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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