Downbearing

[gordon stelter]

--- David Skolnik <davidskolnik@optonline.net> wrote:
> 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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