Instructors’ ‘27 T Roadster
Class Participation Rules

Please Participate
– Agree, disagree, comment, share your
opinion and experiences, ask questions
etc. but just don’t sit there.
– Remember this class is a lot more fun
when I’m not the only person talking.
Seminar Itinerary

Break(s) as needed through out the class.
– How many of you have not had lunch?
150 slides in the alignment theory portion
of the class.
 Class time today 12 to 6 pm. Tomorrow
class time is 1 pm to 7 pm.
 Shop time will be the later part of the class
today and tomorrow.
 Remember please participate.

Class presentation prepared by:
Understanding The Angles
Automotive Wheel Geometry
A Technician’s Guide to Wheel Alignment
Quick, name 15 alignment
related angles/references that
you will deal with on every
wheel alignment!
1. Camber (front)
2. Caster (front) Is there ever rear caster?
3. Toe (front)
4. Camber (rear)
5. Toe (rear)
6. Thrust
15 alignment related
angles/references cont.
7. GCL (Geometric Center Line)
8. VCL (Vehicle Center Line)
9. Scrub Radius
10. RSR (Road Surface Resistance of the tire)
11. Camber Roll
12. Slip Angle
13. Over Steer, Under Steer, Neutral Steer
14. Dominant Force
15. Dynamic loaded angle changes (toe
compliance)
Now name 15 more alignment
related angles/references
What do you mean that you can’t think of another 15
alignment related angles/references that affect the tire
wear and handling of a vehicle? Doesn’t everyone
know about the 2000 model year spindle offset change
GM made on some of their FWD cars and what it does
for handling? Do you know about increased spindle
height? Why Ford is using it on their new SUV’s and
why it’s almost a 100% positive design feature with
virtually no negative features.
16.
17.
18.
19.
20.
21.
22…..
Ride Height
Ride Height
Ride Height is not listed in your student
book as a separate section.
 How ride height affects various alignment
adjustments and setting is covered under
the various individual sections. i.e..
Camber, Caster, Scrub.
 The following Ride Height information is
presented to give everyone the same
foundation about Ride Height problems.

Ride Height
• Check Spec Manual for
Specs and Measurement
Points
Ride Height Measurements Facts

Just measuring for a side-to-side or front-to-rear
comparison is not measuring ride height.

Even within a car family, similar body styles
shared between different name plates, the ride
height specs are not always the same.

Sometimes measuring ride height correctly is a
real pain.
Ride Height Related Problems




Q. Which alignment angle is affected most by a
rear to front ride height problem? (The rear of
the vehicle is lower than the front.)
A. Caster, the suspensions upper pivot points
move rearward.
Q. How much caster change, and which way, +
or -, do you get when the trunk is loaded and the
rear of the car angles 3 degrees downward?
A. A 3 degree positive caster change.
Ride Height Related Problems
Q. Why does a car handle so poorly when
the trunk is overloaded, i.e.. 8 bags of
cement mix, if caster moves positive when
the rear of the car drops (supposedly you
get more stability with increased caster)?
 A. Because of the weight transfer to the
rear there is now less RSR (road surface
resistance) between the front tires and the
road.

Ride Height Related Problems
Q. If you have a vehicle with one corner
lower than the other, sagged or broken
spring, what alignment related angles or
directional influences come into play?
 A. Camber change, caster change (with
rear spring problem), understeer/oversteer
depending on which spring has problem
and at least five other angles.

Vehicle Handling
Characteristics
Oversteer (Loose)
 Understeer (Pushing)
 Neutral Steer

Oversteer
The rear tires drift out when the vehicle is
driven in a circle, when the front wheels
are turned from straight ahead.
 The vehicle will have a tendency to turn
tighter than steering wheel input.
 When power is put to the wheels the car
will turn sharper.
 Usually referred to as being “loose”.

Understeer
Front tire drift causes the vehicle to refuse
to turn as sharply as the wheels are
pointed.
 With the steering wheel held in place and
the throttle constant a vehicle will make an
ever increasing larger circle (pushing)
outward.
 Most FWD (Front Wheel Drive) vehicles
have built in Understeer tendencies.

Neutral Steer
Front and rear tire drift is the same.
 The vehicle goes where it is pointed.
 Would create a driving safety hazard for
most people. Especially those who use a
cell phone and don’t pay attention to where
they are steering.
 Isn’t used on the average vehicle sold in
the U.S.
 Neutral steer doesn’t idiot proof a vehicle.

Camber
Camber Definition

Viewed from the front, camber is the
inward or outward tilt of a tire and wheel
assembly from true vertical.
– Tilted outward = Positive Camber
– Tilted inward = Negative Camber
Camber Example
RF
+ 1 ½ degree
camber
LF
-1 degree
camber
This vehicle has a 2 ½ camber split (spread) and
should have a severe pull to the right.
Camber Facts
All other things being equal a vehicle will
pull/drift toward the side with the most
positive camber.
 Camber is always assumed to be positive.
If you are stating or writing about negative
camber you must say negative or use the
minus sign.
 If no sign minus sign (-) is present the
reading/specification is positive.

Camber Facts
A tire with positive camber is attempting to
roll around the apex of the cone created by
positive camber.
 A tire with positive camber will wear on
the outside edge.
 A tire with negative camber will wear on
the inside edge.

Camber Facts

This RF tire would attempt to roll right. If the
left side did not have an equal amount of
offsetting camber the vehicle would go right.
Apex of the cone
RF
tire
Direction of travel
Road Surface
Proving Camber Facts
Take a Styrofoam coffee cup and lay it on
its side (empty it first).
 Visually extend a line down the side to the
table surface. This is the apex of the cone.
 Gently blow on the side of the cup and
note that is is pivoting around the point
where the line contacts the table top.

Camber Pull Demonstration
This tire/wheel will pull to the right as
it attempts to rotate around the apex of
the cone established by the tilt of the
tire/wheel assembly.
Vehicles direction
of travel.
Imagine that this is the right front tire with
positive camber (outward tilt of the tire).
Camber Pull Demonstration
Right Front
Cup Started
X Here
Apex of the cone
If the left front tire does not have an opposing force the vehicle will
go to the right. Remember that the spindle and knuckle are attached to
the tire/wheel so whatever force is present at the tire will be
transferred to the vehicle.
Camber Facts

Because the entire tire must rotate at the
same RPM, speed, and because the one
side has a smaller diameter than the other
scuffing will occur, on the outside edge.
RF
tire
Direction of travel
Smaller diameter
Road Surface
Camber Facts
If a tire/wheel is angled inward, toward the
engine, camber is negative.
 If a tire/wheel is angled outward, away
from the engine, camber is positive.
 Many late model (1998 & later) vehicles
have set static camber negative.
 There are over 23 million vehicles on the
road that have camber specified negative.
Why?

Camber Effect & Radial Tires

Radial ply tires are not affected by camber
as much as bias-belted or bias-ply tires are.
– On the rear of a FWD vehicle it usually takes
more than 1 degree of camber before any tire
wear is present provided toe is set correctly.
– On the rear of a vehicle if toe is out of
adjustments even slightly any amount of
camber over .5 degree will usually amplify the
misadjusted toe condition.
Camber Effect & Radial Tires
Because camber doesn’t greatly affect
handling on a vehicle equipped with radial
ply tires whenever possible it is best to
compensate for road crown with caster.
 It is best to compensate for road crown
with caster even if you must install an
aftermarket adjustment kit to do so.

Camber Roll
Spindle Height- Spindle Spread

Definition:
– The mid-point of distance between the pivot
points of the spindle (the ball joint tapered
holes at the end of the spindle).

Purpose:
– Greater spindle height/spread minimizes
camber changes during vehicle dive and
chassis roll.
Spindle Height is the midpoint between the upper and
lower ball joints. This midpoint is shown by the RED
arrow on the picture at the
right.
Upper ball
joint
Lower
ball joint
The greater the spindle
height/spread/length the less
side force is placed on ball
joints during jounce, rebound,
vehicle dive and chassis roll.
Refer to the TOTAL ALIGNMENT
AUTOMOTIVE WHEEL GEOMETRY
“UNDERSTANDING THE ANGLES” A
Technicians Guide To “Wheel
Alignment book for further information.
Caster
Caster Definition
Zero Caster
Negative Caster
Positive Caster
Viewed from the side
caster is the forward or
rearward tilt of a line
drawn through the
steering axis compared
to true vertical.
Caster Definition & Facts
Remember that the steering axis is defined
as a line drawn through the suspension
pivot points (ball joints) compared to true
vertical.
 Note that the true vertical line is always
measured straight up from Negative Caster
the center of the tire
contact patch

X
Positive Caster
Caster Reaction
Zero Caster
Negative Caster
Positive Caster
Positive Caster =
Directional stability.
Excessive positive caster
means hard steering but
the power steering easily
overcomes the effect.
Negative Caster = Less
stability especially at
highway speed. Excessive
negative caster gives the
feeling of instability,
wandering and light
steering.
Caster Reaction
Zero Caster
Negative Caster
Positive Caster
Provided there is no other
dominant force a vehicle
will pull to the side having
the least amount of
positive Caster. That is the
same as the most negative
caster.
Caster is not a direct tire
wearing angle until you
turn the wheels from a
straight ahead position
then the camber roll,
caused by positive caster,
can cause tire wear.
0 Degree Caster Load Point
Location
With 0 Caster the point of load is directly in line with the
true vertical line.
Upper BJ
RF
RF RF
Lower BJ
RF
Direction
of travel
Direction
of travel
shown in red.
Positive Caster Load Point
Location
With Positive Caster the point of load is in front of the
true vertical line.
True Vertical Line
Upper BJ
RF
Lower BJ
Direction
of travel
Direction
of travel
Positive Caster Effect

Increased Positive Caster =
– Increased Vehicle Stability.
– Increased Steering Effort. (Easily overcome
by the vehicles Power Steering).
– Increased steering wheel returnability after a
turn.
– Increased road shock from bumps.
– Increase in the effect of bump steer problems.
Positive Caster Effect Without
Positive Caster Angel
The Caster Line is
through the center of the
upper and lower ball
joints.
The actual center of the
spindle has been moved
back from the caster line.
This gives a Caster effect
without a high Caster
angle.
ASE Test Question

Caster causes tire wear:
–
–
–
–
–
A: Directly
B: Indirectly
C. Not at all
D. Depends on the suspension design.
E. This is really a lousy ASE test question.
Caster causes tire wear:
When the wheels of a vehicle with positive
caster are turned from a straight ahead
position caster causes the camber to
change (roll).
 The more positive caster you have the
more camber roll (change) you get.
 The more you turn the wheels the more
camber roll (change) you get.

4 x 4 Trucks & Camber Roll
The larger the
diameter and the
wider the tire the
more camber wear
problems you will
have because of
camber roll.
Visualize a 4x4 truck with “mudder” type of tires and a caster
setting of 5 ½ degrees (positive) being constantly driven in town
making tight turns. You can now understand why the tire will
chunk out and show irregular thread block wear. The only
solution is to lower the caster and this is only a partial solution.
Rotation also helps but nothing is a cure.
Plus Size Tires & Camber Roll
The larger the diameter
and the lower the
profile (aspect ratio)
the more tire wear
problems you will
have because of
camber roll.
Plused sized tires with lower aspect ratios have shorter sidewalls
than higher profile tires. This means that the springing effect of
the tires sidewall is demised. Any force put into the tire will have
a higher degree of reaction on the tire face. In other words the
tires edge will be loaded quicker and with more force because
the sidewall of the tire does not flex as much.
Bump Steer
Bump Steer Definition

An uncontrolled amount of unequal
individual toe change that takes place
when the suspension of a vehicle moves,
either in jounce or rebound. This will
cause a vehicle to dart/dive to the right or
left depending on which tire toes in and
which toes out.
Bump Steer (Orbital Steer)
Bump Steer Causes
R&P Steering

Causes for bump steer on R&P equipped
vehicle:
– Crossmember location, accident, cradle
movement etc.
– Rack mounting bushing worn.
– Body damage, when rack is mounted on the
firewall.
– Severely worn inner or outer tie rod end.
– Ride height problem on one side compared to
the other
Checking For Bump Steer
R&P Steering

With the vehicle on a alignment rack
measure from the R&P steering gear to the
alignment rack air jack.
– Find a similar point on the R&P gear hosing
and measure the right and left sides.
– The measurement should be within ¼” sideto-side.

Be sure that you don’t have a basic ride height
problem that is throwing off the measurement.
Bump Steer Causes
Parallelogram Steering

Causes on Parallelogram Steering systems:
–
–
–
–
–
–
–
–
Frame horn/rail bent from an accident.
Adjustable idler arm not adjusted correctly.
Worn idler arm, up and down movement.
Loose steering gear box mounting to frame rail.
Bent steering linkage.
Wrong tie rod ends installed on vehicle.
Basic ride height problem.
Weak coil springs:


Compress under engine torque
Compress under vehicle load
Checking For Bump Steer
Parallelogram Steering
With the vehicle on a alignment rack and
with the wheels steered straight ahead
measure from the air jack to the grease
fitting on the inner tie rod ends.
 The measurement should be within ¼” side
to side.

Toe
Toe Definition

When measured in inches:
Toe relates to the difference in the distance
between the front of the tires and the rear
of the tires on the same axle.
65”
Always select toe to be
displayed in degrees on your
alignment equipment.
66”
28.625”
66 ½”
67 ½”
If you choose to display toe reading in inches your alignment equipment
assumes a 28.625 (O.E.) size tire is on the car. If the tires has been
“plused” sized you will have an inaccurate toe setting.
Toe Definition

When measured in degrees:
Toe angle relates to the difference in the
angle of each individual wheel from
straight ahead or being at a right angle to
the GCL of the vehicle.
Positive toe is when a tire is toed in.
Negative toe is when a tire is toed out.
Always select toe to be displayed in degrees on your alignment
equipment.
The number of
degrees from
straight ahead
doesn’t change with
tire diameter.
When the toe measurement is selected as degrees tire diameter doesn’t
make any difference. You are measuring the amount of degree variation
from the tire being straight ahead.
Toe Conditions
Toe Facts

When measured by using the center
scratch and toe bar measurement technique
the overall tire diameter doesn’t make any
difference as you are actually measuring
the physical distance between the scratch
marks on the front of the tires and the back
of the tires.
This method is as accurate as measuring
degrees from straight ahead.
Toe Facts
Using a spreader bar will greatly increase
the accuracy when adjusting static toe.
 When you have a vehicle with negative
scrub place the spreader bar rearward of
the spindles.
 When you have a vehicle with positive
scrub place the spreader bar forward of
the spindles.

Toe Facts
The following statements apply to electronic
alignment equipment, not to mechanical or laser
equipment.


Whenever possible select toe to be measured in
degrees.
When toe is measured in inches is assumes a
standard tire size of 28.625”. If the vehicle has “plus
size” tires installed on it the end result is that the toe
setting is not accurate.
Toe Facts
When toe is measured in degrees it is a
measurement of the angle the tire is from
being straight ahead.
 The overall diameter of the tire does not
change this angle measurement.

Toe Facts

Front toe will always equally divide itself
when a vehicle is driven.

On some vehicles toe changes dramatically
as the outer tie rod swings forward
and backwards (articulates).
Toe Facts

Dynamic and static toe settings can be two
completely different things.

When you turn the wheels from a straight
ahead position the total toe changes.

Toe, front or rear is a vehicle handling angle
long before it is a tire wearing angle.
Toe Facts

Rear toe is more critical to vehicle
handling than front toe.

Rear toe when not equally adjusted on both
wheels will create a thrust angle.

When you turn the wheels from a straight
ahead position the individual toe
changes on each wheel are not equal.
Toe Facts

Scrub will affect the amount of dynamic
toe change that takes place from the static
toe setting.

Vehicles with negative scrub tend to toe in
when driven.

Vehicles with positive scrub tend to
toe out when driven.
Toe Facts

Worn inner R&P tie rod ends allow radical
dynamic toe changes.

Outer tie rod ends that have lateral (side to
side) looseness allow radical dynamic toe
changes.

RBS tie rod ends restrict dynamic toe
change.
Thrust Angle
Thrust Angle Definition

Thrust angle is established by a line,
bisecting rear toe, drawn at a right angle to
the mid-point of the rear axle and
compared to the GCL (Geometric Center
Line).
– The GCL is established by drawing a
line connecting the mid-points of the
front spindles and the rear spindles.
Thrust Angle
Geometric
Centerline
Thrust line
Positive Thrust
Negative Thrust
Thrust Specifications

It is generally accepted that the maximum
allowable thrust angle for FWD vehicles is
.125
– For a average vehicle .125 thrust means that
the rear tires will move sideways ¼” for every
vehicle length it moves forward.
– Assumes a average vehicle length of 10’.
Tire Reaction to Thrust
.125 Thrust = ¼”.
 Tire sideways movement of ¼” for each
10’ of forward movement equals 11’
sideways drag per mile of forward
movement.

– 5280’ per mile = 528 vehicle lengths per mile
– 528 vehicle lengths X ¼” per length = 132, ¼”
– 132, ¼” = 11 feet (sideways drag per mile)
Understanding Thrust
Remember any thrust angle means that the
rear tires are not pointed straight ahead.
 On a FWD vehicle the front tires are the
driving force (pulling force).
 The rear wheels are just along for the ride
(following).

Why Thrust Wears Tires
The rear tires will be pulled forward and
will try to go right or left (positive or
negative thrust) following the thrust angle.
 After the tire tread can no longer go in two
directions (forward & sideways) it will
flex as the tire is pulled back under the
vehicle to a straight ahead direction.

 This
Is The Cause Of Rear Diagonal
Tire Wear
A Statement About Thrust

Someone once said:
–“The front wheels, of a vehicle,
steer a vehicle from straight
ahead. The rear wheels
determine what straight ahead
is”!
Thrust Toe Relationship

You can have a toe problem and not have a thrust
problem.
– This would be true if toe were set, inward or outward,
equally out of specifications on both rear wheels.

Even though the tendency is to do so rear
toe can not divide itself equally as a FWD
vehicle is driven.
– Unless the FWD vehicle has 4-wheel steering.
Toe Out On Turns
(Turning Radius)
Toe Out On Turns
(Turning Radius) Definition

The design feature, created by the inward
angling of the steering arms, where when
the front wheels are turned the inner
tire/wheel assembly turns at a tighter angle
than the outer tire/wheel.
Toe Out On Turns
(Turning Radius)
18o turn
20o turn
When a car steers into a turn, the
two steerable tires must turn on
two different size circles. The
entire car turns about a common
center. This means that the two
steerable tires must turn on two
different size angles, with the tire
on the inside of the turn having
the greater angle
All Wheels
Turn on a
Common
Center
Toe Out On Turns
(Turning Radius)
TOOT (Toe Out On Turns)
 Turning Radius
 Turning Angle
 Ackerman Steering


All of the above terms mean the same
thing. When the front wheels are turned
from a straight ahead direction the inner
wheel turns tighter than the outer wheel.
Toe Out On Turns
(Turning Radius)

Can be measured with modern alignment
equipment as a separate measurement.
– Can be viewed as the movement reaction of the caster
indicators on some alignment equipment.


Can be measured by the movement of the
turntables in relationship to each other.
Must have toe first adjusted “in the ball park” or
a erroneous TOOT reading will result.
Toe Out On Turns
(Turning Radius)

When measured with modern alignment
equipment:
– The centering of the wheels on the turn tables
is not critical.
– The squareness of the vehicle on the rack is
not critical.
– The basic toe setting is important (must be
within reading specifications and should be
within 1 degree of specifications).
Toe Out On Turns
(Turning Radius)

When measured by the degree of
movement of the turn tables:
– The centering of the wheels on the turn tables
is extremely critical.
– The squareness of the vehicle on the rack is
very critical.
– The basic toe setting is critical (must be within
¼” (1/2 degree) of specifications.
Toe Out On Turns
(Turning Radius)

When measured by the degree of
movement of the turn tables:
– The turning motion should be done from the
steering wheel not by forcing the tires left and
right.
– It critical that the wheels are blocked tight and
that the parking brake is applied. The vehicle
must not roll when the wheels are tuned.
– The turntables must move without resistance.
Set Back
Set Back Definition
(Front Setback)

As measured from an imagery line
across the front of the vehicle. Front
setback is present when one wheel
of the front axle is farther back or
forward than the other wheel.
Setback
Setback
occurs when
one wheel on
an axle is set
back farther
than the other
wheel on the
axle.
Set Back (+ & -)

Positive set back (+) is present when the
right front wheel is set back farther than
the left front wheel.
– If set back is not indicated as being negative
(-) it is assumed to be positive.

Negative set back (-) is present when the
left front wheel is set back farther
than the right front wheel.
Illustration is of
positive set back
Set Back Facts

Because a vehicle has a set back condition,
positive or negative, doesn’t mean that it
has a problem.
– Some vehicles have been designed and
manufactured with set back (1/2”).
A ½” set back (+ or -) reading is
considered acceptable.
 O.E. set back specifications are
seldom published.

Set Back Facts
Set back differences will affect the wheel
base measurement.
 In theory a vehicle will pull/drift to the
side with the shortest wheelbase (most
positive setback).
 Set back is usually not a “DF” (dominant
force) factor and seldom is the true cause
of a vehicle pull.

Set Back Facts

Caster will affect set back reading but….
– A normal caster split will not cause a set back
reading to be out of the generally acceptable
range of ½”.
– The type of suspension system will cause the
amount of set back in relationship to caster
split to vary.

If you think you truly have a set
back problem try the “three finger
test”.
Cradle Position Adjustment
FWD Engine Cradle
 Camber is High
 SAI is Low
 Included Angle is OK
 Camber is Low
 SAI is High
 Included Angle is OK
Cradle Shifted
FWD Engine Cradle Adjustment

Adjustable front cradles on FWD vehicles can
affect
–
–
–
–
–
–
–
Front camber
Front caster
Front GCL
Front set back
Scrub radius
TOOT
Rear Thrust
FWD Engine Cradle Adjustment

A FWD vehicle with an adjustable front
cradle should have the cradle adjusted
before adjusting rear toe and camber.
The reason this must be done is ………….
Engine Cradle
Adjustment Sequence

The cradle is the front reference point for
the GCL.
– The GCL and individual rear toe (bisected at a
right angle to the rear axle) establishes thrust.

As you move the front GCL side to side
you change the thrust angle.
Engine Cradle Adjustments
Some vehicles use the adjustable feature of
the front cradle to set front caster.
 Cradle adjustments are most controllable
when the vehicle has part of the weight
taken off the tires.

Engine Cradle Adjustments

Alignment angle readings and their
changes, when you move the cradle, may
be read directly through live camber and
caster reading when you select the raise
vehicle/jack & hold feature on your
alignment equipment
Scrub Radius
Scrub Radius Definition

Viewed from the front scrub radius/offset
is the distance between the tire centerline
at the contact area, and a line extended
downward through the steering axis.
Scrub Radius
The dotted lines
represent movement of
the tire outward at the
top because of positive
camber. This movement
does not affect scrub.
True Vertical Line This lines does not
change because of
a camber change.
Shown is an example of
positive scrub. Note that the
intersection of a line
through the pivot points
and the tire road contact
center point is what
establishes scrub.
McPherson Strut
Negative Scrub Radius
Remembering The Facts About
The “True Vertical” Line

True vertical is established upward from a
reference point at the center of the tire
contact patch.

The true vertical line does not move with a
camber change.
Remembering The Facts About
The “True Vertical” Line

The true vertical line does not move when
caster is adjusted.

The true vertical line does move when
wheels with a different offset are installed.
Remembering The Facts About
The “True Vertical” Line

The true vertical line doesn’t move when
plus size tires are installed unless the rim
offset is changed.

The true vertical line doesn’t change when
wider tires are installed on stock offset and
stock width wheels.
Remembering The Facts About
The “True Vertical” Line

The only way you can change the true
vertical lines location is to move the center
of the tire inward or outward.

The true vertical line will move slightly
when the suspension moves through it
range of jounce and rebound.
Wide Rim Effect
Scrub Radius will always change with a change in rim offset.
Scrub Radius effect (not actual scrub) changes with rim width
changes. Remember the true vertical line measurement point.
0 Degree Caster & Camber
Load Point Location
LF
RF
Direction of travel
FWD Vehicle With Negative Scrub
LF
Direction of travel
Load Point With Positive Caster
LF
RF
Direction of travel
Load Point With Positive Caster
LF
RF
The tires are attempting to pivot inward rotating on the POL
POL Point POL Point
Direction of travel
FWD Vehicle With Negative Scrub
Tire To Road Friction Will BE
Called RSR (Road Surface
LF
Resistance)
RF
POL Point
POL Point
POL (Point of Load)
The tires are attempting to pivot inward rotating on the POL
FWD Vehicle With Negative Scrub
Tire To Road Friction Will BE
Called RSR (Road Surface
LF
Resistance)
RF
POL Point
POL Point
100 units
of RSR
100 units
of RSR
In this example both tires are trying to rotate inward equally.Bo
FWD Vehicle With Negative Scrub
When A Tire Goes Flat The RSR Increases
LF
RF
POL Point
Flat tire =
400 units
of RSR
POL Point
100 units
of RSR
The flat tire will make the vehicle pull left, the RSR will make it
pull right. The vehicle may go right, go left or go straight
depending on how much initial scrub it has.
When One Front Brake Fails The RSR Of The
Two Front Tires Is No Longer Equal
LF
RF
POL Point
Working
brake =
300units
of RSR
POL Point
100 units
of RSR
The working brake will make the vehicle pull to the left but the
increase RSR will make it go to the right. In some cases the
vehicle will actually pull right.
The Reason That One Tire Forces The Other To
Move Is That They Are Connected By The Steering
Linkage.
Working
brake =
300units of
RSR
POL Point
100 units
of RSR
Linkage is forced to the left because of the dominant inward
pivoting force of the left tire. This forces the right to turn outward.
Steering Axis
Inclination
Steering Axis Inclination
SAI = Steering Axis Inclination
 KPI = King Pin Inclination
 MSI = McPherson Strut Inclination
 MLI = Multi-Link Inclination

 The
above terms all mean the same
thing. They will be referred to in
this class as SAI.
Steering Axis Inclination
Definition

Viewed from the front it is the angle
established by a line extended through the
suspension pivot points and true vertical.
SAI, KPI, MSI Definition
SAI = Steering Axis Inclination
 KPI = King Pin Inclination
 MSI = McPherson Strut Inclination
 MLI = Multi-Link Inclination

SAI is always viewed from the front. By everyone’s definition it
is a line drawn through the upper and lower suspension pivot
points compared to a true vertical line measure from the center of
the tire upward”.
Checking SAI On Modern
Alignment Equipment
Usually part of the weight is taken off the
tires when taking an SAI reading.
 This eliminates the possibility of a worn
suspension part, I.e. side play in a lower
ball joint, from skewing the readings.

Checking SAI On Modern
Alignment Equipment
When supporting a vehicle on air jacks
while taking a SAI reading you should
lock the air jack in position.
 Any change in vehicle height while taking
a SAI reading will give inaccurate results.
 Follow your alignment equipments
directions for leveling and locking the
heads.

Steering Axis Inclination (SAI)
SAI Line
SAI, KPI, MSI all
measure the same angle,
a line through the upper
and lower pivot points
compared to true
vertical.
How about a multi-link suspension
system?
Steering Axis Inclination (SAI)

Everyone definition says that the load of
the vehicle is carried through the SAI line.
– Remember the SAI line is stated as being
through the suspension pivot points.

So what happens when the load is not
carried through the pivot points such as
………………….
SAI Definition Exceptions
When you have two upper and/or two
lower pivot points (ball joints).
 When you have a multi-link type of
suspension where the load line and the
pivot points are on two different planes.
 Where is the SAI reference line on the
above “exceptions”?

SAI Definition Exceptions
On multi-link suspensions the SAI line is
an imaginary line somewhere between the
load line and the pivot points.
 This point moves in and out as the
suspension moves through it range of
motion.
 In other words the reference points float
along with the influence of SAI as the
suspension moves up and down.

Audi A-4 Suspension System
Features two upper and two lower ball joints. Where is the SAI
measures on this suspension? If it’s through the upper and lower
pivot points which one? The forward or rearward upper and
lower pivot point, (ball joint)?
2003 Ford Expedition
The load line is through the strut/shock, the pivot point is through
the upper and lower ball joint. Where is the SAI line?
Remember the SAI Definition
Exceptions:
On multi-link suspensions the SAI line is
an imaginary line somewhere between the
load line and the pivot points.
 This point moves in and out as the
suspension moves through it range of
motion.
 In other words the reference points float
along with the influence of SAI as the
suspension moves up and down.

Another Example of an SAI
Definition Exception
2002 Ford Thunderbird
Front Suspension
What Happens To SAI When You
Install An Alignment Kit At The
Locations Shown By The Arrows?
Does SAI Change?
Specialty Products Company
2002 Ford Thunderbird
Rear Suspension
Included Angle
Included Angle Definition
Included angle is SAI and camber added
together, when camber is positive, or SAI
minus camber when camber is negative.
 Included angle is used as a diagnostic
angle.
 Included angle specifications are usually
not published.

Included Angle Diagnostics

The following Included Angle diagnostic
charts will assist you determining what
parts are causing camber/SAI problems.

Note that there are different charts for
different types of suspensions.
Rear Thrust on
RWD Vehicles
With Leaf
Springs
Thrust on RWD Vehicles
w/solid rear axles.
Thrust is adjustable on many RWD
vehicles using solid axles and leaf springs.
 Individual toe (thus rear thrust) is changed
by shifting one side of the axle forward
and the other backward.

Thrust on RWD Vehicles
w/solid rear axles.
Thrust angle is measures the same as on
FWD vehicles.
 Thrust is established from the same
reference points.
 Thrust influence is far less on RWD
vehicles with solid rear axles than on FWD
vehicles.
 It usually takes over .5 (1/2 degree) of
thrust before it affects vehicle handling.

RWD Vehicles With Leaf
Springs Can Have The Thrust
Corrected.
Rear Thrust Alignment Plate
For Solid Axle Leaf Spring
Vehicles
A RWD Thrust Plate Is Put
Only On One Side
Rear Thrust on
RWD Vehicles
With Independent
Rear Suspensions
Thrust on RWD Vehicles
W/Independent Rear Suspensions.
Adjusted by O.E. or aftermarket cams,
shims or lateral link adjustments.
 Each individual wheel is adjusted to be at a
specific angle in relationship to the GCL of
the vehicle.
 Can establish oversteer or understeer
handling characteristics depending if the
individual toe is adjusted (equally) inward
or outward.

Idler Arm Facts
Idler Arm Looseness Check
Apply 25 lbs.. of force in a
upward and downward
manner.
1/4” total
movement
is accepted
What three angles/readings/settings, previously shown,
will a loose idler arm affect?
What three angles/readings/settings,
previously shown, will a loose idler arm
affect?
1. Bump steer
2. Total Toe
3. Individual Toe
Vehicle Handling Problems
Caused By Tires

Radial Ply Tire Pulls
– Radial tires create a lateral (side force) when
they rotate.
– This force is at the contact patch of the tire.

This force can be present for two reasons:
– Tire ply-steer
– Tire concinnity
Vehicle Handling Problems
Caused By Tires – (Ply-Steer).

The steel belts under the thread of a radial
ply tire can create a lateral force when:
– They are laid in off-center.
Vehicle Handling Problems
Caused By Tires – (Tire
Concinnity).
If a tire has an unequal amount of sidewall
stiffness side-to-side it has a concinnity
problem.
 A vehicle will go toward the side with the
softest (shortest under load) sidewall.
 Other alignment angles can amplify a
concinnity problem or they may offset a
concinnity problem.
