[pianotech] Grand Backcheck Geometry

[Ron Nossaman]

On 2/28/2012 9:08 PM, jim at grandpianosolutions.com wrote:
> Lets try that with a more focused example.
>
> In determining the radius of the hammer tail for a grand is 2-1/2 -3" .
> WNG uses an even tighter radius, 1.5" I think. Why? THe pivot for the
> shank is at the flange, not 1/2 or 3/4 out on the shank.
>
> This is presumably to deal with the radius described by the backcheck as
> it swings into full key dip, to avoid interference. So my question (more
> focused-ly) What is the geometrical relationship between those two arcs?
>
> Jim Ialeggio

It's not an intersecting arcs thing, it's a spring deflection and 
friction thing. The 72° back check face angle provides the wedge, sprung 
by the back check wire, to jam the tail to a stop. The tail radius 
splits the difference between too shallow an angle (large radius) that 
will grab but the height at which it ends up is very dependent on the 
force of the blow. Too shallow an angle (small radius) and the depth of 
catch is the same at any blow force, but it may bounce and not catch at 
all. A 2.5"-3" tail radius will catch dependably and at a fairly uniform 
height regardless of blow force. That's what you need. Roughing tails is 
an attempt to get poorly set up back checks to work, but it doesn't. The 
WN&G checks have a smaller checking surface and more wire, therefor more 
spring to the wire, and little pad under the leather. I presume (don't 
know for sure) that the tighter tail radius is a compensation for the 
increased spring and firmer check surface that proved to work.

That's how this shade tree engineer sees it anyway.
Ron N