Probably the one we think of first is reduced operating
costs through longer tire life. But that's just
the beginning. Not only do tires on a properly aligned
vehicle last longer, but some manufacturers suggest
that there are significant improvements in fuel
economy, component wear, even less driver fatigue.
Does the whole vehicle have to be aligned?
Ideally, yes. Many of us tend to think of alignment
on trucks the way we think of it on our personal
automobiles. And there, generally, we think only
of aligning the "front end." On a passenger
car, that may be enough. But on a vehicle that has
tandem axles, like a Class 8 truck, aligning only
the front end is only doing a small part of the
job.
Why is that?
Because, while a misaligned front end on a truck
can cause similar problems to one on an automobile,
the other axles can have a huge effect, too. In
fact, we're going to look at alignment in order
of which axle set tends to cause the most problems.
Where do we start?
Let's begin by talking for a moment about vehicle
"tracking." In a perfect world, trucks
would travel only in perfectly straight lines from
one location to another, never having to turn. And
tires, by their nature, would last longest if they
only rolled in perfectly straight lines.
If this
were the case, as the truck went down the road,
all the axles would be absolutely parallel, and
all would be perfectly perpendicular to the vehicle
centerline.
And
this, incidentally, is one of the reasons long distance
line haulers tend to have such long tire life—because
they travel in what amounts to mostly straight lines.
There's very little in the way of side forces acting
to scrub rubber off their tire treads.
But trucks have to turn.
Of course. And whenever a tire turns, side forces
act on it that cause the tread to wear. The trouble
is sometimes a tire is turned, and it's so slight
that we barely notice. And that's what alignment
is all about, trying to remove even the tiniest
turning side forces.
So which axle is most critical?
The drive tandem. Alignment experts tell us that
this is the most important set of axles to have
properly aligned, and the one that can cause the
most trouble if it is not.
That's
because even if all the other axles are correct,
misaligned drives can force the vehicle to track
improperly. And there are two basic ways that drive
axles can be misaligned.
In
one case, if both axles are more or less parallel
but not perpendicular to the vehicle centerline,
we have what is called a "thrust" angle
problem. As the diagram shows, the drive axles are
trying to push the vehicle away from the centerline.
If
the axles are parallel, the problem is described
as a "scrub" angle problem. In this case,
the drive axles are trying to turn the vehicle.
Either
way, to bring the truck's travel back into a straight
line, the driver has to turn the steering wheel.
Because the "thrust" or "scrub"
forces from the drive axles are more or less constant,
the driver's steering imput has to be constant.
