Alignment Information and Vocabulary
PURPOSE FOR PROPER ALIGNMENT
1) Minimize tire wear
2) Ease of handling
3) Maximum safety
Always follow manufacturer's specifications and procedures when performing alignments.
Definition: Inward or outward tilt of the tire. The top of the tire is always used as the point of reference.
· Positive Camber - top of the tire tilting outward (away from the center of the car)
· Negative Camber - top of the tire tilting inward (toward the center of the car)
Effect on tire wear: Moderate. Camber wear tends to be a smooth wear.
Cause lead or pull? Yes, a car will tend to lead or pull toward the side of the car that has the most camber.
Camber is measured in degrees.
True vertical is O degrees. So, camber is the negative or positive degrees from that true vertical position. For example: -2 degrees or 2 degrees. (No sign before the reading on the Hunter equipment indicates a positive number).
Camber has a cone effect that causes two phenomena:
1) The tire tries to go to the direction that is leaned.
2) It causes the tire to wear at the smaller diameter of the tire because of lack of leverage. (The wider the tire, the more susceptible it will be to this type of wear). Camber can wear just one tire unlike toe, which will wear both tires even if only one is out of specifications.
· Rule Of Thumb: Camber reading should be within 1/2 degree of each other. Any looseness in the suspension will tend to make the camber and other alignment angles go negative. This is due to the effect that the weight of the car has on the angle.
Reasons for positive camber:
1) It places the load of the car on the inner wheel bearing, the larger of the two. (Tire patch is the part of the tire that is contacting the road surface).
2) It decreases the leverage effect of the spindle (which increases road isolation).
Ways of adjusting camber:
4) Bending components
5) Aftermarket camber correction kits and procedures
Rear Wheel Camber
Rear wheel camber by definition is the same as the front. The effect of excessive rear wheel camber generates the same potential tire wear as the front, but the pulling characteristic does not apply and, in some cases, the function is different. It would be an over-simplification at this time to state that one position is preferred over the other due to the varied rear suspensions in use today. The design itself will dictate preferred setting.
Definition: The forward and rearward tilting of the steering axis. *The top of the tire is used as the reference point.
· Positive Caster: rearward tilting of the top of the steering axis.
· Negative Caster: forward tilting of the top of the steering axis.
Effect on tire wear: None (with the wheels in the straight-ahead position).
Cause lead of pull? Yes - a car will tend to lead or pull to the side, which has the least caster.
If the top of the steering axis is behind the true vertical position, it will lead the tire and create very stable driving.
If the top of the steering axis is in front of the true vertical position, it will follow the tire and create very unstable driving.
Negative caster's only benefit is that it aids in low speed steering, a benefit for cars without power steering. Positive caster creates harder turning, but the stability is better and most cars have power steering, which compensates for this effect.
Positive caster tends to cause a toe-in effect due to the weight of the car.
Positive caster tends to return the steering to the center position. This is because that is the most relaxed position. As you turn the wheel from side to side, the spindles are lifted and lowered which takes more effort because positive caster is pushing toward center position.
Caster is measured by measuring camber through a range of change. The amount of camber change from one point to another will give you your caster reading. With positive caster, if you turn left, the left wheel will go more positive on camber. When you make a right hand turn, the left wheel camber goes negative. So when turning, caster will lift one side of the car and lower the other side.
Two disadvantages of excessive positive caster:
1) Tends to cause high-speed shimmy since a lot of caster will tend to cause even more toe-in. *Shimmy is rapid toe change.
Any looseness in the suspension tends to snap back and forth. (This is why some vehicles use a steering stabilizer that is similar to a shock and is very important to help eliminate shimmy).
2) Camber roll: which is the camber change during turning due to extreme caster.
This will cause tire wear on the outer and inner edges of the tire. It may look like under-inflation wear, and this can affect only one tire.
An example of extreme positive caster is 6 degrees or more.
Cross caster (difference in caster between each side) should be no more than 1/2 degree. If this specification is exceeded, the side with the least amount of positive caster will tend to lead the car in that direction. This is related to the effect caster has on toe-in. For example: if the left caster is at 4 degrees and the right is at 3 degrees, the left is causing more toe-in pressure than the right side is since it only has 2 degrees caster. The toe will equal out, but since there is more toe-in pressure to the right, the car will lead or pull to the right.
Caster is a dynamic measurement. (Wheels have to be moved to measure it).
When measuring caster only, with Hunter equipment, make the sweep with wheels unlocked or locked and sensors unlocked.
Caster need not be adjustable to insure steering wheel return ability on many cars because Steering Axis Inclination is built into the suspension to aid in this factor.
If caster is out of specification and is not adjustable, then something is bent or the cradle is shifted.
Ways of adjusting caster:
4) Strut rods
5) Bending components
Definition: The angle between the thrust-line and the geometric centerline. This occurs when the rear wheels of the car point in a direction that is not in line with the centerline of the car. Also referred to as the rear average toe.
· Positive thrust angle - is when the angle is formed to the right of the geometric.
· Negative thrust angle - is when the angle is formed to the left of the geometric centerline.
Effect on tire wear: None
Cause lead or pull: No
A thrust-line alignment aligns the front wheels to track along with the rear wheels even though it has a certain amount of thrust angle. A driver will have to steer toward the same direction as the rear wheels are thrust forward.
Definition: The camber change that occurs during turns due to the effect of positive caster.
Effect on tire wear: This is dependent upon the amount of positive caster.
Cause lead or pull? No.
Camber roll is what occurs with extreme positive caster. It is the tilting of the tire during turning.
Camber roll wears the tires on the outer edges and resembles under inflation wear.
STEERING AXIS INCLINATION (SAI)
Definition: The amount the steering axis is tilted from true vertical.
Effect on tire wear: None.
Cause lead or pull? No.
Steering axis inclination is built into the knuckle and is not adjustable.
SAI is typically: 5-9
9-15 degrees on McPherson struts
SAI is in the neutral position with the wheels straight ahead. Turning the spindle tends to pick up the car no matter which way to turn because the spindle moves down regardless of which way the spindle is turned from the center position.
SAI aids in steering wheel return ability and help the car moved down the road straight.
Side-to-side SAI should be within 1.5 degrees.
SAI and Included Angle (IA) are set and not adjustable, but on struts with adjustable camber slots in the bottom, IA is going to change when changing camber. When measuring SAI and IA, the wheels should be locked and the sensors leveled and locked before making sweep.
Caster need not
be adjustable to insure steering wheel return ability on many cars because SAI
is built into the suspension to aid in this factor. This is especially true of strut type
suspensions that have much more SAI built into them than regular
Definition: The distance between the front of the tires compared to the distance between the back of the tires.
· Toe-in (positive toe): The distance between the front of the tires is
closer together than the rear of the same tires.
· Toe-out (negative toe): The distance between the rear of the tires is closer together than the front of the same tires.
Effects on tire wear: Extreme.
Causes pull or lead: No.
Toe specifications for RWD cars usually call for toe-in due to the rear end of the car pushing the front of the car that tends to pull the front wheels out when driving down the road.
Toe specifications for FWD cars usually call for toe-out because the front wheels are pulling the car and tend to pull the wheels in at the front when driving down the road.
If both wheels are parallel, then you have zero toe.
Toe can be measured in inches, millimeters, or degrees.
Total toe (also known as sum toe) is the toe reading for both sides. It is the difference between the front of the tires and the rear of the tires. For example: when measuring the distance between the front of the tires, you get 59 and 3/4". Measuring the distance between the rear of the tires, you get 60". The total toe is 1/4", so if you are given a specification of 1/4" toe, you must set each side at 1/8" toe in. This will give you a total toe of 1/4".
Three Reasons For A Centered Steering Wheel
2) Proper center-to-center steering (gearbox)
3) Proper turn signal cancellation
Individual toe uses the geometric centerline as a reference point. This is more accurate. There is not a definite relationship between inches and degrees. For example: when comparing a 1/4" toe-in for a Honda 12" wheel and a 1/4" toe-in for a 26" truck tire, the smaller tire will register more of a degree change than the larger wheel. To further visualize this effect, draw a complete circle with each tire and compare the sizes. The larger circle will have a larger circumference than the smaller circle. This proves the above statement.
Toe is the last adjustment to be made when doing an alignment.
Toe is always shared. For example: if toe is okay on one side and the other side is off by 1/4", both sides will suffer tire wear at a rate of approximately 1/8".
If a technician runs his hand across a tire, he should be able to tell whether or not a car is toe-in or out and it should be the same on the other side. This is because toe wears tires with a saw-toothed pattern or feathered edge. Whichever way the edges point is the way the toe is off. So, if toe-in, the edges point in, and if toe-out, the edges point out. Running your hand across the tire tread checks this. (Not with it, side to side)
1/8" off of toe will scrub a tire 11 ft. across the road within one mile.
If a technician notices that the front tire of a car has a feathered edge that points to the inside, he can assume two things:
1) He has a toe-in problem.
2) The tire on the other side will have the same type of wear.
Even if he feels this rough edge as he pulls his hand across the tire from the inside of the wheel well out and his alignment machine says that toe is fine or even toed-out, he still knows that he has a toe-in problem that wore the tires the way that they are worn. He needs to start looking for a cause for a hidden toe change such as loose suspension/steering components
Hidden toe change: when the toe is changing while you drive down the road. (This is due to loose parts such as idler arms and tie rods).
Biggest tire wearing angle is TOE.
Biggest tire wearing factor is improper tire inflation.
Sagged steering linkage causes hidden toe change. Idler arms are a big cause of this problem. Any time you adjust caster and camber, you must check/set toe because it may have been affected by the camber/caster adjustment.
Toe-out on turns will turn the inside wheel more than the outside wheel because of the angled steering arms. This works off of the Ackerman Principle.