The Ackerman principle
In 1818, long before the advent of the automobile, German inventor Rudolf Ackermann patented a device based on the principle of geometrically correct steering. He stated that when a vehicle travels in a curved path, its wheels should describe circles around the same centre. A wheel that follows a markedly different path will slide to some extent, resulting in a ‘scrub’, which causes excessive tyre wear.
The application of Ackermann’s principle makes it possible to arrange for imaginary lines through the axes of all the wheels, front and back, to pass through, or very near, the same point — the centre of the curve on which the car is travelling. This is achieved by turning the inner front wheel through a greater angle than the outer front wheel.
However, modern car designs no longer need to follow the Ackermann principle strictly, because of improvements in suspension and tyres. When a car takes a bend at speed, the deflection of the tyres on the road surface creates a sideways force that assists steering.
Toe-in and Toe-out
Although in theory the front wheels should be parallel when pointing straight ahead, the best practical results are usually obtained by setting them slightly out of parallel. This gives the steadiest steering and least tyre wear.
On most cars, when the steering is centralised, the front wheels point inwards by a fraction of an inch at the front. This is known as toe-in. It can be regarded as a compensation for the fact that no steering and suspension can be perfect and no steering linkage is free from a certain amount of ‘give.’ Some cars, usually those with front-wheel drive, have the wheels pointing slightly outwards. This is known as toe-out.
A means of adjusting the amount of toe-in or toe-out is always provided. Wheel alignment is the term used to describe the amount of toe-in or toe-out. Incorrect alignment of the rear wheels, due to wear or accident damage, can also adversely affect steering in cars equipped with independent rear suspension.
The steering ratio is the ratio between the rotation of the steering wheel and the angle of movement imparted to the steering arms — those parts of the steering linkage connected directly to the stub axles and which move the wheels. If, for instance, a full turn (360 degrees) of the steering wheel moves the steering arms through 30 degrees, the steering ratio is 12:1(360:30). Most popular saloon cars have a steering ratio of about 15:1. To move the front wheels from lock to lock (about 60 degrees) takes two-and-a-half turns of the steering wheel. But a heavy car may need four or five turns — a steering ratio of at least 24:1.
All cars are fitted with positive lock stops, to limit the steering movement of the wheels and thereby ensure that the tyres do not rub against any part of the car. The stops may be at the wheel pivots or on the steering gearbox. The minimum turning circle of a car is either the diameter of the circle traced by the extreme outer corner of the car or, more usually, the diameter of the circle traced by the outer front wheel.