Eh, what do you mean actually by "couple unbalances"? Is that dynamic unbalance?
For
static balance, a rotating body's CM must lie exactly on its spin axis. And for
couple balance, the spin axis must exactly parallel one of the body's 3 principal axes of inertia.
Having one kind of balance here does
not guarantee the other. And for
dynamic balance, you must have both. Since pricipal axes always go through the CM, that means a spin axis that
exactly coincides with a principal axis. Otherwise, wobble-free spins will be impossible.
Wikipedia's article on rotating unbalance will get you started here, but expect to do some reading and experimenting (with LEGO, for example) to get what the statements in this reply really mean.
For a top to spin perfectly smoothly, it must have
dynamic balance. And per the tennis racket or intermediate axis theorem, for its spin axis to be stable in direction, the pricipal axis the spin axis must coincide with has to be either the greatest or least principal axis, not the intermediate one. Again, lots of homework for you here.
Practically speaking, in a LEGO top made up of several layers of parts arrayed along the intended spin axis at various levels, the best way to insure couple balance is to force static balance on
every layer separately. It's not enough for one layer's static unbalace to cancel another's at a different level.
Top 1 clearly doesn't meet this criterion. However, the couple unbalance created by its upper white plate cancels that created by the lower white plate, while both contribute to the top's overall static balance. So dynamic balance holds. Top 5 has a slight residual couple unbalance, but it doesn't produce much wobble in this case.
Couple unbalace also arises when the contact point on a spike tip is off the pricipal axis established by the rest of the top. Iacopo works hard on his replaceable tip alignments to avoid this.
Wobble-wise, couple unbalance is often way worse than static. It generally takes a severe static unbalance to make the tip of a wobbling top scrub audibly on the ground. When you hear that, think couple unbalace first.
PS: You're right in thinking that quick-and-dirty kind of goes against my nature. So it's gonna take some practice. But I should have more success with subjects that don't need multiple examples to pin down what I mean.
