The suspension is one of the most important parts of a car. All cars have some sort of suspension system, even golf carts. A suspension has three major jobs: to keep the car's frame from dragging on the ground, to control body movement during cornering and to control wheel movement over imperfections in the road. Suspension design is a complicated science--very nearly art--in its subtlety, complexity and variation.
The suspension is one of the most important
parts of a car. All cars have some sort of suspension system, even golf carts.
A suspension has three major jobs: to keep the car's frame from dragging on the
ground, to control body movement during cornering and to control wheel movement
over imperfections in the road. Suspension design is a complicated science--very
nearly art--in its subtlety, complexity and variation. Even on a fairly smooth
road, small bumps jolt the car as it travels. Without any sort of suspension,
these bumps could force the wheels off the road, damage the car and jolt the
driver. Suspension keeps the wheels in contact with the road surface, and keeps
the car intact and the driver comfortable.
A suspension system comprises five basic
components: the tires, wheel hubs (steering knuckles on the steer axle), the
springs (which bear the weight of the car), the control arm or arms (which link
the wheel hubs, steering knuckle or axle to the chassis) and the dampers
(a.k.a. shock absorbers, which slow the movement of the suspension and inhibit
the springs' natural tendency to oscillate).
Tires and Springs
• The first layer of the suspension system is the tires. A car's flexible,
air-filled tires respond to dips and bumps in the road, bending to absorb some
of the shock. The tires are attached to the wheels, which are attached to
springs. When a bump pushes on the wheels, it compresses the spring. This lets
the wheel move up and down with jolts, keeping the body of the car from
absorbing the knocks directly.
• A problem with springs is that they don't dissipate energy. When a bump in
the road compresses a spring, it stores the energy. It then springs back,
pushing back against the road with almost as much force as the road had
exerted. (Some of the force is dissipated as heat.) If the suspension consisted
of nothing but springs and tires, the car would bounce up and down
continuously, making for an uncomfortable and dangerous ride. Shock absorbers
are the solution to this. There are many different kinds, but they all consist
of a column or flexible container filled with a fluid. When a coil inside is
compressed, a piston pushes down on the shock absorber, compressing the fluid.
This pushes the fluid out of the way, creating resistance. This resistance
turns the energy pushing down on the shock into heat, which leaks out into the air.
Instead of storing the energy from every jolt in the spring, most of it is
dissipated through the shock. The car doesn't bounce back violently--it only
rebounds enough to stay in contact with the road.
• Suspensions fall into one of two basic categories: dependent and independent.
A dependent suspension links the wheels on both sides of the car with some sort
of solid or flexible axle, so suspension movement on one side of the car
directly affects the other side. Independent suspensions utilize one of several
different configurations to keep motion on one side of the car from affecting
the other side. Generally speaking, dependent suspensions are cheaper to build
and engineer, but independent suspensions offer better handling (especially
over rough roads at high speed) and a more comfortable ride.
• Dependent suspensions come in two basic flavors: the "live" axle
(meaning that it has a differential for transferring power, as in most
rear-wheel-drive cars and trucks), the "dead" axle (which simply
serves to support the weight of an un-driven end of a car, as in the rear of a
front-wheel-drive car). Dead axles come in a few different variations,
including the twist-beam semi-independent axle. Twist beam axles utilize a
tube-within-a-tube design. The outer tube connects to the wheel hubs and
springs, and the inner tube keeps the outer tubes together. This allows the
wheels on either side to act a little more independently than a solid dead axle
Lateral Arm Suspensions
• Most steering systems use some sort of lateral-arm (meaning that the control
arm pokes out perpendicular to the frame rail, toward the wheels) independent
suspension. There are several different types of lateral-arm suspensions, but
the two most common are the double-A-arm and MacPhereson strut. Double-A-arm
suspensions are so-called because they use a pair (upper and lower) of A-shaped
control arms to connect the chassis and wheel hub. MacPhereson strut
suspensions replace the upper control arm with a strut, which is a combination
control arm, spring and shock absorber. Strut suspensions are lighter and
cheaper to build, but generally don't handle as well as a proper double-A-arm.
Trailing/Leading Arm Suspensions
• Trailing/Leading-arm suspensions use a control arm connected to the chassis,
either ahead of the axle (trailing-arm, used for many rear suspensions), or
behind the axle (leading-arm, used to control the front axle of many
four-wheel-drive trucks). Trailing/leading-arm suspensions are the longitudinal
equivalent of lateral-arm suspensions; a four- or five-link suspension uses an
upper and lower control arm (like the double-A-arm), and a single-trailing-arm
(a.k.a. "truck arm") suspension uses a single link to connect the
chassis and axle (similar to the MacPhereson strut). Truck-arm suspensions
don't offer the control of a four- or five-link, but are still suitable for
many live-axle applications; Nascar stock-car builders prefer the truck-arm
suspension for its light weight, durability and simplicity.
Olivia Tong is the freelance writer for
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