Ball joint
Definition
A ball joint is a hardened steel ball and stud in a socket, sealed by a rubber boot and filled with grease, that connects the control arm to the steering knuckle and lets the wheel both steer and move up and down. The load-carrying joint (usually the lower one on a strut suspension, or the one nearest the spring on a double wishbone) takes the vehicle's weight and wears fastest; the other acts as a follower. Wear begins when the boot splits and water washes out the grease, producing play that shows as a knock over bumps, a clunk when braking or turning at low speed, vague steering, and uneven inner tyre wear; a separated joint lets the wheel fold under, so any measurable play is a replacement item. Check with the suspension unloaded as the manufacturer specifies (the method differs between strut and wishbone designs), lever the arm with a bar and watch the joint for movement, and compare with the opposite side. Many joints are pressed into the arm and need a press tool or the whole arm replaced; always perform a wheel alignment afterwards. No fault codes; related complaints are clunking noises, wandering, pulling after alignment and ESP steering-angle plausibility codes if the geometry is badly out.