wheel alignment,,CAMBER

Camber

The front wheels of the car are installed with their tops tilted outward or inward (this can best between by looking at the wheels from directly in front).

This is called camber and is measured in degrees of tilt from the vertical. When the top of a wheel is tilted outward, it is called positive camber. Conversely, inward inclination is called negative camber.

Role of camber. In early automobiles, the wheels were given positive camber in order to improve the durability of the front  axle, and to cause the tyres to contact the road surface at right angles to prevent uneven  tyre wear on roads where the centre of the road is higher than the edge.

In modern automobiles, the suspension and axles are stronger than in the past and surfaces are flat, so there is less need for positive camber. As a result, tyres are being adjusted more toward zero camber (and there are some vehicles with zero camber).  Some models are even being given negative camber to improve cornering performance expect commercial vehicles.

Positive Camber. The roles of positive camber are as follows:

When viewed from the front, the tops of the tyres are tilted outward. This inclination is referred to as positive camber. The following is a explanation of its function.

1. Slip-out prevention. Due to the weight of the vehicle, the bottoms of the tyres tend to pull outward making it possible for the wheel to slip out of position. To prevent this, camber is provided. As is shown by F in the figure, the load received on the tyres in perpendicular to the road surface. The load on the spindle, however, is divided between the force F1, perpendicular with the spindle and force F2, in the direction of the spindle axis. At this time, force F2 is acting to push the tyre inward, making slip-out difficult. Conversely, with negative camber (when wheels are closer together at top than at bottom), the tyres can slip out of position rather easily.

The reasons the inner taper roller bearing is larger than the outer is because there is a greater inner side load with positive camber.

2. To lighten the perpendicular load. If there were no camber, the load, in relation to the spindle, would be where the tyre centre line intersects with the spindle, as shown by F in the figure. With camber, however, the tyre surface contacting the road is further inward, placing the load nearer the spindle base so there is less force tending to bend the spindle and steering knuckle

3. Lessen required steering effort. Combined with the kingpin angle, camber serves to shorten the offset. When the steering wheel is turned, the wheels roll with the kingpin as the centre and the offset as the radius. Therefore, the larger the offset is, the greater the tyre travelling distance, resulting in the necessity for greater steering effort. conversely, steering is lighter is the offset is less, but there is a limit to how small the offset can be.

As explained, positive camber is helpful from the aspect of reducing steering effort and prevention of tyre slip-out, but simply started, camber is the inclination of the tyre. Referring to the figure (above), a tilted wheel will always roll in a circle, but is rolled on a side tester, the tyre will force the side slip tester inward, and the tester indicates toe-out, and to off-set this, it is necessary to provide the wheel the toe-in adjustment.

Zero camber. The main reason for adopting zero camber is that it prevents uneven wear of tyres.

If the wheels are given positive camber, the outside of the tyres turns on a shorter radius than the inside of the tyre. However, because the rotational speed of the tyre is the same on the inner and outer sides, the outer side of the tyre must slip on the ground while it waits for the inner side to “catch up” with it. this causes quicker wearing of the outer side of the tyre. In the case of negative camber, the opposite is the case, with the inside of the tyre becoming worn more quickly.

Negative camber. When a vertical load is applied to a cambered tyre, will attempt to move downward. However, because it is blocked by the road surface, the tread will deform as shown in the illustration below. As the same time, the tyre’s elasticity resists this deformation and, therefore, acts against the road surface in direction (A), As a result of the reaction in direction (A), the tyre rolls indirection (B). The force acting in direction (B) is called the “camber thrust”.  The camber thrust increases with increase in the tyre inclination relative to the road surface(camber relative to the road) as well as with increases in the load. Therefore, in general vehicles less gross vehicles weight is given negative camber.

When an automobile turns a corner, the camber thrust on the outside tyres acts to reduce the cornering force due to the increase in positive camber. Centrifugal force tilts the turning vehicle due to the action of the suspension springs, changing the camber. Some vehicle models take advantage of this effect and add a slightly negative camber for straight-ahead driving so that positive camber will be reduced during turning, reducing the camber thrust and yielding sufficient cornering force for the turn.

References

1. Automobile Engineering (Vol. 1 & 2)            -           K.M.Guptha
2. Automotive Mechanics                                   -           Joseph Heitner
3. Automobile Engineering                                 -           Harbans Singh Reyd
4. Automotive Mechanics                                   -           William H. Course