Anyone have any advice on specific questions to ask regarding road force balancing?
I currently have a vibration when travelling over 40mph with my new Michelin LTX tires. I've had them rebalanced, but still the same problem.
Previously had original Bridgestone Dueller H/Ts and got 85K out of them without any vibration or alignment problem.
Yup. I'm one of the four engineers who designed the Hunter GSP9700 Road Force Balancer. There is some of my hardware and software in that product.
Let's take 'me one at a time.
Michelin states to get a representative to examine the tires, they need to be road forced balanced first. Tire place I bought them from states they'll pay for it.
Michelin bought the first six GSP9700's off the production line after a thorough qualification of the product. In essence, Michelin is quite satisfied that the GSP9700 accurately measures what it measures.
So, let's look at just what the GSP9700 measures, shall we?
A tire is not necessarily round. Its tread surface does not necessarily describe a perfect circle. What is most important is, "Is it round while rolling?" The first step in using a GSP9700 is to measure just that.
A tire/wheel is mounted on the shaft of the GSP9700 and a roller is pressed against the tire as the shaft slowly rotates. The roller presses with a constant force of about 700 lbs (early version) or 1,400 lbs (later version). If the tire is perfectly "round while rolling", then the roller will roll with the tire but will not move otherwise, i.e. there will be no measured "runout" of the tread surface. If the tire is not "round while rolling", then the roller, which is mounted on an arm, will move back and forth, toward and away from the shaft of the GSP9700, as it rotates, i.e. it will measure the "loaded runout" of the tire.
Now, how significant is that runout?
Here is what happens: Suppose the tire rolls along a perfectly flat surface (i.e. a really good road), and suppose its axle is mounted to an assembly that keeps the axle at
exactly the same distance from that surface. If the tread surface is perfectly round while rolling, then the force exerted by the axle on the wheel (which, of course, is the same as the force exerted by the tire on the wheel), will be constant as the tire rolls along. As a "high spot" in the tread circumference, i.e. a "high spot" in the loaded runout, rolls into contact with that surface, then the tire pushes upward against the axle with a little
more force than average, right? As a "low spot" in the tread circumference, i.e. a "low spot" in the loaded runout, rolls into contact with that surface, then the tire pushes upward against the axle with a little
less force than average, right? That variation in the upward force the loaded tire exerts against the axle tries to make the axle move up and down as the tire rotates. In a vehicle,
it does make the axle move up and down. It feels like an out-of-balance tire/wheel assembly.
Interesting, huh? The nominal scale factor is that a measured runout of 0.001 inch corresponds to about one pound of "road force".
That's where the term "road force" originates.
It isn't enough to just measure the loaded runout of the tire, though. Why not? Because the wheel isn't perfectly round at the bead seating surface, either. You could mount a perfect tire on a wheel that isn't round and you would get the same effect as mounting a tire that isn't round on a perfect wheel.
So, what's a feller to do?
The GSP9700 can also measure the runout of the wheel. It uses a small roller against the rim on each side to measure its runout. Then, with a bit of arithmetic, it can determine how much of the loaded runout at the tread is due to the wheel and how much is due to the tire.
That's what Michelin wants measured. They want to know the loaded runout (measured in thousandths of an inch or in pounds) of the tire. If it's too much, then the tire is defective. Michelin understands this quite well.
So far, all we've discussed is how road force is measured. What, then is a "road force
balance"?
Once the operator has measured the loaded runout of the tire and the runout of the wheel, the GSP9700 computes how to minimize the net loaded runout of the assembly. The operator makes a chalk mark on the outer sidewall of the tire and another on the rim, as directed by the GSP9700. He then removes the assembly from the shaft, deflates the tire, breaks the beads loose from the wheel, and rotates the tire on the wheel to align the two marks. Then he re-seats the beads, re-inflates the tire, and re-mounts the assembly on the GSP9700.
What this does is mount the "high spot" of the loaded runout of the tire at the "low spot" of the runout of the wheel, which minimizes the net runout of the whole assembly as measured at the tread. The proof is in the puddin', as the saying goes. The GSP9700 re-measures the loaded runout, at which time the operator can see just how much better it is.
It is not necessarily going to be perfect, because the loaded runout of the tire does not necessarily equal the runout of the wheel. The procedure is more correctly described as a "compensation" than a "correction", as one defect is used to compensate for another.
That procedure is called a
"road force balance". Note that a road force balance
has nothing to do with mounting weights on the wheel. Once the road force balance is completed, then the whole assembly has to be spun by the balancer, its imbalance measured, and weights applied to correct the imbalance, in the usual manner.
The result is that you can have a wheel/tire assembly that has significant road force imbalance but is in perfect weight balance, and you can have a wheel/tire assembly that has significant weight imbalance but is in perfect road force balance.
Either condition can cause vibration that you can feel. You gotta get 'em
both right, and it takes a separate procedure for each one to do that.
But, you can't get road force balance perfect, as I noted above. How good is good enough? Usually about 25 lbs of road force imbalance is a threshold above which you can begin to feel vibrations from it.
Is this real? Damned right it is. EVERY wheel and tire put on new vehicles is road force balanced at the factory.
Sound simple? Here's the problem after visting a few places to talk about the process:
Tire warehouse - $17/tire - road force balances by placing 700 lb load on tire to 'flatten' any high spots in tire? Huh?! Another place tells me this just reseats the bead of the tire.
Pressing the roller against the tire does help to seat the beads of the tire. The tire should be mounted using
lots of high quality tire mounting lubricant.
Don't skimp on lubricant. Lube the tire bead and lube the wheel bead seat. When the tire runout is measured, the whole assembly rotates more times than is necessary, with the extra rotations being at the start; their only purpose is the help seat the beads properly.
But, the purpose of the roller is not to flatten any high spots in the tire. If that worked, then the roller wouldn't measure any runout, would it? The vehicle would do that when it rolls down the road, wouldn't it? If that worked, then there wouldn't be any need for a road force balancer, would there? But
every wheel and tire put on new vehicles is road force balanced at the factory. You can bet your last dollar that it's necessary.
So, Tire Warehouse needs some training, right?
Town fair tire - $13/tire - will road force balance only if machine states is necessary. Single iteration of matching high spot on tire to low spot on rim. Tech states that you can still have a vibration problem since no tire will balance perfectly.
Bingo! Dead right. This shop knows how to use the machine.
Sullivan tire #1: $13/tire - machine has two buttons, one for regular balance, the other for road force. Road forcing is just as simple as hitting the button. But, what about matching high spot on tire to low spot on rim? Oh, that's extra. $30/tire
This shop is clueless. They
REALLY need training.
Sullivan tire #2: $25/tire - performs multiple iterations of matching high spot on tire to low spot on rim. States no such thing as a defective tire unless road force balancer cannot provide a correction to balance weight.
Amost right. The tire's loaded runout is fixed and the wheel's runout is fixed. There is only so much "compensation" that can be done. That's why tires don't have to be perfect. They have to be good enough. That's why I use ONLY Michelin tires. They are better, in my opinion, than all the others.
Why are there soooo many variations on how road force balancing is done.
The GSP9700 is a complex and sophisticated machine, and the whole process is subtle and ingenious. Some people understand it and some don't. The problem is that those who don't understand it often don't know that they don't understand it. You should not necessarily conclude that they are trying to cheat anyone, rather you should simply identify them and go elsewhere.