Could you show me the formula for the critical speed? If I am not too scared by the first look of it, I just might try to understand some of it and draw conclusions.
Not missed, just deferred, as the formula deserves some explanation.
For a top in steady precession (no wobble) at tilt angle
θ, the formula you usually see is for the square of the critical angular speed
ωC in rad/s:
ωC² = 4
M g H I1tip cos(
θ) /
I3²
where
M is the mass in kg,
g = 9.81 m/s² is the acceleration of gravity,
H is the CM-contact distance in m,
I1tip is the transverse moment of inertia (TMI)
about the tip (contact point) in kg m², and
I3 is the axial moment of inertia (AMI) in kg m². When you have a sleeper with
θ = 0, the cos(
θ) factor goes away.
Technically,
ωC includes both the pure spin rate and a component of the precession rate. But for design purposes, you can interpret
ωC as the critical spin rate to a good approximation, as the precession rate involved is usually much smaller.
Making that which is hidden seenThe problem with using
I1tip is that it buries a hugely important dependency on
H. To get the latter out in the open where it belongs, I prefer the "central TMI"
I1cm -- i.e., the TMI about the CM. An added advantage is that
I1cm is the TMI you see in tables of formulas like
the one on Wikipedia. The parallel axis theorem shows how the two TMIs are related:
I1tip =
I1cm +
M H²
With
H exposed in all its glory, the critical speed is then
ωC² = 4
M g H (
I1cm +
M H²) cos(
θ) /
I3²
So your plan to minimize
H and maximize
I3 for a given scrape angle is well founded, as
H is the most influential parameter here, with
I3 not far behind. You can also see why we sometimes talk about the TMI/AMI ratio.
Forget the massBut the mass
M turns out to be just confusing clutter, as both moments are also proportional to
M. To give
M the boot it deserves, I prefer to use the "specific moments"
Ji (aka moments per unit mass), like so...
J3 =
I3 /
M and
J1 =
I1 /
MFor example, in the formula
I3 = 1/2
M R² for the AMI of a thin disk of radius
R , the specific AMI is just the 1/2
R² part.
The critical spin rate then reduces to
ωC² = 4
g H (
J1cm +
H²) cos(
θ) /
J3²
In words, critical speed has nothing to do with mass. To minimize it, you need to focus on maximizing
J3 and minimizing both
J1cm and
H. As an added bonus, you can then get all the AMI you need with the least weight on the tip.
Practical applicationsUnlike the regular moments, the specific moments are purely geometric in nature. For example, the specific AMI
J3 measures the efficiency of a top's mass distribution about the spin axis without regard to how much mass there actually is.
Suppose you have two lumps of clay, A and B, of the same mass
M. You mold A into a solid disk and B into a hollow toroid of the same maximum radius
R. Since B will get more AMI out of the same mass, it will have the greater specific AMI.
I find it useful to think in terms of specific moments when I'm optimizing top designs -- especially
J3. But you also have to play a delicate trade-off with the absolute AMI
I3, as AMI is the inertia that opposes
any change in spin rate -- during both spin-up and spin-down. Too much AMI, and you'll limit the release speed your fingers can attain in the brief time available during a single twirl. Too little, and air and tip resistances will chew up your hard-won release speed that much faster. Test, test test!