Longitudinal Caster Angle Theory

Wheels will meet with an equal counteracting force, due to the weight of the vehicle, acting in a transverse direction along the axis of the oblique kingpin, and thus contributing enormously to conserving the stability of the drive of the vehicle.

Castor angle

Definition

The castor angle is the angle, measured in degrees, formed between the axis of the kingpin and the perpendicular to the ground, looking at the vehicle from the side,

As this angle is formed longitudinally relative to the vehicle, it’s more exact definition is: longitudinal castor angle, In practical usage it is known more simply as the castor angle,

It has been established by convention that, if the extension of the kingpin axis falls in front of the point where the wheel stands on the ground, the castor angle is defined as positive, and if it falls behind this point the castor angle is defined as negative, the castor angle is zero when the kingpin is perfectly vertical,

The castor angle given to the kingpin creates two very important phenomenon for the ride of the vehicle, the first is related to stability, in maintaining the straight line travel of the vehicle, with the relative return of the steering after steering round a bend, and the second is the tilt of the wheel which occurs during steering, and which is observed by the inclination of the wheel being turned,

Stability Phenomenon

This is created on the basis of the distance between the point at which the kingpin axis extension falls (in relation to the direction of travel) and the point of contact between the tyre and the ground,

Two wheels with positive castor angle (the kingpin extension falls before the point of contact with the ground) using two different systems, one is to incline the kingpin, and the other is to move the kingpin position in relation to the wheel axis, the straight line travel stability exists in both cases,

In fact, in the case of the positive castor angle, the wheel is pulled, as it is the line of application of the force applied to the axis that passes in front of the wheels mid-point, without taking the direction of travel into consideration, each attempt made by the wheel to deviate from straight line travel will be counteracted by the straightening couple generated by the force and by the wheel rolling resistance,

However, in the case of negative castor angle, the wheel is pushed, as it is the line of application of the force applied to the axis passes behind the mid-point of the wheel, without taking the direction of travel into consideration,

Each attempt made by the wheel to deviate from straight line travel will be helped and increased by the couple generated by the force and by the wheel rolling resistance,

Consequently, the best stability condition for straight line travel is obtained with a positive castor angle and with the wheel being pulled, in fact, in this case, the phenomenon of wheel-wobble and the consequent effects on the steering are avoided,

Let us now examine what happens to the wheel during steering, if the castor angle value is zero, the turning axis will coincide with the centre-point of the wheel’s contact with the ground, in this case, the wheel’s behaviour is neutral, and it is sensitive to all the perturbation forces which attempt to modify the straight line travel position, ( and consequently has no steering return action),

If the wheel has a positive castor angle, when it is turned about the kingpin axis it shifts it’s point of contact with the ground and, consequently, the point at which the force representing the rolling resistance is applied, the force of the kingpin axis extension, however, always acts in the same direction,

This leads to a return couple being created by the forces “rolling resistance” and “kingpin axis extension” which tends to take the wheel back to the straight line position,

If the wheel has a negative castor angle, when it is turned about the kingpin axis it shifts its point of contact with the ground and, consequently, the point at which the force representing rolling resistance is applied, the force of the kingpin axis extension, however, always acts in the same direction,

This leads to a couple being created by the forces “rolling resistance” and “kingpin axis extension” which , in contrast to the previous case, tends to increase the steering effect on the wheel and impede the steering return,

The different behaviour of the wheels with positive and negative castor angles can be verified practically by driving the same vehicle in forward gear and in reverse gear, if the castor is positive in forward gear, it will be negative in reverse gear,

In the two cases it can be seen that the bigger the castor angle (either negative or positive) the greater will be the longitudinal offset, and the greater will be the shift of the point of application of the resistance force during steering, therefore, the conclusion can be drawn that the couple “rolling resistance and the kingpin axis extension” is both straightening and self - centring, and directly proportional to the longitudinal offset,

In the most common case, with a positive castor angle, the couple, which creates the steering return, will contrast the steering movement when a steering action is performed by the driver, thus, it could be said that, the bigger the positive castor angle, the greater will be the force exerted on the steering wheel to turn the wheels but, at the same time, there will be an increased conservation of the direction of the vehicle under the effect of perturbation forces against straight line travel,

If the castor angle is different on the two wheels causing different reactions to variations in the vehicle will tend to deviate from its trajectory by itself, from the side where the angle is smaller in absolute terms, this effect is counter- productive for vehicle stability, both when the vehicle is under acceleration, the thrust effect or braking,

The phenomenon of steering return in the straight line position is guaranteed by the transverse inclination of the kingpin, the fact that the castor angle also contributes to the same effect helps in the resolution of the most desperate cases, according to the various needs created during the design stage, when the geometric attitude of the vehicle is set, in traditional motor vehicles, the castor angle tends towards zero, and it is bigger only in vehicles where the weight acting on the directional wheels is low,

Tilt State Phenomena

In the chapter dealing with the camber angle of the wheels, it can be seen that the camber angle varies according to the positioning or length of the steering arms under the effect of the compression and release of the suspension,

This effect is very useful when steering around bends, when the centrifugal force deforming the parallelograms formed by the suspension units gives the outer wheel on a bend a negative camber and the inner wheel a positive camber,

The same result is obtained on the wheels with the castor angle, but this is generated on the basis of the steering width and the vertical movement of the suspension arms, so the following conclusions can be drawn: when the wheel turns about a kingpin with a positive caster angle, if it is in the outer position on a bend it will take on a negative camber that increases with the strength of the steering action, and will thus act against the overturning of the vehicle, if on the other hand, it is in the inner position on the bend it will take on a positive camber which follows and assists the turn,

Consequently, when the vehicle takes a wide bend (small steering action) at a very high speed, it will be the same centrifugal force which, by acting on the de-formability of the suspension parallelograms, will create the advantageous position for the wheels: however, when the bend taken is very tight (strong steering action) and the speed is moderate, it will be the castor angle that will create the advantageous wheel camber angle,

Other factors influencing the castor angle, low-pressure tyres, in modern vehicles, contribute to reducing the caster angle given to the kingpin during construction, in fact, when under the influence of active or braking thrust, the tyres deform and tend to increase the longitudinal offset by shifting their point of contact with the ground,

The castor angle also varies with the variation in the weight distribution of the vehicle, If the load is shifted to the rear axis this will increase the positive castor angle, and if it is shifted to the front axis it will decrease it,

True vertical Kingpin axis extension
Theoretical pivot point
Trail distance

Longitudinal castor angle/ set as positive, the kingpin axis falls in front of the theoretical pivot point generating the trail distance, this gyroscopic affect guarantees stability, and can be defined as the castor angle.