Wheel Alignment

Written by Don Stewart | 01 May 2006

Passenger car wheel alignment is the topic for May. What is it? Why do our cars need it? What are some of the symptoms of poor alignment? What are some of the terms used when talking about alignment? I’ll touch on those topics in no particular order.

If you boil it down to basic terms, wheel alignment consists of adjusting the angles of the wheels so that under dynamic loading, they are perpendicular to the ground and parallel to one another. Don’t confuse alignment with wheel balancing. Out of balance wheels cause vibration (see the Pony Tricks article for September 2002). Poor alignment can cause tire wear plus steering problems and tracking problems. Camber, caster, and toe are the three settings normally associated with alignment. There are others but we’ll look at those three first. Each car has specific settings that are optimum. Alignment shops refer to those specifications when they work on your car.

Camber

Camber is an angle measured in degrees from vertical. Looking at the front tires from the front of the car, camber refers to how much the top of the tires tilt in toward the center of the car (negative camber) or how much they tilt away from the center (positive camber). Correct camber angle keeps the tires in maximum contact with the road. In other words it maximizes the tire patch. Correct camber may be either positive or negative depending on the design geometry of the front suspension.

If camber is out of adjustment, tires will wear rapidly. If camber adjustments are different for each side of the car, the steering may pull one way or the other. Our Mustangs can be adjusted to bring the camber to the correct angle.

Caster

Caster is also an angle measured from vertical. Correct caster enhances the car’s ability to track straight down the road.

The caster angle applies when viewing the front wheels from the side of the car. You can’t actually see caster because you can’t see the axis called the steering pivot; the steering pivot passes through the upper and lower ball joints. Positive caster has the upper ball joint slightly behind the lower ball joint; negative caster places the upper ball joint ahead of the lower ball joint. The correct caster angle is always a positive value greater than zero.

Alignment machines can measure caster and our Mustangs are adjustable so that caster can be set to specification. As with camber, different caster settings from one side to the other can cause the steering to pull. Too much, but equal, positive caster can cause the steering to be heavy and the wheel may kick when the tire(s) hit(s) a bump. Negative caster will make the steering too light and the car will wander. Caster has little, if any, affect on tire wear

Toe

Toe is a measure of the difference in distance between the leading edges of the front tires and the trailing edges. The measurements are taken on the horizontal plane that passes through the spindles. Unlike camber and caster which are expressed in degrees, toe is usually expressed in fractions of an inch, although degrees from straight ahead may be used. When the leading edges are closer together than the trailing edges, toe is positive and it’s called toe-in. Conversely, when the trailing edges are closer together, toe is negative and it’s called toe-out. Incorrect toe can cause rapid tire wear because the tires are being dragged sideways. Correct toe is always a positive value. Typically, toe will be set positive by 1/16” to 1/8^”. The idea is to have toe approximately equal to zero under dynamic loading when the car is traveling down the road. At highway speeds, aerodynamic forces will cause positive toe (toe-in) to open up slightly so that the tires run parallel to each other (zero toe). Radial tires require less toe-in on the same vehicle than bias-ply tires do. The reason is that radial tires are influenced mostly by aerodynamic forces and very little by drag. Bias-ply tires, on the other hand, react to a drag component between the rubber and the road as well as to the aerodynamic forces.

With camber, caster, and toe being the three adjustments that most people think about when they think about alignment, this would seem to be the end of the story. But there other important considerations such as riding height, thrust angle, steering center, and set back.

Riding Height

Riding height is the distance from the bottom of the rocker panels to the ground. Measured on a level surface the height should be within one inch side to side and also one inch front to back. If the difference is more than one inch, springs should be replaced in pairs to bring everything into spec. Anybody got a pair of rear springs for a first generation Mustang?

Thrust Angle

Thrust angle is the direction that he rear wheels are pointing with respect to the centerline of the car. If they’re not pointing exactly parallel to the centerline, they’ll thrust the car toward one side or the other causing it to “dog track.” Thrust angle can be adjusted in some cars but in others it cannot. If it’s adjustable, it should be set to zero before proceeding with front wheel alignment. If it’s not adjustable, the front toe must be set to compensate

Steering Center

Any good alignment technician will make toe adjustments so that the steering wheel is straight when the car is going straight down the road. It’s done by first setting the thrust angle to zero and then clamping the steering wheel in the straight ahead position while setting the toe. If the thrust angle cannot be set to zero, then it’s up to the technician to make the proper compensation. Failure to set the steering wheel straight means the technician did a poor job.

Set back

Set back is when one front wheel is further from the front of the car than the other wheel. Anything more than 1/4” is abnormal and probably means something is bent. Up to a point, a technician can compensate. But there is a limit where the source of the problem will need to be determined and corrected. Failure to compensate or correct will cause an off center steering wheel.

There are three other points to be made. First any worn parts in the suspension will compromise alignment and they will interfere with the ability of an alignment technician to make proper corrections. Worn parts need to be replaced for safety’s sake and to provide a proper starting point for correcting alignment issues. Secondly, unequal tire pressures can cause some of the same symptoms that poor alignment causes. Keep tires properly inflated to eliminate that variable as well as for safety reasons. Finally, don’t waste time and money on an alignment if your tires have uneven wear patterns. Replace the tires and then get the alignment.