Q: Thrust angle alignment

A thrust angle alignment, which I will explain momentarily, is a mundane, routine alignment procedure which is in fact performed every time a vehicle has a 4 wheel alignment. A quick digression though: I am surprised to hear that it is a "line item" in a Carfax report, or any report frankly, because typically the "thrust angle" is, again, just simply part and parcel, or "built-in", to a routine 4 wheel alignment. Consequently, my concern would be if perhaps something "special" was done, and thence recorded on service papers, due to damage or accident damage. So, the alignment "technique" itself is NOT unusual but you just want to be sure there is no undisclosed damage. YourMechanic does offer a pre-purchase vehicle inspection in the event you want a really comprehensive, in depth examination of he vehicle.

The track of a vehicle, that is the direction it will tend to drive down the road, will be along an imaginary line drawn longitudinally, on or near the vehicle centerline, perpendicular to the rear axle. If that imaginary line coincides exactly with the vehicle centerline (on the frame), you have a "zero thrust angle" and that would be preferred and is indeed "perfect". On the other had, if the rear axle is not perfectly perpendicular to the centerline of the frame of the car, that is if it is slightly offset, at a small angle, the vehicle body will not track straight down the road. Adjusting the thrust angle corrects for that and adjusting the thrust angle is often necessary because in vehicle assembly there can be minor imperfections in the position of the suspension. If the position of the rear axle is fixed (not adjustable) relative to the frame, what is done during a thrust angle alignment is the front wheels are adjusted such that the front wheels are running completely parallel to the rear wheels. Once so adjusted the car will travel in a straight line. If the vehicle has an adjustable rear suspension, usually the rear wheels can be adjusted such that those wheels are parallel to an imaginary longitudinal centerline of the car and then the front wheels are simply adjusted to be parallel to the rear wheels. A thrust angle of zero is of course preferred because that means that the rear axle is perfectly perpendicular to the centerline of the car. In practice, the axle can be offset a little bit, with acceptable results, and that is why the front wheels will just simply be set parallel to the rear wheels no what the final thrust angle is. The bottom line is, if the car is perfectly aligned, you should be able to drive it straight on a flat road and the steering wheel should be perfectly centered. The vehicle should not pull to either side AND you should be able to let go of the wheel and the car should travel straight ahead on its own for a reasonable distance, which would typically be at least a tenth of a mile. The point is the car should generally track straight ahead, all by itself with only very, very minor steering input. If it doesn’t track in this fashion, then it’s not aligned well and/or it might have other deficiencies, perhaps as simple as uneven tire pressure but extending on up to worn suspension parts. Please let us know if we can assist you further as you consider this purchase.