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I need to do this on my wagon, but didn't know why...
This is the how and why (snipped from another car enthusiast site):
You have to remove the wheel and tire from the vehicle to have it road force balanced. What the machine does is spin the wheel/tire slowly while pressing a roller against the tread with about 1400 pounds of force. It measures the "loaded runout" of the wheel/tire combination. Then it measures, using other rollers, the runout of the wheel where the bead seats. Then it instructs the user to mark the tire and the wheel, remove them from the machine, break the beads loose, rotate the tires on the wheel to match the marks, then re-inflate and re-mount the wheel/tire on the machine. Then it spins it again to measure the results, then spins it up so balance weights can be applied in the usual manner.
OK, now what's it REALLY doing?
It models the wheel bead mounting surface as a circle that is not necessarily concentric with the rotational axis of the wheel. For the technically literate, that's the first order radial harmonic. It models the tire as a circle that's not necessarily concentric with its beads. It then computes how to mount the tire on the wheel so that the "high spot" of the tire is at the "low spot" of the wheel. Again for the technically literate, it puts the first order harmonic of the tire out of phase with the first order harmonic of the wheel such that they cancel as much as possible.
What that does is minimize the net loaded radial runout of the wheel/tire combination. In layman's terms, it makes the tread surface, under load, as round as possible, relative to the axis of rotation of the wheel.
Does it work? Absolutely. Very expensive machines are used to do the same thing with the tires and wheels that are mounted on new vehicles at the factory. The Hunter GSP9700 does it just as well and it does it in a tire shop. Ask Michelin -- their engineers bought the first six of them Hunter produced.
You can road force balance wheels at any time. I've (the author of the article) done it with my own tires at 50,000+ miles. Tires sometimes change their loaded runout with wear, and road force balancing can improve their performance.
posted by 192.28....
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