22.11.
Rigid-axle-beam Suspension
The beam-type axle (Fig. 22.53A and B) is the oldest and simplest of sprung axle arrangements. It still offers certain advantages to justify its use over more sophisticated suspension systems. This design permits the road-wheels at the front to be interconnected by a rigid solid forged axle-beam, which is a non-drive axle and at the rear by a rigid tube-section beam at the centre of which the drive half-shafts pass to transmit the drive to the rear wheels. Stub-axles are hinged or pivoted on to each end of the front axle-beam to support mechanisms for steering the front road-wheels.
The semi-elliptic springs seat on flat spring-saddles or spring-beds, formed on the under face of the axle-beam (Fig. 22.54A). A countersink hole at the centre of each spring-saddle provides a location point for the centre-bolt head. The springs and the axle are clamped together using two U-bolts. Sometimes rubber pads are fitted on each side of the spring-blade pack on car axles to damp out small vibrations and noise from the unsprung axle.
With large trucks, the axle and saddle are clamped directly on to the steel main leaf (Fig. 22.54B). Axle clamp blocks and separate clamp bolts in this case replace the U-bolts for greater grip. In addition, the spring leaves in large springs are dimpled in the centre to avoid sliding movement between the leaves and to minimize shear force on the centre bolt.
The T section, which provides the greatest resistance against bending, is used for the front axle-beam span between the spring-saddles. Between the spring-saddles and the stub-axles the I-section changes to a round section, which is more resistance to twisting, produced due to acceleration and braking torques. The sprung chassis transmits the vehicle’s weight and payload to the front and rear mounting brackets, shackle-pins, leaf springs, and to the unsprung axle-beam. Finally, the axle-beam divides the vertical forces between the two road-wheels.
During body roll, wheel camber roll does not take place because the axle-beam holds the road-wheels perpendicular to the ground under normal driving conditions (Fig. 22.55A). If one of the wheels goes over an obstacle or falls into a pot-hole, the road-wheel moves vertically up and down depending upon contour of the road surface without changing the average sprung body height (Fig. 22.55B). However, the vertical motion of the wheel unfortunately tilts the axle-beam and transmits some of the bump or rebound movement from one end of the axle-beam to the other. This deflection can cause an oscillating tilting motion of the axle-beam, known as axle ‘tramp’, which is felt as a tremor of vibration on the steering wheel. The bump stops in the form of rubber blocks are usually clamped on top of the axle or spring using the U-bolts to stiffen the suspension during violent bouncing or excessive body rolls.
Fig. 22.53. Semi-elliptic spring with rigid-axle-beam.
Fig. 22.54. Vehicle axle and leaf-spring location and clamping. A. Car and va^ B. Heavy commercial vehicle.
Fig. 22.55. Effect of body roll and irregular road surface or rigid axle-beam with semi-elliptic springs. A. Body rolls, wheels remain upright. B. Wheel enters pot-hole, axle and wheel tilt.
Advantages of Beam Axle.
(a) The axle-beam stub-axle and semi-elliptic springs constitute a simple and compact suspension and steering system using only few parts, and is capable of taking up considerable rough handling. It is reliable, easily serviced and cheaper than other types of suspension.
(6) The axle provides rigid support for each spring, both in the vertical plane against bouncing of the sprung body and in the horizontal plane against side forces.
(c) The alignment of both road-wheels to the road in this case depends not on the chassis and body stiffness but on the rigidity of the axle-beam, which supports both stub-axles and wheels.
(d) As the road-wheels are attached to the axle-beam by the stub-axles, the wheels always remain approximately perpendicular to the road so that best possible tyre tread contact with the road is achieved. This provides very good road grip and consequently prolong tyre life.
(c) The height of the axle-beam above the road does not alter between the unladen and laden state or when the body, rolls or goes over a bump or pot-hole. This constant ground clearance is desirable for the vehicle travelling over rough ground.
Disadvantages of Beam Axle.
(a) Hard springing takes place due to small maximum spring deflection. The vertical axle movement is limited by clearance between the axle and the engine.
(6) Steering geometry is not accurately controlled. Figure 22.56A illustrates the change in chamber angle that occurs when one wheel strikes a bump. Due to sudden alteration in chamber angle the wheels, which are acting as a form of gyroscope, ‘flap’ around the king pin, known as wheel shimmy. The difference in castor angle when the spring is deflected is shown in Fig. 22.56B.
(c) Due to high unsprung weight maximum wheel adhesion is not obtained.
(d) Normally the engine has to be located behind the axle to provide clearance. In case the engine can be moved forward, there is possibility of the accommodating all the passengers within the wheelbase providing greater comfort.
(e) Poor ‘roll stiffness’ at the front may produce oversteer. The front springs have to be mounted close together.
Fig. 22.56. Steering geometry alterations. A. Change in camber angle. B. Change in castor angle.
