road force balancing - what questions to ask?

[simon barsinister]

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.

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.

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.

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.

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

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.

Why are there soooo many variations on how road force balancing is done.

Anyone have any comments on what exactly I should be asking and looking for in my quest?

[DJ]

Originally Posted by simon barsinister

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.

Originally Posted by simon barsinister

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.

Originally Posted by simon barsinister

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?

Originally Posted by simon barsinister

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.

Originally Posted by simon barsinister

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.

Originally Posted by simon barsinister

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.

Originally Posted by simon barsinister

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.

[Eddie]

I usually write the roadforce results on the inner sidewall of each tire I balance,ask them to do the same. Your front tires should have the least amount of roadforce.

[simon barsinister]

Thanks for the explanation DJ! Hopefully, now, I can communicate more effectively with these shops. Although, based on your explanation, I'm not quite sure how the GSP9700 or any other road force balancer can tell the tech if road force balancing is required when a standard balance is performed. The two types appear to be mutually exclusive.

[HOGWILD]

Hunter Engineering Wheel Balancer Family how does the road force work on the more agressive MTR Style tire and wheel combos? Can ya get a big old Mudder tuned in with the road force machine or is it a no go? 160.00 bucks for Road Force balance too high? i have been checking the site out and just trying to educate myself due to me using the tripod floor mount balancers years ago at my dads service station. I was pretty good at it and i am talking early 70's when thikngs were LOW tech!

[DJ]

Originally Posted by HOGWILD

Hunter Engineering Wheel Balancer Family how does the road force work on the more agressive MTR Style tire and wheel combos? Can ya get a big old Mudder tuned in with the road force machine or is it a no go? 160.00 bucks for Road Force balance too high? i have been checking the site out and just trying to educate myself due to me using the tripod floor mount balancers years ago at my dads service station. I was pretty good at it and i am talking early 70's when thikngs were LOW tech!

I would not try to tune a "big old mudder". Such a tire is, in my experience, very non-uniform in its construction and especially in its sidewall stiffness. They are made to be tough and have big treads, but they are not made to be smooth and quiet, and they aren't.

The question thus is, why can't road force balancing fix that, or at least make it better?

To understand that, we have to dive headfirst into just what the road force balancer does. At this point, simplifications and generalizations won't explain it.

To help think of it, stand facing the tire and (in your mind, I mean; we're just illustrating things here) paint numbers like a clock face on the side of the tire. That will let us talk about stiffness at various points around the tire without confusion.

A perfect tire has uniform stiffness all around the clock face. If we plot that stiffness as a function of the angular position around the tire, the plot is a circle. On the plot, the distance of a point from the center of the plot indicates the stiffness at that point, and the "time of day" position on the plot indicates the corresponding position on the wheel.

Now, suppose the tire had a "hard spot", i.e. a slightly stiffer spot in the sidewall, in the two o'clock position. The plot of stiffness would be a circle with a slight bulge in the two o'clock position. Makes sense, right?

The road force balancer, in layman's terms, requires the operator to mount the tire on the wheel such that the bulge is located at the lowest point of the rim runout. The intention is to make the net surface of the tread, when loaded, as circular as possible.

But what actually is done is much more complex than that. Consider what happens if the tire has more than one stiff spot. Consider what happens if the rim has more than one low spot. What does it match with what?

This is where "mathematical modeling" comes in. The balancer "models" the varying stiffness of the tire as if its plot were a circle whose center is not quite at the center of the plot, i.e. not quite at the center of the tire. It "models" the varying runout of the rim as if its plot were a circle whose center is not quite at the center of the plot, i.e. not quite on the axis of rotation of the wheel. It requires the operator to mount the tire such that the highest point of the model circle of the tire matches the lowest point of the model circle of the wheel. On average, this produces a tire/wheel assembly with the lowest peak loaded runout.

This method produces marvelous results when used with ordinary tires. Indeed, it is used on every tire/wheel assembly that is put on a new vehicle at the factory, including the spare.

But it often produces poor results on "big old mudders" and such because modeling the loaded runout of such a tire is usually not very accurate. Such a model is called "first order". A much more accurate model for such a tire is "second order" or higher, in which the tire is more nearly shaped like a football (i.e. second order), or a triangle (i.e. third order), or a square, (i.e. fourth order), and so on. If it were, exactly which point on the model is the "high point" to match with the "low point" on the model of the wheel?

Also, it often produces poor results on the wheels used with "big old mudders" and such because modeling the rimi runout of such a wheel is usually not very accurate. A much more accurate model for such a wheel is "second order" or higher, in which the wheel is more nearly shaped like a football (i.e. second order), or a triangle (i.e. third order), or a square, (i.e. fourth order), and so on. If it were, exactly which point on the model is the "high point" to match with the "low point" on the model of the tire?

The bottom line is that the imperfections of the wheel are used to compensate, to the extent they can, for the imperfections of the tire. With big old mudders and their wheels, the imperfections are usually large but irregular, and only limited such compensation results.

Then, the first time your big old mudder hits a big old hole, the wheel bends slightly and you have to start over.

[HOGWILD]

Yea i was looking at the smart weight technology thats new from hunter and that is why i asked because i knew the bigger tires have more irregular imperfections but was thinking on going with NEW rims and new Goodyear MTR's if the US ARMY would let me have a set new design has made them hard to come by because Army is buying them all!! Thought about the Dunlop Mud rovers but prices are the same and i would be more inclined to go MTR's if i can find 4, this is on my 93 PU and only used for Gila Trips or Pueblo trips but mielage is terrible on them but out in the middle of BF NM ya want a tire that is not going to put ya on a winch bar!! or walking!! Guess they can only do so much with what they are given as to how good a balance they can get!! Thanks for info and since i read the stuff you explained it makes more since about not wasting my $$$ for the road force procecure on some 31 10.5 x 15R LT's just trying to get a few more miles and guess it was a stretch to think the cost wold be worth the effort. Thanks DJ!

[shorepatrol]

My brain hurts

[etravis]

Mine too!

[SuspensionMAXX MAN]

Ever heard of tire truing?