The suspension system creates the physical link between the road and the vehicle. Safety, performance and overall automotive reliability are a few of the concerns that stem from this region of the automobile. Equally, road shock, vibration, uneven terrain, inconsistent friction and highly fluctuating thermal variation are but a few of the problems that this system has to deal with on a frequent basis.
Where it all starts
The front suspension system has two primary purposes: to isolate the driver from irregularities in the road surface and to define the ride and handling characteristics of the vehicle.
The front suspension is designed to absorb the impact of the tires traveling over irregular road surfaces and dissipate this energy throughout the suspension system. This isolates the vehicle’s occupants from the road surface.
The vehicle’s ride characteristics are defined by the rate at which the suspension dissipates energy and the amount of energy that is absorbed. Ride characteristics are designed into the suspension system and are typically not adjustable.
The suspension system must allow for the vertical movement of the tire and wheel assembly as the vehicle travels over irregular road surfaces while maintaining the tire’s horizontal relationship to the road. On a typical independent suspension system, the steering knuckle is suspended between an upper and lower control arm or a lower control arm and a strut assembly, depending on design.
Short-Long Arm (SLA) suspension: SLA is a common type of suspension that uses upper and lower control arms of unequal length. The upper arm is usually shorter than the lower arm to control camber changes during jounce and rebound. As many late model vehicles use a strut type suspension, let’s briefly discuss this system.
Strut-type suspension: The strut-type suspension uses a lower control arm that attaches to the steering knuckle at the outermost point of the control arm. The attachment is through a ball joint. The innermost end of the control arm is attached to the vehicle through semi-rigid bushings. The upper portion of the steering knuckle is attached to the strut assembly, which then connects to the vehicle body by way of an upper bearing. The steering knuckle is allowed to travel up and down independent of the vehicle body structure and frame.
This up-and-down motion of the steering knuckle as the vehicle travels over bumps is absorbed primarily by the coil spring. The spring is retained under tension over the strut assembly. A strut (shock absorber) dampens the oscillations of the coil spring and acts as the connection point of the suspension system to the vehicle. Since it also includes the coil spring seat, the strut utilizes the shock’s dampening action to reduce the recoil of the spring.
Front suspension systems also utilize a stabilizer bar. It connects between the left and right lower control arms through the stabilizer link and stabilizer shaft insulators. This bar controls the amount of independent movement of the suspension when the vehicle turns.
The critical angles
Now, in order for the suspension to function as designed and prevent premature tire wear, we must understand the critical alignment angles.
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 can create excessive wear on the suspension parts. Too much negative camber will result in premature wear on the inside of the tire and can create excessive wear on the suspension parts. Unequal side-to-side camber of 1 degree or more will cause the vehicle to pull or lead to the side with the most positive camber.
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 tilt to the rear is positive (+) and a forward tilt is negative (-). Caster influences directional control of the steering but does not affect the tire wear. Caster is affected by the vehicle height; therefore, it is critical that the body is at its specified trim 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 speeds and wheel return 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-in/toe-out) 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 offsets the small deflections of the wheel support system that occur when the vehicle is rolling forward. In other words, a non-moving vehicle that has the wheels set with toe-in, will cause the wheels to roll parallel on the road when the vehicle is moving.
Improper toe adjustment will cause premature tire wear and can cause steering instability.
The thrust angle is the path that the rear wheels take. The front wheels aim or steer the vehicle. The rear wheels control tracking. This tracking action relates to the thrust angle. Ideally, the thrust angle is geometrically aligned with the body centerline. In the illustration, toe-in is shown on the left rear wheel, moving the thrust line off center. The resulting deviation from the centerline is the thrust angle.
The non-adjustable angles
Although there are some alignment angles that are not adjustable, these angles should still be considered during diagnosis. As an example, a vehicle that has been in a collision should have all angles inspected prior to routine services.
Setback applies to both the front and the rear wheels. Setback is the amount that one wheel may be aligned behind the other wheel. Setback problems may be the result of a road hazard or a collision. The first clue is typically a caster difference from side-to-side of more than 1 degree.
Steering Axis Inclination (SAI) is the angle formed by the rotational axis of the front suspension in relation to a vertical angle through the center of the wheel spindle. SAI causes the wheels to return to the straight-ahead position after cornering, and is therefore a steering control angle. It is not adjustable, and if incorrect, requires replacement of suspension parts to correct it.
A true plumb line, placed at the center of the wheel at the point of road contact, and the projected line created by the strut or the upper and lower ball joints determine SAI. The two lines will intersect at a point just below the road surface on most vehicles, but on some front-wheel-drive vehicles the point of intersection will be just above the road surface. The distance between the projected line and the vertical line at the road surface is called scrub radius.
Ideally, the scrub radius should be as small as possible. Normally, the SAI angle, the centerline of the tire and the wheel intersect below the road surface, causing a positive scrub radius. With struts, the SAI angle is much larger than the long-arm/short-arm type of suspension. This allows the SAI angle to intersect the camber angle above the road surface, forming a negative scrub radius. The smaller the scrub radius, the better the directional stability. Installing aftermarket wheels that have additional offset will dramatically increase the scrub radius. The newly installed wheels may cause the centerline of the tires to move further away from the spindle. This will increase the scrub radius.
A large amount of scrub radius can cause severe shimmy after hitting a bump. Four-wheel-drive vehicles with large tires use a steering damper to compensate for an increased scrub radius. Scrub radius is not directly measurable by the conventional methods.
Turning angle, also called toe-out on turns, changes the toe of the front wheels during turns. This toe change is necessary because while turning, the outside wheel travels farther than the inside wheel. The actual difference between the toe angles of the front tires is typically only a degree or so during turns.
If the turning angle is incorrect, the usual result is tire squeal at low speeds. The tires squeal as they try to travel the different distances around a turn. Incorrect turning angles force the tires to slip in order to make up the difference between their actual setting and the specified turning angle. Low speed tire squeal can be the result of this slippage.
Mounting the steering arms onto the knuckles at an angle allows one wheel to turn a little more than the other wheel.
The included angle is a combination of two other angles: camber and SAI. The included angle is calculated as SAI plus or minus camber.
Add positive camber to the SAI to determine included angle. Subtract negative camber from the SAI to determine included angle.
The included angles on each side of the vehicle should normally be within 0.5 degrees of one another. If the included angle is incorrect, the front strut or spindle assembly is bent. This damage is usually the result of a front-end collision. Bent spindles affect included angle.
When the included angle is incorrect, the vehicle can wander, weave and have rapid tire wear or a steering wheel that does not return to center after completing turns. The only way to correct included angle is to replace the damaged spindle or strut.
Installing shims between the upper control arm shaft and the mounting pads is how caster and camber adjustments are typically made. However, specific procedures depend upon how the control arms are attached. To adjust a vehicle with a cross shaft, shims are added, subtracted or removed altogether.
The symptoms
Now that we have an understanding of the basic angles, let’s explore some of the symptoms we may experience when diagnosing complaints.
Lead/pull is the deviation of the vehicle from a straight path on a level road, without hand pressure on the steering wheel. It is usually caused by poor tire construction, uneven brake adjustment or power steering valve unbalance.
Tire construction can be a major cause of lead in a vehicle. An example of this is placement of the belt. Off-center belts on radial tires can cause the tire to develop a side force while rolling straight down the road, and the tire will tend to roll like a cone.
Always check the basics first in order to verify that improper wheel alignment or worn components are not mistaken for tire lead.
Torque steer is present when a vehicle pulls or leads in one direction during hard acceleration, and in the other direction during deceleration.
Many factors may cause torque steer to be more apparent on a particular vehicle:
- A slightly smaller diameter tire on the right front increases a right torque lead. Inspect the front tires for differences in the brand, the construction or the size. If the tires appear to be similar, change the front tires from side-to-side and retest the vehicle. Tire and wheel assemblies have the most significant effect on torque steer correction.
- A large difference in the right and left front tire pressure.
- Left-to-right differences in the front axle angle may cause significant steering pull in a vehicle. The pull will be to the side with the most downward sloping axle from the differential to the wheels. Axles typically slope downward from the differential. The slope of the transaxle pan to level ground may be used as an indication of bias axle angles. The side with the higher transaxle pan, shown on the left side of the illustration, has the most downward sloping axle angle.
Memory steer is when the vehicle wants to lead or pull in the direction the driver previously turned the vehicle. Additionally, after turning in the opposite direction, the vehicle will want to lead or pull in that direction.
Memory steer can be the result of improper tire inflation, lack of lubrication to tie rod ends and ball joints, binding strut bearings and wheel alignment.
Wander is the undesired drifting or deviation of a vehicle to either side from a straight path with hand pressure on the steering wheel. Wander is a symptom of the vehicle’s sensitivity to external disturbances, such as road crown and crosswind, and accentuated by poor on-center steering feel.
What’s the cause?
Problem vehicles that keep coming back because they continue to wear out tires or won’t steer straight no matter what you do, can often be diagnosed as collision damage. A bent strut, steering arm, control arm, mislocated strut tower, sub-frame or engine cradle will all affect alignment. Other times, a problem vehicle may turn out to have misaligned structural members that position the front or rear suspension.
When in doubt, tell technician customers to check for a service bulletin on a tire wear or handling problem, and then follow the recommended cure.
Suspension and steering systems have come a long way in recent years and at best, we have barely scratched the surface of these highly sophisticated systems. But hopefully you’ll be a little better informed the next time a customer calls and needs assistance on a front suspension problem.n
