A tie rod is an integral part of any suspension system because it ties your vehicle’s steering rack to the steering arm, which is attached to the wheel. In short, tie rods are needed to turn a vehicle’s wheels left and right.
Tie rods attach to one of two steering systems, a simple rack and pinion steering system or a mechanical ball system
In a rack and pinion system, you when turn the steering wheel, this turns a pinion gear that rolls along your steering rack. This turning force is applied to the tie rods at the end of the steering wheel and is translated into lateral motion at the wheel.
In a re-circulating ball system, turning the steering wheel will set several gears in motion that control a pitman arm that converts the angular motion of the steering rack into the linear motion required to turn the wheels. Just like in a rack and pinion system the tie rods are connected to the end of this assembly and turn the wheels.
Both of these systems would not be able to function without tie rods because they are the pivot system for your wheels. Vehicle alignment can be adjusted by manipulating the outer tie rods