During a severe frontal collision, a vehicle will often sustain damage to its suspension and steering systems. The insurance adjuster as well as the body shop estimator will visually check for bent or broken suspension and steering components. However, a mere visual inspection is not enough. Bent, broken or misaligned suspension or steering components can cause a vehicle to handle poorly, as well as cause excessive tire wear and poor fuel economy.
With many newer vehicles, the same types of problems can occur with the rear suspension components when they’re involved in a rear-end impact. On some modern vehicles, manufacturers have specifications for the rear wheel alignments as well. Misaligned or broken rear suspension components can cause the same type of problems as found on the front suspension and steering systems.
A wheel alignment machine is the best tool for checking suspension and steering component damage after an accident.
A wheel alignment is the adjustment of the suspension and steering systems that allows the tires to roll smoothly and correctly over the surface of the road. It’s also the measurement of the suspension angles and the adjustment of the suspension and steering components. It consists of properly aligning the wheels so that they’re parallel to each other and perpendicular to the ground. This ensures straight wheel tracking and minimizes tire wear.
The alignment of the wheels should be checked periodically. In most cases, the alignment is checked when new tires are put on the vehicle or when suspension or steering components are replaced either because they’re worn out or damaged in an accident.
Caster: Caster is the angle of the steering axis measured in degrees. As viewed from the side of the vehicle, it’s the tilt of the steering axis. Three to five degrees positive caster is typically normal on compact to intermediate vehicles, whereas lower angles are used on heavier vehicles such as pickup trucks and the larger SUVs to reduce steering effort.
Positive caster: Positive caster tilts the tops of the steering knuckles toward the rear of the vehicle. This helps keep the vehicle’s wheels traveling in a straight line.
Negative caster: Negative caster tilts the tops of the steering knuckles toward the front of the vehicle. It makes the wheels turn easier and can produce a somewhat less directional stability. Because of this, the wheels have a tendency to follow imperfections in the road surface such as ruts. Caster has little or no effect on tire wear.
One of the best ways to visualize caster is to picture the caster on a desk chair. When the wheel is behind the pivot at the point where it contacts the ground, it’s in positive caster.
On many front-wheel-drive vehicles, the caster is not adjustable. If the caster is out of adjustment on these vehicles, it indicates that something is possibly bent from an accident and must be repaired or replaced. On most vehicles with an SLA type system, shims can be added or taken out of the upper control arms to adjust the caster angles.
Camber: Camber is the tilt of the wheels inward or outward from the vehicle’s centerline when viewed from the front of the vehicle. It ensures that the tire tread comes in contact with the road surface.
It’s measured in degrees and is usually the second angle adjusted during a wheel alignment. If the camber is out of adjustment, it can cause premature tire wear on one side of the tire as well as a pulling problem to the side with the most positive camber. This is usually the case when the vehicle has been involved in a collision that has caused damage to the strut and/or spindle assembly or structural damage to the vehicle. Camber can also go out of adjustment when ball joints or other parts are worn or defective, or when the springs sag and cause the ride height to change.
Positive camber: Positive camber indicates that the top of the wheel is tilted outward when viewed from the front. The outer edge of the tire tread contacts the road surface.
Negative camber: Negative camber indicates that the top of the wheel is tilted inward when viewed from the front. The inner tire tread contacts the road surface more.
Camber settings are controlled by the control arms and their pivots. Loose, worn or damaged ball joints, control arm bushings or wheel bearings usually affect this setting. It can also be affected by changes in the chassis ride height.
Camber is adjustable on most vehicles. Some manufacturers include a camber adjustment at the top mounting position of the strut or at the spindle assembly area.
Very little adjustment is required if the strut tower and lower control arm positions are in the proper place. If you find an out-of-whack camber error and the suspension mounts have not been damaged, it’s an indication of bent suspension parts. In this case, a diagnostic angle and dimensional checks should be made to the suspension parts. Damaged parts must be replaced.
Toe: Toe is the most important alignment setting relative to tire wear. If the toe setting is just 1/32-inch off of its correct setting, each tire on that axle will scrub almost 3-1/2 feet sideways every mile, reducing tire life.
A front-wheel drive vehicle “pulls” the vehicle, resulting in forward movement of the suspension arms against their bushings. Most front-wheel drive vehicles use negative toe to compensate for suspension movement.
The toe angle indicates the direction of the tires compared to the centerline of the vehicle. A rear-wheel drive vehicle “pushes” the front tires, and as they roll along the road, resistance causes some drag, resulting in rearward movement of the suspension arms against their bushings. Most rear-wheel drive vehicles use positive toe setting to compensate for the suspension movement.
Toe-in: Toe-in results when the front of the wheels are set closer than the rear. The wheels point in at the front direction.
Toe-out: Toe-out results when the front of the wheels are set farther apart than at the rear. The wheels point out. Toe is a critical tire-wearing angle. Wheels that do not track straight ahead have to drag as they travel forward.
Steering axis inclination: Steering axis inclination is a way to check for misaligned or bent suspension or steering components. It’s the measurement in degrees of the steering pivot line when viewed from the front of the vehicle. On a short-long arm (SLA) type suspension system, the line runs through the upper and lower ball joints. (This type of system is usually seen on rear drive vehicles).
On a MacPherson strut type suspension, a line runs through the lower ball joint and upper strut mount or bearing plate. If the steering axis inclination is different from side to side, it will cause a pull at very slow speeds. It’s usually a nonadjustable angle and can be used with structural measurements to diagnose:
• Misaligned uni-body or body-on-frame type constructions
• Shifted engine cradle or transmission crossmember
• Strut tower misalignment
• Control arm mounting damage
Included angle: Included angle is another way to check for bent or broken suspension or steering parts. It’s the sum of both the camber and steering axis inclination angles. If the camber is negative, then the included angle will be less than the steering axis inclination. If the camber is positive, it will be greater.
The included angle must be the same from side to side even if the camber is different. If there is a difference, then something is bent, possibly a steering knuckle.
Scrub radius: Scrub radius indicates the distance between the extended centerline of the steering axis and the centerline of the tire where the tread contacts the road. This distance must be exactly the same from side to side, or the vehicle will pull strongly to either side. If the steering centerline is inboard of the tire centerline, the scrub radius is positive. If the steering centerline is outboard of the tire centerline, the scrub radius is negative. Rear-wheel drive cars and trucks generally have a positive scrub radius, while front-wheel drive cars usually have zero or a negative scrub radius. Using wheels other than the stock type can alter the scrub radius.
Riding height: Riding height is usually measured in inches, from the rocker panel to the ground. A good wheel alignment chart should provide specs, but the main thing is that the measurements should be within one inch from front to rear and side-to-side. Riding height is not adjustable on most vehicles, except those with coil over, torsion bar types and some air suspension systems. On nonadjustable suspensions, spring replacement is the best way to fix this problem. NOTE: Should only be replaced in pairs. Riding height variations can affect both toe and camber. An alignment is warranted after this procedure.
Setback: Setback is a problem when one front wheel is set further back than the other. Setback will cause an un-centered steering wheel. Any good four-wheel alignment machine will reference the rear wheels when setting toe in order to eliminate this type of problem. Good alignment equipment will measure setback and give you a reading in inches or millimeters. Some manufacturers consider a setback of less than ¼-inch a normal setting. If it’s more than a ¼-inch, there is a good chance that something is bent. A collision can cause a vehicle to have setback.
Thrust angle: Thrust angle is the direction that the rear wheels are pointing in relation to the centerline of the vehicle. A vehicle will “dog track” if the thrust angle isn’t zero, and the steering wheel will not be centered.
The best way to correct this condition is to first adjust the rear toe to the centerline and then adjust the front toe. This is done during an all-wheel alignment if the rear toe is adjustable. If the rear cannot be adjusted to within specs, the vehicle may have to be sent to a frame and body alignment machine. If the rear is not adjustable, then the front toe must be set to compensate for the thrust angle, allowing the steering to be centered. However, if the thrust angle is not correct on a vehicle with a solid rear axle, it might require the frame to be straightened to correctly reposition the rear axle.
On vehicles with independent rear suspension, the toe must be adjusted individually until it has reached the appropriate setting for its side of the vehicle. Incorrect thrust angle is often caused by an out-of-position suspension or incorrect toe settings.
So, in addition to the handling problems that are the result of incorrect toe settings, thrust angles can also cause the vehicle to handle differently when turning left versus right.
Center steering wheel: Center steering wheel refers to when the steering wheel is centered when the vehicle is traveling down a straight and level road. However, most roads are crowned to allow for water drainage, which may cause the vehicle to drift to the right. Therefore, the steering wheel will appear to be off-center to the left on a straight road. To compensate for this:
• The left caster should be more negative than the right, but not more than 1/2 degree within the specified range.
• The left camber should be more positive than the right camber. Check the specs to see what the allowable differences are.
Steering Angle Sensor
Technology has now made it a little easier to diagnose front-end alignment problems. The newest device to help technicians in the ever-changing automotive market is called a “Steering Angle Sensor” or (SAS) for short. The SAS not only assists the technician in diagnosing alignment problems but serves as a safety device for the driver and occupants.
The SAS is designed to monitor the driver’s steering style by continuously measuring the position and turning rate of the steering wheel. It then reports this information back to the vehicle’s on-board computer system.
The SAS system basically consists of a resistance circuit and two potentiometers offset by 90 degrees. The sensor, which is designed as a hollow shaft, is mounted on the steering column in the interior of the vehicle.
The SAS also works in unison with many of the other vehicle sensors, such as the wheel speed sensor, electronic stability control system, electric power steering system, parking assist, traction control, ABS system and active steering system. These sensors take into account lateral forces, vehicle roll and other variables. Their measurements, along with the steering data provided by the SAS, are used to control the various driver-assist systems mentioned. The system depends on the driver’s intent in the steering direction of the vehicle as well as the turning force involved. It can control and correct driving problems such as over-steer or under-steer.
The SAS and the other sensors outlined above must be re-calibrated to manufacturers’ specifications after replacing steering parts either due to a collision or the parts or sensors being worn out, as well as a typical trust-line type of wheel alignment.
New technology is great providing you’re properly trained to work on these complicated vehicle steering systems. Automobile manufacturers as well as the alignment machine companies offer seminars or classes to help you understand these new developments.
It’s important to take these steps when diagnosing damaged or bent suspension and/or steering components. This process is the only true way to insure that the vehicle you’re repairing goes back to the customer in its pre-accident condition and is in specs with the vehicle manufacturer.
John D. Lyman, Sr. has been the collision repair instructor at the Forbes Road Career & Technology Center in Monroeville, Pa., since 1991. He has worked in the collision repair industry since 1974, is ASE- and PPG-certified and holds a master’s degree in education. He can be reached at [email protected].