TUNING PERFORMANCE SUSPENSION A PRIMER By Tom O’Rourke INTRODUCTION Designing and tuning the automotive suspension for performance involves fact and science, as well as a good measure of experience and art. In the following monographs I will emphasize the former, but will occasionally use a first person presentation to acknowledge the intertwining of my opinions. Keep in mind that cats can be skinned numerous ways. One man’s stay bar adjustment is an other’s sway bar change. Often differing approaches do not mix well. Accordingly, for the most part, my comments will be more qualitative than quantitative. And particularly keep in mind that "performance" comprises quite a range of cats. Completely opposite objectives and expedients are appropriate for various specialties. My bias tends towards road and circle racing on pavement. Organization of the various subjects under set-up or dial-in reflects the above-mentioned bias. Under set-up I address those items more readily sorted out in the shop or during testing. Set-up items typically alter other settings, or are difficult to set and/or measure. Dial-in items tune the suspension to a particular track or changing conditions. Each topic is developed further in a more focused paragraph.
SET-UP The first concern is basic housekeeping. Chassis rigidity, particularly resistance to twist in torsion, is a necessity. Chassis flex will render the other tunable suspension control mechanisms insensitive or inoperative. The chassis should be square check the wheelbase and wheel diagonal measurements. And place as much weight as feasible low and toward the center of the vehicle, except high for drag racing, somewhat higher for dirt, and towards the left (driver’s side) and maybe a bit more rearward for circle racing. Ride height is the primary suspension setting. Most of the other suspension settings change when ride height is altered. Set ride height first and check it regularly. Static wheel weight should also be set during this process. I prefer to treat wedge as a fixed parameter, though this is a minority view in many circles. Locate and, if necessary, adjust the various pivot axes between the sprung weight and the unsprung weight. These include the roll centers and dive and squat characteristics. The front and rear roll centers define the roll axis which, in conjunction with the Cg, in turn define the roll moment, i.e. the tendency of the vehicle to lean when turned. Dive and squat suspension geometry influence the vehicle pitch attitude, i.e., nose down under braking and tail down during acceleration. While springs and dampers, though often varied somewhat during dial-in, are initially chosen to keep the vehicle from bottoming and oscillation, respectively. Calculating the wheel-rate during set-up aids understand of the effect spring and damper changes will have on handling. Steering geometry is also an early consideration. The front spindle determines the steering inclination angle to produce a reasonable scrub radius. Caster, Ackerman geometry, and bump steer are important parameters which should at least be recorded and tracked. With these items under control, we can proceed to dial-in.
DIAL-IN My preference in "dialing-in" is to first optimize the absolute total adhesion of the tires at speed, and then work on balancing the vehicle. Of course if the vehicle balance is vicious, it is only prudent to kill the biggest and closest snake first. Tire temperatures are an excellent measure of tire adhesion. Tire pressure and camber are adjusted to produce a flat temperature profile across the tire. Roll stiffness at one or the other end of the vehicle is then adjusted to balance the handling in terms of understeer (push) or oversteer (loose). Springs may also be varied in small increments to tune pitch stiffness as well as roll stiffness. At this stage the tires will be flat on the ground and working. The vehicle will have a reasonably balanced response. If a locked rearend is involved in a circle track setting, stagger is provided at the rear tires to facilitate power-on turning. Front to rear brake bias is adjusted to optimize braking effectiveness and stability, and /or enhance corner turn-in. Dampers can be tuned to adjust the rate of weight transfer, either fore and aft or in roll -but usually roll- as well as to control small, rapid wheel movements. Though not one of my favorite expedients, wedge is sometimes fine tuned to balance the vehicle. While wedge changes ride height somewhat, it is often effective as a quick pit stop adjustment to correct an unbalance roll stiffness resulting from tires fading unequally during a race. Of course the trick is to make the best compromises and get all of the above to work together to approach (but never reach) the optimum. Ackerman The Ackerman concept deals with the differing radii followed by the inner and outer steered wheels when turning. By varying the angle between the steering arm (an imaginary line from the tie rod attachment point to the actual steering axis) and the tie rod, the amount of turning of the inner wheel relative to the outer wheel can be varied. For parking lots and wagons, each steered wheel follow its actual turning radius to minimize tire wear. For racing there is a theory for any possibility. My view is that we want to gain as much cornering power as possible from the inside tire. Tire slip angles under maximum turning allow the actual radii traveled by the tires to differ from each other as well as to deviate from the direction in which each tire points! The more heavily loaded outer tire pretty much determines the actual cornering line. Provided the inside tire is not turned so far as to induce it to lose its slip angle and slide (wash), there is worthwhile cornering power to be gained by turning the inside tire even beyond the theoretical Ackerman angle –though the reasons for this are beyond the scope of this discussion. Optimum Ackerman geometry is best determined by testing. With rear steer (the steering arms pointing rearward), start with a modest setting and move the tie rod forward and/or increase the inward steering arm angle until the driver notes a fall off in performance, and then back off a bit. Front steer is a bit more problematic in that the desired steering arm angle often interferes with the tire. Rearward movement of the tie rods is the easier approach. Be sure that toe and/or bump steer don’t wander and influence the results. Brake bias Front braking power relative to rear braking power is significant for two somewhat contrary reasons. Under braking, a rolling tire has more traction than a sliding tire. If rear brakes lock first, the front brakes will have more grip and the vehicle will tend to spin. Thus, to enhance stability, the braking action is usually biased so the front brakes lock first. Over biasing in this manner diminishes the braking effectiveness. Brakes can also be advantageously biased to emphasize instability. According to the traction circle theory, a tire can do only so much total work. If asked to accelerate or brake while turning, less traction is available for turning. Most of us are familiar with "driving with the throttle" in a turn. Similarly, by biasing braking more strongly at the rear and utilizing trailing brake into a corner, more turning traction is available at the front and less at the rear. Thus the vehicle rotates into a corner. Turn-in can be greatly aided by this technique. As can spinning! Bias is controlled in several ways. Foot force on the brake pedal can be selectively divided between front and rear master cylinders by an adjustable bias bar. Or an adjustable proportioning value in the hydraulic circuit will attenuate the pressure down stream of the valve. Braking can also altered by changing the diameter of the master cylinder and/or the slave cylinder –decrease the master and increase the slave for more braking. Tire diameter and/or footprint also affect brake bias. Many race vehicles are running with a lot of bias cranked into the front because it feels more comfortable. Or because the factory would rather error on the side of stability at the expense of total braking. Bump Steer As you might expect bump steer is steering resulting from a wheel hitting a bump. Actually upward movement of the wheel constitutes bump whatever the cause. Roll can similarly cause unwanted steering. And the rear wheels can be steered with suspension movement. While even a good driver may not be able to identify substantial amounts of bump steer, very worthwhile improvements can be had by remedying the condition. Bump steer is identified by jacking a wheel through its normal travel and measuring the thus induced steering movement. This is commonly accomplished by mounting a flat plate on the wheel and measuring the angular movement of the plate with one or two dial indicators as the wheel is exercised up and down. Of course the steering mechanism must be locked. I’ve had good results by hanging plum bobs from the end of the plate and measuring the position changes at the floor by marking segments of tape with a pen and measuring with dial calipers or a good rule. Remember that we’re not interested in the lateral movement of the wheel but only in the direction it points. Correction of bump steer is by changing the relative arcs of the suspension arms and the tie rod by moving the drag link or steering rack position. Both pivot location and tie rod length are of concern. If you find appreciable bump steer, investment in a more comprehensive suspension text (I haven’t published one) would be a wise investment.