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Geometry Theory

by Tony Bones

“Vehicle attitude” is taken to mean the geometric condition of all the parts which contribute to the determination of the position of the wheels when moving, either in a straight line or on bends.

This geometric attitude can be checked only in a static state, when the vehicle is stationary, with the wheels in the straight position and in the cornering position.

When the vehicle is moving, with all the different possible load conditions, multiple forces come into play , created by resistance to travel, weight, the acceleration or deceleration thrust generated by the engine, centrifugal force, brakes, etc, which tend to modify the geometric attitude.

When adjusting the geometric attitude, in accordance with the data supplied by the manufacturer, these variations are taken into account, the correct balance of forces and of the points of application of these forces when the vehicle is moving can be achieved, this means that a real “balancing of moving forces” can be performed, just as if the vehicle was actually in motion.

Vehicle geometry basic conditions

As well as the characteristic wheel angles, which will be dealt with later on, the vehicle must satisfy certain set conditions of symmetry and perpendicular in the axes.

The wheelbases and tracks do not necessarily always have to equal, in fact there are a large number of cases in which the tracks are different and, less commonly, where the wheelbases are not equal.

Typical cases of different wheelbases can be found in some vehicles with front-wheel drive and rear suspension cross torque struts fitted, Renault/ Peugeot trait.  A very important condition to take into account is distribution of the load acting on the vehicle and the consequent longitudinal and transverse levelling affect in relation to the ground.

The vehicle can be used empty (with only the driver on board) or with passengers, and with or without a full fuel tank or luggage, the influence of these very variable load conditions on the compression of the elastic parts of the suspension, plus the effect of various forces acting when the vehicle is moving, lead to variable geometric attitudes in the wheels, which are not perfect in all cases.

From this it can be understood that a thorough knowledge of the geometry of the wheels, and of the effects generated by the conditions of use of the vehicle is needed to make an intelligent interpretation of the data supplied by the manufacturer and the applied tolerances.

Current practice is to measure the geometric attitude of the vehicle in an unloaded state, unless otherwise specified.  This is because the measurement can be done quickly and because this is the state that most approaches the normal use condition of the vehicle, which covers most of its mileage with only the weight of the driver on board.  An exception to this arises when checking whether the variations produced in a fully loaded vehicle caused by the compression of the elastic parts of the suspension alter the elasticity characteristics and the geometric positioning.

Definitions

The word suspension is used to describe the totality of the elastic parts that connect the wheels to the chassis. Suspension systems have the following functions: to absorb the bumps from uneven road surfaces; to ensure a certain degree of smoothness in the ride for passengers, or for objects being transported; to avoid subjecting the mechanical parts to excessive wear; to ensure that the tyres are in constant contact with the ground to achieve good road holding and for the correct functioning of the steering for a safer ride.

In order to meet with the various vehicle construction demands, there are a number of different types of suspension systems in existence: Mac Person; deformable transverse four bar linkage; longitudinal four bar linkage; cross member type.

Technical term for the angles

Characteristic angles

The angles related to the vehicle front axis or directional axis are as follow:

1- wheel camber angle

2- wheel toe

3- wheel toe-out on turns

Kingpin angles

4- transverse kingpin inclination or, in brief form “kingpin inclination”

5- longitudinal castor angle or, in brief “castor angle”

The angles related to the rear axis are

6- camber angle

7- wheel toe

 

Tony Bones and Jason Saunders of Wheels in Motion (http://www.wheels-inmotion.co.uk/index.php) have developed somewhat of a cult following as alignment experts in England. Although their articles are amazingly complicated and there may be a little bit of a language barrier, we asked to reprint these gems for your benefit. Tony Bones did ask us to please advise readers that this information is a global overview of alignment principles and actual practical application to specific vehicles should be left to the professional alignment technician or shop.

 

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