Get it straight

Whether you already perform wheel alignments or are just getting ready to get into the game, a solid understanding of four-wheel alignment principles serves as a solid foundation.


Whether you already perform wheel alignments or are just getting ready to get into the game, a solid understanding of four-wheel alignment principles serves as a solid foundation. Before we begin, it's important to note that performing good alignments requires an understanding of the marriage between vehicle technology and alignment equipment.

Major thrust

The fundamental principle that supports the concept of four-wheel alignment correlates to a characteristic where the rear wheels steer the car, but the front wheels turn it. Since the rear axle sets the direction of vehicle travel, this direction is referred to as the thrust line. It's really a 90-degree angle towards the front of the car, referenced off a straight line through the rear wheels.

The next line for discussion is the geometric centerline. It is an imaginary line extending through the center of the car, all the way from the back to the front. If a car's alignment was absolutely perfect, the thrust line would overlap the geometric centerline, making the two lines look like one. However, since no car is perfect, some allowance for deviation from the geometric centerline is allowed. This difference, the angle formed between the geometric centerline and the thrust line, is called the thrust angle. Nowadays, carmakers specify the maximum amount of thrust angle for their vehicles.

It's interesting to note that the thrust line and the geometric centerline have always been there, going back to Conestoga wagons. Until fairly recently, we just ignored them. Since older cars typically used a full-frame design with a solid rear axle, the rear half was usually assumed to be OK during an alignment.

A new era of autos

When introduced way back in 1978, the Dodge Omni/Plymouth Horizon made a bold statement about the kind of vehicle and suspension technology that would become common in the '80s.

Control arm suspension gave way to MacPherson struts, frames gave way to unibodies, and solid rear axles gave way to rear independent suspension. This drove the last nail into the coffin for two-wheel alignment and set the course for the years to come.

Today, more than 90 percent of the vehicles on the road, foreign and domestic, have rear wheel alignment specifications.

2+2 ? 4

If you're not convinced about the need for two more alignment heads, try using a two-wheel machine first at one end, then at the other. You'll quickly find that the results you get are a waste of time. A single pair of heads simply doesn't give you the point of reference you need, which is provided by the second pair of heads.

Selecting a system

Following are some of the main topics of consideration to hash over before you embark on your aligner shopping spree. Don't forget to ask for input from your colleagues and technicians that already use a certain type of aligner. That's the kind of information you won't find in any sales brochure.

• Shop space.
Naturally, you have or will have a place to put the aligner. Find out how much space a typical machine takes and don't forget the rack that you'll probably need. For shops with limited space, there are aligners that can fit into extremely tight quarters.

• Initial cost.
Add this price to the price of start-up accessories (alignment racks, steering wheel holder, alignment wrenches, etc.) so you know what your initial outlay will be.

• Maintenance.
Ask your equipment rep about any required maintenance and what its annual cost is. Keep this in mind for calculating payback.

• Paybacks.
Without getting into any formulas, payback on an alignment machine will have a lot to do with how often it's used. A machine used 12 times a week will earn its keep a lot faster than a machine used three times a week.

Keep in mind that alignment service also brings a lucrative second profit center, the sales of parts and labor on things like shocks, struts, springs and other front-end parts.

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