Jim   Richard Davenport has this to say. The back check needs to be at a 72 degree angle to the key.  The radius on the hammer tail must be made on a 3 inch arc. The back check face and the hammer face should be parallel at rest. DO this and good checking is mostly positive. So there it is throw out the cad and the microscope. St checking no more or no less than 12 to 14 mm. So get some sleep pal.   Of course a high or low action ratio could change the dynamics but I'm not prepared to jump in on that. <div style="clear:both">Dale Erwin RPT- Mason & Hamlin/Steinway/U.S pianos Pre-hung Ronsen hammer sets/Abel parts Sitka Soundboards & Supplies WWW.Erwinspiano.com 209-577-8397 <tt><tt>     </tt></tt> </div> <div style="font-family:arial,helvetica;font-size:10pt;color:black">-----Original Message----- From: jim <jim@grandpianosolutions.com> To: pianotech <pianotech@ptg.org> Sent: Tue, Feb 28, 2012 5:33 pm Subject: [pianotech] Grand Backcheck Geometry <div id="AOLMsgPart_0_b6ceef42-0e9e-4e28-8ac5-a2f20af0bdc0" style="margin: 0px;font-family: Tahoma, Verdana, Arial, Sans-Serif;font-size: 12px;color: #000;background-color: #fff;"> <pre style="font-size: 9pt;"><tt>Is there someone who could explain the actual geometry, ie intersecting arcs that are necessary to achieve optimal grand backchecking. There is lots of info regarding tail length/radius, height of check in relation to the hammer at various points in the stroke, etc, offered as a "prescription" for good checking. But I' trying to understand the actual geometry that will achieve good checking, ie the shape of the backcheck face, what the actual geometry of the two meeting faces optimally are (radiused tail and backcheck cushion) in relation to the arcs the backcheck and tail are swing in. Also obviously how these arcs are effected by varying amounts of key dip. Maybe the geometry I'm asking for is actually a graduate course in calculus, and the "prescriptions" being empirical explanations of what seems to work? The reason I ask is I have been stymied by 2 unsusual actions of late, actions which don't adhere to normal string heights or key lengths, and am having trouble applying the "prescriptions". I don't like flying blind, and though my drawing in autocad might get me there, that the optimal mating face of the tail/cushion are seem to be something other than what I thought was obvious (I think). Jim Ialeggio -- Jim Ialeggio <a href="mailto:jim@grandpianosolutions.com">jim@grandpianosolutions.com</a> (978) 425-9026 Shirley, MA </tt></pre> </div>  </div>