Ball joints act like hinges and connect the suspension control arms to the steering knuckles with a rotating, spherical stud and a socket, giving the wheels a wide range of movement -- from side to side for steering and up and down for going over bumps. Some vehicles have upper and lower front ball joints, and depending on the suspension design either can be the load-bearing joint that carries the vehicle’s weight (load-bearing joints tend to wear out sooner). Vehicles with McPherson strut suspensions have only lower ball joints, and some vehicles also have rear ball joints. Most vehicles come with sealed ball joints that don’t need periodic lubrication (“greasing” them used to be a routine part of oil changes), but the internal lubrication or seals can wear out over time. When ball joints wear, they allow too much suspension movement, resulting in shimmies or shakes on smooth roads, steering that wanders from straight ahead, or uneven tire wear. They may give audible warnings that they need to be replaced by squeaking or clunking, especially over bumps or when turning. Some ball joints have wear indicators, but mechanics can also determine if they’re worn by jacking up the vehicle and seeing if there’s excessive play by rocking the wheel back and forth.