Collision Wheel Alignment

Wheel alignments are an everyday job done at service facilities across the country. And while vehicle manufacturers provide specifications to align caster, camber and toe, each car and light truck is unique. Alignment angles need to be set to meet the driving conditions of the owner.

For instance, a pick-up truck that is used as a work vehicle will handle differently when the bed is loaded as compared to the bed empty. Also, a minivan that usually has several people in it, will handle differently than one with only the driver. The technician needs to take these parameters into consideration when doing the alignment. A vehicle that has been properly maintained will have a steering wheel that is not off-center and it will not pull, wander or shimmy.

As you can see, achieving a good alignment for your customers’ vehicles can be tricky. But, when the vehicle has been in an accident, the task becomes more difficult. With this in mind, let’s review the steps that must be taken to repair a collision-damaged vehicle in order to obtain a proper wheel alignment.

COLLISION INSPECTION

Whenever a vehicle has sustained collision-damage, especially to the front end, a careful and detailed inspection is required. Remember, when a vehicle is involved in a collision, all of the parts absorb the collision energy, resulting in varying degrees of damage. And based on the location of the damage and the angle at which the vehicle was struck, some components will be damaged more than others.

Cast parts, such as steering knuckles and rack and pinion housings, can be damaged and not show any sign of distress. Never attempt to repair any of these cast steering or structural parts.

In addition, structural damage is likely to have occurred. Strut towers are often damaged in collision events. Subframe damage would more than likely result in the movement of mounting locations for control arms, steering gears and struts. Such damage will make it impossible to properly align the vehicle.

Replace the bad components and straighten the subframe, as required. Frame and body dimensions can be found in the appropriate Body Repair Manual for the vehicle being repaired.

For example, even if you are using a measuring tape or a tram gauge, check the position of the lower ball joints from side-to-side. If the positions are wrong, the Steering Axis Inclination (more on that subject below) will be affected. Also, check the upper strut tower dimensions and be sure these readings are the same on each side of the vehicle. If measurements are found to be out-of-spec, go back and make the necessary adjustments and corrections. Don’t wait until the rest of the car is repaired and it is on the alignment rack.

CHECKING THE CRITICAL ANGLES

In order to determine if the structural damage has been correctly repaired and all the damaged parts replaced, check the critical alignment and steering angles. The first angle to be checked is the Steering Axis Inclination (SAI).

The SAI is the angle between a true vertical line and a line drawn through the center of the upper ball joint, or strut (depending on the suspension design) and the lower ball joint. The vertical line starts at the center of the tire at the road contact point. This angle is shown as (1) in Figure 1. SAI is the critical angle used to determine if the structural dimensions are correct.

If the SAI is on the low side of the spec, look for the problem on the lower sections of the suspension; conversely, if the angle is on the high side, look at the upper sections. Be aware if the SAI is greater on one side of the vehicle compared to the other side. If this is the case, check the position of the engine cradle or crossmember.

Camber is the next angle to be checked. It is the vertical tilt of the wheel (inward or outward) as viewed from the front of the vehicle. This tilt is measured from the same vertical line used to measure SAI. Camber is shown as (2) in Figure 1.

Camberand SAI added together gives you the Included Angle (IA). This angle is shown as (3) in Figure 1. IA can determine if there are bent or incorrectly adjusted parts between the upper and lower pivot points of the suspension.

For vehicles that have a McPherson strut-design front suspension (this is a large number of vehicles), if the SAI is out-of-spec, check for the following: (1) damaged strut tower, (2) bent control arm, (3) bent ball joint stud and (4) a shifted engine cradle. But, if the SAI angles are the same on the right and left sides of the vehicle, the frame is not bent.

It should be mentioned that many technicians often use the camber measurement as the reference point for the other remaining measurements. They believe, incorrectly, that camber mustbe set to specs even if it affects other alignment angles. One alignment angle should never be adjusted out-of-spec to make another angle within spec. If this is the approach you are taking in your repair work, you need to re-examine your shop procedures.

If an alignment angle is out-of-spec, you need to find the reason why it is out-of-spec. In the case of a bad camber reading, it is possible that the SAI angle is incorrect. Find the reason why it is. Furthermore, the IA might be out-of-spec. As mentioned earlier, this means that something is wrong between the pivot points. Check the strut and spindle. One or the other is probably bent.

Caster is the vertical tilt of the steering knuckle (forward or rearward) in reference to the position of the lower ball joint and upper ball joint, or strut. It is viewed from the side of the vehicle ( Figure 2 ). Caster is measured in degrees relative to a true vertical center line.

Although caster does not affect tire wear, it does affect the stability of the vehicle. As a result of a collision, the steering knuckle can be damaged, changing the position of the knuckle and/or ball joints. If there is an imbalance of caster from one side to the other, the vehicle will drift to the side with the least amount of positive caster.

Toe is the inward or outward angle of the wheels as viewed from above the vehicle. It is measured in degrees or inches and illustrated in Figure 3 . Toe has a profound effect on tire wear. If toe is properly adjusted, it allows the wheels to travel in the same direction. However, if toe is not properly adjusted, the misaligned wheels will scuff the tires sideways, causing rapid tire wear.

After a vehicle has been in a crash, it is likely that the components that control toe, such as tie rods, were damaged. As a result, toe will require adjustment. It is the last of the alignment angles to be adjusted when performing a wheel alignment; therefore, all the damage should have been properly repaired by this time. This will ensure an accurate toe adjustment.

While on the subject of toe, it should be remembered that rear toe has an effect on the way a vehicle tracks. On many vehicles, rear toe can be adjusted. If it is, each rear wheel should have one-half of the total toe adjustment. If equipped with a solid axle, be sure the axle is not cocked, especially if the vehicle has been in an accident.

The direction of the rear wheels in relation to the centerline of the vehicle is 
called the thrust angle. When adjusted properly (zero thrust angle), the rear tires travel directly behind the front tires when the steering wheel is in the straight-ahead position. An off-center thrust angle, either negative or positive, can cause abnormal tire wear, a lead or a pull and a crooked steering wheel.