There are very few adjustments on a model racecar that create as much confusion as caster. Depending on where you get your information (whether it be a magazine article, or instructions for a setup board), it seems that there are very different opinions on the effects of caster adjustment.

I do not profess to being an expert on car geometry. However, after racing for 9 years one tends to pick up a fair bit of know-how, especially in the area of car handling. For the purpose of this article, I have borrowed explanations from a variety of sources, and I will try to paint a clear picture of how caster works.

What is caster?
Caster is the angle at which the front steering pivots. The primary purpose of having caster is to have a self-centering steering system. The caster angle affects on- and off-power steering, since it tilts the chassis more or less depending on how much caster angle there is.

For the purpose of model racecars, it is generally recommended that you use a steeper caster angle (more vertical) on slippery, inconsistent and rough surfaces, and use a shallower caster angle (more laid-back) on smooth, high-grip surfaces.

Camber vs. caster
Camber is all about contact patch – keeping as much tire on the ground as possible. Camber and caster are related in that caster can afford an amount of EFFECTIVE CAMBER change when the front wheels are turned in a corner.

Caster has the effect of progressively leaning the front tires into the direction of the corner. The more the caster angle is laid-back, the greater the effective camber change when the wheels are turned. This happens because the tops of the wheels BOTH TILT towards the inside of the corner; the wheels “dig in” more, counteracting the centrifugal forces pushing the car to the outside.

Compare that to the static camber of the wheels, which is adjusted with the car sitting on a level surface and the wheels pointed straight ahead. Static camber adjustments primarily affect the outside wheels, since these are the wheels that bear the majority of the load during cornering.

Hence, the amount of front camber required to maintain maximum tire contact largely depends on the amount of caster. A steeper caster angle requires more camber, while a shallower caster angle requires less camber.

It all depends on your point-of-view...
“More caster increases off-power steering” or, “less caster increases off-power steering.” Why is there such a difference of opinion? Is one right and the other wrong? No, they are both right… it just depends on your point-of-view.

The first statement refers to the steepness of the caster angle; therefore MORE caster means a more vertical angle. The second statement refers to the difference between the caster angle and true vertical (see the figure above); therefore LESS caster also means a more vertical angle. The same thing said in completely different ways, but both correct. So the next time your racing buddies are talking about caster, ask them what they mean by “more” or “less”. Maybe referring to caster as “steeper” or “shallower” would make more sense.

Steeper caster (more vertical)
Increased OFF-power steering INTO a corner.
Why? Imagine that the caster angle is vertical. Now imagine that you turn the steering; the wheels turn to the side. The steeper the caster angle, the more that the wheels deflect to the side, giving you more turn-in into a corner.

Increased suspension efficiency.
Why? The inboard suspension pins are, for the sake of discussion, parallel the chassis (horizontal) which means that the suspension arms move up and down vertically. Now, imagine that the caster angle is vertical, meaning that the top and bottom of the steering “kingpin” is directly aligned with the motion of the suspension arms. And finally, acknowledge that shock absorbers are pretty much horizontally aligned (the top is no further ahead of or behind the bottom), running perpendicular to the long axis of the car. Since bumps in the racing surface cause vertical deflections of the wheel, the more vertically oriented the steeringblock is, the better the front suspension can soak-up bumps without binding.

Decreased ON-power steering OUT of a corner.
Why? When you increase the power coming out of a corner, the weight bias shifts from the front wheels to the rear wheels. The more vertical the caster angle, the less the effective camber change of the wheels, so that ONLY the static camber of the outside wheel is affecting how much the wheels “dig in”. Since the wheels cannot “dig in” effectively, the reduced weight on the front wheels will cause the front to lose traction more easily, causing the car to understeer.

Decreased wheel-centering.
Why? Imagine that the caster angle is vertical. Now imagine that you take hold of the forward edge of a front tire and move it from side-to-side. The wheel deflects an amount proportional to how much you move it with your hand. Vertical caster is highly unstable because there is little in the way of forces to want to keep the wheels pointing straight ahead.

Shallower caster (more laid-back)
Decreased OFF-power steering INTO a corner.
Why? Imagine that the caster angle is so laid-back that it is horizontal (though this would be impossible). Now imagine that you turn the steering; the wheels would not turn to the side anymore, but rather the tops of the wheels would now tilt to the side. The shallower the caster angle, the less the wheels deflect to the side, giving you less turn-in into a corner.

Increased ON-power steering OUT of a corner.
Why? The more laid-back the caster angle, the more effective camber you get when you turn the front wheels. When you increase the power coming out of a corner, the weight bias shifts from the front of the car to the rear. Normally this would cause front to lose traction and understeer. However, since there is more effective camber at more laid-back caster angles, the “tilted” front wheels are more able to “dig” into the corner, allowing the car to resist centrifugal force and giving it a greater amount of control when exiting a corner.

Increased wheel-centering, but decreased straight-line stability.
Why? Imagine that the front wheels of a shopping cart (which have extremely shallow caster). Push the cart forward, and the front wheels will always try to center themselves. The shallower the caster angle, the more the steering is always fighting to get back to center. However (you knew this was coming, right?), the shallower you make the caster angle, the greater the amount of force trying to center the wheels. Eventually the forces become so great that the wheels will start to shimmy, decreasing straight-line stability.

Not the whole story
I hope that this article helps you to understand caster a little bit better. However, these explanations do not tell the whole story about how a car handles. Caster alone will not determine how your car handles on- and off- power, but it is definitely a contributing factor.

Car handling is a complex interaction of numerous factors; caster, camber, anti-roll bars, shock absorbers, and spring rates to name only a few. So keep in mind that there is not one “master” adjustment to make your car perform like a Formula 1 car; it is all a matter of compromise.

So keep the adjustments small, and one-at-a-time. That magic combination is out there for you.