One of the most basic, but also most important, settings on the front of the car is caster
Caster settings are set relative to the type of car and the design of the racetrack. When you determine the correct settings, you need to maintain those settings. We will discuss what effects caster has in our race cars, as well as how to properly measure it.
It is important to know how to properly measure for the amount of caster in your race car. All teams need to learn the proper procedure for determining the correct that exists in the front end geometry of their race cars because it is many times a driver preference.
Caster is a design condition that, in addition to the spindle king pin angle, serves to cause a wheel to want to track straight ahead. A common example is a bicycle front wheel and fork assembly. The tube that the handlebars are mounted to is mounted in a set of bearings above the fork and from a side view this tube is angled so that the bottom bearing is ahead of the top bearing. If we turn the front wheel to the side from the direction of travel, it will want to return to straight ahead by the effect of caster. The same effect is present in the front wheel assemblies of our race car.
What Caster Does
1 . Positive caster in the front wheel assembly is created when the ball joints are offset
To ease the amount of effort it takes to turn the wheel in our race cars, we introduce caster split into the design. Split means that we set different caster amounts into each wheel assembly so that the car will want to turn to the left naturally 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 that the left front wheel will have less positive caster than the right front 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 gauge. This tool attaches to the wheel hub. To check the amount of caster, we need to follow these instructions:
1. Attach the caster gauge to the right front wheel hub first.
2. Turn the steering wheel to the right so that the right front wheel has turned exactly 20 degrees from straight ahead.
3. For a manual gauge, level the gauge and set the adjustable caster bubble vial so that the bubble is at the zero mark on the caster side of the tool. For a digital gauge, set zero once the wheels are turned and the tool leveled.
4. Turn the steering wheel to the left so that the right front wheel is turned past straight ahead and ending up left of straight ahead by 20 degrees.
2 . To measure caster, set the bubble at zero on the caster side of the gauge with the whe
3 . For a digital gauge, turn the wheels to the right so that they turn 20 degrees, level
4 . Turn the wheel past zero degrees on the turn plates and go past that to 20 degrees lef
5. Again level the gauge and then note the location of the bubble on the scale of the manual gauge and record the amount of caster in the right front wheel. For a digital gauge, just read the display and that is the caster amount for the RF wheel.
6. While the wheels are still turned left 20 degrees, remove the caster gauge and place it onto the left front 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 bubble scale or display to know how much caster is in the LF wheel.
5 . It is a good idea to use turn plates in order to know exactly when you have turned the
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 or upper control arm shaft 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
6 . To measure on the floor to get exactly 20 degrees of steering, cut a 30-inch piece of
Normal caster splits for most short track asphalt applications are 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. Also, 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.