--- 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 > > __________________________________ Do you Yahoo!? Yahoo! Mail SpamGuard - Read only the mail you want. http://antispam.yahoo.com/tools
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