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Double wishbone suspension
From Wikipedia, the free encyclopedia
In automobiles, a double wishbone (or "upper and lower A-arm") suspension is an independent suspension design 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 and more.
The double wishbone suspension can also be referred to as double 'A' arms, and short long arm (SLA) suspension if the upper and lower arms are of unequal length.
SLAs are very common on front suspensions for medium to large cars such as the Honda Accord or Chrysler 300, pickups, SUVs, and are very common on sports cars and racing cars.
A single wishbone or A-arm can also be used in various other suspension types, such as MacPherson strut and Chapman strut.
The suspension consists of a pair of upper and lower lateral arms. The upper arm is usually shorter to induce negative camber as the suspension jounces (rises). When the vehicle is in a turn, body roll results in positive camber gain on the outside wheel. The outside wheel also jounces and gains negative camber due to the shorter upper arm. The suspension designer attempts to balance these two effects to cancel out and keep the tire perpendicular to the ground. This is especially important for the outer tire because of the weight transfer to this tire during a turn.
Between the outboard end of the arms is a knuckle with a spindle, hub, or upright which carries the wheel bearing and wheel. Knuckles with an integral spindle usually do not allow the wheel to be driven. A bolt on hub design is commonly used if the wheel is to be driven.
In order to resist fore-aft loads such as acceleration and braking, the arms need 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.
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 centre line. If they are set at an angle, then antidive and antisquat can be dialed in.
The advantage of a double wishbone suspension is that it is fairly easy to work out the effect of moving each joint, so you can tune the kinematics of the suspension easily and optimize wheel motion. It is also easy to work out the loads that different parts will be subjected to which allows more optimized lightweight parts to be designed. They also provide increasing negative camber gain all the way to full jounce travel unlike the MacPherson strut which provides negative camber gain only at the beginning of jounce travel and then reverses into positive camber gain at high jounce amounts.
The disadvantage is that it is slightly more complex than other systems like a MacPherson strut.
Prior to the dominance of front wheel drive in the 1980s, many everyday cars used double wishbone front suspension systems, or a variation on it. Since that time, the MacPherson strut has become almost ubiquitous, as it is simpler and cheaper to manufacture. Double wishbones are usually considered to have superior dynamic characteristics, load handling capability and are still found on higher performance vehicles.