In some front suspension systems, torsion bars replace the coil springs.
However, instead of compressing like coil springs, a torsion bar twists during
wheel jounce and straightens out to its original position during wheel rebound.
A torsion bar is capable of storing a higher maximum energy compared to a loaded
coil or leaf spring.
One end of a torsion bar - made of heat-treated alloy spring steel - is
attached to the vehicle frame. The other end is attached to the lower control
arm. When the wheel moves up and down, the lower control arm is raised and
lowered. This twists the torsion bar, which causes it to absorb road shocks. The
bar's natural resistance to twisting quickly restores it to its original
position, returning the wheel to the road.
During the manufacturing process, torsion bars are prestressed to provide
fatigue strength. Because of directional prestressing, torsion bars are
directional. Torsion bars are marked right of left, and they must be installed
on the appropriate side of the vehicle. Left and right on a vehicle is always
viewed from the driver's seat.
Torsion bars have a riding height adjustment screw at the end where they are
attached to the frame.
Many late-model pickups and SUVs use longitudinal torsion bars in their front
suspensions. They are used in this type vehicle because they require less space
compared to coil or leaf springs. They can also be mounted low and out of the
way of the driveline components. Transversely mounted torsion bars were used in
some front suspensions on older cars.
A variety of devices are added to basic suspension components to provide
additional stability. One of the most common in the sway bar, which is also
known as the anti-sway or stabilizer bar. This is a metal rod running between
the opposite lower control arms. As the suspension at one wheel responds to the
road surface, the sway bar transfers a similar movement to the suspension at the
other wheel. For example, if the right wheel is drawn down by a dip in the road,
the sway bar creates a downward draw on the left wheel as well. This produces a
more level ride, and sway or lean during cornering is also reduced.
If both wheels go into a jounce, the sway bar simply rotates in its insulator
bushings. It is a different matter when only one wheel goes into jounce. The
stabilizer bar twists, just like a torsion bar, to lift the frame and the
opposite suspension arm. This action reduces body roll.
The sway bar can be one-piece, U-shaped rod fastened directly into the
control arms with rubber bushings, or it can be attached to each control arm by
a separate sway bar link. The arm is held to the links with nuts and rubber
bushings and is also mounted to the frame in the center with rubber bushings. If
the sway bar is too large, it causes the vehicle to wander. If it is too small,
it has little effect on stability.
On suspensions that use single-housing lower control arms instead of wishbone
types, the sway bar can also be used to add lateral stability to the control
arm. Strut rods are used on models that do not use the sway bar for this
purpose. Strut rods are attached to the control arm and frame with bushings,
allowing the arm a limited amount of forward and backward movement. Strut rods
are directly affected by braking forces and road shocks, and their failure can
quickly lead to failure of the entire suspension system.