Low Inertia

[David Love]

As I said, it would nice to establish the relationship because weight and
distance ratios currently are not the same.  That was what I took Jude to be
referring to.  I assume you meant you weren't sure this was possible rather
than "accurate" since I made no claim one way or the other.

With reference to change in SW Ratio and change in regulation.  I didn't
qualify whether the change would be significant or not.  The issue is that
any change in the action ratio will result in a change in regulation specs.
You can compensate for that change by adjusting the blow distance, the
aftertouch or the dip and the amount of the compensation may in fact be
minimal if the change to the ratio is minimal.  So a change in the SW ratio
does mean a change in the regulation specs, necessarily.  It may not be a
significant change but there will be a change.  

David Love
davidlovepianos at comcast.net 
www.davidlovepianos.com

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Richard Brekne
Sent: Monday, October 06, 2008 12:30 PM
To: pianotech at ptg.org
Subject: Low Inertia

Greetings:


    It's only logical that weight and distance ratios are related.  You
    can't change the weight ratio without creating the need to alter
    regulation specs. While problem actions we're talking about that
    have excess lead have, by definition, a mismatch between ratio and
    hammer or strike weights, they also generally regulate with too
    shallow key dip (<10 mm), at least by modern standards.  Changing
    the capstan or knuckle position to improve the ratio to strike
    weight relationship will entail increasing the dip but that's
    usually a good thing, or at least a perfectly acceptable thing.  If
    you want the action to regulate by older standards with shallower
    key dip you will need to use very light hammers to go with a higher
    action ratio (or compromise the blow distance).

    One thing that would be nice would be to establish the relationship
    between the Stanwood weight ratio and the distance ratio (since they
    don't currently match) so that regulation specs could be targeted
    using weight ratio as the standard.  However, since both numbers are
    easy to calculate it doesn't present that much of a problem. 

I'm not sure all this is all that accurate. One has to remember that 
Stanwoods weight ratio is all in all an entirely different puppy then 
the distance ratio as given by for example Ron Overs on his website. 
Stanwood does two things that are not really compatible with the 
distance ratio and can explain why the SWR can be the same for two 
actions of different distance ratio.  Number one, he throws out the 
individual ratios of the top two levers in the action and combines them 
into one quantity. Then this quantity is never really used directly in 
his formula but is rather factored out to arrive at his equation of 
balance ratio. (see my article on dissecting his equation from a couple 
three years back in the Journal)  Secondly... his equation is that of 
the ratio of  the SW to that of the combined weight of BW + FW - WW 
where WW is the whippens radius weight times the key ratio. It is not a 
direct ratio such as the distance ratio which is the ratio of hammer 
movement to that of the key movement.

It is clear that one can achieve identical distance ratios for the upper 
two arms using various combinations of the individual arms. Choice of 
individual arm lengths affects the speed of each of the parts in each 
individual arm and the speed of the individual arms themselves. This 
illustrates part of the difficulty in attempting a translation from one 
type of ratio to the other. The end balance weight ratio... or SW ratio 
as Stanwood has termed it is not porportional to either a standard 
distance ratio or any given speed ratio.

The only relationship that does exist without further ado is that if one 
increases or decreases any given action ratio through some or another 
manipulation, one will indeed alter all other action ratio measurements 
in the same direction. That is to say if you increase the SW ratio, then 
you will increase the distance ratio and the speed ratio as well.  How 
much in each case is a bit more complicated.

Another thing,... a change in the SW ratio by no means necessarily 
implies a significant change in action regulation specs. One can alter 
the SW ratio quite a bit and end up requiring no more then a couple mm 
change in blow distance to achieve same aftertouch for same key dip and 
same letoff/drop.

Cheers
RicB