Lowell Gauge...was Down Bearing

[David Skolnik]

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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:

>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)

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:

>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.

and
Date: Sat, 21 Feb 2004 21:16:24 -0600
From: Ron Nossaman <RNossaman@cox.net>
Subject: Re: Sohmer Agraffe Bridges

>How?
(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


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