[CAUT] Key ratio measurement (was Re: moving capstans question)

[Fred Sturm]

On 9/22/07 1:17 AM, "Richard Brekne" <ricb at pianostemmer.no> wrote:

> It should be mentioned tho
> that the ratio measured is not the same ratio as the Stanwood ratio...
> nearly every instrument will end up yeilding two reasonbly significant
> different  results when both ratio measurements are the same.  And tho I
> have yet to check it... I dont think either of them are the same ratio
> as the Overs method.. which is more akin to what designers operate with.

    Yes, I have noticed marked discrepancies when I have done these three
ways of determining key ratio, plus another rather simple one that I think
may have originated with Bill Spurlock: add a known weight (I like 2 gm) to
a hammer (binder clip or the like) and see what that does to DW and UW. Why
do these four methods yield such different results? Anyone know?
    I figured that the friction component could muddy the waters with weight
based tests, but figured the Erwin style would at least come close to
mirroring the Overs style ("Overs style" meaning measuring the 6 levers: key
front and back, the two wipp levers (capstan/center and center/jack top),
and the shank levers(center/knuckle jack contact and center/strike point),
and doing the simple calculation). But they don't seem to come nearly as
close as I would suspect, even with the most precise measurements I can come
up with for Overs style.
    [BTW, in a slight variant on Erwin's, developed independently, I set dip
on a sample to exactly 10 mm, lower the capstan to just above cushion, raise
letoff so that it doesn't start during that 10 mm, then measure hammer rise
for full dip. I like dividing by 10 <G>, it's easy, having more or less full
dip makes me feel better than a sample portion, and the larger distance
seems like it might yield more accuracy. Maybe, anyway.]
    At any rate, I tend to agree with Keith Roberts and, I guess, Dale Erwin
(I wasn't directly aware of what he was doing) that "input/output" or
"distance down/distance up" (key/hammer) is the one I would be most likely
to take to the bank. Though I admit I am intrigued by Ed Foote's comments
about ratio changing in relation to the convergence pattern of capstan to
wipp heel. I just don't get why the down/up ratio should vary so much from a
simple, lever measurement based calculation.
Regards,
Fred Sturm
University of New Mexico