"Steve W." wrote in message
...
Ed Huntress wrote:


This discussion is always hampered by not having illustrations --
animated ones are best -- and I've had a heck of a time over the years
discussing it just in words. But I'll try. Bear with me.

What you're describing, if I understand you correctly, is a static
balancing system that makes its measurement by rotating the wheel around
its axis, allowing a pointer to swing through a circle. The radius of
that circle is the measure of relative imbalance.

But that's still static balance you're measuring -- the displacement of
weight around the wheel, in just one plane, which is a plane that is
perpendicular to the wheel axis (the axis of an axle through the wheel.)

Dynamic balance is something completely different. Some automotive tech
literature actually describes it incorrectly. There are several ways to
describe it correctly but I'll try an example:

Say you start with a perfectly balanced wheel and tire, and you put it on
your static balancer. Now put a wheel weight in one spot on the outside
rim. Now put the same amount of weight on the inside rim, 180 degrees
away from the first weight. Check it with your static balancer; if you
did it right, it still indicates that the wheel is perfectly balanced.

Not going to happen. The heavy spot on the tire will still show heavy. The
180 out weight will only offset the weight you added. I proved this to
someone who sells dynamic balancers by doing your exact test in front of
him. Tire started off 1 oz off. Added 1 oz to just the one side. then
added a duplicate weight 180 out on the back of the rim. Put it back on
the bubble and guess what. It showed the tire was STILL 1 oz. out of
balance.at the same spot on the rim.

Hey, Steve, take a look again at the example. I said that the wheel and tire
started out PERFECTLY BALANCED. In other words, there was no "heavy spot" to
begin with in the example.

In your example, the two weights balanced each other, but they didn't
balance the original out-of-balance condition.



But put it on a dynamic balancer and spin it at the (typical) 300 rpm,
and you'll see that it's way the hell out of whack. The wheel is wobbling
from side to side like a drunk.

And it should wobble. You have NOT balanced the tire properly.

Ah, it was balanced properly before we even started. g



The reason is that centrifugal force is acting on those weights, forcing
them to seek the plane of the wheel's rotation. When the weight on the
outside rim is at the top of rotation, the centrifugal force vector is
trying to push it as far from the axis of rotation as possible. What's
"possible" is that the weight wants to swing in an arc to one side, so
that it's farther from the axle. For this example, say that it's being
pushed to the right.

And if you had properly used the static balancer it would show it being
balanced. Like I said it requires a brain to use on properly.

Yes, it WOULD show that it was properly balanced on a static balancer, after
the two weights are put on it. But it isn't. Put it on the car, and the
front end will wobble all over the place.

snip

--
Ed Huntress