In automotive suspension, a control arm, also known as an A-arm, is a hinged suspension link between the chassis and the suspension upright or hub that carries the wheel.
The inboard (chassis) end of a control arm is attached by a single pivot, usually a rubber bushing. It can thus control the position of the outboard end in only a single degree of freedom, maintaining the radial distance from the inboard mount. Although not deliberately free to move.
Lower Control Arms
If a vehicle is in MacPherson strut style front or rear suspension, this car only use lower control arms. The upper control arm isn’t needed because the strut takes its place.
Upper Control Arms and Trailing Arms
For the rear of a vehicle with a solid axle, any style of control arms might be used to connect the rear axle to the frame or unibody. Mostly, it is three or four control arms with rubber bushings on each end. These control arms are called “trailing arms” or “rear trailing arms”. Only a vehicle has independent rear suspension, it may have upper and lower A-Arms, trailing arms, or some other unique design that fits the shape of the vehicle.
Adjustable Control Arms
Adjustable control arms are used to adjust wheel camber. Camber is the vertical alignment of the wheels. Negative camber means that the top of the wheel is tipped inward toward the center of the vehicle. Positive camber means that top of the wheel is tipped outward, away from the center of the vehicle. Adjusting camber is a huge factor when it comes to racing, stance, and the lowering or lifting a vehicle.