AUTOMOBILE RUNNING SYSTEM AND POWER TRAIN 5. SUSPENSION SYSTEMS
The frame and the body of an automobile are not mounted directly on the front and rear axles but on the suspension system which is installed between the axle and the frame or body of the vehicle. The suspension system consists of springs and hydraulic dampers, which are also known as shock absorbers. The suspension system prevents road shocks from being transmitted to the frame and body of the vehicle so that these do not get transmitted to the passengers or goods when the vehicle is moving on the road.
5.0 Objectives After going through this lesson, you will be able to: i. Classify suspension systems ii. List the components of suspension systems and give the function of each iii. Compare different types of suspension systems
5.1 Introduction A good suspension system affords a comfortable ride to passengers in the car. Hence car manufacturers constantly try to develop better and better suspension systems. Suspension systems are classified into conventional and independent. Earlier the conventional system was used at the rear axle. It is used even now in heavy commercial vehicles. Passenger cars now have the independent suspension system at the rear also, because it gives a smoother ride. The independent suspension is used at the front axle in cars. This reason is that the front axle also performs the function of steering and the independent suspension affords better stability since with the independent suspension system only one wheel tilts while the other executes only up and down motion.
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A suspension system performs the following functions: It prevents the road shocks from being transmitted to the vehicle frame, thereby providing a comfortable ride to the driver and other occupants. It reduces the stress on various vehicle parts. It preserves the stability of the vehicle while braking or during a sharp turn.
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It maintains the proper steering geometry. It damps the oscillations down that are caused by irregularities in the road.
5.2 Classification of Suspension Systems Suspension systems are of two types: i. Conventional, and ii. Independent. An independent suspension system is used for front and rear axles in cars nowadays to improve passenger comfort.
Self-check Questions 1. Suspension system is installed between the _________ and the ___________ of the vehicle. 2. Suspension system consists of _________ and ___________________. 3. Give the function of the suspension system. 4. Name the 2 main types of suspension systems 5. Which suspension system is invariably used at the front in passenger cars?
5.3
Components and Working
A good suspension system should have spring and damping actions. Spring action provides an elastic resistance to the load while damping is the ability to absorb some of the energy of the spring after it has been compressed. If this energy is not absorbed, the spring will overshoot its original position and will continue to bounce up and down. The springs act as reservoirs of energy and cushion the body of the car and its occupants from road shock. There are three types of springs that are used in automobiles: i. Leaf springs, ii. Coil springs, and, iii. Torsion bar. A leaf spring has the two ends, called eyes, attached to the car frame by bolts supported in rubber bushes and the middle of the spring is clamped to the axle casing by U-bolts. The best shape for storing energy for a given weight of spring is circular, and a coil spring stores the energy produced by the up-and-down movement in the most efficient way.
Torsion bar stores energy when it is twisted and has one end anchored to the car frame and the other to a component that is subjected to loading.
Fig. 5.1: (a) Spring action and damping. (b) Leaf spring. (c) Coil spring. (d) Torsion bar.
Dampers, which are often also called ‘shock absorbers’ or plain ‘shocker’ damp the vibrations so that the springs do not bounce up and down continuously. The most commonly used damper is the telescopic hydraulic damper, which has a piston and the movement of this piston causes oil to be displaced through small holes which resists the flow of oil. The telescopic damper consists of a cylinder containing a piston on a rod. The closed (lower) end of the cylinder is attached to the axle, and outer end of the rod, which passes through a sealing gland on the
Fig. 5.2: Shock absorber.
cylinder, is connected to the body of the car. The two ends of the damper move towards each other when the spring compresses and they move away from each other when the spring expands, thereby preventing oscillations and providing damping action.
Self-check Questions 6. 7. 8. 9. 10.
5.4
What are the 2 main requirements of a good suspension system? What will happen if hydraulic damper is not provided? Name the 3 types of springs used. How is the leaf spring attached? Which spring is considered best for storing energy?
Types of Suspension Systems
The suspension system mounted on the front axle is more complicated than the one on the rear because the front wheels not only move up and down but also need to be turned sideways for steering. 5.4.1 Independent Suspension An independent suspension system is usually used for each front wheel, where each front wheel is connected to the body of the car using its own linkage and spring so that the up and down movement does not affect the other wheel.
Fig. 5.3: Types of suspension systems.
Double-wishbone type: A type of independent front suspension system is the doublewishbone type. It consists of two wishbone links that are hinged at their broad ends to the body or sub frame of the car, and at their narrow ends to the swivel members which have stub axles to carry the wheels. The wishbone links are known by this name owing to their shape, which resembles the bones of a chick.
Fig. 5.4: Double wishbone system.
A spring and a hydraulic damper to absorb road shocks are installed between the wishbone on each side and the car frame. In this kind of design of the suspension system, the damper is usually installed within a coil spring. The braking forces are taken up by an oblique strut from the lower wishbone to the body or to the cross-member carrying the suspension. The wish bones are generally not parallel and they are of unequal length, with the upper one being of a shorter size. The wheel, therefore, does not remain in the upright position as it moves up and down over bumps, but leans slightly inwards. This results in better cornering characteristics. McPherson strut type: Besides the double wishbone type, the other type of suspension system is the one that has the McPherson strut layout. This suspension system consists of a single control arm and a telescopic strut for each front wheel. The knuckle carrying the wheel is connected to the lower part of the strut. The entire strut is turned by the steering linkage and it is connected at the top of the body structure of the car with the help of a support. To be able to absorb road shocks, a coil spring surrounds the upper part of the strut, which contains a hydraulic damper.
Fig. 5.5: McPherson assembly.
The main advantages of this kind of suspension system are: • It is mechanically simple. • It has moving parts that are light in weight and which help the wheel to follow the irregularities of the road. • It does not cause the camber to vary by a large angle when the wheel moves up and down.
5.4.2 Conventional Suspension
Fig. 5.6: Conventional suspension system.
The conventional suspension systems comprise a rigid axle beam, leaf springs and hydraulic dampers. When one wheel rides over a bump or a pothole, the entire axle beam is tilted, which also affects the other wheel and, in turn, the stability of the car. Hence, the conventional suspension system is not used in the front of the cars and is generally limited to the rear of the cars and is used in the front and rear of trucks. The front of the car invariably uses the independent suspension system.
Self-check Questions 11. Which suspension system (front or rear) is more complicated? Why? 12. Name the 2 types of independent suspension systems. 13. Coil spring and hydraulic damper are installed between the 2 control arms in the wishbone type independent suspension system. State whether True/False. 14. Which control arm is longer in the wishbone type independent suspension system? 15. Give the main disadvantage of the conventional suspension system.
5.5
Activity 1. Investigate how bicycle seat provides cushioning effect. 2. Push down and leave quickly the front ends of different cars. Note how many times they move up-down. What inference do you draw regarding their suspension systems?
5.6
Summing Up
This lesson also taught us how the suspension system works to prevent shocks on the road from being transmitted to the passengers or goods in the vehicle. The two main types of suspension systems are the independent suspension system and the
conventional suspension system. The independent system is usually used nowadays because of the advantages it offers. There is also a McPherson strut type system that is used in front wheel drive vehicle.
5.7
Possible Answers to Self-check Questions
1. axle, frame or body 2. springs, hydraulic dampers 3. to prevent road shocks from being transmitted to the frame and body of the vehicle 4. independent, conventional 5. independent 6. spring and damping action 7. spring will oscillate for long 8. coil, leaf, torsion bar 9. by bolts through spring eyes at front and rear end to the frame of the vehicle and to the axle casing by U-bolts 10. coil spring 11. front; the front wheels also have to be steered 12. double wishbone type and McPherson type 13. False 14. lower 15. if one wheel goes over a bump or pothole the other wheel is also affected because the whole beam tilts
5.8
Terminal Questions
1. 2. 3. 4.
5.9
Where is the suspension system placed? What does it comprise of? Classify suspension systems and discuss their suitability for front and rear axles. List the components of the suspension system and give the function of each. Differentiate between the two types of independent suspension systems.
References and Suggested Further Reading
1. AA BOOK OF THE CAR. Drive Publications Ltd, London. 2. Maruti 800 Service Manual 3. http://www.howstuffworks.com
5.10
Glossary
Coil spring
Steel wire in the form of coil which absorbs force by compression.
Conventional suspension system
Suspension system built on a rigid axle so that all wheels on the axle are affected if wheel(s) on one side go over a bump.
Double-wishbone type independent suspension system
Suspension system having 2 v-shaped control arms fitted to the frame and at the outer end of which wheel is mounted.
Eye
Circular hole at the end of the leaf spring.
Frame
Structure with longitudinal and cross members on which chassis is built.
Hydraulic dampers
Shock absorbers as they are commonly called.
Independent suspension system
Suspension system in which all wheels are mounted independent of each other.
Knuckle
Component by which the road wheel is steered by the tie rod.
Leaf spring
Steel strip(s) which act like springs to absorb force by flexing.
McPherson strut
Telescopic strut and spring system used in front wheel drive cars.
Oscillations
Repeated extension and contraction of spring after removal of force.
Steering geometry
Settings of steered (usually front) wheels to obtain good steering stability and tyre life.
Suspension system
System of springs and dampers to cushion road shocks.
Telescopic hydraulic damper
Shock absorber in which one tube slides within the other.
Torsion bar
Rod which absorbs force by twisting.
U-bolt
U shaped bolt which is used to hold leaf spring to axle casing.