Click Any Link Below to Jump Straight to the Subject:
Wheel Alignment :: 4 Wheel Alignment :: Tire Balancing :: Tire Rotation :: CV Axle, Joint & Boot

What is Wheel Alignment?

• Unusual tire wear - tires are worn in a cupped, scalloped or diagonal stripe pattern at edges or across the tread, or wear is uneven with "feathered" edges on the treads.
• Unusual steering feel - the steering feels stiffer than it should, or the wheel does not return to the center position when released.
• The steering wheel is cocked to one side when the front wheels are pointing straight ahead.
• The vehicle tends to pull to one side while driving, or tends to wander or weave, or is subject to front end "shimmy."

The following are settings that should always be checked when doing a wheel alignment.  A good technician will check these measurements when he is trying to diagnose a front-end problem.  Very quickly they are:

Thrust Angle:  The angle between the thrust line and centerline. If the thrust line is to the right of the centerline, the angle is said to be positive. If the thrust line is to the left of center, the angle is negative. It is caused by rear wheel or axle misalignment and causes the steering to pull or lead to one side or the other. It is the primary cause of an off-center or crooked steering wheel. Correcting rear axle or toe alignment is necessary to eliminate the thrust angle. If that is not possible, using the thrust angle as a reference line for aligning front toe can restore center steering.

Included Angle:  The sum of the camber and SAI angles in a front suspension. This angle is measured indirectly and is used primarily to diagnose bent suspension parts such as spindles and struts.

Steering Axis Inclination (SAI):  The angle formed by a line that runs through the upper and lower steering pivots with respect to vertical. On a SLA suspension, the line runs through the upper and lower ball joints. On a MacPherson strut suspension, the line runs through the lower ball joint and upper strut mount or bearing plate. Viewed from the front, SAI is also the inward tilt of the steering axis. Like caster, it provides directional stability. But it also reduces steering effort by reducing the scrub radius. SAI is a built-in nonadjustable angle and is used with camber and the included angle to diagnose bent spindles, struts and mislocated crossmembers.

Kingpin Offset/Scrub Radius:  Is the distance from the center of the wheel contact face to the intersection point of the kingpin extension. The line through the center point of the spring strut support bearing and the control arm ball joint corresponds to the "kingpin". The scrub radius is influenced by camber, kingpin angle and wheel offset of the wheel rim. This is set at the factory and is not adjustable.

Set Back:  The amount by which one front wheel is further back from the front of the vehicle than the other. It is also the angle formed by a line perpendicular to the axle centerline with respect to the vehicle's centerline. If the left wheel is further back than the right, setback is negative. If the right wheel is further back than the left, setback is positive. Setback should usually be zero to less than half a degree, but some vehicles have asymmetrical suspensions by design. Setback is measured with both wheels straight ahead, and is used as a diagnostic angle along with caster to identify chassis misalignment or collision damage. The presence of setback can also cause differences in toe-out on turn angle readings side-to-side.

Ride Height:  The distance between a specified point on the chassis, suspension or body and the ground. Measuring ride height is an indirect method of determining spring height, which is important because it affects camber, caster and toe. Low ride height indicates weak or sagging springs. Ride height should be within specifications before the wheels are aligned.

back to top  >> return to Maintenance page

4 Wheel Alignment

In a four-wheel alignment the rear wheels are adjusted to specification before any front wheel adjustments are made. In relation to the added work, the four-wheel alignment does costs more.  All vehicles should be 4-wheel aligned.  Two-wheel alignments have become obsolete because they align only the front wheels to the vehicle's centerline.  A 2-wheel alignment assumes the rear wheels are already aligned with the geometric centerline.  Two-wheel alignments may save you a little money up front, but your vehicle's handling and tire tread life will be compromised.

Now, before any wheel alignment is done, the car should be road tested so the technician can see what the vehicle is doing. Then the front end should be thoroughly inspected for worn, bent or broken parts.  He should check that the tires and wheels match and that the tires pressures are set correctly.

Older cars could have the camber, caster and toe adjusted. On most modern cars, this is no longer true. On all cars toe is adjustable but front McPherson struts set caster and camber. The theory is that as long as these components are not bent or broken, the alignment will always be set properly. Notice I said "in theory." In the real world, other things besides the struts can be thrown slightly out of adjustment causing the wheel to be thrown out of line. In cases such as this, some aftermarket manufacturers make kits that can be installed to allow caster and camber adjustment.

How often should you have your wheels aligned? Some manufacturers recommend as little as 10,000 miles. Personally, I feel once a year is a good interval between alignments. A yearly alignment can add thousands of miles to your tires over the years. That's money in your pocket in tires not bought and extra fuel not burnt. Of course if you buy new tires, that's an excellent time to have your wheels aligned.

back to top  >> return to Maintenance page