Why 4 wheel alignment is necessary
Many companies offer 'tracking checks', which to the unsuspecting or uninformed motorist may seem to be good value. In many cases however, attention is only paid to the front wheels. Additionally, very few establishments have the correct equipment required for fine measurement. Because of the high investment required for 'state of the art' alignment systems, most garages use very basic equipment which is wholly unsuitable at best, and inaccurate at worst. Only true 4 wheel alignment checks the important dimensions and accurately pinpoints the adjustments which should be made, depending upon specific vehicle design.
Thrust Axis...
The four wheel thrust line of a vehicle (or thrust axis as it is also known) is the direction/position that the vehicle is in when being driven on the road. The rear wheels directly follow the front wheels. Ideally the thrust line of the vehicle should be exactly the same as the centre line of the vehicle. The centre line is the imaginary line between the centre of the front axle and the centre of the rear axle. When measured the thrust line should equal zero degrees.
Thrust line deviation occurs when the thrust line and the centre line of the vehicle are at different angles (see Fig. I.) A positive deviation angle will cause the rear wheels to push the rear of the vehicle to the near-side and a negative angle the rear of the vehicle to the off-side. In very bad circumstances the deviation manifests itself as the vehicle appearing to be twisted sideways, 'crabbing', when driving on the road.
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The cause of the thrust line deviation is usually accident damage or rear wheel/suspension misalignment, causing an incorrect rear wheel toe angle. It is important to remember that it is the rear wheels that determine the angle at which a vehicle drives on the road.
Thrust line deviation can cause tyre wear and handling difficulties. The Caster and SAl angles (Steering Axis Inclination ) on the front wheels are designed to make the front wheels self-centre about the true centre line of the vehicle. If there is a thrust line deviation the driver will feel the vehicle trying to pull to one side while attempting to keep it running in a straight ahead direction.
When cornering to the right with a positive deviation, handling will become unbalanced as the front wheels are turning to the right and the rear of the vehicle is being pushed to the left (near-side), creating an oversteer effect. This will cause the driver to correct the steering lock while going around the corner in order to keep on the correct side of the road.
To cure any deviation you need to be able to adjust the rear suspension settings (especially the toe and camber angles) in the same way as you can the front. Before any adjustments can be made it is therefore necessary to check and note the existing settings of all of the road wheels not just the front, and it is for this reason that it is necessary to check the alignment using a four wheel alignment system.
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