Valvoline.com

How Sway Bars Work

It's only rock 'n roll but you'll like it

Whatever type of vehicle you have, excessive body roll can interfere with its performance—if not make you carsick. Selecting the right sway bars (also called stabilizer bars or anti-roll bars) can make a big difference in settling things down.

Overview

Starting with an overview of the basics on body roll, when a vehicle goes around a corner, it tends to lean toward the outside of the curve. The amount of body roll is called the roll angle. The greater the roll angle, the more the tires change their camber angle in relation to the road surface (tilting inward or outward). With too much roll, the tire's contact patch becomes markedly smaller, reducing its grip. The goal is to keep the tire as close to perpendicular as possible.

Several different components can affect body roll. A vehicle's suspension basically consists of springs, shocks, linkage, bushings and sway bars. The springs support the weight of the car and absorb vehicle load while traveling over bumps or during cornering. The shocks help to reduce the unwanted spring motion in the suspension. The linkage or control arms control the motion of the wheels relative to the vehicle body. The bushings work in conjunction with the linkage and other components by acting as pivot points and/or cushions (ranging from soft to hard). Lastly, they control the roll stiffness by torsional resistance.

All of these components work together as a system, and changing one can affect the performance of another. For instance, if you modify your springs (such as when lowering or lifting the chassis), body roll can increase. The best way to control changes in tire camber caused by body roll is to limit the roll angle by changing the roll stiffness of the suspension vis-a-vis sway bars. These U-shaped tubes are transversely mounted beneath a vehicle's frame. When both wheels are connected to each end of a sway bar, they tend to twist the bar as one wheel moves up and the other moves down. The stiffer the bar, the more resistance to body roll it can provide.

Roll Control

Increasing the thickness of a sway bar is a good way to control changes in tire camber caused by body roll, and allows for fine-tuning of a vehicle's suspension for precise, crisp and predictable handling. (Note that a swaybar should not to be confused with the purpose of a brace between the strut towers, which is designed to stiffen the chassis.) The stiffness of a sway bar must be properly transmitted into the chassis to do any good. The length of the arms that transfers loads into the chassis has a dramatic effect on how much roll stiffness a given bar can provide. The shorter the bar, the more stiffness it supplies. Roll resistance depends on the stiffness of the frame mounting, control arms, and drop links, and also where those links connect to the control arms. The point being, simply bolting on a sway bar may not improve your handling if you don't consider all of the above factors.

For instance, since a sway bar functions by twisting, the bushings used in the pivot points and mounting hardware are important to keep in mind. A stiffer sway bar won't do its job if its rubber bushings are too soft or worn out. Polyurethane bushings are practically impervious to oil and other road contaminants, and won't crush down or wear out like rubber bushings. They are also designed to be free-floating, rotating 360 degrees, so the sway bar (and other suspension components that also are fitted with them) can articulate fully without binding. In contrast, rubber bushings are often bonded to a metal shell and sleeve, and function with a twisting action that, when pushed to its limit, binds up instead of rotating freely.

Adding a bit more complexity to sway bar installation is the fact that varying the size and effectiveness of the front versus rear sway bars can change the understeer/oversteer characteristics of a vehicle. You can tune the suspension so that more of the cornering forces feed either the front- or rear-outside tire. The result is a vehicle with more balanced steering characteristics, along with less body roll.

Resources

Eibach Springs, 17817 Gillette Avenue, Irvine, CA 92614-6501, 949/752-6700, www.eibach.com

Energy Suspension, 1131 Via Callejon, San Clemente, CA 92673, 949/361-3935, 888/913-6374, www.energysuspension.com