trunnion

A trunnion is a cylindrical bearing pin or shaft that allows a component to rotate or pivot about a fixed axis. In automotive suspensions, it typically serves as the rotational joint between the upper control arm and the steering knuckle (or between other suspension members), permitting the wheel assembly to move up and down while the steering geometry remains controlled.

The trunnion itself is usually a hardened steel pin, often between 15 and 25 millimeters in diameter depending on the vehicle class, pressed or bolted into a bore in one component and supported by a bushing or bearing in the mating part. The bushing is the critical wear item: it handles radial loads and the rocking motion during suspension travel. Most modern designs use rubber-bonded metal bushings that damp vibration while allowing rotation; some heavy-duty or performance applications employ ball joints instead of traditional bushings for lower friction and longer service life.

Wear at the trunnion manifests as play in the suspension, which degrads steering feel, increases tire wear, and can eventually cause clunking noises during cornering or bumps. Inspection involves lifting the vehicle, gripping the control arm, and checking for lateral movement at the trunnion connection. Replacement requires pressing out the old pin and bushing and installing new ones, a task that typically requires a hydraulic press and alignment equipment afterward.

The term "trunnion" comes from older military and mechanical engineering usage, where it referred to the pivoting pins on cannon mountings. Automotive engineers borrowed the term because the function is identical: a pin that permits controlled rocking motion in one plane. Though the word is less common in modern service manuals (which often call the assembly a "control arm shaft" or simply identify it by the bushing type), trunnion remains the precise technical term used by suspension designers and in engineering specifications.