The first question we should first be asking is what are control arms and what do they do?
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 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.
This is in contrast to wishbones which are triangular and have two widely spaced inboard bearings. These constrain the outboard end of the wishbone from moving back and forth, controlling two degrees of freedom, and without requiring additional links.
Most vehicles use either one or two control arms per wheel, on both the front and rear suspension. Many front-wheel drive vehicles only use a lower control arm, while trucks and SUVs often have both an upper and lower control arm. A control arm connects the wheel hub and steering knuckle to the frame of the vehicle. They are typically equipped with bushings on the frame side of the vehicle and a ball joint on the wheel side of the vehicle that allow flex and controlled movement according to road conditions and steering input from the driver.
Control arms are most commonly encountered as part of the MacPherson strut independent front suspension. The control arms are perpendicular to the axis of the vehicle and are termed track control arms. A diagonal radius rod constrains the strut from moving forward and back.
In MacPherson's original design, an anti-roll bar also acted as the radius rod. This requires the bar to be attached through a ball joint, so as to also provide longitudinal control. In most contemporary designs, still commonly termed MacPherson struts, the radius rod and anti-roll bar are now separate, with the anti-roll bar mounted in a sliding bush.
Most factory control arms are set at only one position. Which leaves the advantage of adjustability to aftermarket control arms, because when you tune your car, tuning the suspension is almost always a must.
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.
When a lowering kit or a lift kit is installed on a car or truck, adjustable control arms are needed to correct the negative or positive camber that goes along with them. If not corrected, the tires won't have the proper traction, and they will wear unevenly and prematurely.
So, to make the world a better or a safer place, replacing your control arms when tuning your car – or when symptoms start popping up – is a very good idea.
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