On a vehicle with Independent Rear Suspension or IRS, undertaking a front-wheel-only alignment is considered to be an inadequate procedure.
The thrust angle refers to all four wheels and their relationship to each other in addition to their relationship to an imaginary centerline that runs from each pair of wheels down the center of the vehicle. The term "thrust line" refers to the direction in which the rear wheels are pointing. The thrust angle can be adjusted on vehicles with adjustable rear suspensions.
On vehicles that do not have adjustable rear suspensions, the thrust angle can be compensated for by aligning the front wheels to the rear wheels.
Referencing the front steering geometry to the rear is very important. A Quadralignment procedure involves aligning all four wheels in relation to each other.
The thrust line may be in the same position as the vehicle's geometric “centerline”, however there are variations to this. Ideally the thrust line and the vehicle’s geometric centerline should line up closely.
The “centerline” is drawn through point’s midway between each pair of wheels; however the thrust line is normally in the perpendicular position of the rear axle on solid axle cars, or, with IRS, is a line derived by splitting the toe angle of the rear wheels on the vehicle.
For instance, if the rear wheel on the right rear wheel side of the vehicle is toed-in six degrees, and the rear wheel on the left is at zero degrees, the thrust line will veer off three degrees to the left of the vehicle centerline at the rear wheels when the vehicle is moving forward.
An ideal situation is where the thrust and centerlines coincide. However, given the size of a vehicle, the tolerances during manufacture, operational stresses, and component wear, it's rare that they do.
If the deviation is very small, then remedial action is normally unnecessary. However, a large deviation can cause considerable concern when the vehicle is being driven. And the cause of this condition needs to be identified and corrected.
Under such conditions the rear wheels are steering the car away from its centerline and the driver has to turn the steering wheel to one side to keep the car going in a straight line.
In extreme circumstances, the tracks the rear tires make are beside those of the front. This condition is known as "dogtracking" or “crabbing” and can cause diagonal tire pattern wear as well as vehicle instability in some driving conditions.
Summary
The thrust angle refers to all four wheels and their relationship to each other and to an imaginary centerline that runs from each pair of wheels down the center of the vehicle. The term "thrust line" refers to the direction in which the rear wheels are pointing.
Source: CDX Global & Wikipedia - en.wikipedia.org
