The steering knuckle or "spindle" pivots on either a kingpin or on the upper and lower ball joints. The disc brake caliper is attached to the spindle, as well as the tie rod ends and the upper and lower ball joints, or a MacPherson strut bottom clamp maybe attached, instead of an upper ball joint. In rear-wheel drive cars, the front axle (the shaft which supports the wheel bearings) is part of the spindle. Front wheel drive spindles have bearing pockets only.