Suspension Alignment

With the tremendous beating that many Canadian roads have taken over the last winter, not to mention challenged municipal budgets, the record count of potholes big enough to swallow a subcompact is sure to play havoc with suspension systems.

When the wheel-to-pothole collision is great enough, parts will break and those customers will end up arriving at a service facility. But when the damage is less obvious, it can be just a question of pushing the suspension out of alignment, sometimes severely, which could signal a more expensive repair down the road.

For the counterperson, it is important to know the elements that go into an alignment, knowledge that can help you explain to car owners why their car doesn’t drive the way it used to.

First, it is important to know that suspension alignment is a simple concept, even if the actual job of ensuring it can be quite particular. It involves three key measurements: caster, camber, and toe.

The vast majority of alignments focus only on toe, as caster and camber are often set at the factory and are not adjustable, a consequence of widespread use of the McPherson strut.

However, the lack of factory adjustability does not guarantee that caster and camber will remain perfect.

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, helping to provide “automatic centring” of the steering wheel. It also provides more stable handling over road bumps.

However, when a car has sagging springs or is overloaded, the effective caster of the front wheels can change.

In general, an increased positive caster is not so noticeable for the driver, but if too little positive caster is dialled in–if the rear of the vehicle is higher than stock, for example–steering may be skittish at highway speeds and the steering wheel may not want to return to a straight-ahead position without prompting.

Most noticeable is if one wheel has more positive caster than the other, as that wheel will pull toward the centre of the vehicle. This condition will cause the vehicle to pull or lead to the side with the least amount of positive caster.

Such a pulling condition can be evidence that a car has suffered damage severe enough to shift a suspension or damage a strut.

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 wheels tilt inward at the top, the camber is negative (-). The amount of tilt is measured in degrees from the vertical. Camber settings influence directional control and tire wear and are the most visible of suspension values.

Camber is often the culprit when rapid tire wear is experienced. For the more sensitive driver, improper camber settings can cause some serious handling anomalies.

Normally, however, it can be considered that 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 with the most positive camber, lead to the side.

Even on vehicles where camber would not be considered adjustable, in that there are no built-in adjustments, aftermarket solutions do exist.

While certainly sagging springs or other worn components could be to blame, sometimes returning camber to factory settings might be best accomplished through the use of strategically placed shims or “cam bolts,” that provide adjustments where none existed before.

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.

There are also a number of other suspension measurements that can be referred to when executing a suspension alignment.

Thrust Angle

This is the angle between the thrust line and centreline. If the thrust line is to the right of the centreline, the angle is said to be positive. If the thrust line is to the left of centre, 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-centre 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 centre steering.

Included Angle

This is 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)

SAI is the angle formed by a line that runs through the upper and lower steering pivots with respect to vertical. On an SLA suspension, the line runs through the upper and lower ball joints. On a McPherson 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 poorly located cross members.

Kingpin Offset/Scrub Radius

Kingpin offset is the distance from the centre of the wheel contact face to the intersection point of the kingpin extension. The line through the centre 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.

Setback

Setback is 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 centreline with respect to the vehicle’s centreline. 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 measured side to side.

Ride Height

Ride height is 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.