Car Parts and Accessories

Ackerman principle of correct Steering

Long before the advent of the car, the German inventor Rudolf Ackerman patented a device based on the principle of geometrically correct steering. He stated the principle that when a vehicle travels in a curved path, its wheels should describe circles round the same centre. A wheel which follows a markedly different path will slide to some extent; and this will cause tyre wear.

The application of Ackerman’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 designers no longer need to follow the Ackerman principle strictly, because of improvements in suspension and tyres. When a car rounds a bend at speed the deflection of the tyres on the road surface creates a sideways force which assists steering. Designers consider it more important to obtain the smallest turning circle.

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 centralized, the front wheels point inwards by a fraction 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’. Cars front-wheel drive usually have the wheels pointing slightly outwards. This is known as toe-out. The amount of toe-in or toe- out can be adjusted.

The term ‘wheel alignment’ describes the overall steering geometry, including toe-in / out as well as castor and camber angles. The amount of toe-in or toe-out rarely exceeds 5 mm. Incorrect rear-wheel alignment can also affect steering in cars with independent suspension.

Vehicle Steering Ratio

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 lightweight saloon cars have a steering ratio of about 15:1. To move the front wheels from lock to lock (about 60 degrees) might take 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 so 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 box.

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.

Camber Angle

The front wheels of most cars, when seen from the front, lean slightly—either inwards or outwards. The 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 which are closer together at the top than at the bottom have a ‘negative’ camber.

The object is that, 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 designer because he cannot relieve stress on the steering linkage by placing the pivot directly over the wheel, as on a bicycle. The best the designer can do is to used a ‘dished’ (saucer-shaped) wheel and slope either the wheel or the steering pivot. or both.

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.

If the camber angle is too great the tyre’s outer edge 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.

Wheels with a permanent negative camber are used on some continental cars and on some sports-car conversions.

Castor Angle

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 makes 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 the 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 mad 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.

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