12-04-2011, 03:55 PM
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1. INTRODUCTION
Independent suspension is a broad term for any automobile suspension system that allows each wheel on the same axle to move vertically (i.e. reacting to a bump in the road) independently of each other. This is contrasted with a beam axle, live axle or de Dion axle system in which the wheels are linked - movement on one side affects the wheel on the other side. It is common for the left and right sides of the suspension to be connected with anti-roll bars. The anti-roll bar ties the left and right suspension spring rates together but does not tie their motion together.
Most modern vehicles have independent front suspension (IFS). Many vehicles also have an independent rear suspension (IRS). IRS, as the name implies, has the rear wheels independently sprung. A fully independent suspension has an independent suspension on all wheels. Some early independent systems used swing axles, but modern systems use Chapman or MacPherson struts, trailing arms, multilink, or wishbones.
Independent suspension typically offers better ride quality and handling characteristics, due to lower unsprung weight and the ability of each wheel to address the road undisturbed by activities of the other wheel on the vehicle. In the case of straight line drag racing though, it can be more of a burden because of the design, IRS may cause the vehicle to experience wheel hop on a hard launch. Independent suspension requires additional engineering effort and expense in development versus a beam or live axle arrangement. A very complex IRS solution can also result in higher manufacturing costs.
The key reason for lower unsprung weight relative to a live axle design is that, for driven wheels, the differential unit does not form part of the unsprung elements of the suspension system. Instead it is either bolted directly to the vehicle's chassis or more commonly to a sub-frame.
The relative movement between the wheels and the differential is achieved through the use of swinging drive shafts connected via universal (U) joints, analogous to the constant-velocity (CV) joints used in front wheel drive vehicles.
2. THE NEED FOR SUSPENSION
The study of the forces at work on a moving car is called vehicle dynamics. Some of the concepts are needed to be understood in order to appreciate why a suspension is necessary in the first place. Most automobile engineers consider the dynamics of a moving car from two perspectives:
• RIDE - a car's ability to smooth out a bumpy road
• HANDLING - a car's ability to safely accelerate, brake and corner
These two characteristics can be further described in three important principles - road isolation, road holding and cornering. These principles are explained below and also how engineers attempt to solve the challenges unique to each.
ROAD ISOLATION: The vehicle's ability to absorb or isolate road shock from the passenger compartment. Allow the vehicle body to ride undisturbed while traveling over rough roads. Absorb energy from road bumps and dissipate it without causing undue oscillation in the vehicle.
ROAD HOLDING: The degree to which a car maintains contact with the road surface in various types of directional changes and in a straight line (Example: The weight of a car will shift from the rear tires to the front tires during braking. Because the nose of the car dips toward the road, this type of motion is known as "dive". The opposite effect "squat" occurs during acceleration, which shifts the weight of the car from the front tires to the back. Minimize the transfer of vehicle weight from side to side and front to back, as this transfer of weight reduces the tire's grip on the road.
CORNERING: It is the ability of a vehicle to travel a curved path. Minimize body roll, which occurs as centrifugal force pushes outward on a car's center of gravity while cornering, raising one side of the vehicle and lowering the opposite side. Transfer the weight of the car during cornering from the high side of the vehicle to the low side.
3. TYPES OF SUSPENSION
Conventional suspension system
Independent suspension system
Air suspension system
Hydro-Elastic suspension system
Under the independent suspension system, it is further classified into:
Swing Axle
Sliding Pillar
MacPherson Strut
Double Wishbone
Multi link
Trailing and Semi-trailing
3.1 SWING AXLE
A swing axle is a simple type of independent suspension first used in early aircraft (1910 or before), such as the Sopwith and Fokker, usually with rubber bungee and no damping. Some later motor-car rear swing axles have universal joints connecting the drive-shafts to the differential, which is attached to the chassis. They do not have universal joints at the wheels: the wheels are always perpendicular to the drive shafts. Swing axle suspensions traditionally used leaf springs and shock absorbers. This type of suspension was considered better than the more typical live axle for two reasons:
It reduced unsprung weight since the differential is mounted to the chassis
It eliminates sympathetic camber changes on opposite wheels
However, there are a number of shortcomings to this arrangement:
A great amount of single-wheel camber change is experienced, since the wheel is always perpendicular to the driveshaft
"Jacking" on suspension unloading (or rebound) causes positive camber changes on both sides, which (In extreme cases) can overturn the car.
Reduction in cornering forces due to change in camber can lead to over steer instability and in extreme cases lift-off over steer
Mercedes-Benz addressed the inherent handling issues by producing swing axles with a single-pivot point located under the differential, and thus well below the axle. This configuration markedly reduced the tendency to "jack-up" and the later low pivot swing-axle equipped cars were praised in contemporary publications for their handling.
Swing axles were supplanted by de Dion axles in the late 1960s, though live axles remained the most common. Most rear suspensions have been replaced by more modern independent suspensions in recent years, and both swing and de Dion types are virtually unused today. One exception is the Czech truck manufacturer Tatra, which uses swing axles and a central 'backbone' tube instead of more common solid axles. This system is claimed to give greater rigidity and better performance on poor quality roads and off road.
Another use of the swing axle concept is Ford's "Twin I-Beam" front suspension for trucks. This has solid axles (so they do not transmit power). Though it is touted as an independent suspension system in that each tire rises and falls without affecting the position of the other, the parallelogram action of the A-arm suspension system is not present. Each tire in fact moves with a similar camber change to that of the powered swing axles for the rear wheels listed above. But the pivot point of the axles is located not in the middle of the car but nearly on the other beam of the chassis, so the effect is far less hazardous.
