Although leaf springs were the first type of suspension spring used on
automobiles, today they are generally found only on trucks, vans, SUVs, and a
few passenger cars. There are two basic types of leaf springs: multiple leaf and
Multiple-leaf springs consist of a series of flat steel leaves of varying
lengths that are bundled together and held with clips or by a bolt placed
slightly ahead of the center of the bundle. One leaf, called the main leaf, runs
the entire length of the spring and has an eye on each end. The next leaf is a
little shorter and attaches to the main leaf. The next leaf is shorter yet and
attaches to the second leaf, and so on. The more leaves and the thicker and
shorter the leaves, the stronger the spring. This arrangement of leaves means
that nearly any number of leaves can be used to support the vehicle's weight. It
also give a progressively stiffer spring.
The front eye of the main leaf is attached to a bracket on the vehicle's
frame with a bolt and bushing. The rear eye is secured to the frame with a
shackle, which permits fore and aft movement in response to spring compression
and other forces resulting from acceleration, deceleration, and braking. An
insulating bushing is pressed into each main leaf eye.
When a leaf spring is compressed, the length of the leaves changes and the
leaves slide on each other. This sliding action could be a source of noise and
can also produce friction. The friction can produce a harsh ride as the spring
flexes and can also dampen the motion of the spring. These noise and friction
problems are reduced by interleaves, or spacers, made from zinc and plastic that
are placed between the steel leaves.
Each spring leaf is curved in the manufacturing process. If this curve were
doubled, it would form an ellipse. Leaf springs are referred to as
semi-elliptical or quarter elliptical. The ellipse designation refers to how
much of the ellipse the spring actually describes. The vast majority of leaf
springs are semi-elliptical.
A center bolt extends through all the leaves to maintain the leaf position in
the spring. In addition to absorbing blows from road forces, they also serve as
suspension locators by fixing the position of the suspension with respect to the
front and rear of the vehicle. Leaf springs are usually mounted at right angles
to the axle. Because of this location, they provide excellent resistance to
lateral wheel movement.
The head on the spring center bolt fits into an opening in the axle to
position the axle. If the center bolt is broken, the axle position may shift and
alter vehicle tracking and alignment.
Some leaf springs are made of a single steel leaf, and these springs are
referred to as mono-leaf springs. Monoleaf springs are usually tapered with a
heavy or thick center section tapering off at both ends. This provides a
variable spring rate for a smooth ride and adequate load-carrying capacity. In
addition, single-leaf spring do not have the noise and static friction
characteristic of multiple-leaf springs. Mono-leaf springs may be mounted
longitudinally or transversely and may be used in front or rear suspensions.
Some cars, such as the Corvette, use a fiberglass-reinforced plastic mono-leaf
spring to reduce weight.
an air spring is used in electronically controlled systems that provide
automatic front and rear leveling. The air springs are located in the same
positions where coil springs are found. Each spring consists of a reinforced
rubber bag pressurized with air. The bottom of each bag is attached to an
inverted piston-like mount that reduces the interior volume of the air bag
during jounce. This has the effect if increasing air pressure inside the spring
as it is compressed, making it progressively stiffer. A vehicle equipped with an
electronic air suspension system is able to provide a ride that is about
one-third softer than conventional coil springs. At the same time, its variable
spring rate helps absorb bumps and protect against bottoming.
Some late-model pickups and SUVs offer air suspension systems. These systems
are added to existing leaf-spring suspensions. The air spring is positioned
between the center of the leaf spring and the frame of the truck. The air spring
serves as an adjustable and additional spring at each end of the axle.