DOUBLE WISHBONE suspension is the simplest, most effective way to maintain a constant camber angle in an independent suspension.
The problem with using a torsion bar suspension (such as that featured on the Datsun 240Z) or a swing axle suspension (As on the original Volkswagen Beetle) is that the camber, or the angle at which the plane of the wheel meets the ground, changes as the wheel moves up and down. This leads to inconsistent handling. While it is not a major factor in small cars moving at low speeds, especially with narrow, rounded tires, in larger vehicles and with more rubber, it becomes a serious liability as grip changes radically with tire motion.
Double wishbone suspension solves this problem by using a parallelogram design. There is an A-arm or a similar link at the top and of the hub carrier (or suspension upright), to which the wheel is mounted. The slope of the line described by the mounts on the upright remains the same as the slope of the line described by the mounts attached to the chassis. This keeps camber constant. As in other types of suspension, the links consist either of a bushing, usually made of rubber or polyethlene, and sometimes filled with a fluid, or a ball joint. Typically, you will only find ball joints on the wheel side of the A-arms, and furthermore only on wheels involved with steering (usually the front, but some cars do have four wheel steering.)
There is another type of suspension which performs the same function, known as a multilink suspension. Instead of using two suspension members, such a system may use as many as five links. This reduces weight and allows mounting in different locations, saving considerable space, and is especially useful on small cars.
Double Wishbone suspension may be used on both the front and rear suspension.
Depicted here is an inequal-length double wishbone suspension:
| Tire |
|_ _| Upper Link / A-Arm
| \_________/ | ____________ __ Mount
Wheel | _/ | / \ `--/ \ Point
----> |/ ___|__\__/~~~~-,___ | | <------
|_\ / ______| `~-\__/
|X|______/ / |
|X|| | |
|X|| | <---- Hub Carrier / Upright
|X|| | |
|X|| | | Lower Link / A-Arm
|X|`----- \ |__
|X| \ \/ \`\____________,------.
|| \ \__/. / \
| \ \____| `------------,__\___/
| \________/ |
| Tire |
~~~~~ Ground ~~~~~~~~~
Originally, the upper and lower suspension members were always the same length, but this leads to "scrubbing", or the lateral motion of tires across the road surface. This is because as the tire moves up or down, the distance between the mounting location for the inner link and the center of the wheel decreases. To solve this problem, the inequal-length double wishbone was created. This instead slightly increases camber when the wheel moves up, which will happen when it is loaded, and decreases it when it moves down.
For example, if you make a right turn, the body rolls left. The suspension on the left side moves upward, increasing camber to maximize the grip from your tires. The suspension on the right side moves downward, decreasing camber, but since the body has rolled, the camber of the right side increases to the same degree as it has increased on the left side. Your camber remains the same on both sides of the vehicle, and rather than the tire moving sideways, it simply changes camber, which in this one case is actually beneficial. You can minimize body roll with a sway bar, which is simply a torsion bar which can be added to any kind of suspension.