The chassis is connected to the road wheels through the medium of springs and axles, and it is this medium which, with the wheels and tyres constitutes the suspension system of any vehicles. Although the axles are an integral part of the suspension system, they also incorporate other functions, i.e., steering in the case of front axle, and final drive in the case of rear axle; they are therefore dealt within other chapters. However, their respective function in the suspension system should not be overlooked. The springs as contrasted with the wheels and tyres, may be considered in this respect first. Basic requirement of suspension system are as follows
(a) To protect the occupants from road shocks.
(b) To reduce the stresses due to road shocks on the mechanism of the vehicle and,
(c) To maintain the body even when travelling over rough ground or while taking a turn.
TYPES OF SUSPENSION SYSTEM
(a) Road spring Type
(b) Coil spring Type
(c) Air bellows Type
(d) Independent type
Most of the vehicles depend for their main springing on the laminated leaf spring assisted by pneumatic tyres, shock absorbers and upholstery. The springs are usually the means by which axles are secured to the frame and they prevent any sideways or lengthwise movement between the axles and the frame. Furthermore, the front springs resist the braking torque of the front wheels, and in the case of Hotchkiss drive, the springs take both the braking and driving torque.
CONSTRUCTION OF ROAD SPRING
(a) It consists of a number of steel blades of varying length mounted together at centre by a bolt, known as the centre bolt. This forms a steel beam thickest at the centre and thinnest at the ends. This form gives the steel beam uniform strength at all parts and this is the highest possible construction of beam for its strength. The object of number of blades is to allow more vertical movement, and to introduce a number of frictional surfaces in order to dampen the oscillations of the whole spring. The top leaf is known as “Master leaf” the ends of which are turned over to house a phosphor bronze bush or silent block for connecting the spring to the chassis frame through shackles.
(b) To avoid the lateral sliding of the blade, they are held together by “U” shaped metal straps. In order to prevent leafs rusting, and squeaking thin layers of special material, such as zinc or some other material, is inserted between the blades. Some time, brake lining material, is inserted between the blades. In some models the whole spring is enclosed in grease retaining cover, known as spring gaiters.
SERVICING OF ROAD SPRING
(a) Lubrication points are provided at all shackles, centre bolts, fulcrum pins and true-union bearings which should be lubricated periodically. It should be ensured that the lubrication penetrates to the bearing surfaces which are indicated by the presence of fresh lubricant at the ends of the bush or bearings. “Silent blocks “don’t need lubrication but attention should be given periodically to check for wear between shackles and springs eyes.
(b) Leaf springs should be kept clean, dust, girt and mud should be removed by scraping and wire brushing, examined for cracks or other defects. These are usually lubricated by the application of penetrating oil (engine oil and kerosene mixed). The spring to be lubricated, should be relieved of the vehicles weight by jacking up under the chassis which enables the leaves to be separated, so that the lubricant can reach all surfaces. If difficulty is experienced in getting the leaves separated, a large screw driver can be used and lubricant applied by means of broken hacksaw blade. During cleaning process an examination of the spring should be made for security of shackles, broken leaves, and tightness of bolts securing the spring to the axle chair. Worn out centre bolts, shackle bolts, bushes and shackles should be renewed.
(c) During major servicing of a vehicle, it is necessary to check the spring camber. This is usually measured perpendicularly, from a line through the spring eye, to the surface of the main leaf, which should agree with the makers’ specification.
(a) In any suspension system the aim is to obtain the maximum comfort to the driver and passengers, with the minimum stress on the frame and springs. When deflected all the springs have the tendency to oscillate about the normal static system. The energy, given to the spring by the wheel hitting bump is absorbed by the shock absorbers, the two methods used are friction dampening and hydraulic dampening.
(b) The shock absorbers in use on service vehicles is of the hydraulic type, and the two main types of hydraulic shock absorbers are the radial lever piston type, and the direct acting telescopic type. In both cases they operate on the principle of hydraulic displacement through a restricted orifice from one chamber to another brought about by the relative movement between the axle and the chassis of the vehicle.
OPERATION OF SHOCK ABSORBER
(a) Bound Stroke. As the shock absorber is compressed by the rising wheel, the piston rod assembly moves down in relation to the cylinder, thus creating a pressure below the piston. The oil flows through the outer ring of the holes lifting the flap valves against its spring ,the volume of the piston rod entering the cylinder displaces an equal volume of oil which is forced through the holes in the valve past the spring disc and in to the reservoir.
(b) Rebound Stroke. On the rebound stroke the shock absorber is extended, reversing the flow of oil .The lower flap valve moves against the helical spring, uncovering the inner ring of the holes and allowing oil to flow through. As the piston rod relatively squeaking is withdrawn from the cylinder, and equal volume of oil is filled from the reservoir through the central orifices in the valve assembly.
REMOVAL & FITMENT OF ROAD SPRING
(a) Park the vehicle on hard level ground.
(b) Chock the wheels.
(c) Disconnect the battery.
(d) Place the jack underneath the axle.
(e) Remove the wheel.
(f) Jack up chassis frame in such a way so that load of the vehicle can be relieved from the road spring.
(g) Remove the shackle pin after removing the locking bolt from the front end of the road spring.
(h) Remove the ‘u’ bolts which are mounted to the road spring and axle.
(j) Remove the pivoting pin from the rear end of the road spring after removing the locking bolt and take out the road spring.
(k) Fitment of the road spring is to be carried out in reverse order.
REMOVAL & FITMENT OF ROAD SHOCK ABSORBER
(a) Loosen the upper mounting lock nut of shock absorber and remove it.
(b) Remove the upper mounting nut and remove the upper end of shock absorber and collect rubber pads.
(c) Loosen the lower mounting lock nut of shock absorber and remove it.
(d) Remove the lower mounting nut and remove the lower end of shock absorber and collect rubber pads.
(e) Take out the shock absorber.
(f) Fitment of the shock absorber is to be carried out in reverse order.