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Know...
How Your Car Works? |
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Photo courtesy
Hunter Engineering Co. | In its most basic form, a wheel alignment consists of
adjusting the angles of the wheels, so that they are perpendicular to
the ground and parallel to each other. The purpose of these
adjustments is maximum tire life and a vehicle that tracks straight
and true when driving along a straight and level road. This article begins with information that any motorist should know; however, if you are interested in learning more about
this topic, click on the underlined words for more detailed
explanations of each term. We will cover various levels of detail
with the deepest levels containing information that even a wheel
alignment technician will find informative. Wheel Alignment is often confused with
Wheel Balancing. The two really have nothing to do with each
other except for the fact that they affect ride and handling.
If a wheel is out of balance, it will cause a vibration at highway
speeds that can be felt in the steering wheel and/or the seat.
If the alignment is out, it can cause excessive tire wear and
steering or tracking problems. If you know anything about wheel alignment, you
have
probably heard the terms Camber, Caster and
Toe-in.
Camber
 Camber is the angle of the wheel, measured in
degrees, when viewed from the front of the vehicle. If the top of
the wheel is leaning out from the center of the car, then the camber
is positive ,if it's leaning in, then the camber is negative. If the
camber is out of adjustment, it will cause tire wear on one side of
the tire's tread. If the camber is too far negative, for instance,
then the tire will wear on the inside of the tread.
 Camber wear patternIf the camber is different from side to side it can
cause a pulling problem. The vehicle will pull to the side with the
more positive camber. On many front-wheel-drive vehicles, camber is
not adjustable. If the camber is out on these cars, it indicates
that something is worn or bent, possibly from an accident and must be repaired or replaced. Caster  When you turn the steering wheel, the front wheels
respond by turning on a pivot attached to the suspension system.
Caster is the angle of this steering pivot, measured in degrees,
when viewed from the side of the vehicle. If the top of the pivot is
leaning toward the rear of the car, then the caster is positive, if
it is leaning toward the front, it is negative. 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 affect on tire wear.
 The best way to visualize caster is to picture
a shopping cart caster. The pivot of this type of caster, 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. Picture yourself trying to
push the cart and keep the wheel ahead of the pivot. The wheel will
continually try to turn from straight ahead. That is what happens
when a car has the caster set too far negative. Like camber, on many
front-wheel-drive vehicles, caster is not adjustable. If the caster
is out on these cars, it indicates that something is worn or bent,
possibly from an accident, and must be repaired or replaced. Toe-in  The toe measurement is the
difference in the distance between the front of the tires and the back of the tires. It is measured in fractions of an inch in the US
and is usually set close to zero which means that the wheels are
parallel with each other. Toe-in means that the fronts of the tires
are closer to each other than the rears. Toe-out is just the
opposite. An incorrect toe-in will cause rapid tire wear to both
tires equally. This type of tire wear is called a saw-tooth wear
pattern as shown in this illustration.
If the sharp edges of the tread sections are pointing to the center of the car, then there is too
much toe-in. If they are pointed to the outside of the car then
there is too much toe-out. Toe is always adjustable on the front
wheels and on some cars, is also adjustable for the rear
wheels.
Four Wheel Alignments
There are two main types of
4-wheel alignments. In each case, the technician will place an
instrument on all four wheels. In the first type the rear toe and
tracking is checked, but all adjustments are made at the front
wheels. This is done on vehicles that do not have adjustments on the
rear. The second type is a full 4-wheel alignment where the
adjustments are first made to true up the rear alignment, then the
front is adjusted. A full 4-wheel alignment will cost more than the
other type because there is more work involved.
Following are the other facts that every driver should
know about wheel alignments:-
1) A proper wheel alignment should always start and end
with a test drive. 2) The front end and steering linkage should be checked
for wear before performing an alignment.
 3) The tires should all be in good shape with even wear patterns. If you have a tire with
excessive camber wear, for instance, and you correct the alignment
problem that caused that wear, the tire will now be making only
partial contact with the road. (see illustration on right) 4)
Pulling problems are not always related to wheel alignment.
Problems with tires (especially unequal air pressure), brakes and
power steering can also be responsible. It is up to a good wheel
alignment technician to determine the cause.
While Camber, Caster & Toe-in are the settings
that are always checked when doing a wheel alignment, they are not
the only settings. Below is a list of the alignment settings that
are important for a wheel alignment technician to know about in
order to diagnose front end problems.
To find out more about each of these measurements,
read below :- 1) Camber
2) Caster
3) Toe
4) Steering
Axis Inclination (SAI)
5) Included
Angle 6) Scrub
Radius 7) Riding
Height 8) Set
Back 9) Thrust
Angle 10) Steering
Center
11) Toe
Out on Turns
When camber specifications are
determined during the design stage, a number of factors are taken
into account. The engineers account for the fact that wheel
alignment specifications used by alignment technicians are for a
vehicle that is not moving. On many vehicles, camber changes with
different road speeds. This is because aerodynamic forces cause a
change in riding height from the height of a vehicle at rest.
Because of this, riding height should be checked and problems
corrected before setting camber. Camber specs are set so that when a
vehicle is at highway speed, the camber is at the optimal setting
for minimum tire wear. For many years the trend has been
to set the camber from zero to slightly positive to offset vehicle
loading, however the current trend is to slightly negative settings
to increase vehicle stability and improve
handling.
Positive caster improves straight line tracking because the caster line (the line drawn through the steering pivot
when viewed from the side) intersects the ground ahead of the
contact patch of the tire. Just like a shopping cart caster, the
wheel is forced behind the pivot allowing the vehicle to track in a straight line. If this is the case, then why did most cars have
negative caster specs prior to 1975 ? There are a couple of reasons
for this. In those days, people were looking for cars that steered
as light as a feather, and cars back then were not equipped with
radial tires. Non-radial tires had a tendency to distort at highway speed so that the contact patch moved back past the centerline of
the tire (Picture a cartoon car speeding along, the tires are
generally drawn as egg-shaped). The contact patch generally moves behind the caster line causing, in effect, a positive caster. This
is why, when you put radial tires on this type of car, the car
wanders from side to side and no longer tracks straight. To correct
this condition, re-adjust the caster to positive and the car should steer like a new car.
Like camber, toe will change depending on vehicle speed. As aerodynamic forces change the riding height, the toe setting may change due to the geometry of the steering linkage in
relation to the geometry of the suspension. Because of this, specifications are determined for a vehicle that is not moving based
on the toe being at zero when the vehicle is at highway speed. In
the early days prior to radial tires, extra toe-in was added to
compensate for tire drag at highway speed. On some older alignment machines, toe-in was measured
at each wheel by referencing the opposite wheel. This method caused
problems with getting the steering wheel straight the first time and
necessitated corrective adjustments before the wheel was straight.
Newer machines reference the vehicle's centerline by putting
instruments on all four wheels. For more information on this see
Steering
Center and Thrust
angle.
 SAI is the measurement in degrees of the steering pivot
line when viewed from the front of the vehicle. This angle, when
added to the camber to form the included
angle (see below) causes the vehicle to lift slightly
when you turn the wheel away from a straight ahead position. This
action uses the weight of the vehicle to cause the steering wheel to
return to the center when you let go of it after making a turn.
Because of this, if the SAI is different from side to side, it will
cause a pull at very slow speeds. Most alignment machines have a way
to measure SAI; however it is not separately adjustable. The most
likely cause for SAI being out is bent parts which must be replaced
to correct the condition. SAI is also referred to as KPI (King Pin
Inclination) on trucks and old cars with king pins instead of ball
joints.
 Included angle is the angle formed between the
SAI and the camber. Included angle is not directly measurable. To
determine the included angle, you add the SAI to the camber. If the
camber is negative, then the included angle will be less than the
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 it is not the same, then something is bent, most
likely the steering knuckle.
 Scrub radius is the distance between where the SAI
intersects the ground and the center of the tire. This distance must be exactly the same from side to side or the vehicle will pull strongly at all speeds. While included angle problems will affect
the scrub radius, it is not the only thing that will affect it.
Different wheels or tires from side to side will cause differences
in scrub radius as well as a tire that is low on air. Positive scrub
radius is when the tire contact patch is outside of the SAI pivot,
while negative scrub radius is when the contact patch is inboard of
the SAI pivot (front wheel drive vehicles usually have negative scrub radius).
If the brake on one front wheel is not working, with
positive scrub radius, stepping on the brake will cause the steering
wheel to try to rip out of your hand. Negative scrub radius will
minimize that effect.
Scrub radius is designed at the factory and is not
adjustable. If you have a vehicle that is pulling even though the
alignment is correct, look for something that will affect scrub
radius.
Riding
Height  Riding height is measured, usually in inches,
from the rocker panel to the ground. Good wheel alignment charts
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 adjustable except on vehicles with torsion bar type springs. The best way to fix this problem is to replace the springs (Note: springs should only be replaced in matched pairs).
Changes in riding height will affect camber and toe so if springs
are replaced or torsion bars are adjusted, then the wheel alignment
must be checked to avoid the possibility of tire wear. It is
important to note that the only symptom of weak coil springs is a sag in the riding height. If the riding height is good, then the springs are good.
 Set back is when one front wheel is set further back than the other wheel. 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. A set back of less
than 1/4 inch is considered normal tolerance by some manufacturers.
More than that and there is a good chance that something is bent.
 Thrust angle is the direction that
the rear wheels are pointing in relation to the center line of the
vehicle. If the thrust angle is not zero, then the vehicle will "dog
track" and the steering wheel will not be centered. The best solution is to first adjust the rear toe to the center line and then
adjust the front toe. This is normally done during a 4-wheel
alignment as long as 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.
 Steering center is
simply the fact that the steering wheel is centered when the vehicle
is traveling down a straight and level road. A crooked steering
wheel is usually the most common complaint that a customer has after
a wheel alignment is performed. Assuming that the steering wheel
stays in the same position when you let go of the wheel (in other
words, the car is not pulling), then 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. Then the steering
wheel is locked in a straight ahead position while the front toe is
set. Before locking the steering wheel, the engine should be started
and the wheel should be turned right and left a couple of times to
take any stress off the power steering valve. After setting the toe,
the engine should be started again to be sure that the steering
valve wasn't loaded again due to the tie rod adjustments. Of course,
you should always road test the vehicle after every alignment as a
quality control check. Another problem with
steering center has to do with the type of roads that are driven on.
Most roads are crowned to allow for water drainage, and unless you
drive in England, Japan or another country where they drive on the
wrong (sorry) left side of the road, you usually
drive on the right side of the crown. This may cause the vehicle to
drift to the right so that the steering wheel will appear to be
off-center to the left on a straight road. The best way to
compensate for this is as follows:
If there is a
difference in caster, it should be that the left wheel is more
negative than the right wheel, but not more than 1/2 degree.
Check the specs for any specific recommendations on side-to-side
differences. If there is a
difference in camber, then the left wheel should be more
positive than the right wheel. Check the specs to see what the
allowable difference is.
 When you steer a car through a turn, the
outside front wheel has to navigate a wider arc then 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.
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