Dave wrote:
On Sat, 03 Feb 2007 09:04:46 -0700, Milton Waddams
said:

Maybe
someone here can explain why the dynamic balance showed what static
balancing couldn't.

Sure. I've seen it many times with a car's tires. Take the tires off and
have them balanced, put them back on, and you still feel a shimmy. Have them
balanced with one of those devices that spins the tire without removing it
and the tire gets properly balanced. The difference is that the dynamic
balancing balances the entire bunch of moving parts as a single mass. It's
no good to balance just the tire if your brake drums or disks are out of
balance.

Its a lot more complicated than simply adding in additional
components. Static balancing only tries to balance the distribution
of mass. Dynamic balancing has to take into account the moment of
inertia around an axis.

Its hard to explain in words, but consider a tire that has more rubber
at "12 o'clock" on the inside part of the tread, and more on the
outside part at 6 o'clock. The tire may be balanced statically, but
will wobble when rotated.

Also, the common way to statically balance is to add weights on the
rim, but the real cause of dynamic unbalance is the amount of rubber
on the tread, several inches further out. Without balancing at the
same distance the moment will be off,