<html> <body> Dear Bearing Heads- There are a few questions afoot (sorry Ed) right now within this thread. I think it might be easier to combine my replies in one post, except for John Hartman's, which deserves its own. To begin by clarifying a point raised by Jim Bryant: <blockquote type=cite class=cite cite>Date: Sat, 21 Feb 2004 21:54:17 -0500 (EST) From: JIMRPT@aol.com Subject: Re: Lowell Gauge...was Down Bearing David;  "Negative front bearing" is most difinitely an issue of concern...don't confuse a discussion of bridge agraffe thingees to apply to convential bridges...or am I reading you wrong? Jim Bryant (FL)</blockquote> You are and I'm not. (my attempt at a Nossmanesque reply).    This current thread derives, to varying degrees (sorry) from the following: Date: Tue, 17 Feb 2004 22:38:11 -0500 (EST) From: A440A@aol.com Subject: hearts on fire Date: Thu, 19 Feb 2004 11:11:08 -0500 (EST) From: Alpha88x@aol.com Subject: what is downbearing? Date: Sat, 21 Feb 2004 15:02:10 -0800 From: Joseph Garrett <joegarrett@earthlink.net> Subject: Re: Lowell Gauge...was Down Bearing My concerns about the use of the Lowell gauge and about negative downbearing are not being directed to the Sohmer bridge agraffe configuration, at the moment.  While that is where the discussion began, the present thread  "Re: Lowell Gauge...was Down Bearing" addresses the use of the gauge in examining conventional bridges. At 12:48 PM 2/22/2004 +0100, RicB  wrote: <blockquote type=cite class=cite cite>Skolnik seems to raise the point (as I read it) that the difference between the ideal flat surface top of the bridge... (yeilding two seperate angles) and the more real life condition of the string approaching a condition where it is actually curved around the bridge top is significant in terms of figuring downbearing.  Is it ?? I would have thought you could have figured the middle spot on the highest point of the string on the bridge (no matter how long) as a single point where the front and back lengths meet and create a single angle... more or less forming a simple triangle (with the straight line of the undeflected string) which could be interpreted as tension instead of distance and solved with simple vector geometry.</blockquote> and Date: Sat, 21 Feb 2004 21:16:24 -0600 From: Ron Nossaman <RNossaman@cox.net> Subject: Re: Sohmer Agraffe Bridges <blockquote type=cite class=cite cite>How?</blockquote>(how Nossmanesque!) Ric'n Ron 'N - There are three, somewhat separate concerns...Force (positive or negative),  Termination, and Interpretation.  My primary concern, at the moment, is the last of these, which is, in itself, composed of two parts.  First would be the actual, real world bridge configuration that is there, and second...the accurate use of the gauge to interpret that configuration.  As you alluded to in your question, the bridge surfaces, specifically beneath the actual bridge-string segment is rarely flat. The distortions could derive either from the planing process or from the compression caused by the string, either in the building process, or over time. The other factor to consider is the contribution the bridge pins make in creating these distortions. Let me try to give you a fairly simple example. First, imagine the sounding length, the (flat) bridge-segment length and back length all to be in one continuous plane.  If you measure the front and rear segments, you will, correctly infer that there is no deflection angle and no downbearing force.  If, however, that same bridge surface had some domed curvature, the front and rear string segments would still appear the same to the gauge, because the bridgepins are masking what would happen if they were not there...that is, a string deflection angle wood be apparent, from that high spot.  In such a perfectly hypothetical situation, the bridge pins are creating some negative front and rear bearing (because, without that pin, the string would elevate from the actual edge of the bridge), but the net force is positive downbearing.  Even if you measured the bridge-string segment using the "traditional" method of locating the feet of the gauge proximate to the front and rear bridge pins, your data would not take into account the elevation of the bridge curvature, thus giving you a false zero, or possibly even false negative reading. The most common configuration I measure generally shows a negative angle when measuring the smallest possible Bridge-string segment behind the front bridge pin and comparing to sounding length string plane.  Next, when moving the gauge in the closed-foot position from contact with front bridge pin to contact with rear pin, the bubble will move dramatically towards the front, by anywhere, up to .036" or more.  The rear segment is commonly negative to the plane of the sounding length. There is a difference between negative downbearing in general, which addresses questions of soundboard loading and impedance, and negative pin bearing, (my term) which would address string termination issues at the front pin, and at the rear, questions regarding the effect of any upward force upon the bridge and board. Arguing about the relative importance of these phenomena is not the point here, at the moment.  What is important is to acknowledge  the existence of a physical model that is more varied and complex than is generally envisioned, and how that revised model, along with the proper tools and techniques, can inform our understanding of what we see before us, not to mention our ability to communicate with each other with greater precision.  My comment in a previous post expressing a perceived lack of concern, in general for the ramifications of negative downbearing is material for another time. David Skolnik </body> </html>