A-arm

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

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. It can thus control the position of the outboard end in only a single

degree of freedom Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...

, 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 A radius rod (also called a radius arm, torque arm, torque spring, and torsion bar) is a suspension link intended to control wheel motion in the longitudinal (fore-aft) direction. The link is connected (with a rubber or solid bushing) on one end t ...

. This is in contrast to the wishbone, 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 is a Japanese public multinational conglomerate manufacturer of automobiles, motorcycles, and power equipment, headquartered in Minato, Tokyo, Japan. Honda has been the world's largest motorcycle manufacturer since 1959, reaching a producti ...

products from the 1990s -- feature what's known as a

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 knuck ...

. A double wishbone design features both upper and lower contro