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Wheel Alignment
should be checked whenever new tires are installed, suspension
components installed, when the vehicle has encountered a major road
hazard or curb and any time unusual tire wear patterns appear.
Wheel Alignment is the Measurement of complex suspension
angles and the adjustment of a variety of
suspension components. It is a suspension-tuning tool which greatly influences the vehicle's handling
and tire wear.
Wheel alignment consists of adjusting the angles of the wheels so that
they are parallel to each other and perpendicular to the ground, thus
maximizing tire life and ensures straight and true tracking along a
straight and level road.
The primary static suspension angles that need to be
measured and adjusted are caster, camber, toe and thrust angle.
The following are
definitions Conditions
and Possible Causes of each
angle and its influence on a vehicle and its tires.
Camber
Camber
is the angle of the wheel, measured in degrees, if the top of the wheel
is tilted out then the camber is positive, if it's tilted in, then the
camber is negative.
If the camber is out of adjustment, it will cause
premature tire wear on one side of the tire's thread. When the camber is
out of adjustment it can cause a pulling problem to the side with the
more positive camber.
This usually happens when the vehicle has been involved in an accident
which has caused structural damage or damage to the strut and / or
spindle assembly. Camber also goes out of adjustment when the springs
sag and causes ride height to change, or when ball joints and or other
attached parts are worn or defective. It also varies depending on speed
as aerodynamic forces changes riding height.
After repair and alignment,
pulling problem could persist due to the insufficient and or uneven tire
to road contact. If a tire shows camber wear pattern, moving it to the
rear might be effective but replacement might be best.
Whenever camber
changes, it directly affects toe.
On most front-wheel-drive vehicles, camber is not
adjustable, however there are
aftermarket kits that allow sufficient adjustment to compensate for
accident damage or the change in alignment due to the installation of
lowering springs.
Caster
Caster is the angle of the steering
pivot, measured in degrees.
Viewed from the
side, the caster is the tilt of the steering axis. When the wheel is in
front of the load the caster is positive.
Three
to five degrees of positive caster is the typical range of settings,
with lower angles are being used on heavier vehicles to reduce steering
effort.
If the caster
is out of adjustment, it can cause problems in straight-line tracking.
If the caster is different from side to side, the vehicle will pull to
the side with the less positive caster. If the caster is equal but too
negative, the steering will be light and the vehicle will wander and be
difficult to keep in a straight line. If the caster is equal but too
positive, the steering will be heavy and the steering wheel may kick
when you hit a bump.
Caster has little or no effect on tire wear.
One of the best ways to visualize caster is to picture the caster on a
shopping cart. The pivot while not at an angle intersects the ground
ahead of the wheel contact patch. When the wheel is behind the pivot at
the point where it contacts the ground, it is in positive caster.
Like
camber, on many front-wheel-drive vehicles, caster is not adjustable. If
the caster is out of adjustment on these vehicles, it indicates that
something is possibly bent from an accident, and must be repaired or
replaced.
Toe
The vehicle's
toe is the most critical alignment settings relative to tire
wear. if the toe setting is just 1/32-inch off of its appropriate
setting, each tire on that axle will scrub almost 3 1/2 feet sideways
every mile, therefore reducing tire life.
Like
camber, toe will change depending on vehicle speed, as aerodynamic
forces changes the riding height hence affecting camber and toe due to
the geometry of the steering linkage in relation to the geometry of the
suspension.
The
toe angle identifies the direction of the tires compared to the
centerline of the vehicle. Rear-wheel drive vehicle "pushes" the front
tires, as they roll along the road, resistance causes some drag
resulting in rearward movement of the suspension arms against their
bushings. Most rear-wheel drive vehicles use positive toe to compensate
for suspension movement.
Front-wheel
drive vehicle "pulls" the vehicle, resulting in forward movement of the
suspension arms against their bushings. Most front-wheel drive vehicles
use negative toe to compensate for suspension movement.
Toe can also be
used to alter a vehicle's handling traits. Increased toe-in will reduce
oversteer, steady the car and enhance high-speed stability.
Increased
toe-out will reduce understeer, free up the car, especially during
initial turn-in while entering a corner.
Before
adjusting toe outside the vehicle manufacturer's specification to
manipulate handling, be aware that toe setting influences tire wear.
Excessive toe settings often causes drivability problems,
especially during heavy rain. This is because most highways have tire
groves from the daily use by loaded tractor trailers. These heavy
vehicles leave groves that fill with water. When one of the vehicles
front tire encounters a puddle, it loses some of its grip, the other
tire's toe setting will push causing excessive toe-in, or pull causing
excessive toe-out. This may cause the vehicle to feel unstable.
Steering Axis Inclination (SAI)
Steering Axis Inclination (SAI)
is the
measurement in degrees of the steering pivot line when viewed from the
front of the vehicle. On a SHORT-LONG ARM (SLA) SUSPENSION the line runs
through the upper and lower ball joints.
On a MacPherson strut
suspension; the line runs through the lower ball joint and upper strut
mount or bearing plate. This angle (SAI), when added to the camber to
forms the
included angle and causes the vehicle to lift slightly when the
wheel is turned from a straight position. The vehicles weight pushes
down and causes the
steering wheel to return to the center when you let go of it after
making a turn.
Like caster, it provides directional stability and also
reduces steering effort by reducing the scrub radius.
If the Steering
Axis Inclination (SAI) is different from side to side, it will cause a
pull at very slow speeds. SAI is a nonadjustable angle, it is used with camber and
the included angle to diagnose bent spindles, struts and mislocated
crossmembers.
The most likely cause for Steering Axis Inclination (SAI)
being out of specification
is bent parts, which has to be replaced to correct the condition. On
older vehicles and trucks with king pins instead of ball joints,
Steering Axis Inclination (SAI) is referred to as (KPI) King Pin
Inclination.
Included Angle
Included angle is the sum of
the Camber and Steering Axis Inclination (SAI)
angles Included angle is not directly
measurable. It is used primarily to diagnose bent suspension parts.
If
the camber is negative, then the included angle will be less than the
Steering Axis Inclination (SAI), if the camber is positive, it will be
greater.
The included angle must be the same from side to side even if
the camber is different. If there is a difference, then something is
bent, possibly the steering knuckle.
Scrub Radius
Scrub Radius
is the distance between the extended centerline of the
steering axis and the centerline of the tire where the tread contacts
the road. This distance must be exactly the same from side to side or
the vehicle will pull strongly.
If the steering centerline is inboard of the tire
centerline, the scrub radius is positive. If the steering centerline is
outboard of the tire centerline, the scrub radius is negative.
Rear-wheel drive cars and trucks generally have a positive scrub radius
while FWD cars usually have zero or a negative scrub radius because they
have a higher Steering Axis Inclination (SAI),
angle.
Using different wheels other than stock can
alter the scrub radius.
Riding Height
Riding height
is usually measured in inches, from the rocker panel to the ground. A good
wheel alignment charts should provide specs, but the main thing is that the
measurements should be within one inch from side to side and front to
rear.
Riding height is not usually adjustable except on vehicles with
torsion bar type springs, coil-over and some air suspensions.
On a
nonadjustable type suspensions, springs replacement is best way to fix
this problem.
Note: Springs
should only be replaced in pairs. Changes in riding height affect camber
and toe, so if springs are replaced or torsion bars are adjusted, the
wheel must be aligned to avoid tire wear.
Set Back
Set back is
when one front wheel is set further back than the other. With alignment
equipment that measures toe by using only the front instruments, any
setback will cause an uncentered steering wheel. Any good 4-wheel
aligner will reference the rear wheels when setting toe in order to
eliminate this problem.
Some good
alignment equipment will measure set back and give you a reading in
inches or millimeters.
Some manufacturers consider a set back of less than 1/4-inch normal
tolerance. More than that and there is a good chance that something is
bent.
Setback is Caused By: Manufacture or Collision.
Thrust Angle
Thrust angle is
the direction that the rear wheels are pointing in relation to the
centerline of the vehicle.
The vehicle will "dog track" if the thrust
angle is not zero and the steering wheel will not be centered.
The best
solution is to first adjust the rear toe to the centerline and then
adjust the front toe. This is done during a all wheel alignment if the
rear toe is adjustable. If the rear is not adjustable, then the front
toe must be set to compensate for the thrust angle, allowing the
steering to be centered.
If the thrust
angle is not correct on a vehicle with a solid rear axle, it often
requires a frame straightening shop to correctly reposition the rear
axle.
A vehicles with
independent rear suspension, the toe must be adjusted individually until
it has reached the appropriate setting for its side of the vehicle,
incorrect thrust angle is often caused by an out-of-position suspension
or incorrect toe settings.
So in addition to the handling problems that
are the result of incorrect toe settings, thrust angles can also cause
the vehicle to handle differently when turning left vs. right.
Alignment
Ranges
The vehicle
manufacturers' alignment specifications usually identify a "preferred"
angle for camber, caster and toe (with preferred thrust angle always
being zero). The manufacturers also provide the acceptable "minimum" and
"maximum" angles for each specification. The minimum and maximum camber
and caster specifications typically result in a range that remains
within plus or minus 1-degree of the preferred angle.
If for whatever
reason your vehicle can't reach within the acceptable range, replacing
bent parts or an aftermarket alignment kit will be required. Fortunately
there is a kit for almost every popular vehicle due to the needs of body
and frame shops doing crash repairs and driving enthusiasts tuning the
suspensions on their cars.
Steering Center
Steering center
is that the steering wheel is centered when the vehicle is traveling
down a straight and level road. However most roads are crowned to allow
for water drainage, this may cause the vehicle to drift to the right so
the steering wheel will appear to be off-center to the left on a
straight road. to compensate for this
· The
left caster should be more negative than the right, but not more than
1/2 degree within the specified range.
· The
left camber should be more positive than the right camber. Check the
specs to see what the allowable differences.
A crooked steering wheel is one of the most common complaints after a
wheel alignment. Steering center is controlled by the front and rear toe
settings, when setting steering center, the rear toe should be set first
bringing the Thrust Angle as close to the vehicle centerline as
possible. the steering wheel is then locked in a straight-ahead position
in order to set the front toe. Please note; before locking the steering
wheel, the engine should be started and the wheel turned right and left
a couple of times. This will take any stress off the power steering
valve. Repeat the above starting and turning of the steering after
setting front toe to ensure that the steering valve wasn't loaded again
due to the tie rod adjustments.
Toe Out on Turns
When you steer
a car through a turn, the outside front wheel has to navigate a wider
arc than the inside wheel. For this reason, the inside front wheel must
steer at a sharper angle than the outside wheel.
Toe-out on turns is
measured by the turning angle gauges (turn plates) that are a part of
every wheel alignment machine. The readings are either directly on the
turn plate or they are measured electronically and displayed on the
screen. Wheel alignment specifications will usually provide the
measurements for toe-out on turns. They will give an angle for the
inside wheel and the outside wheel such as 20º for the inside wheel and
18º for the outside wheel. Make sure that the readings are at zero on
each side when the wheels are straight ahead, then turn the steering
wheel so that the inside wheel is at the inside spec. then check the
outside wheel. The toe-out angles are accomplished by the angle of the
steering arm. This arm allows the inside wheel to turn sharper than the
outside wheel. The steering arm is either part of the steering
knuckle or part of the ball joint and is not adjustable. If there is a
problem with the toe-out, it is due to a bent steering arm that must be
replaced.
Wheel Offset
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The offset of a
wheel is the distance from its hub mounting surface to the
centerline of the wheel. The offset can be one of three types.
Zero Offset
The
hub mounting surface is even with the centerline of the wheel.
Positive
The
hub mounting surface is toward the front or wheel side of the
wheel. Positive offset wheels are generally found on front wheel
drive cars and newer rear drive cars.
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Negative
The
hub mounting surface is toward the back or brake side of the
wheels centerline. "Deep dish" wheels are typically a negative
offset.
If
the offset of the wheel is not correct for the car, the handling
can be adversely affected. When the width of the wheel changes,
the offset also changes numerically. If the offset were to stay
the same while you added width, the additional width would be
split evenly between the inside and outside. For most cars, this
won't work correctly. |
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