Originally Posted By: geeman789
Think of this example. You have a slightly bent wheel / tire assembly, and you take it in to get balanced. The shop can usually get the "balance" pretty close, as in the center of gravity (cg) is in the exact middle of the wheel. So the tire is "balanced". It will still vibrate rolling down the road, because the wheel is bent, and there is a hop. If you took it back to be re-balanced, it would spin up as being "balanced"' but it is not " round..."
Road force balancing does two jobs. Yes, it will "balance" the assembly, just like any spin balance does. But the machine will also help "match" up the wheel and tire in the best possible position, the "roundest" assembly. EVERY wheel/ tire has some out of roundness, and road force balancing works well to match the stiff/high spot of the tire, with the low spot on the wheel, so the unit will be balanced and as round as possible.
Go look at a new car lot. Check out the wheels/tires. You will usually see a green sticker or some sort of mark on the wheel, and you will usually see a red dot on the tire matched up to this spot. The car maker takes extra care to measure the wheels and test the tires, so they can be matched for the smoothest ride. That is why when you replace the factory tires, it can be hard to get as smooth a ride as the factory set.
Remember, "balanced" is not always smooth. If you "stack" the tolerances, ie you end up with the high /stiff spot on the tire matched with the high spot on the wheel, you might still have a vibration even with a perfect balance job. If a tire comes back with a vibration, shops without roadforce balancing equipment should check the balance again, and if it shows as "balanced" , then rotate the tire 180 * on the wheel and re-balance, Often, this takes care of the problem. If the tire guy says " I dont know, the machine says its balanced...?", GO SOMEWHERE ELSE...!
Some cars are more sensitive to out of round / balance issues, and the front axle is wayyyyyyy more sensitive than the rear, because the front has a moveable steering axis to transmit vibration, the rear does not. It takes a large imbalance/ out of round issue to feel it through the rear ( and your rear...!) , but not much in the front.
I certainly understand all of this, but my tires always start out as smooth drivers....they become shaky after they've been on the vehicle for a few thousand miles. Problem is, I don't know if it's the pot hole covered roads I travel that's making them shaky, or if it's a problem with my vehicle. I guess I could have a 4 wheel alignment done and see if that cures the problem...