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

[Richard Brekne]

Hi Fred

I have to admit that my motivation for measuring the ratio as I do is 
motivated from the same sense of uncertainty connected to the need for 
taking UW and DW measurements, which can IMHO be quite iffy.  Further... 
getting a ball park figure measuring one key is a much quicker affair. 
Those who are comfortable with Stanwoods way no doubt find it 
comfortable enough and that is of course just dandy for whomever.  But 
as Keith pointed out in is opener on this,  if you know what you are 
doing can indeed start with just about any reasonable way of measuring 
the actions ratio and achieve a fine result. If you are after a certain 
BW, then you will have to find a way of compensating for what ever built 
in  discrepancy there is between the Strike Weight Ratio, and the actual 
ratio you have measured, but that is easy enough.  One thing that hasnt 
been mentioned about doing it the EAR way, or like you do it, or as I do 
it is that whatever discrepancy there is is absolutely uniform for the 
whole action, and thus any compensator in a formula for figuring key 
radius weights can be just as uniform.

I generally install a Strike Weight curve I know I can counter weight 
with moderate key leading amounts... then temporarily lead  4-5 C's to 
see how close I am to my target BW.  My spreadsheet for this takes  
input values for Ratio, Hammer Radius Weight, Whippen Radius Weight and 
Target BW and outputs Hammer Radius Weight's.  You can actually adjust 
any of the inputs to compensate for whatever discrepancy in the Target 
BW from the actual resultant one that emerges from my 4-5 samples and 
then recalculate all FW's to get dead on your BW target. 

Eds point about the ratio changing through the key is valid regardless 
of how you measure the ratio when it comes down to it. I believe on 
Overs site he mentions something about the need to take into 
compensation the angles at which the parts are designed to be in their 
rest positions to get the correct reading... because it will be 
different if you measure same in another position. That is to say it 
will be different if you first figure in the angles to begin with. Again 
tho... as long as you compensate appropriately somewhere along the 
line... you can achieve exactly the BW you set out to.

Cheers
RicB


        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