Also known as a torque arm or radius arm, a radius rod is part of the suspension system of an automobile and is used to control the movement of the wheels. The rod connects an axle or wheel carrier to the frame of the car. It restricts the wheel's movement to a range of movement within a specific arc to keep the wheel from moving too much in one direction.
The radius rod adds weight to the car and must be created out of lightweight materials to keep it from affecting the car's movement. Typically it is constructed from stamped steel, meaning steel frames that are manufactured in bulk and the design is stamped into the metal, or out of aluminum. These lightweight materials add little weight to the car, allowing the radius rod to work without hindering the functioning of other parts of the vehicle.
If the rod were connected to the wrong place on the vehicle, undesirable results could occur. The motion created when the car must stop or accelerate can cause unwanted motion in the radius rod. When the car stops abruptly, this can cause brake dive, a term used to describe the feeling of suddenly being thrown forward as the vehicle comes to a stop. In contrast, when a car accelerates suddenly, it can cause the wheels to move up and down enough to come off the ground for a few seconds.
To prevent brake dive and wheel hopping, the radius rod is mounted in front of the wheel. From this position the sudden impact will not affect the rod the way it would if it were mounted behind the wheel. This is yet another design feature to make the ride as safe and as comfortable as possible for the passengers in the car.
Certain vehicles have springs to help control the motion of the wheel and absorb the impacts from the road so the passengers won't feel them. Problems arise with this method, however. Stronger cars, such as drag racers, are too strong for the springs to work. They also may not be able to keep the wheel from moving under hard braking and can cause the wheels to bounce during braking. For this reason, an actual, separate radius rod must be used.