The correct geometrical configuration of a automobile frame provided with wheels is achieved by measuring and correcting the position of each wheel relative to the others, and relative to the longitudinal and transverse axes of the frame. The whole of the angles defining the wheel positions is called the attitude of the vehicle.
Each wheel position is defined by a certain number of characteristic angles well known to the expert of the art, which are measured in known manner by goniometers or angle transducers fixed to a wheel rim and interacting with analogous instruments fixed to the rims of the two closest wheels in the transverse and longitudinal directions.
Interaction between said goniometers or angle transducers can be achieved either mechanically by wires or springs, or by electrical or optical means.
Among the most advanced of the known goniometers or angle transducers are illustrated in U.S. Pat. Nos. 4,341,021 in the name of BEISSBARTH and 4,302,104 in the name of HUNTER.
The instruments described and illustrated in said patents are intended to be applied in a total number of at least six to the wheel rims of an automobile frame and to mutually cooperate to identify the convergence angles of the wheels of an automobile frame and the angles between those wheels on one and the same side, deriving from the wheel track difference.
By subjecting said angles to known calculation systems, the characteristic attitude angles of the wheels can be obtained, namely:
right, left and total front convergence PA1 right, left and total rear convergence PA1 front wheel misalignment PA1 front and rear wheel line of thrust PA1 convergence angles to the thrust line and/or frame geometrical axis.
For better understanding, the aforesaid quantities are defined as follows.
The geometrical axis of the automobile frame is the axis passing through the centre points of the frame front and rear axles.
The frame thrust axis is the axis defined by the bisector of the angle formed by extending the rear wheel equatorial planes.
The front wheel misalignment is the angle between the straight line joining the centres of the front wheels and a straight line perpendicular to the geometrical axis.
The front and rear total convergence is the angle formed between the extension of the equatorial planes of the respective front or rear wheels.
However notwithstanding the sophisticated calculation systems available for processing the effected measurements, the known art is deficient in the sense of not allowing complete and immediate data acquisition.
In this respect, using known instruments and measurement-systems it is not possible to measure the automobile frame wheel tracks or mutually integrate the characteristic attitude angles.
Moreover it is not possible to obtain for all wheel positions the data relative to geometrical position defects between the wheel rotational axis and the wheel geometrical centre or any coaxiality error between the actual wheel rotational axis and its geometrical axis, as required for subsequent calculations.
It is also difficult to calculate the variation in the attitude angles as a function of the steering angle without using complicated additional equipment such as goniometric platforms on which to rest the wheels of the automobile frame.
Nor is it possible to measure the transverse or longitudinal wheel offset.
The aforesaid limitations result in practice not only in the impossibility of knowing the corrections to be made to the measured attitude angles, but also in the impossibility of checking correct steering behaviour at the various steering angles.