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Camber Angle Theory

by Tony bones

 

Definition

The wheel camber angle is the angle, measured in degrees, between the centre line of the wheel and the perpendicular to the ground, looking from the front.

Background

Let us now look at the reasons why wheels have a camber. If the tyre is to be perfectly positioned on the ground, and the wear on the tread is to be symmetrical, the wheel should have a zero camber (perfectly perpendicular to the ground) for all the conditions that are created during the ride, as the conditions are extremely variable. However, the existence of a correctly set positive or negative angle must be acknowledged, in certain specific cases.

Wheel camber was created to meet needs which stem from olden times, as far back as the horse-drawn carriage. In those times, wheels were large-diameter to overcome the bumpy road surfaces, and at the same time, for weight reasons, they were fitted with spokes, attached to a central outer wheel hub.

Impact against an obstacle

To compensate for the fragility of the wheel spokes, which could break quite easily when bumping against an obstacle, the spokes were inclined towards the inside of the vehicle, giving them a concave shape. The effect of the weight could cause breaks at the point of attachment to the hub or at the circle attachment point.

To remedy this problem, the hub was pointed downwards, so that the weight of the vehicle was on the spoke when it was in a vertical position. At the same time, the concave shape of the wheel, very resistant to side impact caused by bumping against obstacles, was maintained, and because of the reaction generated by the weight of the vehicle, it was impossible for the wheel to detach itself from the hub, as it was held against the hub inner shoulder.

Motor vehicle wheels straight away followed the method for cart wheels. This method took advantage of the fact that the reaction generated by the weight of the vehicle had the effect of pushing the inner bearing of the wheel against the steering axis of the wheel itself, but at the same time, other needs were created.

In the first place, because of the speed of motor vehicles, the need arose to have a very small rotating mass in order to eliminate the gyroscopic effect This effect, according to specific law of physics, creates a strong resistance when attempting to change the direction of a rotating mass, and the bigger mass, the greater the velocity, the stronger the effect.

In the second place, the necessity arose to turn the wheels only, and not, as is the case for carts, the whole axle. This small variation in the wheelbase between the axis (to avoid turning the vehicle over at high speed) is the result of the weight force and the centrifugal force and can easily fall outside the support perimeter of the four wheels or, at the same time, reduce the under body dimensions of the front axis, which became fixed. These problems are overcome by fitting the directional wheels to two kingpins which act as steering pins.

The kingpin is used for steering, but the resistance encountered in turning the wheel increases with the distance between the point of contact between the wheel and the projected ground point of the kingpin, known as the” kingpin offset.” In fact, a strong resistance effect is created, proportional to the kingpin offset, when the resistance due to the friction between the wheel and the road acts on the wheel, or a perturbation force generated by impact against an obstacle.

The product is the moment of resistance which must be overcome in order to turn the wheel. The greater the value of the offset, the greater effort to turn or maintain the direction of the wheel.

At the same time, as the offset increases, the radius of the curvature of the vehicle increases and consequently the risk of a break in the hub.

In the past the hub has been inclined downwards in order to reduce the kingpin offset. This, however, is not sufficient to reduce the perturbation force effect, which affects the steering, as the wheel camber angle could not be increased to the extent necessary to cancel the kingpin offset.

To reduce the amount of interference with the steering, the use of high pressure tyres was replaced with the use of low pressure tyres or tyres with inner tubes which, because of their larger cross-section area, supported the same weight with a low inflation pressure, thus having a softer contact with the ground.

However, the effect of the strong camber on the wheel and the softer tyre is to give negative results, as the outer part of the tyre becomes deformed, and turns with a smaller rotation radius then the inner part, with the consequent increased wear on the outer strip of tread.

It can be said that the tread goes into a conical shape against the ground, which tends to turn the wheel towards the outside, causing irregular wear of the tread and directional instability, if the camber angles of the two wheels are not the same.

Therefore, the camber angle must be reduced, but this would once more increase the length of the kingpin offset, and there would be a definite deterioration when the wheels are fitted with brakes. In fact, if the brakes are uneven, this would have the same negative effects on the steering as the perturbation forces have on the direction, creating serious road-holding problems.

The solution to the problem can only be found when the wheel is tilted in the opposite way to the kingpin, so that the kingpin projection falls within the contact surface between the ground and the tyre.

Effects

This means that the wheel camber angle can be reduced considerably, leaving a small angle to compensate the axis deformation caused by the load in rigid-axle vehicles. At the same time, this would lead to a reduction in the kingpin offset and the negative effects this has on the steering. In fact, it can be seen that the wheels tend towards a zero camber angle under the effect of the deforming load.

With improvements in construction techniques and with the introduction of independent-arm suspension, the wheel camber angle tends towards a value very close to zero under the most common use or load conditions It should be taken into account, however, that the camber angles of the wheels will tend to vary as the vehicle is jolted about.

When the suspension is in compression, the bump position of the wheel will be higher relative to the body. With the release, however, the bump position of the wheel will be lower relative to the body. During these movements the wheel, with its ideal position being perpendicular to the ground, will take on a negative camber angle during compression, and a positive camber angle during release. This is created by the combination of factors related to the de-formability of the parallelogram formed by the upper and lower suspension arms and the wheel kingpin.

This was one of the many factors that led to the independent-arm suspension being preferred to the rigid-axle suspension. The benefits of this effect is most apparent on bends, where the compression of the suspension on the outer wheel on the bend caused by centrifugal force, produces a negative camber on the wheel itself that acts against the overturning of the vehicle. This does not occur with rigid-axle suspension systems.

Another positive aspect of independent suspension systems is as follows. With reference to the distance between the elastic reactions of the two systems, with independent suspension, this distance has the same value as the impact of the wheels on the ground, thus giving greater stability and increased elasticity to the system, as compared to rigid-axle suspension.

The de-formability of the parallelogram formed by the upper and lower suspension arms and the wheel kingpin is linked to a series of well-defined dimensional ratios. The ratio between the lengths determine the degree of angular variation in the camber of the wheel, either positive or negative, according to the vertical bump or jolt movement. The bigger the bump, the bigger the variation in the camber angle.

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Positive or negative camber angle is determined by the distances ‘A’ and ‘B,’ depending on whether or not these two distances are the same, and by the position of the arms relative to the ground under normal use conditions of the vehicle. If, for example, it is decided that the wheel should have a zero camber angle under normal load conditions, then obviously it must be at the mid-point in the range of movement between complete compression and complete release. The camber will then tend towards positive when empty, and negative when fully loaded. When the suspension system is in a classical position, the condition described above will be satisfied.

In fact, with the suspension system in this arrangement, which means that the distance ‘A’ will be different to distance ‘B’ for any minimum upward or downward movement, the wheel camber angle will inevitably change. For a large number of reasons, it is not always possible for the manufacturer to follow the classical arrangement rules, and a complex number of possibilities arise from this regarding the geometrical arrangement of the suspension systems.

Tony Bones and Jason Saunders of Wheels in Motion (http://www.wheels-inmotion.co.uk/index.php) have developed somewhat of a cult following as alignment experts in England.  Although their articles are amazingly complicated and there may be a little bit of a language barrier, we asked to reprint these gems for your benefit. Tony Bones did ask us to please advise readers that this information is a global overview of alignment principles and actual practical application to specific vehicles should be left to the professional alignment technician or shop.

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