Long ago, we worked in a very small and ill equipped shop. When
we got a complaint about tire wear or pulling, or after installing new
steering or suspension parts, we set camber to zero using a modified carpenter´s
level. For caster, we just gave a generous amount commensurate with camber
and the range of adjustment available. Our slip plates of sheet metal
with grease between them, an a telescoping rod measuring toe. If we suspected
a cocked rear axle, wheelbase and diagonal measurements were taken with
a measuring tape.
With a thorough understanding of alignment theory, such methods could
get the job done (albeit slowly) on cars with solid rear axles and in
the days when things were not so critical. But just as with everything
else automotive alignment is no longer simple. Accurate settings and productivity
are much more important than ever before, and IRS (Independent Rear Suspension)
is now a huge factor in the vehicle population, all of which makes old
fashioned front-wheel-only alignment a woefully inadequate procedure.
As a marketing executive for one of the major alignment equipment manufacturers
says, "Doing two wheel alignment is essentially a disservice to the
customer. If you don´t know which way the rear wheels are pointing,
you might just make things worse than they were when the car came in".
So, referencing to the rear is becoming mandatory.
Dynamics
Before we go any further, we had better make sure you realize that wheel
alignment is a delicate dynamic balance of many different factors: tire
type, size, tread design, and inflation, weight distribution, the condition
of the suspension and steering comonents, ride height, the proper positioning
of mounting points, the adjustments themselves, etc. If you alter any
one factor, you are apt to affect the others.
You need a firm understanding some terms before you can consider yourself
competent at alignment, so here is a review of the traditional, plus a
few other angular relationship that may be new to you, but which are becoming
more an more important. Try to visualize the explanations that follow.
Tilt
Camber is the tilt of the wheels when viewed from the front. If the top
of the tire leans outward from the car camber is said to be positive,
and if it leans inward, it is negative. Zero camber , where the wheel
is exactly perpendicular to the road surface, is the ideal setting for
longest tire life, positive or negaive settings may be used to achieve
specific results. A small amount of plus camber, for instance, helps compensate
for future sagging of the suspension, while in racing cars, a minus adjustment
is often used to produce better tire adhesion in curves. It is also common
to employ an extra ¼ to ½ degree of positive camber on the
wheel closest to the center of the road to keep the car tracking straight
on crowned pavement.
All other things being equal, a vehicle will lead or pull toward the side
with the most positive camber. This is because a tilted tire acts like
a section of a cone, and a cone, of course always rolls in a circle.
Master caster
The idea of caster is more difficult to grasp. It is the fore and aft
tilt of the steering axis (an imaginary line drawn through both ball joints
on suspension with upper and lower control arms, or through the lower
ball joint and the top pivot point of a MacPherson strut) when viewed
from the side of the vehicle. If this line intersects the road ahead of
the point of tire contact, caster is said to be positive, and vice versa.
To illustrate an ordinary furniture caster trails behind the spot at which
a line drawn down the leg would touch the floor, so it has positive caster.
Also, the front fork of a bicycle has a plus setting.
Positive caster contributes to the tendency of a car to track straight,
and for the wheels to return to the straight-ahead position setting, however,
will reslut in hard steering, increased road shock transmission, and wheel
shimmy. Negative caster makes a vehicle easier to steer, but may cause
it to wander from a straight path. A car may lead toward the side with
the most negative caster, and, since rolling resistance tends to pull
the point of tire contact back, a zero setting may actually become positive
in motion.
Using gravity to steer straight
Steering Axis Inclination (S.A.I., also called "ball joint inclination"
or "king-pin inclination") is the angle between the steering
axis (as described above) and the vertical when viewed from the front.
This is designed into the suspension geometry to cause the spindle to
describe an arc when the wheels are turned. The high point of the arc
is the straight-ahead position, so when you steer to either side, you
are actually lifting the car slightly. Therefore, gravity helps return
the vehicle to a straight course. This angle also reduces road shocks.
Parallel?
Toe is the parallelism of the front wheels when the car is viewed from
the top. Tires last longest if they run in parallel paths, so you would
expect a zero setting to be best. Lash and flexing in the suspension and
steering components, however, allow the leading edges of the tires to
spread away from each other under rolling stresses, so usually the static
setting is slightly toed-in. On the other hand, in front wheel-drive cars
the tires tend to pull in under power, so sometimes a small toe-out setting
is specified.
In a curve, the outside wheel makes a bigger circle than the inside wheel.
If both were steered the same amount, sideways tire scuffing would result.
To avoid this condition, steering geometry usually includes a feature
known as "toe-out on turns". This causes the outer wheel to
be turned fewer degrees than the inner one so that they run in concentric
arcs.
Some authorities claim that this is not necessary with radial tires because
they flex enough to compensate for small angular discrepancies, but most
cars still have it designed in.
Thrust line
Those are the basics involved in two-wheel alignment, but today it is
important to take the relationship among all four wheels into account.
Thrust line (also called "thrust angle") is the first thing
to understand. Is it the same as a vehicle´s geometric centerline?
Ideally yes, but not necessarily. The centerline is drawn through points
midway between each pair of wheels, but the thrust angle line is the perpendicular
of the rear acle on solid axle cars, or, with IRS, a line derived by splitting
the toe of the rear wheels (if the right side is toed-in four degrees,
and the left is zero, the thrust line will veer off two degrees to the
left of the centerline). In other words, it is the direction in which
the rear wheels roll.
If the thrust and centerlines coincide, all is well. but given the size
of a vehicle, the tolerances of manufacture, stresses, wear, and mishap,
it is rare if they do. The question is, how big is the deviation? If it
is barely discernible, it is nothing to worry about. But if it is considerable,
it will throw the dynamics off. To illustrate: You align the front wheels
to perfection, but without reference to the rear. Unbeknownst to you,
the frame or a control arm is bent. You send the car out, and, like a
boomerang, it comes right back. The complaint? The steering wheel is not
centered, a condition that irritates people and is considered by many
experts to be the most common cause of alignment comebacks. What has happened
is that the rear wheels are steering the car away from its centerline
and the driver has to tujrn the wheel to one side to keep the car going
straight (if he did not, he would go around in circles). To put it another
way, the tracks the rear tires make are beside those of the fronts, and
you are dealing with what has traditionally been called a "dogtracker"
( a name derived from the canine habit of loping along sideways).
On all fours
Thrust line has always been important, but with solid axles both rear
wheels were necessarily parallel, and dogtracking only existed if the
frame was bent from contact with an immovable object, or the spring shackles
or rear control arm bushings were worn. With FWD and IRS, you are seeing
more and more cases of rear toe mislalignment, which will result not only
in a cocked steering wheel, but also in accelerated tire wear and poor
driving characteristics. Of course, you can set rear toe, then swap ends
and align the front, but if your equipment, cannot reference to the thrust
line, you are not doing a real four-wheel job.
Between basic two-wheel alignment (toe set to the centerline) and the
full four-wheel treatment (thrust line and centerline brought together
by setting rear toe, then front toe adjusted to this reference), there
is antoher level of service called "thrust line alignment".
That is where the thrust line is found (but not adjusted) then front toe
is set to it rather than to the centerline. It will get that steering
wheel straight, but obviously will not cure dogtracking. It is often the
thing to recommend for solid axle cars and those with non-adjustable IRS,
or as a less-expensive alternative for rear suspentions that can be altered.
Tires first
With those principles in mind, we will move on to some of the things you
will encounter in the real world that affect alignment. The most basic
of these is tires. It makes no sense to try to align away problems caused
by mismatched, unevenly inflated, or badly worn tires. If the vehicle
shimmies, check the tires for proper balance and runout before suspecting
alignment setttings or loose front end parts. Pulling to one side may
be due to a faulty adjustment, but if the car has radials, test for "plysteer"
first by switching the rear tires to the front one at a time.
Sagging springs?
Ride height, while frequently overlooked, is crucial to proper alignment
and vehicle dynamics. With short-and-long arm or strut suspension, camber
changes occure on spring compression and rebound. So, when the springs
sag, the tires are forced to work at an improper angle to the road and
will wear at their edges. Also, this change in height affects the inclination
of the tie rods, altering their effective length and changing the toe
setting.
Returning a vehicle to its original height often brings alignment back
into specifications without any adjustment being needed. The installation
of new springs, of course, is the most straightforward method of accomplishing
this, but it is also the most expensive. Spring-boosted shock absorbers
are an alternative for non-strut suspension, as are rubber spring mounting
pads. On many torsion bar designs, all that it necessary is to turn a
bolt that increases preload.
The condition of all the front suspension components is very important.
You will never get satisfactory results if you try to align a front end
that has excessive play in its joints. Lash in the steering box is a related
area.
Today´s trouble
What the most common alignment-related problems on new cars? Besides the
dreaded uncentered steering wheel, there is diagonal rear tire wear. Because
of radials and light weight, cars are far more sensitive to the accuracy
of any adjustments than were their predecessors, so you have got to be
especially careful. Doing rear camber and toe without benefit of slip
plates is another potential source of trouble.
But to many auto service technicians, the most frustrating complication
in alignment today is suspensions that have little or no provision for
adjustment - not only on front struts, but also on numerous types of IRS.
In these cases, the first step should be finding the reason for any misalignment
your equipment reveals. This entails, first, examining all the suspension
components from the struts and springs to the pivot and mounting points.
Something as simple as replacing a deteriorated bushing or sagging springs
will often put the car back within specs.
If there is nothing wrong so far, acquire a chassis/body specification
book and start taking measurements to see if something is no longer where
it is supposed to be, then have the frame straightened accordingly to
re-establish dimensional perfection.
Of course, our helpful aftermarket has not failed us where adjusting the
non-adjustable is concerned. Tapered shims, wedges, camber kits, etc.
that really work are all available from reputable companies, and you would
be making life more difficult than it has to be by not using them.