What Is Caster?
Caster is the measure of how far forward or behind the steering axis is to the vertical axis, viewed from the side. This is measured by drawing a line between the top and bottom pivot points of the front upright. The angle between the drawn line and vertical is the caster angle.
There are 3 types of caster angle:
Positive caster is when the top upright mounting point is more rearward of the car than the bottom mounting point when looking at the side view.
Another way to get positive caster is when the two mounting points are on the same vertical line as each other but are both infront of the centre line of the wheel. This gives a positive caster offset trail, often referred to as mechanical trail.
Neutral caster is when the upright mounting points are on the same vertical plane as each other as well as being on the same vertical plane as the centre line of the hub and wheel.
Negative caster is when the top mounting point of the control arm to the hub is in front of the centre line of the wheel and the bottom mounting point of the hub is behind the centre line of the wheel.
Another way to obtain negative caster is for both mounting points to be on the same vertical line as each other but behind the centre line of the wheel.
The Purpose Of Caster
Caster is a member of steering geometry and works during cornering and in a straight line. Positive caster is the most used type of caster in the automotive industry due to its many benefits. There are 3 types of caster. Each has their own advantages and disadvantages.
Negative caster gives no feedback to the driver in terms of wheel position. It is also the type of caster that is used on a shopping trolley and is the reason why wheels can wobble and turn with ease. A further disadvantage of negative caster is that it will increase under braking conditions making the steering more unstable due to the car pitching forward. This can however be solved with anti-dive steering geometry.
The only notable advantage to having negative caster is that the steering input effort required is very low making the steering very easy to turn.
One advantage of using zero caster is that it retains the slight reduction in kingpin torque before reaching the limit of adhesion, which is useful for a driver to know when the limit is near. Also, zero caster provides easy steering input without the extreme unstability of using negative caster.
Positive caster creates a self-aligning torque. This increases the stability of the vehicle in straight line situations, increasingly important at high speeds. The firm and stable steering provides the driver with confidence and allows them to drive at higher straight line speeds as they are not fighting the steering wheel. This self-aligning torque also pulls the wheels back to a straight line when exiting a corner allowing the driver to be much smoother on corner exit, controlling the rate that the wheels return to centre.
The main disadvantage to positive caster is that the steering requires a higher amount of input effort to turn the wheel. However with power steering systems this is not an issue. Also, for a racing driver where there often is no power steering, the heavy input is tolerated for the increased stability and smoother corner exits.
Positive caster also creates a jacking effect when the wheels are turned. The front inside wheel rises and the front outside wheel falls. This creates a jacking effect putting more mass on the rear outside wheel when cornering. This can create a positive turn in effect as it helps to rotate the car on corner entry. However, too much caster can lead to oversteer due to the jacking effects.
The Effects Of Caster On Camber
Caster angle affects the camber of the wheel during steering. However, unlike KPI, the effects are beneficial. If a car is set up with positive caster, the outside wheel will gain negative camber during cornering. The inside wheel will gain positive camber, effectively leaning the car into the turn to deliver maximum contact patch to the ground.
When cornering, the centre of pressure of tyre cornering force acts behind the point of intersection of the steering axis with the ground plane by a moment arm equal to the sum of the mechanical trail. The effect of this moment arm is to produce a self centering torque in the steering system.