In automotive suspension, an automobile's control arm or wishbone (aka. A-arm or A-frame) is a nearly flat and roughly triangular suspension member (or sub-frame), that pivots in two places. The base of the triangle attaches at the frame and pivots on a bushing. The narrow end attaches to the steering knuckle and pivots on a ball joint.
The upper control arm can clearly be seen at the top portion of the suspension components in the attached photo, where it is the silver part horizontally attached to the frame inside the red body portion and connecting to the steering knuckle near the side of the tire's wheel rim. Note the roughly A-shaped design with the top of the A near the tire and the bottom two points connected to the frame inside the body's space. In the photo, the A-shape is reinforced with a solid triangular plate near the top of the A.
Two such devices per wheel make up a double wishbone suspension, while one control arm per wheel makes up a part, usually the lower part, of a MacPherson strut suspension or of various other configurations.
- Car suspension at Howstuffworks.com
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