. Thus the passenger compartment, roof side rail door and side body are protected from side impactcollisions. In addition the front, back, left and right side of the floor pan are made uneven in the stampingprocess, increasing the rigidity of the floor pan, which reduces the vibration.Defects in framesThe early failure of axle alignment or repeated problems with steering or axle geometry are usuallyindications that the chassis frame has been subjected to overloading or unusual stress forces and is probablymisaligned. The frame misalignment is generally the result of damaged or loose frame components, whichcould have been caused by the vehicle being involved in an accident or through overloading or throughabuse or rough use. A bent or twisted frame can adversely affect the track of the vehicle, can cause drivelinevibrations through misalignment, and cause abnormal wear of chassis parts that are bolted to the frame.Another tell-tale signs that a frame is bent include driver complaints about uncomfortable steering and rapidtire wear.The prominent defect that may usually occurs in the frames due to accident or rollover fall into one of thefive basic categories. 1. Sideway, where the chassis frame is bent or bowed horizontally in one direction or the other. 2. Diamonding, where one frame rail has moved ahead of the other and with the two remaining still parallel. 3. Twist, where the vehicle’s front end rolls to one side, while the rear rolls to the opposite side. If a frame is twisted, one corner of the frame will be higher than the opposite side. 4. Sag, where the frame is bent down the middle like an old swayback horse. 5. Bowed, where a frame rail or side rail is bent up from where it should be.Prior to checking a frame for alignment, inspect the frame for signs of damage or loosen components.Inspect all frame members and cross members for cracks, twists or bends and all bolts and rivets for signs oflooseness. Check component support brackets for looseness and make any repairs required prior to the checkfor misalignment. Once all these items have been checked and repaired, make sure that all tire sizes are sameand they are inflated to the correct pressure. It is also required to check for any variation in the suspensionsystem height both longitudinally and vertically from the specifications.The frame alignment can be checked without removing the truck body from the frame by carrying out adiagonal or X- checking method. This method involves transferring four or more different datum pointsfrom each side of the frame side rails to a corresponding mark on the clean shop floor using a plumb line.The vehicle is then taken away and the diagonals are measured between corresponding points. Anyinequality of measurements more than 7 to 8 mm indicates a misalignment. The possible causes are 4
  5. 5. 1. The dumb irons or side members may be bent 2. Cross members may be buckled. 3. Some rivets may be loose or broken 4. Broken welding 5. Defective front axle, steering system, suspension, wheels, hubs and tyres etc.Repairing and straightening the frame:Small cracks on the frame most probably near riveted brackets or cross members can be welded and ifnecessary can be plated for extra strength. But when the bending is too great, a frame can be straightened byusing a heavy beam, chains and hydraulic jacks. Use of heat is not recommended when straightening framesas heat weakens structural characteristics of heat- treated frame members. If a frame flange has buckledbadly, it is possible to install a reinforcing sleeve either inside or outside the rail where the frame failureoccurred to return the component to its original strength. Bolting, rather than welding would be used for thispurpose on all heat-treated frames. Similarly if a weld cracks or fails, then it can also be repaired by boltedreinforcement channel. Special straightening tools and equipments are also available to carryout accuraterepair works. But if the damage is beyond the repair limit, then the damaged portion of the frame has to bereplaced with a new one.UNIBODY OR INTEGRAL BODY STRUCTUREThe unibody was a design concept developed for the bodies of aircraft, and the egg shell is often cited as thebest example of this type of structure. Even if you press hard on an egg shell, it is comparatively difficult todestroy since all the force applied by the fingers is not concentrated in one place but is dispersed effectivelythrough the entire shell. In mechanics this action is called stressed hull structure.In a car body there is no complete hull structure, but generally a body with a structure which integrates theframe and body to receive and hold outside forces is called a unibody, and has the following characteristics. 1. A unibody is made by combining pieces of thin sheet metal pressed to form panels of various shapes and joined to an integrated structure by spot welding. This structure is light in weight and highly rigid against bending or twisting. 2. Since there is no frame, the bulk taken by the frame can be used to make the car more compact. 3. Vibration and noise from the drive chain and suspension enters the floor pan and is amplified by the body which acts as an acoustic chamber making it necessary to add extra components to the body to suppress the vibration and noise. 4. Since a unibody is constructed by combining panels that are pressed into complex shapes, once it is deformed, a great deal of time is needed to restore it to its original shape. 5. Since the thin sheet metal body is close to the road surface, it is necessary to take adequate measures to prevent the deterioration in strength due to corrosion. This is particularly important when dealing with reinforcing materials which makeup the underbody. 5
  6. 6. SUSPENSION SYSTEMSAll the components, which perform the function of isolating the automobile from the road shocks arecollectively called a suspension system. Broadly speaking the suspension system consists of a set of springsand dampers. The energy of road shock causes the spring to oscillate. The oscillations are restricted to areasonable level by the damper, which is more commonly known as a shock absorber.The objectives of a suspension system: 1. To safeguard the occupants in a vehicle against road shocks and provide ride comfort. 2. To keep the body of the motor vehicle in the even level while traveling over rough ground, or when turning in order to minimize the rolling, pitching or vertical movement tendency. 3. To minimize the effects of stress due to road shocks on the vehicle components and provide a cushioning effect. 4. To isolate the structure of the vehicle from road shock loading and vibration due to irregularities of the road surfaces without impairing its stability. 5. To absorb the torque and the braking reactions by providing a particular height to body structure.Basic considerations or requirements of a suspension system:1. Vertical loading: When the road wheel comes across a bump or pit on the road, it is subjected to verticalforces, tensile or compressive depending upon the road irregularity. These are absorbed by the elasticcompression, hear or twisting of the suspension spring. The mode of spring resistance depends upon the typeand material of the spring used. It is seen that to reduce the pitching action of the vehicle, the frequency ofthe front springing system should be less than that of the rear springing system. For human comfort pointalso it is seen that it is desirable to have low vibration frequencies.2. Rolling: The center of gravity of the vehicle is located considerably above the ground. Due to this reason,when the vehicle is taking turns, the centrifugal force acts outwards on the C.G of the vehicle and the roadresistances acts inward at the wheels. This gives rise to a couple turning the vehicle about a longitudinalaxis. This is called rolling. The manner in which the vehicle is sprung determines the axis about which thevehicle will roll. The tendency to roll is checked by means of a stabilizer.3. Brake dip: On braking the nose of the vehicle has a tendency to be lowered or to dip. This depends uponthe position of the center of gravity relative to the ground, the wheelbase and the suspension characteristics.In the same way torque loads during the sudden acceleration tends to lift the front of the vehicle. Theseforces on account of braking and driving are carried directly by the deflection of the spring and othersuspension components.4. Side thrust: Centrifugal forces during cornering, cross winds, cambering or banking of roads cause a sidethrust to be applied on the vehicle. Such forces are usually absorbed by the rigidity of the suspension springsor the stabilizer.5. Unsprung weight: Unsprung weight is the weight of the suspension components between the suspensionand the road surface. This includes rear axle assembly, steering knuckle, front axle in case of rear drive rigid 6
  7. 7. axle suspension, wheels, tyres and brakes. The sprung weight ie, the weight supported by the vehiclesuspension system includes the frame, body, engine and the entire transmission system. It is seen that greaterthe weight of the unsprung parts, greater will be the energy stored due to vibration in the suspension springsand consequently greater the shocks.6. Miscellaneous: When a small shocks results in the larger movements of the wheel, the suspension is saidto be soft. Such a suspension is more comfort to the occupants. However excessively soft suspension willresult in the loss of contact between the road and the wheel due to which the driving or the braking effortsare decreased. Thus a good suspension system should be an optimum compromise between softness andhardness of the springs.The components in a suspension system 1. Springs, which absorbs and neutralize the shocks from the road surface. 2. Dampers (shock absorbers), which act to improve riding comfort by limiting the free oscillation of the springs. 3. Stabilizer (sway-bar or anti-roll bar), which prevents lateral swaying of the car. 4. A linkage system, which acts to hold the above components in place and to control the longitudinal and lateral movements of the vehicles.Suspension types and characteristicsBased on the differences in their construction, the suspension system can be divided into two types.a). Rigid axle suspension system. b). Independent suspension system.Characteristics of Rigid axle suspension system: 1. The number of parts composing the suspension system is less and the construction is simple. Therefore the maintenance is also simple and less expensive. 2. It is durable enough for heavy-duty applications. 3. While the vehicle is negotiating a curve there will be less tilting of the body. 4. There is little change in the wheel alignment due to the up and down movement of the vehicle wheels. Therefore there is less tyre wear. 5. Since the unsprung weight is more there is less riding comfort. 6. Since the movements of left and right wheels mutually influence each other, vibration and oscillation occur rather easily.Characteristics of independent suspension system: 1. Unsprung weight can be kept low and the road holding characteristics of the vehicle wheels are good. So the riding comfort and the handling stability are good. 2. In independent suspension, the spring only supports the body and they do not help to position the wheels. (This being done by the linkages) 7
  8. 8. 3. Since there are no axles connecting the left and the right wheels, the floor and the engine mounting position can be lowered. This means that the vehicle center of gravity will be lower and the passenger compartment and the boot space can be made larger. 4. In case of independent suspension it is possible to locate the springs apart enough to obtain the under steer conditions, which is preferred. 5. The construction is rather complex. 6. Tread and alignment change with the up and down motion of the wheels. Suspension spring characteristics:a). Spring rate or spring constant: The deflection of a spring varies in proportion to the force (load)applied to it. That is the value obtained by dividing the force (W) by the amount of deflection (a) is aconstant. This constant (k) is called the spring constant or spring rate. Otherwise the weight necessary forthe unit deflection in spring is called spring rate.That is, W / a = k. Higher the spring rate, stiffer the suspension will be and hence the heavier the load it cansupport. But the ride comfort is reduced. The rear spring rate is about 120% of the front suspension springrate. Hence the frequency of the front suspension springs are less than that of the rear springs. A spring withlow spring rate is said to be soft spring and a spring with a higher spring rate is said to be firm or hard.b). Elasticity & frequency: The springs used in automobiles use the principle of elasticity to cushion thebody and occupants of a vehicle from road shocks, and in doing so temporarily store the energy created(Jounce) by the applied stress. Steel spring store this energy by bending (leaf spring) or twisting (in case ofcoil springs and torsion spring). The jounce energy stored in the spring forces the spring to return to itsoriginal position. But while returning, it overcomes the neutral position, which is called rebound. The springoscillates form jounce to rebound at a rate of about 60 to 70 cycles per minute. If the up and downoscillation were left uncontrolled, it would cause not only an uncomfortable ride, but would also lead tostability and control problems.Types of suspension springs:1. Steel spring a) Leaf spring b). Tapered leaf spring c). Coil spring d). Torsion bar2. Rubber springs a) Compression spring b) Compression shear spring c) steel reinforced spring d) Progressive spring e) Face shear spring f) Torsional shear spring3. Plastic spring4. Air spring5. Hydraulic spring. 8
  9. 9. Leaf Springs:Leaf springs aremade up of anumber of curvedbands of springsteel called leavessticking together inorder from shortestto longest. Thisstack of leaves isfastened togetherat the center with a center bolt or U- bolt to prevent the longitudinal movement. Similarly sometimes theleaves are made with pips or projections at the bottom and recess at the top surface. The leaves are arrangedin such a way that the projection of the upper spring should mesh in the recess of the lower spring. Also tokeep the leaves from slipping out of place, they are held at several places with the clips. Both ends of thelongest or main leaves are bent to form spring eyes, used to attach the spring to the frame. To adjust thevariations in length of the master leaf while the vehicle move across the road irregularities, one end of thespring is connected to the fame through a shackle and the other end is mounted directly on the frame with apin. For the front suspension, it is a usual practice to provide the shackle in the front side of the spring toreduce the wheel wobble.Generally, the longer a leaf spring, the softer it will be. Also the more leaves in a leaf spring, the greater theload they will withstand. But on the other hand as the spring will become firmer, the riding comfort willsuffer.The curvature of each leaf is called a nip. As the nip of the leaf is greater, shorter the leaf will be. Each leafcurves sharply than the one above the stack. When the center bolt is tightened, the leaves flatten somewhatand causing the ends of the leaves to press very lightly against one other.The suitable steels that have been used for the manufacture of leaf springs are chrome-vanadium steel(C-0.46%, Cr-1.4%, Va-0.18%), silico-manganese steel (C-0.52%, Si-1.95%, Mn-1.05%) and carbon steel(C-0.55%, Mn-0.6%, Si-0.2%).Types of leaf spring:a) semi-elliptic type spring b) Quarter elliptic spring c) Transverse type d)Helper springsThe semi-elliptic type leaf spring is the most common type in use where, the spring is attached to the frameat its middle to the axle. One end is connected through a shackle and the other end is connected to the framethrough a pin. 9
  10. 10. The quarter elliptic type spring is a cantilever type spring, which is pivoted at its one end and the other endis shackled or pivoted to the axle. Theshort leaves in this type of springs arearranged in at the top. This type is not incommon use now.Transverse type spring is arrangedtransversely to the vehicle or parallel tothe axle. This spring is rigidly bolted tothe frame at its center. Both the ends ofthe spring are connected to the axlethrough the shackles. The disadvantagein using this type of spring is that thevehicle tends to roll at the turns since theframe is clamped only to their centers.The helper springs or auxiliary springsare provided in addition to the main leafsprings when the vehicle is meant tocarry heavy loads. This will allow a widerange of loading. Helper springs are anadditional set of leaf springs clampedwith the same U– bolt on the top of the main spring. Generally the helper springs are used in the rear sideonly. When the vehicle is lightly loaded, these helper springs will not take any loads and will come in actionand share the loads only after certain deflection of the main leaves.Coil springs:The coil springs are generally used with independent suspension. Since theenergy stored per unit volume of the coil spring is almost double that of theleaf springs, they can be accommodated in restricted space. The coil springsare made from rods of special spring steel formed into the shape of a coil.When a load acts on the spring, the entire spring rod is twisted as the springcontracts to absorb the energy.Progressive coil spring: If a coil spring is made from a rod of spring steelhaving uniform diameter, the entire spring will flex uniformly in proportion to changes in load. This restrictsthe application of this type of spring for the wide range of loading. However, if a spring rod having aconstantly changing diameter is used, the ends of the springs will have a smaller spring rate than the center,thus varying the spring index under different loading conditions. Thus the spring ends will contract and 10
  11. 11. absorb light loads and the center part of the spring will allow a wide range of loading. The progressivesprings can be made by using unequal pitch spring (low pitch at the ends and high pitch at the middle),conical spring or by providing the helper coil springs with the main spring like in the case of leaf spring.Characteristics 1. The energy absorption rate per unit weight is greater. 2. Soft springs and springs with varying spring rate can be made. 3. Since there is no inter-leaf friction like the leaf springs to control the oscillation by the spring itself, it is necessary to use a shock absorber along with them. 4. The coil spring will take shear as well as the bending stresses. However can not take torque reaction and side thrust, for which linkage mechanisms are necessary.Torsion bar springs:A torsion bar spring, usually called as a torsion bar is a spring steel rod that uses its torsional elasticity toresist twisting and takes only the shear stresses. One end of the torsion bar is anchored to the frame or otherstructural member of the body and the other end to a component that is subjected to torsional load.The amount of energy stored per unit weight of materialis nearly the same as that of the coil spring. Torsion baris oftenly used with the independent suspension. Asshown in the figure, the bar is fixed at one end to theframe, while the other end is fixed to the end of thewheel arm and supported in the bearing. The other endof the wheel arm is connected to the wheel hub. Whenthe wheel strikes a bump, it start vibrating up and down,thus exerting torque on the torsion bar, which acts as aspring.Torsion bar is lighter as compared to the leaf springs and so it occupies less space. As the torsion tubes aremuch stiffer than the bars, it is preferred. The main disadvantage of the torsion bar is that it does not take thebraking or driving thrust so that additional linkages has to be provided for this purposes. The seconddisadvantage is that the absence of friction force to damp out the vibrations and hence additional dampersare to be provided.Rubber springs:In some cases the rubber can be used satisfactorily as a spring. The main advantage in the use of rubber is itsvibration damping properties. It can also store more energy than the steel of the same weight and it need notbe lubricated. With the use of spring less bearing surface is needed. Rubber has more life and it is also morereliable because the rubber suspension will not fail suddenly as the metal springs. The common types ofrubber springs are 11
  12. 12. a). Compression rubber spring: It is a long solid orhollow rubber spring having the provisions at eachends to connect two units and have a better dampingquality. It is very simple in construction and can resistoccasional overloads of large magnitude. The maindisadvantage of this spring is that it needs mechanicalguide which is generally undesirable.b). Compression shear rubber spring: In this systemthe rubber spring is arranged to take the compression, tension and shearing stresses since the rubber hasexcellent shearing strength. Hence large strain may be allowed in the rubber spring.c).Steel reinforced rubber spring: This spring is the combination of the rubber and steel helical spring. Thefunction of the steel springs is to guide and stabilize the spring and to respond well on heavy shocks andloads. In this spring one fifth of the full load is shared by the steel spring and the remaining load by therubber itself.d). Disc type rubber spring or face shearspring: This spring consists of a disc of rubberwhich is bonded to metal plates on both thesides. One metal plate is fixed to the frame andthe other to a moving component. The load isabsorbed by the twisting of the spring. Thecross section of the rubber disc is made in sucha way that it keeps the stress fairly uniform inthe rubber.e).Torsional shear rubber spring: This spring consists of a solid or hollow shaft and an outer hollowmetallic shell. In between the shaft and the shell rubber is bonded. The shell is fixed in the frame and theshaft is attached to the road wheel assembly with the help of the linkages. When the vehicle negotiates theroad irregularities, then the energy due to the up and down movement of the road wheels are absorbed by theshearing of this spring. 12
  13. 13. Air Springs: (Bellows type, Piston type and elongated air bellows type air springs) Bellows type Air spring Piston type Air springBellows type air spring consists of rubber bellows, which are generally of circular in section. The bellowshas two convolutions for proper functioning. This type of spring is good replacement for the coil springs.Piston type air spring consists of a metal air container in the form of an inverted drum. The drum is fixed tothe frame and the sliding piston is attached to the lower wishbone. For making the system leak proof a sealis provided by flexible diaphragm. The diaphragm is secured at its outer circumference to the lip of the drumand its centre to the piston.Plastic Suspension:This suspension is similar to the compressiontype rubber spring but here plastic is usedinstead of rubber. It consists of a metalliccylindrical container which is fixed with thechassis. Inside the container there are twocentering rings, one is located at the lower endof the cylinder and the other at the top positioninside the cylinder. A suspension shaft isprovided, which is fixed to the axle of thevehicle and is free to slide the plastic rings. Theshaft has side supports at the upper and thebottom centering ring.Plastic rings are provided above and below theupper centering ring. These plastic rings absorbthe vertical dynamic loads. The plastic ringsare supported on a disc in the shaft and whenthe suspension is in rebound position, the discseats on the bottom centering ring. The centering rings are located in the container with help of the circular 13
  14. 14. pressed steel plates and sleeve assemblies. When the vibrations in the axle is transmitted to the spring shaftdue to the road irregularities, then the plastic rings will be compressed or and elongated and the sleeveassembly, carrying the centering ring and surrounding the plastic rings will move up or down in the cylinder.A flexible gaiter is provided at the bottom to prevent the system from dust or other foreign particles. Themain advantage of this system are light weight, silent operation, simple construction, less maintenance, lowwear cost, better cornering characteristics and less chances of sudden collapse.Hydraulic or Hydro-elastic suspension:The main component of the hydraulic suspension is thedisplacer unit, which is attached to the individual wheelsof the vehicle. The displacer unit consists of twochambers. Chamber A is just above the flexiblediaphragm and the other B is above the separatingmember and connected by the other displacers by a hosepipe. The stem is usually connected to the lower link ofthe double wishbone. The diaphragm is connected withthe piston and bears the wheel load. The fluid in chamberA can all the time pass into the chamber B through thebleed holes provided in the separating member. When thepressure of liquid in B rises sufficiently above that in A, then the rubber flap valve which is loaded by thespring will open downwards thus allowing the fluid to pass from B to A through the holes. Similarly to passthe fluid from A to B the damper valve functions accordingly. The fluid in B acts on the under side of therubber element and through the hose pipe is transmitted to the other wheel unit. The Canister Displacerunit in Hydraulic suspension is provided at the outside of the rubber element, while the pot member atinside. The canister is fixed to the body structure of the vehicle.Inter connecter Air & Liquid suspension: In the air suspensions the compressed air is delivered by thecompressor to the reservoir and then from the reservoir the air under pressure is passed to different airsprings. The air springs of front axle and rearaxle are inter-connected and the air pressurein the springs remains constant. When theroad wheel moves upward, then the force onthe spring increases even though the airpressure in the unit is constant.As shown in the figure, when the wheel risesor falls relative to the body of the vehicle,the effective area of the air spring piston ordiaphragm remains constant. The pressuresinsides of both the spring and the forceexerted by them are equal when they areinter connected. Therefore the resultantvertical force at the mid-point between the wheels will be 2P = W. This resultant force may or may not lie ata point of centre of gravity of the body. Therefore due to shifting of centre of gravity one air spring unitmoves to the maximum upward position, while the other to its lowest. 14
  15. 15. Air Suspension:Air suspension systems are becoming increasingly popular because of certain advantages they possess overthe conventional metal springs. These are 1. A variable space for wheel deflection is put to optimum use by virtue of the automatic control devices. 2. Since the vehicle altitude remains constant, the changes in head lamp alignment due to varying load are avoided. 3. The spring rate varies much less between the laden and un-laden conditions, as compared that of that of conventional leaf springs, reducing the dynamic loading. 4. The improved standard of ride comfort and noise reduction attained by the use of air springs reduces both driver and passenger fatigue. In the lay out shown the four air springs are mounted in the same positions where generally the coil springs are mounted. An air compressor takes the atmospheric air though an air filter and compresses it to a pressure of about 240 Mpa. The same pressure is maintained in the accumulator tank, which is provided by a safety relief valve. This high pressure air goes through the lift control valve and the leveling valves to the air springs as shown. The lift control valve is operated manually by means of a handle on the control panel through a cable running from the valve to the handle. The initial height is adjusted according to the loading conditions. Front wheel Independent suspension systems: 1. Mac Pherson strut type: In this layout only the lower wishbone is used. A strut containing shock absorber and the coil spring also carries the stub axle on which the wheel is mounted. The wishbone is hinged to the cross member and positions the wheel as well as takes the accelerating, braking and side forces. This system is simpler in construction and will also keep the unsprung weight lower. The camber angle does not tend to change as the wheel moves up and down. This system will give maximum room in the engine compartment and therefore commonly used in the front wheel drive cars. This system with an anti roll bar provides increased road safety, improve ride comfort, light and self stabilizing steering. This system is used in Maruti-800cc cars. 15
  16. 16. 2. Wishbone Type independent suspension:It is the most common type of independent suspensionsystem. The use of coil springs with a damper in frontaxle is common in this type of suspension. The upper andlower wishbone arms are pivoted to the frame member.The spring is placed in between the lower wishbone andthe under side of the cross member. The vehicle weight istransmitted from the body and the cross member to thecoil spring through which it goes to the lower wishbonemember. The shock absorber is placed inside the coilspring and is attached to the cross member and to lowerwishbone member.The wishbones not only position the wheels and transmit the vehicle load to the springs, but these also resistacceleration, braking and cornering or side forces. The upper wishbone arm is generally kept shorter inlength than the lower ones to keep the wheel track constant and there by avoiding the excessive tyre wear.However a small change in the camber angle does occur with such an arrangement.3. Vertical guide suspension:In this suspension the king pin is directly attached tothe cross member of the frame. It can slide up anddown as shown corresponding to the motion of thewheel and there by compressing or elongating thesprings. In this type, the wheel track, wheel base andwheel altitude remain unchanged. But the system is having the disadvantage of less stability.4. Trailing link suspension:In this type of suspension, a helical coil spring isarranged in horizontal position rather than thevertical position. One end of the spring is fixed withthe cross member of the frame and the other endwith one end of the trailing link. The other end ofthe trailing link is attached to the shaft which carriesthe wheel hub. When the wheel moves up anddown, it winds and unwinds the helical spring toabsorb the wheel vibrations due to road irregularities. To check the vibrations of the spring a multi-plate 16
  17. 17. shock absorber is also provided at the end of the cross member. In some cases of designs torsion bars areused instead of the helical springs.5. Swinging half axles suspension:In this type of suspension the wheels are rigidlymounted on the half axles, which are pivoted on theirends to the chassis member at the center of the car.The half axles are connected with the coil springsand shock absorbers to the frame. It is very simple inconstruction but the main disadvantage is the up anddown movement of the wheel causes the camberangle to vary.Stabilizer or anti-roll device:When a vehicle pass over a bump, one roadwheel may deflect more than the other andthere will be a tendency for the vehicle toroll. To reduce this tendency a stabilizer oranti-roll bar is used in the form of a torsionbar. The stabilizer ends are connected to therear leaf springs or the lower wishbonesand is supported by two bearings, which arefixed to the side members of the frame. Bythis arrangement when one road wheel spring isdeflected than the other, the energy which would havecaused the rolling of the vehicle is converted into thetwisting of the torsion bar.Shock absorberA shock absorber is a device used to check or damp outthe vibrations of the suspension springs to a comfortablelevel. The resistance to the free oscillation of the springsis obtained in the damper by causing a fluid to pass athigh speed through small holes. The energy absorbeddepends upon the viscosity of the fluid and appears asheat in the fluid. The advantage of the fluid type is thatthe fluid resistance is proportional to the square of thespeed of flow through the orifices and so increasesrapidly with the speed of the suspension movement.Telescopic shock absorber:Description: The constructional detail of a common typetelescopic shock absorber is shown I figure. It consistsof an inner cylinder fitted with head at the open mouth.The head is also screwed into the outer reservoir, whichis fixed to the eye by means of which the reservoir issecured to the axle. The piston slides inside the innercylinder. The piston is secured to the piston rod, whichis connected to the other eye and this eye will beconnected to the chassis frame. The outer portion of thepiston rod and the reservoir are covered with a protecting cover which is fixed with the eye and the rod. Thegland provided at the head prevents the leakage of the fluid by scrapping around the piston rod and then 17
  18. 18. passing it the reservoir through the drain hole. The inner cylinder is provided with two valve systems. Onefoot valve is fixed at the bottom and the other with the sliding piston. Both the piston valve as well as thefoot valve assembly has outer ring of holes and inner ring of holes covered by the disc valves.Working: The inner cylinder is completely filled with the fluid and the reservoir is half filled. When the axleeye is moved upward then the fluid is displaced from the bottom side to the top side of the piston. Under thiscondition the fluid must pass through the disc valve by opening the outer ring of holes. In this conditionwhen the upper portion of the cylinder is fully filled or has less space, then the fluid will also be displacedthrough the inner rings of holes of the foot valve and the level of fluid will rise in the reservoir. Similarlywhen the axle eye will move downwards, then the fluid will be displaced from the top side to the bottomside of the piston. In this condition the fluid will pass through the disc valve by opening the inner ring ofholes. The vacuum left by the piston rod inside the inner cylinder during this movement is filled by the fluidfrom the reservoir to the inner cylinder by opening outer ring of holes of the foot valve. This process will berepeated until the vibrations are damped out.Lever type shock absorbersA lever-type damper is shown diagrammatically in Fig. The damper unit is fixed to the frameand joined by a lever and link to the axle. The horizontal cylinder uses two pistons, whichare equipped with recuperator and pressure valves. A thin, mineral-base, damper oil is filledto the level of the bottom of the filler plug.The damper pistons are operated by the movement of the axle due to riding over a bump. Themovement of pistons displaces oil from one chamber to the other. Oil exert pressure to openthe pressure valve, then flows through an orifice to provide resistance, and finally passesthrough the open recuperator valve to neutralize the depression created in the other chamber.A similar action in the opposite direction is produced during rebound of the spring. Theactual damper differs in various ways from the type shown in Fig. The valve constructionand its location are different, and in some designs the cylinders are mounted vertically.The shock absorbers are classified according to their structure and operation as follows.Classification by operation:a). Single action shock absorber b). Multiple action shock absorber.Classification by construction:a). Mono tube shock absorber b). Twin tube shock absorberClassification by working mediuma). Hydraulic shock absorber b). Gas filled shock absorber.INDEPENDENT REAR SUSPENSION 18
  19. 19. The rear wheels of the general vehicles are power driven therefore considerable difficulties are there toprovide independent suspensions. But looking some advantages of independent rear suspensions over therigid axle type it is used in some vehicles. Referring Figure, M and N are two coil springs in verticalpositions and are mounted on the suspension arms. The arms are pivoted on rubber bushes carried by thesub- frame. The sub-frame incorporates the final drive casing and which is mounted on the body structure onthe four rubber mountings A, B, C and D. The other ends of the suspension arms are connected with thedrive shafts in such a way that the shafts may be housed inside the ends with universal drives. One slidingjoint is also provided between the two universal joints. To release the stresses gradually two shock absorbersare provided at the tail of the suspension arms. SUSPENSION SYSTEM TROUBLESHOOTING.Following are the major defects that occur in the suspension system of the vehicle(i) Harsh suspension: It may be due to the following reasons: (a)Normal or conventional suspension systems: (a) The spring leaves corroded, causing excessive friction remedy is to lubricate them. (b) Extra load or rebound leaves fitted wrong way up, remedy is to adjust them properly. (c) Shackle pins seized, remedy is to replace with new bushes. (d) Lubricating pads at the tips of leaves worn out, remedy is to replace them. (e) Shock absorber defective, remedy is to replace the concerned shock absorber. (f) Tips of leaves have worn depressions in leaves, remedy is to get repaired the spring assembly. (b) Independent suspension systems; (a) Suspension arm pivots seized, remedy is to lubricate them. (b) Coil spring tension too great, remedy is to replace with a proper spring. (c) Torsion bars incorrectly adjusted, remedy is to adjust it properly. (d) Any other defects mentioned in the defects of normal suspension system.ii) Over flexible suspension: This defect may be due the following reasons:(a) Lack of friction in the system due to applying undue lubricants. Remedy is to clean the undesiredlubricant.(b) Weakened springs, remedy is to replace them.(c) Broken few leaves, remedy is to replace the broken leaves. 19
  20. 20. (d) Shock absorbers defective, remedy is to replace them.(e) In case of independent systems, the coil spring incorrectly adjusted or weakened. The remedy is to adjustproperly or replace the spring.(f)In case of independent systems the torsion-bar might have weakened, remedy is to replace the bar.(iii) Sagging of vehicle to one side: The following are the main possible causes, of this trouble:(a) Weak spring remedy is to replace the spring.(b) Broken spring leaves remedy is to replace them.(c) In case of independent systems, incorrect adjustment of torsion bars or springs remedy is to adjust them(iv) Breakage of springs: The following are the main possible causes of this(a) Overloading, the remedy is that the vehicle must not be loaded beyond its capacity.(b) Violent braking or grabbing brakes, remedy is to always apply brakes very gradually.(c) Lowe u-bolts, remedy is to tighten them.(d) Loose spring clips, remedy is to tighten them.(e) Head of the centre bolt or dowel does not fit into recess in axle remedy is to set them(f) Seized shackles, remedy is to repair the shackles.(g) Shock absorbers defective, remedy is to replace them(v) Noises: The trouble of noises in the suspension system may be due to any of the following reasons.(a) Loose U-bolts, remedy is to tighten them.(b) Worn shackle pins and bushes remedy is to replace them.(c) Lack of lubrication may also a cause of noise.(d) Shock absorbers defective, remedy is to replace them.(e) In case of independent systems worn suspension arm bushes or bearing and .worn torsion bar splinesmay also be the causes of noise. 20