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Wheel Alignment

    What is a wheel alignment and how does it effect your vehicle?

I was driving to work one morning and I noticed how many potholes have cropped up in the last few days. Temperature changes and snowplows have a tendency to breed a whole new generation of potholes. New York City has to be the world leader in pothole production. I have heard stories of cars being lost in some of those potholes, never to be seen again. We swerve around them; slow down to go over them and pray that nothing broke after hitting that last one.

Even though you didn't break anything, there can still be damage. Hitting potholes, rubbing curbs and bumping concrete parking stops knock things in your steering and suspension around and, eventually, things are not straight anymore. You notice your car is pulling one way or another or it seems to fight you when you turn or try to maintain a straight course.

Keeping your wheels aligned will prevent tire wear, increase fuel mileage by reducing road friction and improve the vehicles handling. Even without abuse, front wheel alignment will change under normal, everyday driving conditions. The change may be so gradual that it is not noticed at first. The first sign of something wrong usually shows up on the front tires, which develop peculiar wear patterns that will severely shorten the life of the tire. When these appear, the vehicle should have its alignment checked.

Chances are you need a wheel alignment. You know your car needs an alignment every once in a while and you might even get one whenever you buy new tires. But how many people understand what is done in an alignment? What exactly is adjusted and why? I'm going to try and explain what is changed, what it is and why it's important.

Let's start by understand the three main points in a wheel alignment. They are Camber, Caster and Toe. These are the most important of the angles in the geometry of the front end.

  • Camber:
    Camber is the tilting of the wheels from the vertical when viewed from the front of the vehicle. When the wheels tilt outward at the top, the camber is positive (+). When the wheel tilts inward at the top, the camber is negative (-). The amount of tilt is measured in degrees from the vertical. Camber settings influence the directional control and the tire wear.

    Too much positive camber will result in premature wear on the outside of the tire and cause excessive wear on the suspension parts.

    Too much negative camber will result in premature wear on the inside of the tire and cause excessive wear on the suspension parts.

    Unequal side-to-side camber of 1° or more will cause the vehicle to pull or lead to the side with the most positive camber.

  • Caster:
    Caster is the tilting of the uppermost point of the steering axis either forward or backward (when viewed from the side of the vehicle). A backward tilt is positive (+) and a forward tilt is negative (-). Caster influences directional control of the steering but does not affect the tire wear and is not adjustable on this vehicle. Caster is affected by the vehicle height, therefore it is important to keep the body at its designed height. Overloading the vehicle or a weak or sagging rear spring will affect caster.

    When the rear of the vehicle is lower than its designated trim height, the front suspension moves to a more positive caster. If the rear of the vehicle is higher than its designated trim height, the front suspension moves to a less positive caster. With too little positive caster, steering may be touchy at high speed and wheel returnability may be diminished when coming out of a turn. If one wheel has more positive caster than the other, that wheel will pull toward the center of the vehicle. This condition will cause the vehicle to pull or lead to the side with the least amount of positive caster.

  • Toe:
    Toe is a measurement of how much the front and/or rear wheels are turned in or out from a straight-ahead position. When the wheels are turned in, toe is positive (+). When the wheels are turned out, toe is negative (-). The actual amount of toe is normally only a fraction of a degree. The purpose of toe is to ensure that the wheels roll parallel. Toe also serves to offset the small deflections of the wheel support system that occur when the vehicle is rolling forward. In other words, with the vehicle standing still and the wheels set with toe-in, the wheels tend to roll parallel on the road when the vehicle is moving. Improper toe adjustment will cause premature tire wear and cause steering instability.

Now these are the three settings that are always checked when doing a wheel alignment, there are other measurements that are just as important. 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.

Now should you get a two-wheel or four-wheel alignment? At any rate the technician will put a sensor on each wheel. In a two-wheel alignment rear toe and thrust angle is checked. Then all the adjustments are made on the front wheels. This happens on cars that don't have rear wheel adjustments, but can determine if there is something wrong with the back wheels.

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.

For most standard American cars a wheel alignment will cost about $55.00. There is usually an extra charge for vans, 4x4's or 4-wheel steer cars and it takes about an hour to do.

Additional Information provided courtesy of
ALLDATAdiy.com and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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