ratio question

[Michael Spalding]

Ed asks:

>
>     Why is there is a distinct difference in feel between two actions
that 
> achieve the same ratio with different knuckle distances?  Ie.  you can
arrive at 
> the same overall action ratio with a 15.5 mm knuckle distance and a short
key 
> ratio as you can with a 17 mm knuckle distance and a longer key ratio,
but 
> the response of these two actions are NOT the same. 

I think I can see why, in general.   Friction and compressibility.  Look at
what happens at the capstain/cushion interface, and the jack/knuckle
interface.  For a hammer blow of equal power, the capstain must transmit a
larger force through a longer distance, and the jack must transmit a lower
force through a longer distance.  Jack/knuckle friction during let-off
would decrease.  Also, the amount of keydip consumed during let-off is
decreased.   It seems to me that these changes might make the action feel
crisper and lighter, even if the overall ration and balance weight remained
the same.  But this is all conjecture, since I haven't done any real world
testing.  Yet.

Ed, I would be very interested in hearing your "ideas about finding the
optimum combination".  this is of particular practical interest to me right
now, because I am replacing hammers and shanks on an action for which I am
having trouble matching the original knuckle distance - in fact, the
easiest route would be to replace 15.5mm knuckle distance with 17 mm. 
Should I do it?  How should I compensate?  What side effects should I
expect?


Mike Spalding RPT