The Tire Doctor Responds:
Why should I balance my tires?
First of all, you dont really balance "tires." You balance tire and wheel assemblies, since virtually all balancing methods work with mounted tires.
If you could, youd balance the entire wheel-end, including the hub, brake drum and axle. But thats not always practical.
And the reason for balancing is that an out-of-balance condition can cause vibration that may lead to wear problems or ride disturbances.
What puts things out-of-balance?
Its not a perfect world, and manufacturing isnt perfect either. A tire cannot be made perfectly uniform nor can a wheel, an axle, a hub or brake drum.
Variations in weight can occur within a component. One part of a wheel or tire can be heavier than another. A valve stem might unbalance an otherwise well-balanced assembly. Each part in a wheel-end can contribute to an unbalanced condition.
Spinning an unbalanced object makes the force created by the imbalance spin around too. The result can be bouncing, wobbling or vibrating.
Static balancing treats an assembly as though it were a disk operating in a single plane at the center of the assembly.
Appropriate- sized weights bring the assembly into static balance.Standard practice puts half the weight on each side of the wheel, to help minimize dynamic imbalance.
Dynamic balancing considers assembly balance in two planes, located at the rim flange edges. This assembly is still statically balanced, but not dynamically balanced.
Dynamic balancing systems tell you how much weight to put on each side, and at what location to place it.
for clarity. Actual balancing situations are generally
far more complicated.
A tire and wheel assembly that isnt balanced can bounce, wobble, or vibrate, causing wear and ride disturbance problems. Static imbalance tends to cause "hopping," while dynamic imbalance tends to cause "wobbling."
What is "static" balance?
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 cant really do that, but thats 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 thats corrected by static balancing?
It may be. Unfortunately, treating a wheel-end as though it were a flat disk can oversimplify the situation.
Lets 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 t he 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 whats 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.
Its 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. Its fairly fast, fairly simple and inexpensive, and can alleviate many wear and ride problems.