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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 vehicle 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 the above 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.

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.

Axle Alignment:

The final part of wheel alignment is the alignment of wheels in respect to each other across the vehicle. For the majority of race vehicles there will be a fixed axle either at the front or rear of the car, sometimes both. The exception to this will be vehicles that have independent suspension all round, which allows the alteration of the fore and aft position of the wheel without any other wheel. With fixed axles though, if you move one wheel, the other is effectively moved in the opposite direction. The axle will pivot about its centre point, affecting the direction the wheels point. Therefore, if you move the outside rear wheel back the rear wheels would point outwards. As the vehicle goes down the straight, the steering would need to be off centre, making the vehicle 'crab' along the track.
The angle of the axle can be set to aid cornering. By moving the outside front wheel (solid front axle) forward, the wheel will be partially pointing around the corner. By pulling the outside rear (solid rear axle) back the wheels will point slightly outward, but give the effect of rear wheel steering, helping to get the back round the corner. Too much axle angle will have adverse affect down the straight, but some is better than none. It is more important to get around the corners quickly and smoothly than it is to drive quickly in a straight line.
The above techniques describe how to set the wheel geometry up when the vehicle is stationary. By experimenting with different amounts of each variable, any vehicles cornering characteristics can be improved. If taken seriously these variables will be different for each track and surface.
The final thing to note about wheel alignment is that when the vehicle is moving, there are dynamic forces caused by body roll that can change some of the settings. Knowing what these are and whether it is helping or hindering could lead to a breakthrough in a vehicles performance.

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.
 
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