Using excessive camber in either of the front wheels is a common crutch. Usually it means the car has geometry problems causing too much camber change during cornering. One of the most important parts of the total handling package is a front end that is set up for proper camber-change characteristics.
Most of us know that if the right front tire is hot on the outside, we need more negative camber. We can usually adjust our front tire cambers so the heat will be relatively even across the tire. But how do we know if we have the best design for camber change?
Camber change is the number of degrees of camber that the front wheels lose or gain from static (down the straightaway) to dynamic (in the middle of the turns). There is an optimum amount of camber change for each of the front wheels. The amount of track banking is an important factor in determining the best design for your car to produce this change.
Before we get into the specifics of what makes up the best design for a particular track, it is important to understand some basic information about why the camber will change on your race car as you negotiate a lap. Here are the five most important effects that cause camber change: chassis roll, chassis dive, control arm lengths, control arm angles, and spindle height. Let's look at each of these one at a time.
As the car rolls in the turns, there are two things that happen to affect camber change. First the right front wheel is in bump (moves up in relation to the chassis) and the left front wheel is in rebound (moving down in relation to the chassis). Also, though often ignored, the chassis itself is rolling. As the chassis rolls, the upper chassis mounts are moving to the right (in a left-hand turn). If the chassis mounting points are moving, so is the control arm and upper ball joint. If the upper ball joint moves in relation to the lower ball joint, we have a change of camber caused by chassis roll.
Many racers try to determine the amount of camber change by bumping the wheel with the car at static ride height. This will not give you a true picture of your camber change characteristics. The true camber change results from a combination of roll effect and vertical wheel movement.
As the car dives, the upper ball joints are drawn in toward the center of the car. The lower ball joints are either drawn in or pushed out, depending on whether the chassis mounts are higher or lower than the lower ball joints. Since the lower control arms are longer and have less angle than the uppers, the amount of camber change is much less. The upper arms have the greatest influence. Both of the front wheels will gain negative camber (top moving inward toward the center of the car) as the car dives. At high-banked, high-downforce tracks, close attention should be paid to the amount of camber change due to chassis dive.
The lengths of the upper control arms will influence the amount of camber change that occurs in each front wheel. The correct arm lengths for your car will depend on the overall design of the front end relative to the racetrack you intend to compete on. The best way to know how the arm lengths will affect the camber change in your car is to use a geometry software program that will allow you to install different-length arms. Then you can see the camber change effects of different lengths so you can decide which length arms are best for your car.
The smaller the angle (from horizontal) that the upper control arms have, the less camber change will result from chassis dive. A chassis with less upper-control-arm angle will also have more camber change resulting from chassis roll. The opposite is true of higher angled upper control arms. So, the degree of angle we have in the upper control arms will influence the amount of camber change, and the optimum control arm angles are determined mostly by the amount of track banking angle. We will tell you more about this later.
The height of the spindle is the measured distance between the centers of the ball joints. Spindles come in many different lengths. But generally, the greater the spindle length, the less camber change your car will experience from dive and roll. It is reasonably easy to understand why. If the upper ball joint moves one inch, the spindle will change its angle more with a 10-inch spindle than with a 12-inch spindle. The greater the distance between the ball joints, the less the angle that is produced with the same amount of upper ball joint movement and therefore the less amount of camber change. How do we put all of this information together into a front end design we can use? Let's draw some simple conclusions from what has been presented.
As a rule, a flatter track will produce more of a chassis roll angle with less chassis dive. As the track banking angle increases, the amount of chassis roll decreases and the amount of chassis dive increases due to more downforce effect. The overall goal is to produce the least combined camber change in each wheel for each type of racetrack.
The left front wheel will always lose some of its positive camber as the car negotiates the turn. The right front wheel will either gain or lose negative camber, depending on the arm angles and lengths. What we really want is less gain in the negative direction. We never want the right front wheel to change camber in a positive direction. That way, we can enter the turn and brake on a tire that is more in contact with the track surface and then gain some of the negative camber we will need in the middle of the turn as the car dives and rolls.
At the left front, we want to lose as little positive camber as possible so we can start with the least amount of positive camber. You will need to have about half to three quarters of a degree of positive camber in the left front wheel after the car dives and rolls. For both front wheels, the final tuning for the best static cambers is done by running the car and measuring the heat across the tires. Be sure to also look at the tire wear to help determine the best static cambers.
Throughout this whole process, don't forget to track where your roll center is located from the centerline of the car, and keep it where it should be. The height of the roll center will change as you increase or decrease the upper control arm angles. You can change arm angles and arm lengths and still keep your correct roll-center location from centerline. Roll-center software will allow you to make the correct changes to arm angles so the roll-center distance from centerline does not change. As we have learned in past articles (see the May '98 issue on correct roll-angle locations), the location of the roll center in relation to the centerline of the car is critical. Do not start changing arm angles and lengths without tracking how each change affects your roll-center location.
How important is camber change in the overall scheme of things? To give you an example, I have seen front end designs that produced 51/2 degrees of camber change at the right front wheel on a 12-degree banked racetrack. From straightaway to midturn, the tire went from 31/2 degrees of negative camber to 2 degrees of positive camber. The strange thing was that the tire temperatures looked fairly normal. Every part of the tire was being worked, but not at the same time. The inside on entry, the middle at quarter-turn (they added excess pressure because the middle was initially cool), and the outside at midturn. The problem was that the car pushed, because it never had a chance to use 100 percent of the tires' traction potential.
You can get the most traction potential out of your front tires by making sure the camber-change design is correct. It will make a huge difference to how the front of the car performs.