A Spindle, also called a steering knuckle or upright, is the structural component that holds the wheel hub and bearings, mounts to the suspension control arms or strut, and provides the steering pivot points. Spindle geometry — specifically the relationship between the upper and lower ball joints, the steering arm, and the wheel center — determines fundamental suspension behavior including scrub radius, kingpin inclination, and steering effort. Griggs Racing manufactures performance spindles for Mustang SLA and strut applications.