What is thrust angle, and what causes changes in thrust angle?

You’ve probably heard of vehicle alignment factors like wheel camber, maybe even toe and caster, but thrust angle? That’s diving pretty deep into the mysterious world of vehicle alignment. Yeah, vehicle alignment technicians are basically wizards.

But let’s shed some light on the basic concept of thrust angle. What it is, and why your vehicle might be out of alignment due to thrust angle. Here’s the scoop:

What is thrust angle?

The concept of “thrust” is usually one that is associated with jet engines, rockets, and the like, not necessarily automobiles. But you can think of the vehicle acceleration and forward motion as a much less dramatic example of thrust.

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Thrust angle is a measurement involving the front and rear axles, and the wheelbase (the four wheels and tires) of the vehicle.

In a standard automobile, the front and rear axles of the vehicle are parallel, and a perpendicular line drawn straight forward from the center of the rear axle should intersect in that same center position on the front axle of the vehicle. The very practical result of this front to rear axle arrangement is that when the accelerator is depressed with the front wheels pointed straight (“thrust” applied), the vehicle will accelerate straight ahead.

To emphasize the importance of thrust angle, consider an extreme example:

Let’s say instead of the rear axle being parallel with the front axle, it was “tilted” at a 45° angle. In this case, the trajectory of the vehicle would be dramatically altered even though the steering wheel and front wheels are pointed dead straight ahead.

“The thrust line should run parallel to the vehicle’s geometric centerline.”

 – ASE (Automotive Service Excellence)

A visual explanation of thrust
FreeASEStudyGuides.com
A visual explanation of thrust

Thrust angle is therefore a degree measurement of the angle off of this imaginary centerline between the front and rear axles of the vehicle. In certain (out of) alignment circumstances, the line drawn straight forward from the center of rear axle might intersect with the front axle to the left or right of center. In this case, the trajectory of the vehicle is altered even though the front wheels are pointed straight ahead.

Thrust angle is also a reference to the wheelbase of the vehicle, and confirmation that the wheels on both sides of the vehicle are angled within specification (straight ahead with little variation). Even in a circumstance where the axles are parallel, if the wheels are angled with extreme toe in or out, the end driving result is the same as a crooked axle.

How does a vehicle handle with incorrect thrust angle?

If your vehicle is experiencing an out of specification thrust angle, drivability will be altered. The driver is likely to sense handling abnormalities like a pull in one direction, and/or different vehicle behavior when turning left and right.

Also, a crooked steering wheel is a telltale sign of a thrust angle problem. When traveling straight ahead, the steering wheel is angled to compensate/counteract the thrust angle. In a negative thrust angle situation (angle to the left), the car will naturally veer right. A positive thrust angle (angle to the right) creates a veer to the left.

What causes changes in thrust angle?

In the case of a vehicle with a solid rear axle, a thrust angle is most likely to have been caused by an impact or collision. The rear axle and associated componentry were jarred by an impact that created the angle.

More commonly, with modern vehicles that have individual 4-wheel alignment adjustability, a thrust angle is caused by incorrect toe settings.

A visual explanation of thrust
NissanTechnicianInfo.mobi
A visual explanation of thrust

Toe settings can become altered over time and during routine vehicle use. Repeat road impacts from potholes or even a firm bump up against a curb can alter the toe settings of one or more wheels.

We recommend having your alignment checked once or twice per year, and alignment check frequency should be according to the conditions of the roads that you typically drive. See How often do you need an alignment? for more info.

Thrust angle diagnosis and repair is a job best left to the pros

The elimination of an improper thrust angle is no easy task. In the case of a solid rear axle and collision damage, sometimes significant repair will be necessary to bring the axle(s) back into proper alignment.

In the case of incorrect toe settings, alignment technicians rely upon advanced wheel alignment systems that serve significant data to help them both diagnose the thrust angle and bring the vehicle back into alignment.

Basic changes to alignment factors like camber and caster are DIY automotive projects for some enthusiasts, but the elimination of a thrust angle is likely to require both the advanced capabilities of an alignment system, as well as the learned capabilities of a skilled alignment technician.

Last but not least, with thrust angle or any number of other alignment variables that are out of vehicle specification, abnormal tire wear and reduced tire life is almost a certainty. Address your thrust angle or other alignment issues in short order to get your vehicle rolling straight and your tires in optimum condition.

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