Lesson 3/Learning Event 1
Another variable-load suspension system provides a spring arrangement that increases the
effective strength of the springs as the load is increased. The springs are made with flat
ends which bear against curved bearing plates. With a light load, the spring ends make
contact with the outer edges of the curved bearing plates (the part of the bearing plate
that is farthest away from the center of the spring). As the load is increased, the spring
compresses, causing the points of contact to move toward the inner edges of the bearing
plates. This decreases the effective length of the spring, giving it a higher load rating.
This method is referred to as a variable-load spring arrangement.
Two rear axles are used on many heavy vehicles to reduce the load on each rear wheel.
In addition, the use of two axles decreases effects of road shocks and increases traction.
A typical rear end of a heavy vehicle consists of an axle mounted at each end of the rear
springs. The load of the vehicle is applied at the center of the springs by means of a
spring seat which is supported on the frame. The rear springs carry the same load as they
do with a single rear axle. However, the load is divided between two axles instead of
being applied to one. The drive is usually applied to both rear axles on military vehicles.
Torque rods transmit the driving force to the frame and are arranged so that none of the
turning force is applied to the springs.
The rear suspension of a truck with two rear axles is usually called a bogie suspension
unit. It consists of two axles joined by springs that pivot on bearing-mounted spring
seats. The spring seat bearings fit around a trunnion axle which is rigidly attached at the
frame through mounting pads. The ends of each spring rest on hardened-steel bearing
plates on the two axle housings. Both springs are clamped to the spring seats by U-bolts.
When the vehicle travels over uneven surfaces, the springs pivot on the tapered roller
bearings. As a result, the weight remains divided between the four wheels even though
one wheel may be on a lower or higher surface. This allows the vehicle to carry heavier
loads without exceeding the safe tire load. Also, when one wheel hits a bump, the spring
pivots on the seat bearings so that both ends of the spring absorb the shock. Thus, the
effects of road shocks to the vehicle frame and body are reduced by one half. When
only one axle is deflected up or down, the trunnion axle and the vehicle frame are raised
or lowered only half the amount. In this manner, bogie axles reduce by half the shock
impact not only to the vehicle frame but also to the tires.