The geometry of alignment

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The geometry of alignment

With the harsh winter we have been having here in the Northeast and spring right around the corner, I have been experiencing a bit of my own “spring fever” — and the burst of motivation that goes along with it.

While sitting on the front steps admiring the fine job I did of washing my wife’s car, I could not help but notice the front tires I had been neglecting (for what was not, I’m sure, as long as my wife seemed to claim) and decided to investigate that all-to-familiar look of front-end geometry gone awry.

I have been in the automotive industry more than 23-plus years, 14 of which I worked as a service manager and Master Technician for Sullivan Tire Co. Inc., which, in my opinion, excels in its approach and commitment to training.

Throughout my career, I have been advising my customers as to the benefits and necessity of having their alignment checked and, if necessary, re-aligned every six months for optimum tire wear and handling. So, why were the tires on my wife’s car showing such extreme shoulder wear? Could she be right? Could I have been neglecting her pride and joy, as she said?

I have since started teaching at a vocational technical high school and decided since we were already covering alignments, it would be a good time to get the car in and check it out — and get a shot at real or imaginary redemption.


Pre-alignment routine

I try to stress the basics when instructing my students, and a thorough road test is the starting point for most if not all automotive services. Proper verification of a customer complaint or any problems that may be occurring is crucial to the final repair. I mean, how can you verify you actually corrected any problems if you are not aware of them to begin with?

Next, after the road test, comes a thorough check of the front end and the tires for any wear that might indicate what type of problem to look for. Checking the front end for possible mechanical wear may seem like common sense; however, with the inception of the latest alignment machine technology, we no longer are forced to raise the wheels to compensate the camera heads on certain machines. That can and has led to overlooking this very important, basic step. Again, can you really align a vehicle properly if the steering or suspension components are worn?

After we performed the road test and the front end check, we replaced a worn inner tie rod that showed significant play. We also checked and set the tire pressure to manufacturer’s specifications, and checked for any ride height irregularities before proceeding. Satisfied with the basics, we were now ready to align the car.

Alignment geometry

We compensated our heads, performed our caster sweep and printed out our “before” alignment readings. Now we could determine what the actual geometry was, and in what direction the tires were pointing.

There are three major angles, or geometry, of an alignment.

1. Camber: the inward or outward tilt of the wheel when viewed from the front. If the top of the tire is tilted out from vertical, it is said to be positive. If it is tilted inward from vertical, it is said to be negative.

Excessive deviation in this angle from the manufacturer’s specification will cause wear to the inside or outside tread of the tire. Excessive deviation between left and right side also can cause the vehicle to pull in the direction of whichever side is most positive.

2. Caster: the forward or rearward tilt of the steering knuckle when viewed from the side. Caster is used for directional stability and, for the most part, is not a tire wearing angle. However, the least amount of caster still will cause a directional pull to the side.

Caster is also used to offset “road crown” (slope built into the road to help shed water).

The forward tilt of this angle is said to be positive (like a bicycle) and the rearward tilt is said to be negative.


3. Toe: the inward or outward angle of the wheels when viewed from the top. You can visualize toe by pointing your toes together. This is said to be positive toe. If angled out, toe is negative.

Excessive toe will cause rapid tire wear to either the inside or outside tread of the tire due to the tire being dragged.

4. Thrust line: the imaginary line drawn through the center of the rear axle projecting forward.

5. Geometric center line: the imaginary line running through the vehicle center from front to rear.

To thrust or not to thrust

Basically, there are two types of wheel alignments: a thrust alignment and a four-wheel alignment.

The thrust alignment is performed using the rear wheels as a reference. Since the reference is measured at 90 degrees to the rear axle, it should theoretically be in line with the geometric centerline. If this is the case, then the front wheels can be accurately adjusted.

So if the steering wheel is centered, the car will drive straight down the road with no improper tire wear.

However, if the rear axle is not aligned with the geometric centerline for whatever reason — bent parts, broken or worn suspension components — then none of the tires will actually be true to the geometric centerline, and all the tires will wear somewhat (see Figure 1 on page 50).

Due to the advent of front-wheel drive and independent rear suspension and adjustments, the four-wheel alignment allows us to make adjustments to the rear wheels and bring them in alignment with the geometric centerline.

Essentially, since we can adjust the rear wheels so that the thrust line and the geometric centerline are one and the same, all of our angles are set to the true center line of the vehicle running front to rear, giving us an optimum and accurate wheel alignment far superior to that from a thrust alignment.

After we made our adjustments to the rear, we proceeded to the front and set up our adjustments so that all wheels are true to the vehicle center line.

Always follow the manufacturer’s recommended procedure for alignment. In my wife’s case, the procedure for her 2005 Dodge Magnum was to jounce the suspension, as is true in most, if not all, alignments.

Next, we recorded the current readings and compared them to manufacturer’s specs.


Camber and caster are not adjustable on this particular vehicle (although I have found that in this business, most things can be adjusted if you really want to take the time to figure it out).

Since our rear camber and caster readings were within specification, we adjusted the rear toe, which was out of spec.

Satisfied with the rear, we proceeded to the front where, like our rear, camber and caster were within manufacturer’s specs while toe needed a little help to get the tires pointed in the right direction (remember, we replaced one inner tie rod during our pre-alignment inspection).

I should point out that with this particular vehicle platform, camber and caster settings were built into the vehicle by the position of the suspension components. This is sometimes referred to as “net build.”

With a net build, if an adjustment is needed, you will be instructed to first try and reposition the cradle slightly to try and bring the readings in. If this procedure does not work, you may have to install an adjustment kit consisting of an offset bolt package, which enables a technician to make an adjustment of close to 0.3 degrees if necessary.

Since we were fortunate that our camber and caster readings were within specification, we then simply set our front toe so that our shoes were pointed in the right direction.

Then we tightened everything down and printed our final readings.

Post-alignment road test

Satisfied with our resounding success, it was not long before we were on the road for our post-alignment road test, which confirmed a job well done!

I returned the car to my wife, and elaborated on all my skill, and the hard work that went into completing the task at hand. She said “thank you,” and handed me the rest of my “to do list.”

Spring, then summer will be here before you know it, and thanks to a bit of spring fever, I can neglect my “mechanic husband” duties for at least another six months.

Scott Kessell instructs automotive theory at the South Shore Vocational Regional Technical High School in Hanover, Mass.

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