In automotive suspension, a control arm, also known as an A-arm, is a hinged

suspension link 300px, 5-link live axle suspension In automotive suspensions, a suspension link, control link or link is a suspension member, that attaches at only two points. One point being the body or frame of the vehicle and the other point attaching to th ...

between the

chassis A chassis (, ; plural ''chassis'' from French châssis ) is the load-bearing framework of an artificial object, which structurally supports the object in its construction and function. An example of a chassis is a vehicle frame, the underpart ...

and the suspension upright or hub that carries the wheel. In simple terms, it governs a wheel's vertical travel, allowing it to move up or down when driving over bumps, into potholes, or otherwise reacting to the irregularities of a road surface. Most control arms form the lower link of a suspension. The inboard (chassis) end of a control arm is attached by a single pivot, usually a rubber

bushing Bushing may refer to: *Bushing (bearing), a type of plain bearing *Bushing (electrical), an insulated device that allows a conductor to pass through a grounded conducting barrier *Bushing (isolator), a mechanical device used to reduce vibrational ...

. 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, the single bushing does not control the arm from moving back and forth; this motion is constrained by a separate link or radius rod. This is in contrast to the

wishbone Wishbone commonly refers to: * Furcula, a fork-shaped bone in birds and some dinosaurs Wishbone may also refer to: * Wish-Bone, an American salad dressing and condiment company * Wishbone formation, a type of offense in American football * Wish ...

, 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. Certain vehicles -- notably, many Honda products from the 1990s -- feature what's known as a double wishbone suspension. A double wishbone design features both upper and lower control arms that work in tandem with each other to properly locate the wheel. The additional radius rod is then attached to the upper arm.

MacPherson strut

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 Track or Tracks may refer to: Routes or imprints * Ancient trackway, any track or trail whose origin is lost in antiquity * Animal track, imprints left on surfaces that an animal walks across * Desire path, a line worn by people taking the shorte ...

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.

Spring attachment

Citroen front suspension (Autocar Handbook, 13th ed, 1935)

A control arm may be used to carry the suspension load and transmit them to the spring or

shock absorber A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Most sh ...

. Torsion bar suspension commonly does this, with the outboard end of the torsion bar attached to the inboard bearing of the control arm..

References

{{Reflist Automotive suspension technologies

MacPherson strut

Spring attachment

See also

References