- A car's suspension, with its various components, provides all of the solutions described.
- The suspension of a car is actually part of the chassis, which comprises all of the important systems located beneath the car's body.
- Frame - structural, load-carrying component that supports the car's engine and body, which are in turn supported by the suspension
- Suspension System - setup that supports weight, absorbs and dampens shock, and helps maintain tire contact
- Steering System - mechanism that enables the driver to guide and direct the vehicle
- Tires and Wheels - components that make vehicle motion possible by way of grip and/or friction with the road
• So the suspension is just one of the major systems in any vehicle.
- Today's springing systems are based on one of four basic designs.
• Coil springs - This is the most common type of spring and is, in essence, a heavy-duty torsion bar coiled around an axis. Coil springs compress and expand to absorb the motion of the wheels.
• Leaf Springs - This type of spring consists of several layers of metal (called "leaves") bound
together to act as a single unit. Leaf springs were first used on horse-drawn carriages and
were found on most American automobiles until 1985. They are still used today on most
trucks and heavy-duty vehicles.
Torsion Bars - Torsion bars use the twisting properties of a steel bar to provide coil-spring-like
performance. One end of a bar is anchored to the vehicle frame. The other end is attached to a
wishbone, which acts like a lever that moves perpendicular to the torsion bar. When the wheel
hits a bump, vertical motion is transferred to the wishbone and then, through the levering action, to the torsion bar. The torsion bar then twists along its axis to provide the spring force.
Air Springs - Air springs, which consist of a cylindrical chamber of air positioned between the
wheel and the car's body, use the compressive qualities of air to absorb wheel vibrations. The
concept is actually more than a century old and could be found on horse-drawn buggies. Air springs from this era were made from air-filled, leather diaphragms, much like a bellows; they
were replaced with molded-rubber air springs in the 1930s.
Springs: Sprung and Un-sprung Mass - The sprung mass is the mass of the vehicle supported on
the springs, while the un-sprung mass is loosely defined as the mass between the road and the suspension springs. The stiffness of the springs affects how the sprung mass responds while the car is being driven.
- Loosely-sprung cars, such as luxury cars, can swallow bumps and provide a super-smooth ride; however, such a car is prone to dive and squat during braking and acceleration and tends to experience body sway or roll during cornering.
- Tightly- sprung cars, such as sports cars, are less forgiving on bumpy roads, but they minimize body motion well, which means they can be driven aggressively, even around corners.
- So, while springs by themselves seem like simple devices, designing and implementing them on a car to balance passenger comfort with handling is a complex task.
- And to make matters more complex, springs alone can't provide a perfectly smooth ride. Why? Because springs are great at absorbing energy, but not so good at dissipating it. Other structures, known as dampers, are required to do this.
Dampers: Shock Absorbers
- Unless a dampening structure is present, a car spring will extend and release the energy it absorbs from a bump at an uncontrolled rate. The spring will continue to bounce at its natural frequency until all of the energy originally put into it is used up. A suspension built on springs alone would make for an extremely bouncy ride and, depending on the terrain, an uncontrollable car.
- Enter the shock absorber, or snubber, a device that controls unwanted spring motion through a process known as dampening. Shock absorbers slow down and reduce the magnitude of vibratory motions by turning the kinetic energy of suspension movement into heat energy that can be dissipated through hydraulic fluid.
A shock absorber is basically an oil pump placed between the frame of the car and the wheels.
The upper mount of the shock connects to the frame (i.e., the sprung weight), while the lower
mount connects to the axle, near the wheel (i.e., the un-sprung weight). In a twin-tube design, one of the most common types of shock absorbers, the upper mount is connected to a piston rod, which in turn is connected to a piston, which in turn sits in a tube filled with hydraulic fluid. The inner tube is known as the pressure tube, and the outer tube is known as the reserve tube. The reserve tube stores excess hydraulic fluid.
- When the car wheel encounters a bump in the road and causes the spring to coil and uncoil, the energy of the spring is transferred to the shock absorber through the upper mount, down through the piston rod and into the piston. Orifices perforate the piston and allow fluid to leak through as the piston moves up and down in the pressure tube. Because the orifices are
relatively tiny, only a small amount of fluid, under great pressure, passes through. This slows down the piston, which in turn slows down the spring.
- Shock absorbers work in two cycles -- the compression cycle and the extension cycle.
• The compression cycle occurs as the piston moves downward, compressing the hydraulic fluid in the chamber below the piston.
• The extension cycle occurs as the piston moves toward the top of the pressure tube, compressing the fluid in the chamber above the piston. A typical car or light truck will have more resistance during its extension cycle than its compression cycle. With that in mind, the compression cycle controls the motion of the vehicle's un-sprung weight, while extension controls the heavier, sprung weight.
- All modern shock absorbers are velocity-sensitive --the faster the suspension moves, the more resistance the shock absorber provides. This enables shocks to adjust to road conditions and to control all of the unwanted motions that can occur in a moving vehicle, including bounce, sway, brake dive, and acceleration squat.