Double wishbone suspension

A double wishbone suspension is an independent suspension design for automobiles using two (occasionally parallel) wishbone-shaped arms to locate the wheel. Each wishbone or arm has two mounting points to the chassis and one joint at the knuckle. The shock absorber and coil spring mount to the wishbones to control vertical movement. Double wishbone designs allow the engineer to carefully control the motion of the wheel throughout suspension travel, controlling such parameters as camber angle, caster angle, toe pattern, roll center height, scrub radius, scuff (mechanical abrasion), and more.

The double-wishbone suspension can also be referred to as "double A-arms", though the arms themselves can be A-shaped or L-shaped. A single wishbone or A-arm can also be used in various other suspension types, such as variations of the MacPherson strut. The upper arm is usually shorter to induce negative camber as the suspension jounces (rises), and often this arrangement is titled an "SLA" or "short, long arms" suspension. When the vehicle is in a turn, body roll results in positive camber gain on the lightly loaded inside wheel, while the heavily loaded outer wheel gains negative camber.

Between the outboard end of the arms is a knuckle. The knuckle contains a kingpin for horizontal radial movement in older designs, and rubber or trunnion bushings for vertical hinged movement. In newer designs, a ball joint at each end allows for all movement. Attached to the knuckle at its center is a bearing hub, or in many older designs, a spindle to which the wheel bearings are mounted.

To resist fore-aft loads such as acceleration and braking, the arms require two bushings or ball joints at the body.

At the knuckle end, single ball joints are typically used, in which case the steering loads have to be taken via a steering arm, and the wishbones look A- or L-shaped. An L-shaped arm is generally preferred on passenger vehicles because it allows a better compromise of handling and comfort to be tuned in. The bushing in line with the wheel can be kept relatively stiff to effectively handle cornering loads while the off-line joint can be softer to allow the wheel to recess under fore-aft impact loads. For a rear suspension, a pair of joints can be used at both ends of the arm, making them more H-shaped in plan view. Alternatively, a fixed-length driveshaft can perform the function of a wishbone as long as the shape of the other wishbone provides control of the upright. This arrangement has been successfully used in the Jaguar IRS. In elevation view, the suspension is a 4-bar link, and it is easy to work out the camber gain (see camber angle) and other parameters for a given set of bushing or ball-joint locations. The various bushings or ball joints do not have to be on horizontal axes, parallel to the vehicle center line. If they are set at an angle, then anti-dive and anti-squat geometry can be dialed in.

In many racing cars, the springs and dampers are relocated inside the bodywork. The suspension uses a bellcrank to transfer the forces at the knuckle end of the suspension to the internal spring and damper. This is then known as a "push rod" if bump travel "pushes" on the rod (and subsequently the rod must be joined to the bottom of the upright and angled upward). As the wheel rises, the push rod compresses the internal spring via a pivot or pivoting system. The opposite arrangement, a "pull rod", will pull on the rod during bump travel, and the rod must be attached to the top of the upright, angled downward. Locating the spring and damper inboard increases the total mass of the suspension, but reduces the unsprung mass, and also allows the designer to make the suspension more aerodynamic.

A short long arms suspension (SLA) is also known as an unequal-length double wishbone suspension. The upper arm is typically an A-arm and is shorter than the lower link, which is an A-arm or an L-arm, or sometimes a pair of tension/compression arms. In the latter case, the suspension can be called a multi-link, or dual-ball joint suspension.

The four-bar linkage mechanism formed by the unequal arm lengths causes a change in the camber of the vehicle as it rolls, which helps to keep the contact patch square on the ground, increasing the ultimate cornering capacity of the vehicle. It also reduces the wear on the outer edge of the tire.