Alignment problems and their effects

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This info is partly courtesy of the Specialty Products folks in Longmont, Colorado. They manufacture all kinds of kits, shims, eccentrics, etc. for the chassis alignment industry, including the Taurus rear camber/toe bushings. It was provided by Gary Morrell


Caster

Measured in degrees. It is defined as the forward or backward tilt of the upper ball joint, or top of strut in our case, relative to the lower ball joint. Caster affects steering and steering wheel return-to-center.

Zero caster means the top of strut and lower ball joint are vertical as viewed from the side of the vehicle. Positive caster means the top of strut is rearward wrt/the lower ball joint as viewed from the side of the vehicle. Negative caster means the top of strut is forward with respect to the lower ball joint as viewed from the side of the vehicle.

Symptoms of too little (negative) caster are:

Steering wheel shows lack of return ability after a turn. Steering is touchy at high speed. Effect: The vehicle will pull to the side with the least negative CAMBER.


Camber

Measured in degrees. It is defined as the inward or outward tilt of the wheel. Its purpose is to center the vehicle's load on the tire. Negative camber is often added to compensate for the camber loss that occurs as a strut suspension goes into bump (compression). This helps to keep the outside tire contact patch maximized when turning.

Zero camber means the wheel/tire assembly are vertical as viewed from the front or rear of the tire. Positive camber means the top of the wheel/tire assembly is tilted away from the vehicle center. Negative camber means the top of the wheel/tire assembly is tilted toward the vehicle center.

Too much positive camber can cause wear on the outside of the tire. Too much negative camber can cause wear on the inside of the tire. Excess negative camber will also tend to increase straight line stopping distances.

Camber imbalance from side-to-side (>0.5 degrees) may cause the vehicle to pull to one side, e.g., the vehicle may pull to the side with more negative camber.


Toe

Measured in inches, sometimes degrees, but not very often. Defined as the difference between the leading edges and trailing edges of the tire/wheel assembly, measured at spindle height.

Looking down at the tire from the top, if the tire is parallel to the vehicle centerline, toe is zero. If the front of the tire is closer to the vehicle centerline then the rear, the tire is toed-in (positive toe), if the rear of the tire is closer to the vehicle centerline then the front, the tire is toed-out (negative toe). When, in the dark, you trip over a wheel you've carelessly left on the garage floor, that's 'stubbed toe'.

For a particular axle, toe is additive. If both wheels are toed-in 0.1 , then total toe is +0.2 . If one tire is toed-out 0.1 and the other is toed-in 0.1 , total toe is zero, but if the steering wheel is aligned with this condition, the car will be turning when the steering wheel is centered.

Too much toe-in (positive toe) causes rapid outside edge wear; the wear patterns will be saw-toothed or scuffed. If sharp edges are felt when rubbing your hand from the inside to the outside of the tire, toe-in is excessive. Extreme toe-in will cause steering instability.

Too much toe-out (negative toe) causes rapid inside edge wear; the wear patterns will be saw-toothed or scuffed. If sharp edges are felt when rubbing your hand from the outside to the inside of the tire, toe-out is excessive. Extreme toe-out will cause steering instability, especially at high speeds.

A small amount of front toe-in is desirable, as it helps the car track straight down the road. Rear drive vehicles are usually setup with some toe-in, front drive vehicles are usually set for zero toe, as the driveline torque tends to pull the front wheels forward, toeing them in.

A lowered SHO (Eibach springs) will tend toward bump steer, this is because the tie rods are just about level when the suspension is nominal, and when it's bumped, the tie rod effectively gets shorter, causing the tire to toe-out, which decreases steering stability. The only way I've found around this is to raise the steering rack, so the tie rods have a bit of downward slant again. I've raised my rack 0.3 and found that bump steer is just about eliminated. ( damn, there goes one of my secrets...)

For the rear of a front drive vehicle, small amounts of toe-out will tend to induce oversteer. The Taurus rear suspension tends to toe-out when cornered hard, due to bushing compliance in the tension struts, so a bit of toe-in is desirable to compensate for this. Toe is pretty stable thru bump and rebound, but a lowered SHO (Eibach springs again) will loose about 0.25 degrees of camber for every inch of suspension bump.


Turning angle

Measured in degrees. Defined as the relative position of the front wheels during a turn. Boy, it would sure be nice to have a drawing to describe this. Essentially, turning angle is the difference in the angles of the outside and inside front tires while they're turned. The outside tire will be at a smaller angle then the inside tire, us racer weenies refer to this as Ackerman steering. The outside tire in a turn is more heavily loaded, due to the vehicle's weight transferring to the outside, so we turn the outside tire slightly less to reduce its slip angle and keep it from squealing. Most vehicles will have several degrees of Ackerman built into the steering, it isn't generally adjustable. If its way off, a tie rod may be bent. Auto crossers often toe-out their vehicles to increase the Ackerman effect.


Setback

Measured in inches. One wheel is set back further then the other. Can be the result of a collision or manufacture. Small amounts of setback usually won't cause any problems other than some steering wheel misalignment on some alignment machines.


Thrust Angle

Measured in degrees. Defined as the direction the rear wheels are positioned with respect to the vehicle centerline. More common on solid axle rear drive vehicles, especially ones that have been rear-ended. Draw a vertical axis thru the rear differential and rotate the rear axle so one rear tire is forward of the other, the rear thrust angle is no longer parallel to the centerline of the vehicle. Excessive thrust angle (>0.5 degrees) can cause tire wear, steering wheel misalignment, or pulling to one side. On the Taurus, the equivalent would be if one tire was badly toed-out, and the other badly toed-in:


Steering Axis Inclination, Included Angle, and their relationship to Camber and Scrub Radius

Steering Axis Inclination is the angle between a true vertical line starting at the center of the tire at the road contact point and a line drawn thru the center of the strut and the lower ball joint.

Included angle is the S.A.I. angle, plus or minus the actual camber. If camber is positive, its added to the S.A.I. If camber is negative, it's subtracted from the S.A.I.

S.A.I., A.I., and camber are used to locate areas of the strut system on unibody chassis which may be damaged or misaligned. Additionally, a bent lower control arm can change S.A.I.

S.A.I.'s relationship to Scrub Radius: If the S.A.I. line intersects outside of the centerline of the tire where the tread meets the road surface, the scrub radius is negative. Front drive vehicles are set up for zero or slightly negative scrub radius. A large positive scrub radius (S.A.I. intersects inside of the tire centerline) can exacerbate torque steer. Witness those funky Hondas with the tires sticking way outside the wheel wells, this is extreme positive scrub radius, extremely stupid too, kills wheel bearings. Scrub radius is not a parameter that can be aligned, it is set by suspension design and wheel offset.