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volume 12 issue 2 . another look
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Can "slop" ever be an advantage?

Engineers don't like the term "slop." They prefer the word "tolerance" instead. But either way, what you're talking about is the difference between the nominal measurement and reality.

A steel bar isn't exactly one inch in diameter, even if that's what the label on it says. It's one inch, plus or minus the tolerance. The plus or minus might be just a few thousandths of an inch for an expensive bar or many thousandths for one whose dimension isn't that critical.

Every manufactured item has dimensional tolerances. What we're about to explore is how to use them to your advantage.



In spite of what we found on the vehicle, the radial runout of the tire and wheel assemblies on the blancing machine was small, and both assemblies had nearly identical runout.
Normallly, there is about 0.018" clearance between the locating pads on a hub-piloted axle end and the opening in an aluminum wheel.


So how can tolerance be an advantage?

Consider this story. Recently, we heard from a customer who was experiencing steering wheel wobble at about 65 mph. We jacked up the vehicle, and measured the runout of the steer tires.

The left front came in at 0.080", while the right front had only about 0.040" of radial runout. Neither of these numbers is huge (a typical wheel might have runout of about 0.025" and a typical tire about 0.050"), but what was suspicious was that the runout measurements were so different from one another.

Lateral runout on both tires seemed to be minimal.

When we pulled the tire and wheel assemblies and put them on the balancing machine, we again measured the radial runout. This time, both tires were almost identical, at about 0.030-0.035", a pretty low figure.

When we measured the radial runout of the tire and wheel assemblies on the vehicle, the left front tire had twice the runout of the right front.

What did that mean?

It suggested pretty strongly that the real problem was at the vehicle. So, we measured the runout of the studs (these were hub-piloted wheel ends), and discovered that the studs had considerable runout.



Could that be fixed?

Not easily, and not inexpensively. So, we decided to see if we could make tolerances work in our favor. We knew that there's a tolerance between the wheel hole and the hub "tangs," or "pads."

What are those?

They're the metal projections that come out from the hub to help you guide the wheel onto the hub and studs.

Generally, with an aluminum wheel and a hub-piloted axle end, there's about 0.018" of tolerance or clearance between the wheel opening and hub. (The actual tolerance, per SAE specifications, is between 0.012" and 0.028". For steel, the SAE spec is 0.008" to 0.024".)

So, that means we typically have about 0.018? of "slop," or tolerance, between centering the wheel perfectly on the hub and moving it as far to one side as possible.

How do you know how to position everything?

We marked the tire and wheel assembly with a mark indicating its radial runout low point. Then, we marked the radial runout high point of the studs.

If we positioned the high point of the hub at 12 o'clock and the low point of the tire and wheel assembly at 12 o'clock, we knew we'd be offsetting tire and wheel runout with stud runout.

And, by positioning the hub high point at 12 o'clock, we could let gravity pull the wheel down to use up the available tolerance.

Then what?

The next step was to put on some lug nuts - very carefully - by hand, to avoid disturbing the positions of the various parts of the assembly.

We ran the nuts down finger-tight and tightened them gently with a hand wrench. Then, we set all the nuts to the proper torque.

How did it turn out?

Very well. When we measured the radial runout with our gauge, we found that the left front tire and wheel now showed a runout of just 0.040", identical to thaton the right front tire and wheel.

Best of all, when we applied the acid test, having the driver take the vehicle out on the road, he reported that there was little or no wheel wobble, not even at 65 mph.

Why not try balancing or remounting the tire?

If the vibration is coming from runout, balancing is not likely to solve the problem. No matter how much weight you put on there, the tire and wheel simply aren't running "true."

And, we could have tried remounting the tire, but the very low level of runout we found when we put the tire and wheel on the balancing machine suggested that the tire and wheel runouts were probably fairly well matched to each other already.

What if you hadn't improved runout enough?

There's another trick we could have tried: There are usually four to six mounting "pads" on each hub. We could have positioned these so that the 12 o'clock position was between two of these. This would have gained us an additional few thousandths, perhaps another 0.004".

This technique doesn't work with ball-seat systems because the wheel is postioned primarily by the mating of the ball seats on the lug nuts and wheel, and gravity plays a much smaller part in determining clearances.

Would this work with stud-piloted wheels?

Unfortunately, not as well. By design, stud-piloted wheels "center" themselves by means of the machined edges of the lug nuts and ball seats around the wheel stud holes.

So, the best you could do there would be to try to match the high point of the studs and the low point of the tire and wheel assembly, and hope for the best.

But, it might work well enough to solve the problem.

Where can we read more about this?

You'll find more details in the TMC Recommended Practice RP 214B. You can obtain copies from TMC by calling 703-838-1763.

A. By positioning the low point of the wheel and tire and the high point of the hub at 12 o'clock, the clearance between the hub and wheel cancels out about 0.018" of the total radial runout.
B. By turniing the pads so that the 12 o'clock position is between them, we can gain about another 0.004" of clearance between wheel and hub, potentialls reducing runout even more.


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