The front wheels of most cars, when viewed from the front, lean slightly, either inwards or outwards. This tilt of the wheel is called the camber, and the amount that it tilts is called the camber angle. The camber is usually ‘positive’, with the wheels leaning slightly outwards, so that they are further apart at the top than at the bottom. Wheels that are closer together at the top than at the bottom have a ‘negative’ camber. When viewed from the front, the centre of the tyre’s area of contact with the road is brought close to the point where an imaginary extension of the steering pivot’s axis cuts the ground. This is called centre-point steering.
Camber is a condition forced on the car, as engineering does not allow for the relief of stress on the steering linkage by placing the pivot directly over the wheel, as on a bicycle. It has been found that a small amount of offset, which reduces steering effort when parking and eliminates feedback judder from the road wheels to the steering wheel at high speeds, is a desirable feature and is incorporated on practically all modern cars. The effect of this offset is that each wheel tries to turn outwards. But, provided each wheel has the same amount of offset, this tendency will be cancelled out by reaction through the steering linkage connecting the two wheels. This keeps the joints under positive load in one direction all the time, and tends to reduce any rattle. Any error in camber angle or in the angle of the steering pivot will show up either in adverse handling of the car or in tyre wear, or both. If the camber angle is too great, the outer edge of the tyre will wear excessively.
Cars with front-wheel drive often have less offset. With many independent suspension systems, the camber changes from positive to negative as a wheel rises or falls on hitting a bump or pothole.
A car should have an inbuilt tendency to travel straight and to return to the straight-ahead position after a turn. This tendency, which makes a car stable in motion and the steering wheel spin back after a corner, is controlled by many factors, including the suspension and resilience of tyres. One of the most important direction-controlling factors is castor angle. The effect of this is most simply seen on the castor used on furniture. On a tea trolley, for example, the castor wheels, trailing behind their pivots, swing round to follow the direction in which the trolley is pushed, so that it travels in a straight line unless it is deliberately steered.
In a car, the castor angle has exactly the same effect of making a wheel trail behind its steering pivot. The central point of contact of the wheel on the road is behind that of an imaginary line extending the steering-pivot axis to the road. The castor angle is the angle between the line extending the steering-pivot axis and a vertical line through the centre of the wheel. As with camber angle, it needs checking after accident damage. An excessive amount of castor angle or trail, coupled with very freely moving joints in the steering linkage, could lead to violent front-wheel wobble.