22.6.
Rubber Suspension
As rubber can store more energy per unit mass than any other type of spring material, considerable weight can be saved with rubber suspension. Rubber springs, if works on compression or shear, can be used as the main suspension spring, otherwise can be fitted along with metal springs to improve the suspension characteristics. Large rubber ‘bump’ stops used in many suspension layouts stiffens the suspension spring against maximum deflection.
Figure 22.25 represents a rubber suspension system in a simplified form, that is similar to the one used on a popular small car. The spring is installed between the frame and the top link of the suspension system. When the spring is connected to a point near the link pivot, deflection of the spring reduces to a minimum, without affecting the total wheel movement. This arrangement of spring provides a rising-rate characteristic, which is ‘soft’ for small wheel movements but becomes harder as the spring deflects.
The energy released from the rubber spring after deflection is considerably less than that imparted to it. This internal loss of energy is called hysteresis, which is an advantage, because lower-duty dampers may be used. Some rubber suspension systems have a tendency to ‘settle down’ or ‘creep’ during the initial stages of service, therefore allowance for this must be provided.
Fig. 22.25. Rubber spring.
22.6.1.
Rubber Springs Installed on Balance Beam with Stabilizing Torque Rods
Rubber spring suspensions are constructed from alternatively bonded layers of rubber blocks and steel reinforcement plates sandwiched between inclined mounting plates due to which the rubber is subjected to both shear and compressive forces. The rubber springs are installed between the chassis spring cradle and a wedge shaped load transfer member pivoted centrally (Fig. 22.26). To equalize the load between the two axles, a box-sectioned balance beam is mounted centrally by a pivot to the load transfer member. The upper torque arms am linked between the axles and chassis so that the brake torque reaction is eliminated. If a pair of incli<ned rubber springs is positioned on both sides of the chassis, the increase in the loading of the axles produces a progressive rising spring rate due to the changing of imposed stress into the rubber from shear to compression.
The axles are permitted to take up any variation in road surface unevenness independently of the deflection of the rubber springs caused by the laden weight of the vehicle. Rubber bushes are provided at all pivot joints to eliminate lubrication. These rubber spring suspensions are suitable for non-drive tandem trailers, rigid truck with tandem drive axles and bulk carrier tankers.
Fig. 22.26. Rubber spring mounted on balance beam with leading and trailing torque arms.
22.6.2.
Hydrolastic Suspension
This suspension is intended to improve the vehicle’s resistance to pitch, the tendency of the body to oscillate in a fore-and-aft direction when the front springs are compressed and the rear springs are expanded simultaneously. The continuous forward and backward pitching motion provides a most uncomfortable ride, which may become serious when the frequency of vibration of front and rear springs is the same.
The Hydrolastic suspension layout on a vehicle uses inter-connected rubber displacer units (Fig. 22.27) installed between the frame and the independent suspension linkage controlling the wheel. The interconnection is carried out using two pipes. One pipe links the left-hand side units together and the other on the right-hand side. The system is pressurized with an anti-freeze liquid after removing air. Each displacer unit contains a rubber spring; metal separating member, which holds two rubber damper valves; rubber diaphragm attached to the suspension linkage, which holds the wheel; and a metal body, which is secured to the frame of the vehicle.
Fig. 22.27. Hydrolastic displacer unit.
Since road irregularities normally cause the vehicle to pitch, roll and bounce, the operation of the hydrolastic system under these conditions is discussed.
A sudden upward movement of the front wheel causes the diaphragm to displace the liquid through the damper. This action is turn forces liquid along the pipe to the rear unit where it moves the diaphragm and raises the rear of the car to the level of the front (Fig. 22.28). When the front wheel descends, the liquid returns and the vehicle comes to its normal riding position.
During this sequence the liquid has to pass the damper valve in each unit, and the restriction to liquid flow at the valves and in the pipelines damps out the tendency of pitch oscillation.
Fig. 22.28. Action of hydrolastic units.
When a vehicle is cornering, the body of the vehicle tilts or rolls outwards due to centrifugal force. This tilting action is apparent when ‘soft’ conventional springs are used. The hydrolastic system is ‘soft’ during movement of a single wheel, but if the two outside suspension units are loaded during cornering, a stiffening of the hydrolastic system occurs. Under this type of loading displacement of the fluid from one unit to the other does not occur. Instead the increased liquid pressure deflects the rubber springs, which provide a marked resistance to the tilt of the body.
During bouncing of the vehicle four wheels deflect at the same time. To resist this motion all the hydrolastic units perform in the similar way as to react to roll.
