100 Auto Suspension and Steering
Summary
The job of the front suspension is to absorb road shock
and maintain tire-to-road contact as the vehicle is driven.
Modern suspensions must be as light and compact as
possible, while being strong enough to maintain wheel
alignment and withstand harsh conditions without the
need for frequent repairs.
The MacPherson strut front suspension consists of a
strut containing the shock absorber and a coil spring. The
coil spring is usually installed around the strut assembly.
The top of the strut assembly is attached to the vehicle
body. The lower section of the strut is attached to the
steering knuckle. The spring is compressed between the
upper and lower mounting pads of the strut assembly.
Some vehicles have a separate strut cartridge but most
struts are replaced as complete units. Other MacPherson
strut parts include the lower control arm, ball joint, strut
rod, and sway bar. On a few vehicles, the spring is separate
from the strut assembly.
Conventional independent front suspensions consist
of upper and lower control arms attached to the steering
knuckle through ball joints. The control arms are attached
to the frame through bushings. A strut rod is attached to
lower control arms that have a single control arm bushing.
All conventional suspensions use the short-long arm
design. The major variations in conventional suspensions
are the type of springs used and which control arm they are
placed on. Torsion bar tension is adjustable.
Solid and I-beam axle front suspensions are used on
trucks, vans, and sport utility vehicles. All modern vehicles
use ball joints to transfer vehicle weight at the steering
knuckle. A few older vehicles have kingpins.
The two variations of solid front axle suspension
depend on whether the vehicle has 2- or 4-wheel drive.
Non-driving solid axles have ball joints at the top and
bottom. A driving solid axle resembles the rear axle of a
rear-wheel drive vehicle. Power from the final drive unit
turns internal axles that drive the front wheels. Most solid
axles have control arms or strut rods to keep the axle
in place.
Twin I-beam front suspensions have two separate axle
beams. Each axle is connected to the frame on the side
opposite the wheel it serves. The design of twin I-beam
front axles varies between 2- and 4-wheel drive vehicles.
Ball joint loading on MacPherson strut suspensions,
conventional suspensions, and solid or twin I-beam
suspensions varies with the placement of the spring. On
most MacPherson strut suspensions, vehicle weight passes
through the strut assembly and the upper part of the
steering knuckle to the wheel. The lower ball joint is a
follower joint since the load passes through the strut
assembly. When the spring is separate from the strut
assembly, the lower ball joint is a load-carrying joint.
Ball joint loading on conventional suspensions
depends on where the spring or torsion bar is placed.
When the spring is on the lower control arm, the lower ball
joint is the load-carrying joint and the upper ball joint is
the follower joint. When the spring or torsion bar is on the
upper control arm, the upper ball joint is the load-carrying
joint and the lower ball joint is the follower joint.
On solid and twin I-beam suspensions, the ball joints
split the load and there are no load-carrying and follower
joints.
Upper control arm
Coil spring
Lower control arm
Load-carrying ball joint
Steering
knuckle
Lower control
arm
Upper control
arm
Coil spring
Load-carrying
ball joint
Steering
Figure 5-33. In this arrangement, the lower ball joint is the load
carrying joint.
Figure 5-34. In this front suspension layout, the top ball joint is
the load-carrying joint.