Chapter 5 Front Suspension Systems 99
Ball Joint Loading
The basics of compression and tension ball joint load-
ing were discussed in Chapter 4, Common Suspension
System Components. This section covers the determination
of loaded and follower joints on actual suspension systems.
On any suspension system, the weight of the vehicle passes
from the frame to the wheels through the spring or torsion
bar. The spring position in relation to the control arm or
arms determines which ball joint is the loaded joint and
which ball joint is the follower joint. Ball joint loading on
MacPherson strut suspensions, conventional suspensions,
and solid or twin I-beam suspensions is discussed below.
MacPherson Strut Suspensions
On a MacPherson strut, the top of the strut assembly
is attached to the vehicle body and the bottom is bolted to
the steering knuckle. Vehicle weight passes through the
strut assembly and the upper part of the steering knuckle to
the wheel. Weight transfer through a MacPherson strut
assembly is shown in Figure 5-32. The lower ball joint is
the only ball joint used on the MacPherson strut and is a
follower joint since the load passes through the strut
assembly. The purpose of the MacPherson strut lower ball
joint is simply to keep the bottom of the wheel in place.
The only variation to this rule occurs on vehicles on
which the coil spring is separate from the strut assembly.
On this type of design the spring is installed between the
frame and lower control arm. The lower ball joint is a
load-carrying joint since the vehicle weight passes from the
frame through the spring and lower control arm and into
the ball joint. The ball joint then transfers the weight to the
steering knuckle.
Conventional Suspensions
Spring or torsion bar placement on conventional sus-
pensions varies, and this affects ball joint loading as
detailed below.
Spring/Torsion Bar on Lower Control Arm
When the spring or torsion bar is on the lower control
arm, the vehicle’s weight passes through the spring, the
lower control arm, the lower ball joint, and into the
steering knuckle. See Figure 5-33. Therefore, the lower ball
joint is the load-carrying joint. The upper ball joint keeps
the wheel in position.
Spring/Torsion Bar on Upper Control Arm
When the spring or torsion bar is on the upper control
arm, the vehicle’s weight passes through the spring, the
upper control arm, the upper ball joint, and into the
steering knuckle. See Figure 5-34. Therefore, the upper
ball joint is the load-carrying joint. The lower ball joint
keeps the wheel in position.
Solid and Twin I-Beam Suspensions
On solid or twin I-beam suspensions, the ball joints
are solidly attached to the same beam. Therefore, the
weight is evenly split between each ball joint. There are no
loaded and follower ball joints. In these suspension
systems, the ball joints can be tension or compression
loaded.
U-joint
Drive
axle
Road
bump
Steering
knuckle
I-beam
Differential
assembly
Coil spring
Figure 5-31. A four-wheel drive, twin I-beam suspension. Each
side of the suspension flexes individually when traveling over
bumps. (Perfect Circle)
Vehicle
strut mount
Ball joint
Figure 5-32. Weight transfer through a MacPherson strut
suspension.