Topics
Included:
Rear
Camber
Rear Camber is the inward
or outward tilt of the top of the tire/wheel assembly from true vertical.
If the top of the tire/wheel assembly is Tilted Inward, it
has a Negative Camber. If the top of
the tire/wheel assembly
is Tilted Outward, it has a Positive Camber.
If the tire/wheel assembly is straight up and down on a true vertical
line, the Camber is measured at Zero (0°).
Rear camber is not
adjustable on most rear wheel drive vehicles.
These vehicles are
built with zero camber setting and are strong enough not to flex or bend
under normal load. Most front wheel drive vehicles have a
manufacturers specification calling for a slight
amount of rear camber,
usually a small amount of negative camber for cornering stability. If the
manufacturers specification allows, a setting of 0° to -.5° is
preferred for tire wear and ride stability. If rear camber
settings change, most vehicles can be adjusted by using an aftermarket
type of adjustment, such as shims, cam bolts or bushings.
Rear
Toe
Measuring rear toe
using Geometric Centerline
Toe-Out (Negative
Toe) is a condition where the front of the wheel is farther from the
geometric centerline than the rear of the same wheel. Toe-In (Positive
Toe) is a condition where the front of the wheel is closer to the geometric
centerline than the rear of the same wheel.
Measuring rear
toe using Total Toe
Toe-Out (Negative
Toe) is a condition where the distance between the front of both wheels
on a common axle are farther apart than the rear of the same wheels. Toe-In
(Positive Toe) is a condition where the distance of the front of both
wheels on a common axle are closer together than the rear of the same wheels.
Toe can be expressed
in degrees, fractions or decimal inches. Do Not confuse degrees with inches
when selecting your method of adjustment. Rear toe adjustment is the most
critical factor regarding tire wear, mileage, and handling.
Geometric
Center Line
The Geometric
Center Line of the vehicle is established by connecting a
line between the theoretical midpoint of the front spindles and the theoretical
midpoint of the rear axle.
Rear
Setback
Rear setback is a
measurement referencing the rear wheels to an imaginary line perpendicular
to the geometric centerline of the vehicle, and is measured as an angle.
If
a vehicle has rear setback, one rear tire/wheel assembly is further back
from this imaginary line than the other.
Some causes of rear
setback may be from frame, chassis, and rear cradle mis-alignment due to
collision. If the vehicle has a setback condition, the vehicle may pull
to the opposite side of the setback.
Thrust
Line
Thrust line is determined
by bisecting the rear total toe. To bisect the rear total toe, lines that
are parallel to the tire/wheel assembly are drawn until they intersect.
Another line that starts where the geometric centerline and rear axle intersect,
is drawn to the intersection of the tire/wheel lines. This line is the
Thrust Line. When you have a thrust line to the left it is considered Negative
and when it is to the right it is considered Positive.
Thrust
Angle
Thrust angle
is the angle created between the geometric centerline and the newly created
Thrust
Line.
Camber is the
inward or outward tilt of the tire/wheel assembly. This
angle is measured from a true vertical line, i.e. perpendicular to the
ground. A tire/wheel assembly that is tilted outward at the top
is considered to have Positive camber. While a tire/wheel assembly
tilted inward at the top, displays Negative camber. For a
zero setting, the tire/wheel assembly is in the exact vertical position
or perpendicular to the ground. To rephrase, if the top of the tire/wheel
assembly is tilted inward towards the engine, it has a negative camber.
If the top of the tire/wheel assembly is tilted outward from the engine,
the camber is positive.
This is Negative
Camber
 |
This is Zero
Camber
 |
This is Positive
Camber
 |
Effects of Positive
Camber
Slight positive camber
results in a dynamic loading that allows the tire to run relatively flat
against the road surface. Positive camber also directs the weight and
shock load of the vehicle on the larger inner wheel bearing and inboard
portion of the spindle rather than the outboard bearing. Positive camber
in moderation results in longer bearing life, less likely sudden load failure,
and as a side benefit, easier steering. Excessive positive camber wears
the outside of the tire and can cause wear to suspension parts such as
wheel bearings and spindles.
Effects of Negative
Camber
Variations in negative
camber can be used to improve the handling of a vehicle. A setting
of 1/2° negative on both sides will improve cornering without affecting
tire life greatly. This negative setting compensates for the slight positive
camber change of the outside tire due to vehicle roll, thereby allowing
a flatter tire contact patch during cornering. Excessive negative camber
wears the inside of the tire and similar to positive camber, it can cause
wear and stress on suspension parts.
Road Crown and Camber
A crowned road means
that the outside/right hand side of the lane is lower than the left side
of the lane. This improves the drainage of the road but adversely affects
vehicle handling. Road crown must be compensated for in alignment settings
because a vehicle driving on a crowned road leans to the right, causing
some weight transfer to the right, and the camber changes slightly more
positive. This combination creates a pull or drift to the right. Most alignment
technicians adjust the vehicle with a slightly more positive camber, usually
1/4°, on the left to compensate for the road crown. This slightly more
positive camber will not cause a noticeable pull when driving on a flat
road. However, if camber is unequal from side to side with a difference
greater than 1/2°, the vehicle will pull to the side with the most
positive camber. If the specifications allow, 0° to ±.5°
is usually best for tire life and vehicle handling.
Causes of Camber
Changes
Always consult a
ride height specification book prior to beginning alignment. If out of
specification, attempt to correct. Changes in ride height from factory
specifications affect camber. As a vehicle ages, the suspension has
a tendency to sag. The weight of the vehicle can cause springs to weaken.
Springs can also be damaged by excessive vehicle loading or abuse. Another
factor to consider is sagging of center support or crossmember. Modifications
to the vehicle such as raising or lowering the suspension or changing the
total weight of the vehicle can also affect camber.
Front Cone Effect
When a tire/wheel
assembly is tilted it creates a condition called the "cone effect".This
angling of the tire/wheel assembly creates an imaginary cone that rotates
in the direction the wheel is angled. The apex of the cone is created by
the intersection of two lines: 1) the ground and 2) a line projected through
the centerline of the spindle to the ground. The shape of the cone is then
defined by the third line from the top of the tire to the intersection
of 1 and 2. The wheel attempts to pivot around this intersection point.
A positive cambered tire/wheel assembly will roll away from the center
of the vehicle. Conversely, a negative cambered tire/wheel assembly will
roll towards the center of the vehicle. If the vehicle is traveling on
a flat, level road and the amount of camber offset is the same on both
front wheels, the cone effects, although opposite, will offset each other
and the vehicle will travel in a straight line. A maximum side to side
variation of ± .5° is recommended.
Front
Caster
Caster Definition
Caster can be
defined as the forward or rearward tilt of the projected steering axis
from true vertical, as viewed from the side. This line is formed
by extending a line through the upper and lower steering knuckle pivot
points. For vehicles with front control arms, visualize the line extending
through the upper and lower ball joints. On strut equipped vehicles, the
line extends through the lower ball joint to the center of the upper strut
mount. Caster is always viewed from the side of the vehicle.When
the upper pivot point is rearward of the lower pivot point, caster is positive.
If the upper pivot is forward of the lower pivot point, caster is negative.
When the two points are straight up and down from each other, the caster
is zero. A maximum side to side variation of ±.5° is recommended
on most vehicles. Caster is NOT a normal tire wearing angle and is used
as a directional control for stability and steering returnability.
Front of Vehicle
Caster Effect
Caster effect
is necessary so that the load of the vehicle is "carried" through the steering
axis line formed on the upper and lower pivot points. Positive caster gives
a vehicle directional stability because the tire is being pulled along
by the load which is projected in front of the center of the tire contact
area. This causes a vehicle with positive caster (point of load ahead of
the point of contact) to be harder to steer away from the straight ahead
position. With Positive caster, road surface variations have a minimal
effect on the tire, the tire will continue to go straight. When a tire
has a Negative caster condition, where the projected steering axis
point of load is behind the tire point of contact, a vehicle will have
a tendency to be easier to steer but will lack directional stability. A
vehicle with negative caster is affected by any road surface variation
such as small road irregularities or bumps. With the point of load pushing
the tire along (negative caster), any bumps or road irregularities which
are encountered have a tendency to immediately affect directional stability
and vehicle handling.
Front Caster
Effects
Effects of Positive
Caster
Vehicles usually have
some positive caster specified since this promotes directional stability,
however, excessive positive caster can cause two problems. The first is
that excessive caster will cause a high level of road shock to be transmitted
to the driver when the vehicle hits a bump, etc. The second problem is
that a tire/wheel assembly with positive caster has a tendency to toe inward
when the vehicle is being driven. If one side has more positive caster
than the other, this causes it to toe inward with more force than the other
side. This will cause a lead or pull to the side with least amount of positive
caster.
Effects of
Caster on Tire Wear
When set with a substantial
amount of caster, the spindle travels in a vertical arc, causing it to
move up and down and raise and lower the wheels as the steering wheel is
turned. Because of this, camber changes occur. With a high amount of
positive caster, the camber changes that occur, especially at low speeds
in tight turns, cause the tires to show wear on their shoulders. In
high speed cornering, the vehicle tends to continue straight ahead when
the steering is initially turned. Due to this, and the amount of camber
change that takes place when a spindle travels through its arc of travel,
the shoulders of the tires on a vehicle may scrub and wear. When a left
turn is made at a fairly high rate of speed with a vehicle which has positive
caster, the caster of the left front wheel changes toward positive but
the momentum of the vehicle is in a straight ahead direction. This causes
the inside of the left front tire to scrub as it is turned. Just the opposite
effect takes place on the right wheel as the vehicle is turned left at
high speed. The right front wheel's camber will go negative but the outside
edge of the tire is scrubbed because of the vehicle's momentum to go straight.
On some vehicles setting caster more than +2.5° will cause scrub problems.
Front
Toe
Toe Definition
Toe relates to the difference
in the distance between the front of the tires and the rear of the tires
on the same axle, or to the vehicle centerline. Toe-in, or positive
toe, is defined as the front of the tires being closer together than the
rear of the tires. Toe-out, or negative toe, is when the rear of the tires
are closer together than the front of the tires. Zero toe is when the
tires are parallel to each other.
Measuring Toe
Toe on an individual
tire/wheel assembly is understood to be the difference between the distance
of the front and rear of one tire in reference to the vehicle centerline.
Since most alignment specifications show toe as total toe, i.e. both wheels,
it is important to understand two points: (1) 1/2 of the specified total
toe should be applied to each front wheel. (2) a minus(-) sign would
actually indicate a toe-out setting as being specified. It is important
to note that although toe has historically been measured as a distance
in fractions of an inch, and then decimal inches, it is becoming more common
for vehicle manufacturers to express toe in degrees. The idea is that the
angle, rather than an arbitrary distance, determines the side slip of the
tire and the resulting scrub of the tread. This should not be affected
by the tire size, but rather should be constant for a given measurement.
Most alignment equipment displays toe-out as a minus (-) and toe-in as
a positive (+).
Effects of Toe
Excessive toe increases
tire scuffing and results in tire wear and drag on the vehicle. Excessive
toe-in, or positive toe, increases scuffing on the outside of the tire.Excessive
toe-out, or negative toe, increases scuffing on the inside of the tire,
and in some cases can cause a darting or wandering problem. Bias or
bias-belted tires will commonly show a featheredge or saw-tooth toe wear
pattern across the entire tire tread area. Any tire wear pattern caused
by a toe condition can be further affected by an excess camber condition
and may result in irregular wear patterns.
Toe
Out On Turns
When a vehicle
is turned, the inner front wheel must toe-out more than the outer wheel.
The
inner wheel must turn this tighter radius to avoid scrubbing. This is
also known as the Ackerman effect. Viewing the vehicle from the top
as it is turning, the front wheels should turn on two different radii.
Toe Out On Turns,
also known as TOOT, is built into the front steering arms and is
not adjustable. Before checking toe out on turns, make sure that all alignment
settings are within manufacturers specifications. If using degree marks
on the turn tables, make sure that the tire/wheel assembly is centered
on the tables, this will reduce erroneous readings. To check toe out on
turns, steer the wheels to the left so that the inner wheel is at 20°,
the out wheel should be less than 20°, optimal reading is 18°.
Repeat the test in the other direction, and determine if there are any
problems be comparing the manufacturers specifications. A variation from
specifications indicates damaged steering arms and one or both arms should
be replaced.
Front
Setback
Front setback is a measurement
referencing the front wheels to a line placed perpendicular to the vehicle
centerline. This line would be parallel to a line drawn through the centers
of the spindle. If a vehicle has setback, one front tire/wheel assembly
is farther back from this imaginary reference line across the front of
the vehicle than the other.
Positive setback
indicates that the right front wheel is setback further than the left.
Negative setback refers to the left front wheel being further back than
the right. Front setback can be checked during a normal alignment, and
is used to diagnose collision damage or cradle mis-adjustment. If the cradle
is adjusted incorrectly, or damage is present, it is not unusual to also
see a reduced positive caster reading on the side with the setback condition.
Excessive setback can cause an alignment pull to the side with the setback.
If the rear axle is positioned correctly and all other parts and systems
of the vehicle are in good working order, a setback condition will also
create different wheelbase measurement side to side.
Steering
Axis Inclination (SAI)
SAI Definition
The angle between the
centerline of the steering axis and vertical line from center contact area
of the tire (as viewed from the front). SAI is typically not adjustable,
but deviations from specification can indicate vehicle damage. A maximum
variation side to side of ± 1.0° may also indicate vehicle damage.
This topic is covered in detailed charts later.
Effects of SAI
SAI urges the wheels
to a straight ahead position after a turn. By inclining the steering axis
inward, it causes the spindle to rise and fall as the wheels are turned
in one direction or the other. Because the tire cannot be forced into the
ground as the spindle travels in an arc, the tire/wheel assembly raises
the suspension and thus causes the tire/wheel assembly to seek the low
(center) return point when it is allowed to return. Thus, since it has
a tendency to maintain or seek a straight ahead position, less positive
caster is needed to maintain directional stability. A vehicle provides
stable handling without any of the drawbacks of high positive caster because
of SAI.
Included
Angle (IA)
I/A Definition
Included Angle is the
combination of SAI and camber. Viewed from the front, the included angle
is SAI plus camber if the camber is positive. If the camber is negative
the included angle is SAI minus camber. If a side to side variation greater
than ± 1.5° exists, check for vehicle damage.
Angle + Camber
= Included Angle (I/A)
Measuring Procedures
SAI should always
be measured after you have adjusted the camber and caster to the proper
specifications or as close to the specifications as possible. Check for
worn suspension parts. SAI is best measured with the front wheels off the
ground, brakes applied and alignment equipment leveled and locked. Raise
the vehicle underneath the lower control arms but, do not relax the suspension.
Not raising the vehicle from the turntables can cause the control arm bushings
to move when wheels are turned, resulting in an inaccurate reading. Always
refer to the manufacturers SAI specifications and measuring procedures.
If the vehicle has a solid front axle, the measurement can be taken with
the wheels on the turntables.
Steering
Axis Inclination
Troubleshooting
Charts
Short
Long Arm (SLA) Chart
|
SAI
|
CAMBER
|
INCLUDED
ANGLE
|
PROBLEM AREA
|
|
More than
Specs
|
Equal to
Specs
|
Less than
Specs
|
Spindle/Knunckle
or Upper Control Arm and/or Control Arm Mount
|
|
Less than
Specs
|
Equal to
Specs
|
More than
Specs
|
Bent Lower
Control Arm and/or Lower Control Arm Mount
|
|
Equal to
Specs
|
More than
Specs
|
More than
Specs
|
Spindle/Knuckle
Assembly
|
|
Less than
Specs
|
More than
Specs
|
Equal to
Specs
|
Bent Lower
Control Arm
|
|
Less than
Specs
|
More than
Specs
|
More than
Specs
|
Spindle/Knuckle
Assembly
Bent Lower Control
Arm
|
|
Equal to
Specs
|
Less than
Specs
|
Less than
Specs
|
Spindle/Knuckle
Assembly
|
|
More than
Specs
|
Less than
Specs
|
Equal to
Specs
|
Bent Upper
Control Arm
|
MacPherson
Strut Troubleshooting Chart
|
SAI
|
CAMBER
|
INCLUDED
ANGLE
|
PROBLEM AREA
|
|
Equal to
Specs
|
More than
Specs
|
More than
Specs
|
Bent Spindle
and/or
Strut Body
|
|
More than
Specs
|
More than
Specs
|
More than
Specs
|
Strut Tower
IN
at
Top and Spindle or Strut Bent
|
|
Less than
Specs
|
More than
Specs
|
Equal to
Specs
|
Bent Control
Arm or Strut OUT at Top and Bent Spindle or Bent Strut Body
|
|
Less than
Specs
|
More than
Specs
|
Less than
Specs
|
Bent Control
Arm or Strut OUT at Top and Bent Spindle or Strut Body
|
|
Less than
Specs
|
More than
Specs
|
More than
Specs
|
Bent Control
Arm or Strut OUT at Top and Bent Spindle or Strut Body
|
|
Equal to
Specs
|
Less than
Specs
|
Less than
Specs
|
Bent Spindle
and/or Bent Strut Body
|
|
Less than
Specs
|
Less than
Specs
|
Less than
Specs
|
Strut Top
or Bent Control Arm and Bent Spindle or Strut Body
|
|
More than
Specs
|
Less than
Specs
|
Equal to
Specs
|
Strut Tower
IN
at
Top
|
Twin
I-Beam Troubleshooting Chart
|
SAI
|
CAMBER
|
INCLUDED
ANGLE
|
PROBLEM AREA
|
|
More than
Specs
|
Equal to
Specs
|
More than
Specs
|
Bent Spindle
|
|
More than
Specs
|
Less than
Specs
|
Equal to
Specs
|
Bent I-Beam
|
|
More than
Specs
|
Less than
Specs
|
More than
Specs
|
Bent I-Beam
and
Bent Spindle
|
|
Less than
Specs
|
More than
Specs
|
Equal to
Specs
|
Bent I-Beam
|
Mono
Beam Troubleshooting Chart
|
SAI
|
CAMBER
|
INCLUDED
ANGLE
|
PROBLEM AREA
|
|
Equal to
Specs
|
More than
Specs
|
More than
Specs
|
Spindle Bent
Downward
|
|
Less than
Specs
|
More than
Specs
|
Equal to
Specs
|
Axle Housing
Bent Downward
|
|
Less than
Specs
|
More than
Specs
|
More than
Specs
|
Spindle Bent
Downward
and
Axle Housing
Bent Downward
|
|
Equal to
Specs
|
Less than
Specs
|
Less than
Specs
|
Spindle Bent
Upward
|
|
More than
Specs
|
Less than
Specs
|
Equal to
Specs
|
Axle Housing
Bent Upward
|
|
More than
Specs
|
Less than
Specs
|
Less than
Specs
|
Spindle Bent
Upward
and
Axle Housing
Bent Upward
|
Scrub
Radius
Scrub Radius Definition
Scrub radius is the
distance at the road surface between the tire line and the SAI line extended
downward through the steering axis. The line through the steering axis
creates a pivot point around which the tire turns. If these lines intersect
at the road surface, a zero scrub radius would be present. When the intersection
is below the surface of the road, this is positive scrub radius. Conversely,
when the lines intersect above the road, negative scrub radius is present.
The point where the steering axis (sai) line contacts the road is the fulcrum
pivot point on which the tire is turned.
Squirm
Squirm occurs when
the scrub radius is at zero. When the pivot point is in the exact center
of the tire footprint, this causes scrubbing action in opposite directions
when the wheels are turned. Tire wear and some instability in corners is
the result.
Applications in Suspensions
MacPherson strut
equipped vehicles usually have a negative scrub radius. Even though
scrub radius in itself is not directly adjustable, it will be changed if
the upper steering axis point or spindle angle is changed when adjusting
camber. This is the case on a MacPherson strut which has the camber adjustment
at the steering knuckle. Because camber is usually kept within 1/4°
side to side, the resulting scrub radius difference is negligible. Negative
scrub radius decreases torque steer and improves stability in the event
of brake failure. SLA suspensions usually have a positive scrub radius.
With this suspension, the scrub radius is not adjustable. The greater the
scrub radius (positive or negative), the greater the steering effort and
the more road shock and pivot binding that takes place. When the vehicle
has been modified with offset wheels, larger tires, height adjustments
and side to side camber differences, the scrub radius will be changed and
the handling and stability of the vehicle will be affected.
Ball
Joint Inspection
Test ball joints
for radial (horizontal) play by grasping the tire at the top and bottom
and attempting to move the ball joint in and out. Also, test load carrying
ball joint for axial play (vertical) by lifting with an appropriate tool
(pry bar). If any loose or bent ball joints are detected, they should be
replaced. Do Not mistake wheel bearing play for ball joint wear.
For suspensions with
coil springs resting on the lower control arm lift as shown below.
This method will
properly unload ball joint for inspection.
For suspensions
with coil springs resting on the upper control arm, lift as illustrated
at right. This will properly unload the ball joint for inspection.