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Ray,
The thrust angle is defined as the deviation between the bisector of the total rear toe angle of the vehicle (the thrust line) and the vehicle centerline.
This is a tad more complicated than is sounds.. the centerline of the vehicle is defined NOT by the axle midpoints, but rather by the position of each of the vehicle wheels. In an extreme example, imagine a car which is narrower in the front than in the rear. If you draw lines connecting the tires on the left side of the car (front & rear) and the tires on the right side of the car (front and rear), the lines will intersect at some point in space out infront of the car. Bisect the angle formed by the intersection of these lines, and you will have the centerline of the vehicle's wheel system. Usually this is very close to the chassis centerline (as measured by the axle midpoints), but it's not exactly the same.
Next, to determine the total rear toe angle, the toe angle for each rear wheel is determined relative to the wheel system centerline. The sum of these angles is the total rear toe angle. If you project lines parallel to the face of each rear wheel, the two lines will intersect somewhere at a point unless the wheels are exactly parallel to each other. The angle at which these lines intersect is the total rear toe angle. Bisecting this angle with a line yields the thrust line of the vehicle. This is the direction the two rear wheels of the vehicle would travel if allowed to just roll along.
Finally, the angle measured between the vehicle's wheel system centerline (as determined by the position of all four wheels) and the thrust line of the vehicle (determined by the pointing direction of the rear wheels) is measured as the thrust angle.
Hope this helps!
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