AIR SUSPENSION SYSTEM OF AUTOMOBILES MOHD.SHOEBUDDIN DEPARTMENT OF MECHANICAL ENGINEERING SHADAN COLLEGE OF ENGINEERING AND TECHNOLOGY Email Id:
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ABSTRACT
1. INTRODUCTION
The frame as well as body of a vehicle is attached to the rear axle and the front axle by springs. These springs damp the road shock transmitted to the body structure by the wheels, when they travel over the road. In this way the springs are the protecting units supported directly by the frame of the vehicle. Therefore all the parts which perform the function of protection are collectively called a suspension system. These springs are generally of the laminated leaf type, coil type, torsion bar type, hydraulic springs, Plastic springs, and Air springs etc. 1. To prevent the road shocks from being transmitted to the vehicle frame. 2. To preserve the stability of the vehicle in pitching or rolling while in motion. 3. To safe guard the occupants from road shocks. 4. To provide good road holding while driving, cornering and breaking 5. To maintain proper steering geometry. 6. To provide the requisite height to body structure as well as to bear the torque and braking reactions. 7. To minimize the effects of stresses due to road shocks on the mechanism of the motor vehicle and provide a cushioning effect. 8. To keep the body perfectly in level while travelling over rough uneven ground. i.e., the up and down movement of the wheels should be relative to the body.
Besides providing vibration isolation, air springs offer many other user-friendly advantages. Over the traditional leaf and coil springs. There are two basic types of air springs used in vehicle suspensions: reversible sleeve and convoluted. Regardless of whether an air spring is a reversible sleeve or convoluted style, it will operate on the same principle: A column of gas confined within a container allows it to use the contained pressure to generate force. In the case of air springs, the gas is air and the container is a sealed fabric-reinforced rubber bellow or sleeve. Similar to a ball inflated with air, the load an air spring will carry depends on its diameter and therefore, the area of the column of air supported and the pressure of air inside it. The two basic relationships used in determining the load-carrying capability of an air spring are: Force = pressure x area ---------------- (1) diameter2 Area = π x --------------------4 The relationships above demonstrate that increasing the load an air spring can carry (the force) can be accomplished by increasing the pressure inside the air spring, increasing the diameter of the air spring (and therefore increasing the area) or both. The ability to change the load carrying capacity simply by changing the air pressure, rather than changing out the spring, is a major advantage air springs have over steel. Because an air spring consists of a closed volume of air, the compression of the air spring (jounce travel) will cause an increase in pressure, while the extension of the air spring (rebound travel) will cause a decrease in pressure. This allows the air spring to have an automatic tendency to return to the neutral (design) height as it experiences disturbances in the driving surface. The dynamic build-up in Compression also helps protect against "bottoming out" and can be further increased on the reversible sleeve air spring by the addition of a "flare" at the bottom of the piston.
2. COMPONENTS
Although the basic principles behind both the reversible sleeve and convoluted air springs are the same, there are some subtle differences between them. Most notably, the reversible sleeve air spring has a piston which is an additional component that the convoluted air spring does not have. The piston is the component that is fastened to the moving trailing arm or axle mount and, as a result, plunges in and out of the air cavity within the rubber bellows. In general, a piston gives the reversible sleeve air spring an advantage over the convoluted air spring in that spring, rates can be further tuned using a variety of piston profiles. For straight sided pistons, the reversible sleeve air spring has an advantage over the convoluted air spring, in that a constant load for a given internal pressure may be maintained over a range of heights. The two other major components of an air spring are the bead plate(s) and the fabric-reinforced rubber bellows or sleeve. The bead plate allows for a rigid attachment to the mounting surface(s) and the bellow is the dynamically functioning suspension component which contains the air. The main advantages of an air spring over its steel leaf and coil counterparts are variable load-carrying capability, adjustable spring rate, user-friendly height control, low friction action, and road-friendly suspension increasing the pavement life. As already mentioned, the load an air spring carries can be adjusted over a wide range, without changing the air spring height, simply by changing the air pressure. Traditional steel springs need to be replaced if the height must be maintained. In addition to changing the loadcarrying capability, a change in air pressure will also afford the benefit of changing the spring rate without changing the height and without a significant change in the natural frequency. Steel springs exhibit one spring rate for a given height and, once again, will need to be replaced if the height must be maintained. Using air pressure from the compressor, the air spring height can be maintained by a closed-loop control system or adjusted to the other desired heights. This allows for "load leveling" and "squatting" capabilities that steel springs cannot offer. Because there is a flexible rubber member separating the rigid attachment points to the frame and suspension, there is freedom to move about all six degrees of freedom without the resistance and squeaks experienced by the rigid interactions characteristic of steel leaf and coil springs. The components of the suspension system perform six basic functions: 1. Maintain correct vehicle ride height 2. Reduce the effect of shock forces 3. Maintain correct wheel alignment 4. Support vehicle weight 5. Keep the tires in contact with the road 6. Control the vehicle's direction of travel Typically, struts consists of a coil spring to support the vehicle's weight, a strut housing to provide rigid structural support for the assembly, and a damping unit within the strut housing to control spring and suspension movement. The bottom of the strut
body attaches to the steering knuckle, which in turn connects to a lower control arm through a lower ball joint. The top of the strut is connected to the vehicle body through the upper strut mount, in some cases called a bearing plate. This bearing plate allows the strut to pivot as the wheels are turned. It must be flexible enough to handle slight angle changes and dampen movement of the upper end of the strut. This mount or bearing plate transfers vehicle load to the strut and spring, making the upper mount/bearing plate the load carrier and the lower ball joint the follower.
The strut housing holds the damping unit and fluid. It is made of heavy gauge steel so that it is rigid enough to provide structural support and withstand road shock. The piston rod of the strut is much larger in diameter than the piston rod of the typical shock absorber. This is to withstand the side load on the strut shaft. A strut rod will measure up to 7/8 of an inch in diameter while the piston rod of a typical shock measures up to ½ of an inch in diameter. A coil spring is located between the upper and lower spring seats. It is held there by tension. The lower spring seat is welded to the strut housing, while the upper spring seat is kept in place by the upper strut mount. Struts also have a jounce (or compression) bumper located under the upper spring seat. The purpose of this component is to limit suspension travel by not allowing suspension components to hit together. Finally, a large nut at the end of the strut rod holds everything together.
3. Road-friendly suspension In 1993 a study called the "Dynamic Interaction between Vehicles and Infrastructure Experiment" (DIVINE) was initiated by the Directorate of Science, Technology and Industry of
the OECD. Interim results from the study were presented in 1995 and the final report in 1997.
The purpose of the study was to quantify the benefits of heavy vehicles with air suspension on roads. The knowledge gained and appreciated by the co-operating countries can be shared with countries that are rapidly expanding their transportation infrastructures. The end result is a faster payback and reduced costs of maintaining these infrastructures. This study was conducted with participation from European and North American countries, as well as public and private institutions. The purpose of the study was to provide scientific evidence of the effects of heavy vehicles and their suspensions on road systems. Almost 50 per cent of road maintenance costs are associated with effects from heavy vehicles. Further, the amount of dynamic load exerting on roads is directly associated with the type of vehicle suspension. Air suspensions increase pavement life by 15-60 per cent. This corresponds to increased static load of 4-12 per cent. A 15 per cent increase in vehicle mass limit can save upwards of $500 million per annum, while increased pavement life ensures significant reduction in road maintenance cost which forms 90 per cent of the annual road budget in OECD countries. "Road-friendly" suspensions have low spring stiffness and coulomb friction with optimum damping. Well-designed air suspensions best meet these criteria. For a complete copy of the DIVINE report, visit www.oecd.org. Air suspensions are used on a vast majority of heavy duty vehicles in North America and Europe, with a growing penetration worldwide. The advantages of air suspension with regard to the vehicle, driver and transportation systems are appreciated in both qualitative and financial terms. Firestone Industrial Products Company, LLC, a subsidiary of Bridgestone Firestone Diversified Products (BFDP), specializes in air spring manufacturing and technology with a history of more than 60 years of research and development of technologically advanced air springs. With headquarters in Indianapolis, Ind., and six manufacturing plants located on four different continents, the company produces suspension products enhancing the driving experience for drivers of heavy truck/trailer, buses, rail vehicles, passenger cars, SUVs, light trucks, minivans, vans and motor homes.
4. BASIC PRINCIPLE Suspension system that has air as its working fluid and acting as shock absorber is called air suspension system. The detection of causes and remedy for the problems with suspension system are discussed here:
Interconnected suspension system As the name suggests the front and rear suspension system or the suspension units on the two sides of the vehicle are connected with each other. They are also termed collectively as linked system. It was very much effective as compared to independent front and rear suspension units and was able to reduce the tendency of vehicle to bounce, pitch or roll and was able to provide smooth and comfortable drive. Types of interconnected suspension system are: 1) Air suspension system 2) Hydrolastic suspension and 3) Hydro gas suspension system Air suspension system The conventional metal springs faced some drawbacks which were air suspension system overcomes and so they are preferred and used in more these days. Let's see some of the plus points of this system. 1) The automatic control devices installed in the vehicle
2)
allows making optimum use of the variable space for deflection of wheel. The height of the automobile remains steady and so the
3)
changes in the alignment of headlamp due to varying loads are restricted. It helps to reduce the load while the vehicle in motion
i.e. the dynamic loading as the spring rate variation between laden and unladen weight is much less. 4) It gives smooth and comfort ride of the vehicle. Air springs are classified into two types: 1) Bellow type and 2) Piston type. The air springs shown are mounted on the front and rear axle. The atmospheric air first passes through the filter where the dirt is removed and passed on to the compressor. Air is compressed here
and the pressure of air is raised from atmospheric to about 250 Mpa. This pressure is maintained by the accumulator tank. The safety relief valve is provided on the accumulator as a safety device and it opens when the pressure rises above 250 Mpa. This air then moves to lift control valve and through leveling valves to air springs. Hydrolastic suspension In this case the moving part is assembled and fitted at each of the wheel location. These units are interconnected by pipes carrying the fluid. In this moving unit rubber acts as a spring and the fluid under pressure acts as a damping medium. The connecting rod of piston is connected to the wheel through suitable linkage in order to receive the movements of the wheel. The movement of the fluid is controlled by a two way valve assembly. The valves are arranged at right angle to each other. As the pressure of fluid rises it causes the upper valve to open. In the same way the lower valve opens under pressure and allows the fluid to flow in downward direction. Hydro gas suspension system The hydra gas suspension system was designed by Moulton Development Limited and British Leyland Motor Corporation and is manufactured by Dunlop suspension Division of Coventry (England). The drawbacks of hydrolastic suspension system were overcome by this design. It is mainly divided into two parts an internal spring and a damper unit and is assembled at each wheel. The springing action is taken by inert gas like nitrogen. Weight of the car is supported by some fluid like water under pressure. Insulation of the interconnected front and rear system is done generally with hermetic seals. As the wheels in motion come across pit or bumps they cause the piston to move and the diaphragm forces the fluid up which causes the damper valve to open and the fluid moves through the opening.
5. AIR SUSPENSION PROBLEMS •
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Uncomfortable Ride: The reason of the rough ride you might be due to one of the following reasons: 1) Springs might have got rusted and thereby not able to provide proper springing action. Lubrication should be done to minimize the friction. 2) Shock absorbers might got problems repair or replace it. 3) Pins holding the springs together might have got loose tighten it. 4) Pads present at the leaves (curve shaped spring like saucer) might be worn out replace them. Suspension becoming flexible: Some of the main reasons for suspension becoming loose are:
1) Due to long period of continuous use the springs might have become weak they must therefore be replaced with new one. 2) Some springs do not require lubrication yet they might have been lubricated so clean with some cloth. 3) Defects in shock absorber rectify them.
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Noise 1) See the manufactures manual if the springs require lubrication and then provide lubrication for springs. 2) Pins holding the springs and bushes might have got loose. 3) Nuts and bolts holding the springs might be loose tighten them.
6. CONCLUSION Over the last decade or so air suspension has become extremely popular in the custom automobile culture: street rods, trucks, cars, and even motorcycles may have air springs. They are used in these applications to provide an adjustable suspension which allows vehicles to sit extremely low, yet be able rise to a level high enough to maneuver over obstacles and inconsistencies in the roadways (and parking lots). These systems generally employ small, electric or engine-driven air compressors which sometimes fill an on-board air receiver tank which stores compressed air for use in the future without delay. High-pressured industrial gas bottles (such as nitrogen or carbon dioxide tanks used to store shielding gases for welding) are sometimes used in more radical air suspension setups. Either of these reservoir systems may be fully adjustable, being able to adjust each wheel's air pressure individually. This allows the user to tilt the vehicle side to side, front to back, in some instances "hit a 3-wheel" (contort the vehicle so one wheel lifts up from the ground) or even "hop" the entire vehicle into the air. When a pressure reservoir is present, the flow of air or gas is commonly controlled with pneumatic solenoid valves. This allows the user to make adjustments by simply pressing a momentary-contact electric button or switch. The installation and configuration of these systems varies for different makes and models but the underlying principle remains the same. The metal spring (coil or leaf) is removed, and an air bag, also referred to as an air spring, is inserted or fabricated to fit in the place of the factory spring. When air pressure is supplied to the air bag, the suspension can be adjusted either up or down (lifted or lowered). For vehicles with leaf spring suspension such as pickup trucks, the leaf spring is sometimes eliminated and replaced with a multiple-bar linkage. These bars are typically in a trailing arm configuration and the air spring may be situated vertically between a link bar or the axle housing and a point on the vehicle's frame. In
other cases, the air bag is situated on the opposite side of the axle from the main link bars on an additional cantilever member. If the main linkage bars are oriented parallel to the longitudinal (driving) axis of the car, the axle housing may be constrained laterally with either a Panhard bar or Watt's linkage. In some cases, two of the link bars may be combined into a triangular shape which effectively constrains the vehicles axle laterally. Often, owners may desire to lower their vehicle to such an extent that they must cut away portions of the frame for more clearance. A reinforcement member commonly referred to as a C-notch is then bolted or welded to the vehicle frame in order to maintain structural integrity. Specifically on pickup trucks, this process is termed "notching" because a portion (notch) of the cargo bed may also be removed, along with the wheel wells, to provide maximum axle clearance. For some, it is desirable to have the vehicle so low that the frame rests on the ground when the air bags are fully deflated.
REFERENCES 1.
2. 3. 4. 5. 6. 7.
Adams, William Bridges (1837). English Pleasure Carriages. London: Charles Knight & Co.. http://books.google.co.uk/books? id=apw7AAAAMAAJ. Jain, K.K.; R.B. Asthana. Automobile Engineering. London: Tata McGraw-Hill. pp. 293–294.ISBN 007044529X. "Mitsubishi Galant", Mitsubishi Motors South Africa website "Mitsubishi Motors history 1981-1990", Mitsubishi Motors South Africa website "Technology DNA of MMC", .pdf file, Mitsubishi Motors technical review 2005 "MMC's new Galant.", Malay Mail, Byline: Asian Auto, Asia Africa Intelligence Wire, 16-SEP-02 (registration required) "Mitsubishi Motors Web Museum", Mitsubishi Motors website MOHD.SHOEBUDDIN B.TECH MECHANICAL ENGINEERING FROM SHADAN COLLEGE OF ENGINEERING & TECHNOLOGY
