Compared to some forms of racing, caster and camber settings are evaluated a little differently for circle track racing. Because we usually only turn left (as opposed to street driving or road racing), our front-end design is much different and asymmetrical (differing from side to side). The knowledge of what constitutes the optimum design and what the car needs has increased over the past few years.
It is important to know how to properly measure the amount of caster and camber in your race car. From my experience, most teams know little about the proper procedure for determining the amount of each that exists in the front-end geometry of their race cars.
Caster Defined Caster is a design condition that, in addition to the spindle kingpin angle, serves to cause a wheel to want to track straight ahead as a product of weight being applied to the wheel structure. A common example is a bicycle front wheel and fork assembly. The tube on which the handlebars are mounted is placed in a set of bearings above the fork. From a side view, this tube is angled so the bottom bearing is ahead of the top bearing. If we turn the front wheel to 90 degrees from the direction of travel, it will want to return to straight ahead because of the effect of caster. The same effect is present in the front wheel assemblies of our race car.
What Caster Does To ease the amount of effort it takes to turn the wheel in our race cars, caster split was introduced into the design. Split means that we set different caster amounts into each wheel assembly so the car will want to turn to the left and thereby reduce the amount of effort it takes for the driver to hold the steering wheel when negotiating the turns. Proper split for circle track racing means the left-front (LF) wheel will have less positive caster than the right-front (RF) wheel. In some cases, teams have been known to set negative caster in the LF wheel and positive caster in the RF wheel.
To measure caster in each wheel, we use a caster/camber gauge. This tool attaches to the wheel hub. To check the amount of caster, we need to follow these instructions:
1. Attach the caster/camber gauge to the RF wheel hub first.
2. Turn the steering wheel to the right so the RF wheel has turned exactly 20 degrees.
3. Level the gauge and set the adjustable caster bubble vial so the bubble is at the zero mark on the caster side of the tool.
4. Turn the steering wheel to the left so the RF wheel is turned past straight ahead and ending up at left from straight ahead by 20 degrees.
5. Again, level the gauge, note the location of the bubble on the scale, and record the amount of caster in the RF wheel.
6. While the wheels are still turned left 20 degrees, remove the caster/camber gauge and place it onto the LF wheel hub.
7. Level the gauge and set the bubble on the caster gauge to zero.
8. Turn the steering wheel to the right past straight ahead until the LF wheel is turned 20 degrees to the right of straight ahead.
9. Level the gauge and read the caster gauge to see how much caster is in the LF wheel.
Adjusting Caster To adjust the amount of caster in each wheel, you will need to move the upper ball joints fore or aft. To increase the amount of positive caster, move the top ball joint toward the rear of the car. Some cars have slots cut into the upper chassis mounts for this purpose. If you have permanently attached vertical mounting plates for attaching the upper control arms, you can vary the amount of shim spacing for each of the bolts that attach the control arm to the chassis. Wider spacing at the front bolt (control arm shaft inside of the mounting plate) will move the upper ball joint to the front, creating less caster at that wheel and so on. This, however, is not the preferred method. Once you have established the exact caster amounts for each wheel using the above method (if not using slotted control arm shafts), you should order an upper control arm that has the ball joint offset to give the correct amount of caster at each wheel. That way, you can use the same shim spacing for each mounting bolt to connect the upper control-arm shaft to the chassis.
In today's complex world of chassis setup, we cannot overlook theimportance of properly adjusting our car's front geometry settings.Caster and camber are a major part of that equation. |
Positive caster in the front wheel assembly is created when the balljoints are offset (from a side view) so the upper ball joint is fartherto the rear of the car than the bottom ball joint. The degree of casteris related to the angle in degrees that a line through the ball jointsforms from a vertical line. |
Set the bubble at zero on the caster side of the gauge. Some teams useone edge of the bubble so reading the angle is more accurate than tryingto estimate the center of the bubble. |
To measure on the floor to get exactly 20 degrees of steering, cut a30-inch piece of 1-by-6-inch wood or similar straight, flat piece andlay it against the tire. Mark midway on the wood (15 inches) and linethat up with the hub. Mark the outside corner of each end of the wood.Turn the steering wheel until the ends have moved 53/16 inches and youwill have turned the wheel 20 degrees. |
Some cars have the upper control-arm mounts cut with a slot that makesadjusting caster easy. Some upper control arms are also slotted for thesame reason. If your mounting plates or control arms do not offer thismethod of adjusting caster, you can angle (from a top view) the controlarm shaft by moving shims front to rear. This is not the ideal method,but will work until we can find the right caster split that suits thedriver. We can then order offset control arms that will locate the upperball joint for proper caster without needing to angle the shaft. |
Move camber adjustment shims back and forth from each mounting bolt, orslide the control arm shaft if it is slotted, to adjust the amount ofcaster in each wheel. This will move the upper ball joint forward (toreduce caster) or to the rear (to add caster). |
After you have leveled the gauge, read the center of the bubble on thecamber side of the gauge to get your wheel camber setting. Make sure thecar is at ride height with all of the weight in the car including thedriver. Air the tires to operating pressures. These settings may changeonce the car is raced and the tire temperatures have been evaluated. |