The panhard rod and the sway bar, both components designed to enhance the operation of a vehicle’s suspension, are not necessarily included as standard equipment on all vehicles. The two have dissimilar functions. The panhard rod is the simplest of the two so it will be covered first.
The duty of the panhard rod is to locate the rear axle housing laterally under the vehicle. A panhard rod is used primarily on vehicles that use a coil spring suspension as opposed to a leaf spring.
A leaf spring suspension does not need the panhard rod because the leaf springs have a locating pin in the top center of the spring. The axle housing tubes have a flat plate on either end with a hole in the center that is designed to fit over the locating pin. The axle is then bolted to the spring with the use of large U-bolts. The axle housing is held solidly in the center of the vehicle with the only lateral movement being that of the condition of the leaf spring bushings.
The primary advantage of installing a panhard bar on a leaf spring suspension is to eliminate the lateral movement of the leaf springs resulting from the flexibility of their bushings. Insignificant as it may seem, in harsh driving conditions it does give a particular advantage by reducing yaw, the tendency to spin. From a performance standpoint, every advantage, no matter how small, is cumulative.
The coil spring suspension is different. The coil springs have a mounting plate welded to the topside of each axle tube. A corresponding mounting cup is mounted on the body or frame. The springs are compressed between the mounting brackets and a set of shocks mounted to a point on the axle tube. A set of trailing arm rods, one on each side of the axle housing, are mounted both to the frame forward of the axle and to the axle housing. These connect the axle housing to the vehicle, allowing the axle housing up and down movement. Because of the softness of the bushings, if the vehicle were pushed from side to side in the rear, the axle would be stationary while the body of the vehicle would move laterally.
If the panhard rod is installed onto a location on the frame and extended to the top of the axle housing, the axle can no longer move laterally and remains properly centered under the rear of the vehicle.
The sway bar is one of the most important suspension components in terms of handling, yet one of the least understood. The sway bar is a spring steel rod offered in varying diameters that spans the width of the vehicle from lower control arm to lower control arm. It has a 90-degree bend on each end in the same direction creating an arm generally about 1.5 to 2 feet long. The tips of the arms are flattened and have a mounting hole in each.
The main body of the sway bar is mounted to the frame with C-shaped brackets and bushings forward of the front wheels. At this point the arm can be rotated up and down. The tips of the arms are connected solidly to each lower control arm by a sway bar link. This is a solid connection but has bushings, offering a slight flexibility.
To illustrate its function, think in terms of a situation where a vehicle is traveling in a straight line and suddenly changes direction by turning rapidly to the left. Without the sway bar, the vehicle will tend to roll to the right through centrifugal forces. As the vehicle rolls to the right, the right front spring is compressed, the tire’s camber changes negatively, rolling outward, and the right front tire bends to the point of riding on the sidewall.
Enter the sway bar: The same scenario but this time the sway bar is connected to the lower control arms. As the vehicle attempts to roll to the right, the rising of the right lower control arm needs to counteract the tension in the sway bar to bend it. This resistance to bending will not allow the vehicle’s body to roll and keeps the camber in the wheel straight up. This plants more tread on the surface of the road. At the same time, the opposite end of the sway bar preloads the left side lower control arm by applying upward pressure on the control arm.
Changing the diameter of the sway bar changes the amount of body roll. There is also a sway bar for the rear of the vehicle. It attaches to the frame crossmember and the axle tubes.
Mounting a sway bar is much easier than determining what size to use. In brief, the size of the sway bars will differ from front to rear. This is because of a calculation as to the mass, height and center of gravity of the vehicle. The objective is to reduce the rate of roll but keep neutral steering as much as possible. Too large a sway bar in the front can cause oversteer problems while too small a bar will result in understeer.
The proper sway bars will enhance the handling of the vehicle drastically but the application must be appropriate. The discussion on the determining factors is beyond the scope of this article so a site has been included for a more in-depth study.
