To measure on the floor to...
To measure on the floor to get exactly 20 degrees of steering, cut a 30-inch piece of 1x6-inch wood, or a similar straight, flat piece, and lay it against the tire. Mark midway on the wood (15 inches) and line that up with the center of the hub. Mark the outside corner of each end of the wood. Turn the steering wheel until the ends have moved 531/416 inches and you will have turned the wheel 20 degrees.
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 that the upper control arms are attached to, then 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 is not the preferred method, though.
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.
How Much Caster Split
The normal caster split for most short-track asphalt applications is around 2 to 4 degrees of difference. The left-front caster might be 1-2 degrees and the RF caster might be 3-5 degrees. The higher the banking angle of the racetrack, the less caster that is needed because less steering effort is needed due to the banking. On the other hand, the tighter the turn radius, the more caster split is needed.
Driver preference plays a big role in getting the caster split right for your application. Most drivers prefer to have some feel where they need to pull slightly on the wheel to make the turn. What no driver wants or needs is to have more caster split than needed so that they will need to apply backpressure on the steering wheel at midturn.
CAMBER DEFINED
Camber is when a wheel is tilted so that the top of the tire is either closer to or farther from the centerline of the car. Negative camber is when the top of the tire is closer to the center of the car than the bottom of the tire. Positive camber is when the top of the tire is farther away from the center of the car than the bottom of the tire.
From a driver's view, positive...
From a driver's view, positive left-front camber is when the top of the tire leans out, away from the centerline of the car. The opposite is true of the RF tire, which has negative camber where the top of the tire leans toward the centerline.
Circle Track Camber
In circle track racing, we use positive camber on the left-front wheel of the car and negative camber on the right-front wheel. We can easily check the amount of camber by using a caster/camber gauge and reading the amount directly on the camber bubble vial.
We have learned some interesting and important aspects of tire camber for short-track racing. We have always known that a racing tire will flex under the stress of cornering and the tread will move and roll under the wheel when the extreme forces associated with cornering are present. Different brands of tires have different stiffnesses of sidewall construction and therefore roll over more or less.
Tire temperatures tell us more about how much static camber we need than anything else. The overall goal is that we need the tire contact patch to be relatively flat on the racing surface at midturn in order for the tire to be able to provide the maximum amount of traction. This is often referred to as the maximum "footprint."
Tire temperatures can alert us to improperly set static cambers. A front tire that is hotter on the inside edge (side toward the inside of the racetrack) usually has too much positive camber in the case of a left-front wheel, or too much negative camber if it is the right-front wheel.
Move camber adjustment shims...
Move camber adjustment shims back and forth from each mounting bolt, or slide the control arm shaft if it is slotted, to adjust the amount of caster in each wheel. This will move the upper ball joint forward (to reduce caster) or to the rear (to add caster).
Camber Change
The cambers will change as the car dives and rolls when it enters and negotiates a turn. True camber change is a product of both chassis dive and chassis roll. Gone are the days when we would jack up the wheel and measure how many degrees the camber changed in each inch of bump. Those numbers really don't tell us anything. They are only part of the answer. Chassis roll has an effect that adds or subtracts from what dive does. So what we really need to know is what the dynamic camber is after the car dives and rolls, just like it does in the turns.
The left front always loses a lot of camber, so we need to allow for that in setting the amount of static camber. Generally, if we end up with between 11/42 and 1 degree of positive camber at the left-front wheel after the car dives and rolls, then that tire will have the dynamic camber that it needs.
The right-front camber change is a little different. We can design our car so that the right-front camber does not change after dive and roll. This is actually exactly what that tire wants for most short-track applications. The reason for this is that as we enter the turn, the right-front tire takes a set fairly quickly. If the camber continues to change after that initial set, then the tire will give up traction and the car will usually push.
The right upper control arm angle mostly controls the right-front camber change, so we try to work with that control arm angle. Once we have the proper camber change (zero), we leave that angle alone as we further design our front end for moment center location.
SPINDLE HEIGHT
Spindle height affects the amount of camber change at each wheel. The taller the spindle, the less camber change will occur. Trends that have taken place in the past 10 years or so have resulted in excess camber change due to the use of shorter spindles. That trend is in the reverse mode now as the car builders move toward using taller spindles.
Measuring Camber Change
We can measure camber change by several different methods. In the shop, we can set the chassis ride heights just as they would be at midturn on the racetrack and then directly measure the camber at each wheel. To do this, we will need to know the shock travel at midturn, which is very hard to estimate.
If we look at the shock travel indicators on the shaft of the shock, it always tells us total shock travel, which includes braking, going over bumps, banking changes such as exiting the racetrack and driving down onto the apron (this could be quite a lot of left-front shock travel at some high-banked racetracks), or something as simple as steering the car back and forth to warm the tires before running hot laps.