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.
CAMBER
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:
1) Shims
2) Slots
3) Eccentrics
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.
CASTER
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:
1) Shims
2) Slots
3) Eccentrics
4) Strut rods
5) Bending components
THRUST ANGLE
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.
Camber Roll:
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
degrees on
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
TOE
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.
THE RULE
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".
TOE
Three Reasons For A Centered Steering Wheel
1) Psychological
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.