If you
could squash a tire and wheel into a flat disk, then place that flat disk on a point at
the center of the assembly (figure 1), you could add weights until the disk appeared to be
"balanced" (figure 2).
Of course, you can’t really do that, but that’s the idea
behind static balancing.
What happens if static balance is out?
Static imbalance tends to show up as a "hopping," or up-and-down motion.
This can lead to severe irregular wear, sometimes of the "diagonal
wear" type, and in some cases, to serious ride disturbances.
And that’s corrected by static balancing?
It may be. Unfortunately, treating a wheel-end as though it were a
flat disk can oversimplify the situation.
How so?
Let’s look at the previous example, but using a more accurate method.
Imagine that each half
of the assembly is represented by a separate
disk (figure 3). The upper disk represents only
the upper half of the assembly, and the lower
disk only the
lower half.
Neither of these disks is in balance, even
though the whole assembly is still in static
balance. So, even if you use static balancing, you can
still have problems on the road. Such a "dynamic"
imbalance usually causes a wobbling motion, and may
be accompanied by rapid tire wear
So what’s the answer?
Dynamic balancing often helps correct problems that static balancing cannot. The
equipment is more complex, but most dynamic balancing systems actually measure forces
generated by the rotating assembly.
When dynamic balance is achieved (figure 4), static balance is automatic,
so a separate static balance is unnecessary.
Is on-vehicle balancing static or dynamic?
On-vehicle systems spin the entire assembly, which may compensate for non-uniformity in
hubs, brake drums and axles, but unfortunately, they do not dynamically balance the
assembly.
And, to be really effective, any spin balancing system, whether on-
or off-vehicle, must be sensitive enough to detect small
imbalance forces. Truck tires typically turn close to about
500 RPM at highway speeds, which greatly magnifies even
very small imbalance forces.
What about duals?
As far as we know, no one makes equipment to balance dual assemblies.
At the moment, the best approach is to balance each tire
and wheel separately — before bolting them together.
When should a wheel and tire assembly be balanced?
Some fleets routinely balance. Some balance only tractors, and some only the steer
positions of tractors. Others balance only if there has been a wear or ride disturbance
complaint.
It’s best to test it yourself, using two groups of nearly identical equipment,
balancing one and not the other. Track tire performance, being sure to note rate of wear,
irregular wear and ride disturbance complaints. Then, let your own experience be your
guide.
Bridgestone also recommends you make balancing the first step in troubleshooting
any vibration complaint. It’s fairly fast, fairly simple
and inexpensive, and can alleviate many wear and ride problems.
