wishbone

A wishbone is a suspension arm, almost always made from forged steel or aluminum alloy, that connects the wheel hub or steering knuckle to the frame. Its characteristic shape, narrow at one end and split into two prongs at the other, gives it both its name and its structural logic: the two prongs bolt to the frame at widely separated points, while the narrow end pivots at the wheel end. This geometry lets the wheel move vertically while resisting lateral and longitudinal forces.

Most modern vehicles use a pair of wishbones per wheel, stacked vertically or nearly so. The upper wishbone is typically shorter and lighter than the lower one. Together they control the wheel's motion in three dimensions: they permit vertical travel (suspension compression and rebound), they constrain the wheel's horizontal position, and they control camber angle as the suspension moves. The angle between the two wishbones at the frame end directly affects how camber changes with suspension travel, a property called camber gain.

Geometry and material choices

The wishbone's geometry is defined by its pivot points and its length. Typical lower wishbones on passenger cars range from 350 to 500 millimeters center-to-center. The angles at which the prongs meet the frame determine the suspension's roll center height and the wheel's kinematic behavior. Forged steel is standard in volume production because it can be made to precise tolerances and withstands fatigue cycling; aluminum is used in high-performance and luxury cars where unsprung mass matters. Cast iron appears in older designs and some trucks, though it is brittle and prone to sudden fracture if struck.

Wishbone failure typically occurs at the narrow neck where the two prongs converge, or at the ball joint attachment at the wheel end. Fatigue cracking can begin after many kilometers of cornering, pothole strikes, or towing load cycles. A failed wishbone compromises suspension geometry and can cause the wheel to collapse or lock up. Replacement is a common workshop job and usually requires a wheel alignment afterward because the suspension geometry has shifted.

The wishbone design is also called a double-wishbone or A-arm suspension when two are used per wheel, and it is often confused with a control arm (a single rigid link). The wishbone's twin-prong structure is its defining feature and gives it superior stiffness for the weight compared to a single arm of the same material.