INTRODUCTION TO WHEELED VEHICLE DRIVE LINES,
AXLES, AND SUSPENSION SYSTEMS
Automotive drive lines and suspension systems have changed quite a bit since the first automobile was
built. At first, automobile axles were attached directly to the main frame of the vehicle. This caused
many problems. For example, the vehicle produced a very rough ride. Also, rigid construction did not
work well on rough ground because sometimes one of the wheels would not touch the ground. If the
wheel off the ground was a drive wheel, the vehicle lost traction and stopped. This problem proved a
need for a more flexible vehicle.
The problem was corrected by using springs between the axles and the frame. The early springs were
the same type used on the horse-drawn buggy. They allowed the wheels and axles to move up and
down separate from the body. The body moved very little compared to the wheels and axles, and the
ride was much smoother.
Allowing the axles to move separate from the body also kept the wheels on the ground over rough
roads, but this caused a new problem. The old drive train between the engine and the axle would not
work. The train had to be made to move more. This was done by adding movable joints in the drive
shaft known as universal joints. Some early vehicles used only one universal joint on the drive shaft,
while later vehicles used two universal joints on the drive shaft. Drive shafts are now usually called
propeller shafts. Some long-wheel-base trucks now use as many as four propeller shafts between the
transmission and the drive axle. These propeller shafts are connected by universal joints.
Early automobiles were made up of a body, a power plant, and a running gear. The running gear was
made up of the wheels, axles, springs, drive shaft, and transmission. The transmission was often
mounted midway between the engine and rear axle. It was connected to the engine and the rear axle by
The term "running gear" is not used any more. A new term, "chassis," is now used to identify the old
running gear plus the power plant. In modern vehicles, the transmission is generally mounted on the
engine and is part of the power plant.
The chassis of modern military tactical vehicles, especially the frame, spring, and axles, must be very
strong and yet quite flexible. You will learn how this is possible in this subcourse.