Why should ball joints and tie rod ends be changed in pairs?
It is not absolutely necessary, but many professional installers insist on replacing both even if only one is bad. This is because both have more or less the same mileage and sooner or later the other one is going to fail too.
In many instances, the companion part on the opposite side is marginal, so it makes sense to replace both ball joints (uppers and/or lowers) or both tie rod ends (inner and/or outer) at the same time. It also saves the customer the inconvenience of having to bring his vehicle in a second time.
There is some evidence to suggest that suspension and steering components on the right side of a vehicle have a somewhat higher rate of failure than those on the left side. The right side takes more punishment when it comes to close encounters with curbs, potholes and other obstacles, and the typical vehicle makes more right-hand turns than left-hand turns.
Even though all suspension components have the same mileage, those on the right side often need to be replaced at a lower mileage than those on the left.
It is important to check tie rod ends when doing any type of suspension work.
Some components also wear faster than others. Loaded ball joints (those that bear the brunt of the vehicle’s weight) wear out faster than unloaded ball joints. Loaded ball joints are the lower ones on vehicles where the spring rests on the lower control arm, and the upper joints on vehicles where the spring rests atop the upper control arm. The lower ball joints on FWD cars are unloaded.
Outer tie rod ends also tend to wear out faster than inner tie rod ends because the outer ones experience more deflection and are more exposed to road splash, salt and dirt. Inner tie rod sockets on vehicles with rack and pinion steering are protected by bellows, so the rate of replacement for the inner sockets is only a fraction of that for the outer tie rod ends.
The real issue here is not which parts wear out first or why, but how your customer’s needs can be best served. If only one tie rod end or ball joint is obviously shot, but its partner on the opposite side still appears to be in good condition (and the customer cannot afford to replace both at the same time) then do not tell him he has to change both.
Better to sell only what he needs now and get his repeat business when the other part fails than to risk losing his business by trying to sell him parts he is not convinced he really needs.