[Editor’s Note: This article originally appeared in the May/June 1997 issue of Grassroots Motorsports.]
When a racer takes aim at tuning a car’s suspension, one of the first targets is often the anti-roll bar. Anti-roll bars (also known as swaybars or stabilizer bars) are found on virtually every modern car right from the factory—they are important tools used by sus-pension engineers to modify a ear’s ride, handling and response.
On the street, proper anti-roll bar selection can turn a miserable-handling car into a true sport sedan. Racers love tinkering with anti-roll bars because they can provide dramatic performance improvements for competition. Since anti-roll bar modifications are allowed in many classes of autocrossing and roadracing, it’s worthwhile to look into the workings of this device.
The Purpose of Anti-Roll Bars
Anti-roll bars permit two key functions:
- They reduce body roll.
- They provide a means for adjusting the car’s handling by redistributing cornering loads between the front and rear wheels.
Keeping body roll under control is essential in a high-performance car, because doing so restricts the vertical suspension motions that occur when a car is subjected to extreme cornering loads. Due to limitations in suspension linkages, it’s difficult to maintain proper wheel geometry (particularly camber and toe) when the wheels are flopping up and down over large distances.
Reducing body roll keeps the suspension links closer to their optimum zone of operation—in this respect, anti-roll bars serve to compensate for inadequacies in suspension geometry. Adding stiff anti-roll bars to a family sedan that features soft springs and prodigious wheel travel can yield particularly spectacular improvements in handling.
Reducing body roll also improves transient response, which is critical in racing. Transient response can be thought of as the amount of time required for the car to achieve full load transfer (that is, take a set”) following some steering input. Obviously, if the body rolls less, it takes less time for it to take a set—the car is ready to respond to the next steering input that much earlier.
Since autocrossing and road racing require a constant series of rapid steering inputs, slow vehicle response will devastate times—the body will still be rolling in response to one steering maneuver when it’s time to make another. This problem will force the car to either slow down or crush some pylons or Armco.
While reducing body roll is in itself an important function of anti-roll bars, an even bigger reason to use them is that they provide an ability to adjust a car’s handling. An anti-roll bar affects the roll stiffness of the suspension to which it’s attached, which determines the percentage of the total cornering load that will be supported by that end or the car. Thus, anti-roll bars can be used to change the relationship between front and rear cornering loads, which in turn allows the car’s understeer/oversteer characteristics to be adjusted as desired.
How They Work
The main component of an anti-roll bar is a long steel bar or tube with lever arms on each end. The lever arms can be formed by either bending the bar itself into shape, or by fastening separate arms to the bar via splines or some other mechanical means. Anti-roll bars that are bent from a single bar are less expensive to produce, and will work well if clearance around the suspension isn’t a problem. Anti-roll bars with removable lever arms are more adaptable, since the anti-roll bar can be assembled after the individual components have been routed through even very small openings in the suspension or bodywork.
The main length of the bar is attached through bushings to the car’s frame so that it’s free to rotate. The free end of each lever arm is attached to one side of the suspension either directly or through additional linkage known as end links. With this configuration, the right and left suspensions are connected through the anti-roll bar. If the suspension on one side is compressed (as occurs due to body roll during cornering), the lever arm on that side twists the main length of the anti-roll bar and tries to compress the suspension on the other side of the car. In so doing, the anti-roll bar has increased the effective spring rate of the suspension in roll. The extent to which the anti-roll bar increases roll stiffness is dependent on the stiffness of the anti-rollbar itself and where it attaches to the suspension.
Note that if both right and left wheels are moving up or down together at the same rate, such as would happen when driving over a speed bump, the bar remains untwisted. Since the bar isn’t twisted, it simply rotates in its bushings and provides no resistance to vertical wheel motion. The wheels act as if no anti-roll bar were present. This characteristic of anti-roll bars allows a car to be fitted with relatively soft springs to absorb road irregularities, while the anti-roll bar keeps body roll under control.
The roll stiffness that a anti-roll bar contributes to a suspension is a function of many factors. Anti-roll bar stiffness can be expressed by the formula:
where K is the rate of the anti-rollbar in pounds per inch (measured at the end of one arm), D is the main bar diameter, L is the bar length and B is the arm length.
What this handy formula tells us is that the bar diameter has the biggest impact on anti-roll bar stiffness. Since the stiffness increases to the fourth power of the bar diameter, doubling the diameter would yield an anti-roll bar that is 2x2x2x2, or 16 times stiffer! This relationship shows that an increase in diameter of just a few millimeters will produce a significantly stiffer anti-roll bar.
Examining the other parameters in the equation, we find that increasing the length of the arms (B) decreases the stiffness of the anti-roll bar by an amount equal to the square of the change in length; therefore, doubling the arm length will result in one-fourth the stiffness. Finally, we see that anti-roll bar stiffness is inversely proportional to the bar length (L), and that doubling the length of the bar will cut the stiffness in half.
Major changes in roll stiffness can be achieved by fitting anti-roll bars of various diameters, while fine-tuning can be accomplished by designing adjustability into the anti-roll bar. Adjustability is normally attained by making the arm lengths variable. Since the effective arm length is determined by the point at which the end links are fastened to the arms, the end links can be attached to sliding collars that can be secured anywhere along the arms.
Another technique used to create adjustable anti-roll bars is occasionally seen on formula cars. Rather than using a variable arm length, these anti-roll bars feature arms in the shape of flat blades that can rotate in a direction perpendicular to the main bar. When the blades are adjusted so that they are vertical, the arms are very stiff, and all of the anti-roll bar stiffness is determined by the main bar. When the blades are rotated to a horizontal position, they become very flexible, and thus reduce the overall stiffness of the anti-roll bar. Since the arms are used as flexible members in this antiroll bar design, the stiffness equation is no longer accurate; nevertheless, the basic relationships between the various parameters of the anti-roll bar remain the same. The advantages of this anti-roll bar design are that it allows for very fine adjustments, is compact, and is readily adaptable to a remote anti-roll bar adjustment for the driver.
The equation for anti-roll bar stiffness is also valid only if the anti-roll bar bushings, arms and end links are ultra-stiff. Rubber bushings work to reduce anti-roll bar stiffness, since they effectively become an additional soft springing element in the system. To avoid this detrimental effect, competition cars use metal or urethane rather than rubber in their anti-roll bar bushings.
The manner in which the anti-roll bar is mounted to the car has a big effect on its contribution to suspension roll stiffness. For maximum stiffness, the end links (or ends of the arms if end links aren’t used) should be attached as close to the wheel as possible. Since the wheels are effectively out on a lever arm from the suspension pivot points, vertical travel of the suspension links near the wheels is proportionally greater than it is near the pivot points. Greater vertical travel at the end links of the anti-roll bar will cause greater twist in the bar, and thus more resistance to roll; also, placing the end links near the wheels means that the suspension will have less of a mechanical advantage with which to twist the bar. Note, however, that a trade-off can exist-mounting the end links outboard to increase the effect of the anti-roll bar may require a longer bar length, which will reduce anti-roll bar stiffness. The net result of this positioning game will depend on the track width and suspension link lengths of the individual car.
It’s important that the anti-roll bar should not be pre-loaded when the car is resting at its static ride height with the driver on board, since pre-loading causes a difference in handling between right and left turns. Pre-loading of the anti-roll bar occurs when the bar has to be twisted to connect the end links to the suspension. This problem is a result of a mismatch in the geometry of the anti-roll bar and the mounting points of the end links on the suspension. Pre-loading can be avoided by using adjustable-length end links to dial-in the precise length required to avoid twisting of the anti-roll bar. If the anti-roll bar attaches directly to the suspension without using end links, the bar itself must be rebent to the proper position.
Effects on Handling
Since anti-roll bars can be designed to provide a broad range of roll stiffnesses, they are widely used for chassis tuning. When a stiffer anti-roll bar is installed at one end of the car, that end will carry a greater percentage of the total load transferred during cornering. All else being equal, cornering power at the stiffened end will suffer while cornering power at the other end will be enhanced. (The reason for this has to do with the manner in which load transfer affects the torques that are applied about the center of gravity during cornering. This is a subject in itself.) Stiffening the front anti-roll bar will therefore lead towards understeer, while stiffening the rear anti-roll bar will lead towards oversteer.
Occasionally the relationship between roll stiffness and understeer/oversteer will be modified by the effect of reduced camber change due to reduced body roll. By preventing large deflections of the wheels during cornering, the anti-roll bar allows the wheels to remain more upright and provide more traction. The net result of installing a stiffer anti-roll bar may be greater, rather than less, traction at the stiffened end. In general, the increase in traction due to reduced camber change will outweigh the decrease in traction due to higher load transfer only if the car had little roll stiffness and/or poor camber control in the first place.
So we’re now convinced that lots of roll stiffness is a Good Thing. Then why not install truly mondo anti-roll bars and get the car to really corner? The answer to this logical question is that as roll stiffness goes up, the compliance of the suspension goes down. A suspension with little compliance will prevent the tires from responding to road irregularities—the car will skip over the top of even minor bumps, constantly losing and gaining traction as it does so. If the tires aren’t firmly planted on the road, they can’t generate any cornering force. The car will simply patter its way right off the corner.
The optimum roll stiffness for a car will depend on the road surface over which it’s operating, as well as on driver preference. Since autocross courses are typically flat and smooth, an autocross car can in general get away with higher roll stiffness than a road racing car. But the key is to design adjustability into the anti-roll bar so that the stiffness can be adapted to the current conditions.
Testing alternative anti-roll bar designs and stiffnesses can provide any racer with a major performance edge over his or her competition. Since anti-roll bars are relatively inexpensive and are simple to install or change, even a casual competitor should consider experimenting with them.
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