Instructors 27 T Roadster

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Instructors 27 T Roadster
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Class Participation Rules

• Please Participate
• Agree, disagree, comment, share your opinion and
  experiences, ask questions etc. but just dont
  sit there.
• Remember this class is a lot more fun when Im
  not the only person talking.

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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.

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Class presentation prepared by
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Understanding The AnglesAutomotive Wheel
Geometry
A Technicians Guide to Wheel Alignment
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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

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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)

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Now name 15 more alignment related
angles/references
What do you mean that you cant think of another
15 alignment related angles/references that
affect the tire wear and handling of a vehicle?
Doesnt 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 SUVs and why its almost a
100 positive design feature with virtually no
negative features.

• 16.
• 17.
• 18.
• 19.
• 20.
• 21.
• 22..

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Ride Height
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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.

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Ride Height
Check Spec Manual for Specs and Measurement
Points
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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.

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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.

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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.

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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.

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Vehicle Handling Characteristics

• Oversteer (Loose)
• Understeer (Pushing)
• Neutral Steer

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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.

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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.

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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
  dont pay attention to where they are steering.
• Isnt used on the average vehicle sold in the
  U.S.
• Neutral steer doesnt idiot proof a vehicle.

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Camber
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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

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Camber Example
RF
LF
-1 degree camber
1 ½ degree camber
This vehicle has a 2 ½ camber split (spread) and
should have a severe pull to the right.
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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.

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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.

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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
Road Surface
Direction of travel
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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.

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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).
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Camber Pull Demonstration
Right Front
Cup Started Here
X
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.
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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
Smaller diameter
Direction of travel
Road Surface
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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?

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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.

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Camber Effect Radial Tires

• Because camber doesnt 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.

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Camber Roll
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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.

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Spindle Height is the mid-point between the upper
and lower ball joints. This mid-point is shown by
the RED arrow on the picture at the right.
Upper ball joint
Lower ball joint
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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.
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Caster
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Caster Definition
Viewed from the side caster is the forward or
rearward tilt of a line drawn through the
steering axis compared to true vertical.
Zero Caster
Negative Caster
Positive Caster
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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
  the center of the tire
  contact patch

Negative Caster
X
Positive Caster
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Caster Reaction
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.
Zero Caster
Negative Caster
Positive Caster
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Caster Reaction
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.
Zero Caster
Negative Caster
Positive Caster
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0 Degree Caster Load Point Location
With 0 Caster the point of load is directly in
line with the true vertical line.
RF RF
Upper BJ
RF
Lower BJ
RF
Direction of travel shown in red.
Direction of travel
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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
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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.

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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.
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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.

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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.

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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.
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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.
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Bump Steer
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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.

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Bump Steer (Orbital Steer)
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Bump Steer CausesRP Steering

• Causes for bump steer on RP 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

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Checking For Bump SteerRP Steering

• With the vehicle on a alignment rack measure from
  the RP steering gear to the alignment rack air
  jack.
• Find a similar point on the RP gear hosing and
  measure the right and left sides.
• The measurement should be within ¼ side-to-side.
  
• Be sure that you dont have a basic ride height
  problem that is throwing off the measurement.

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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

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Checking For Bump SteerParallelogram 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.
  

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Toe
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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.

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Always select toe to be displayed in degrees on
your alignment equipment.
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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.
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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.

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Always select toe to be displayed in degrees on
your alignment equipment.
The number of degrees from straight ahead doesnt
change with tire diameter.
When the toe measurement is selected as degrees
tire diameter doesnt make any difference. You
are measuring the amount of degree variation from
the tire being straight ahead.
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Toe Conditions
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Toe Facts

• When measured by using the center scratch and toe
  bar measurement technique the overall tire
  diameter doesnt 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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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Toe Facts

• Worn inner RP 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.

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Thrust Angle
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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.

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Thrust Angle
Geometric Centerline Thrust line
Positive Thrust Negative Thrust
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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.

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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)

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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).

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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

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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!

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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.

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Toe Out On Turns (Turning Radius)
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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.

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Toe Out On Turns (Turning Radius)
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
18o turn
20o turn
All Wheels Turn on a Common Center
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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.

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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.

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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).

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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.

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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.

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Set Back
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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.

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Setback
Setback occurs when one wheel on an axle is set
back farther than the other wheel on the axle.
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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
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Set Back Facts

• Because a vehicle has a set back condition,
  positive or negative, doesnt 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.

94
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.

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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.

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Cradle Position - Adjustment
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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
98
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

99
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 .

100
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.

101
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.

102
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

103
Scrub Radius
104
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.

105
Scrub Radius
The dotted lines represent movement of the tire
outward at the top because of positive camber.
This movement does not affect scrub.
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.
True Vertical Line - This lines does not change
because of a camber change.
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McPherson StrutNegative Scrub Radius
107
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.

108
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.

109
Remembering The Facts About The True Vertical
Line

• The true vertical line doesnt move when plus
  size tires are installed unless the rim offset is
  changed.
• The true vertical line doesnt change when wider
  tires are installed on stock offset and stock
  width wheels.

110
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.

111
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.
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0 Degree Caster Camber Load Point Location
LF
RF
Direction of travel
FWD Vehicle With Negative Scrub
113
(No Transcript)
114
LF
Direction of travel
115
Load Point With Positive Caster
LF
RF
Direction of travel
116
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
117
Tire To Road Friction Will BE Called RSR (Road
Surface Resistance)
LF
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
118
Tire To Road Friction Will BE Called RSR (Road
Surface Resistance)
LF
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
119
When A Tire Goes Flat The RSR Increases
LF
RF
POL Point
POL Point
Flat tire 400 units of RSR
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.
120
When One Front Brake Fails The RSR Of The Two
Front Tires Is No Longer Equal
LF
RF
POL Point
POL Point
Working brake 300units of RSR
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.
121
The Reason That One Tire Forces The Other To Move
Is That They Are Connected By The Steering
Linkage.
POL Point
Working brake 300units of RSR
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.
122
Steering Axis Inclination
123
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.

124
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.

125
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
everyones 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.
126
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.

127
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.

128
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?
129
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 .

130
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?

131
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.

132
Audi A-4 Suspension System
Features two upper and two lower ball joints.
Where is the SAI measures on this suspension? If
its through the upper and lower pivot points
which one? The forward or rearward upper and
lower pivot point, (ball joint)?
133
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?
134
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.

135
Another Example of an SAI Definition Exception
2002 Ford Thunderbird Front Suspension
136
What Happens To SAI When You Install An Alignment
Kit At The Locations Shown By The Arrows? Does
SAI Change?
137
Specialty Products Company
2002 Ford Thunderbird Rear Suspension
138
Included Angle
139
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.

140
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.

141
Rear Thrust on RWD Vehicles With Leaf Springs
142
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.

143
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.

144
RWD Vehicles With Leaf Springs Can Have The
Thrust Corrected.
145
Rear Thrust Alignment PlateFor Solid Axle Leaf
Spring Vehicles
146
A RWD Thrust Plate Is Put Only On One Side
147
Rear Thrust on RWD Vehicles With Independent Rear
Suspensions
148
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.

149
Idler Arm Facts
150
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?
151
What three angles/readings/settings, previously
shown, will a loose idler arm affect?
1. Bump steer 2. Total Toe 3. Individual Toe
152
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

153
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.

154
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.