bridges/seating

[Ron Nossaman]

Hi John,

Ambrose Beirce defined apology as "laying the groundwork for a future=
 offense". With this in mind, and looking toward the future, I'm reluctant=
 to accept apologies where there was no offense. This being the case, I'm=
 returning your apology (unused) so you may save it for a more worthy cause.

Doggone you John, I was hoping to get cheap answers without having to do my=
 own R&D. Now I'm going to have to go out in the shop and try your=
 experiment for myself. <G> I can see where the "stagger" and resulting side=
 bearing would have some bearing (sorry) on the phenomenon as it would=
 vector the net bearing more toward the horizontal. With little to no side=
 bearing, the net bearing vector would be vertical, straight down on the=
 bridge. Maybe there's still hope of clarification. Mike Imbler posted on=
 another topic stating that he was a mechanical engineer. Mike help, please!=
 Is the coefficient of friction between music wire and copper plated mild=
 steel high enough to account for this phenomenon with the forces and angles=
 involved here in pianos? In other words, does the math fit the model? What=
 is the high limit of angle at a given tension before the string slides on=
 the pin?=20

While Mike is deciding whether or not to play, I have more questions.

A hard hammer blow (from below) will cause a string to slide up a bridge=
 pin. We seem to have a consensus on this one, yes? Since the hammer blow=
 isn't delivered anywhere near the bridge, isn't it the wave in the string=
 caused by the blow that propagates down to the bridge that does the deed?=
 Isn't the excursion of a string above the rest position balanced by an=
 equal (average-diminishing) excursion below? Why, then, doesn't the=
 excursion below the plane pull the string back down, especially with all=
 the help it's getting from down and side bearing????=20

Lastly, how would a string stuck up on a bridge pin cause a false beat in=
 the first place? If it's hanging in there good enough to withstand all the=
 above mentioned forces to the contrary, the string termination is as solid=
 as one could hope for! If it's because the bridge pin is loose, we are back=
 to my original premise that most of the false beats found in pianos are the=
 result of loose bridge pins and aren't curable by seating strings to=
 bridges. I don't mean this to look like I'm trying to lead the discussion=
 with a bull whip, but this is where the logic process brought me. All=
 enlightenment, clarification, pertinent random musings, empirical=
 observations and universal=20truths cheerfully entertained.


Expectantly,=20
              Ron Nossaman=20





<******* History beyond this point *********>



At 09:17 PM 4/11/97 -0600, you wrote:
>Hi Ron,
>
>My apologies for misinterpreting your question.  I understand now what your=
 asking, and I have tried a very un-scientific, but possibly enlightening=
 experiment this evening.
>
>I am currently doing some work on my personal piano in my shop. I rebuilt=
 this piano (a Bradbury 5'6" reproducer grand) about six years ago, and it=
 happened to be my first bridge recapping job. Because of this some of the=
 bridge pin placement is, well, pretty embarasingly wacky.  A few pins are=
 placed in such a way that there is very little, if any side bearing. The=
 rest of the job is fine: good downbearing, nice tight well angled bridge=
 pins etc. I took some strings off and checked bridge pins and found no=
 *visible* notching in them.
>
>I got to thinking that this would be a great situation to test if side=
 bearing played a major role keeping the strings off the bridge.
>
>Here's what I did: With the piano up to pitch, I simply pulled up on the=
 speaking lenght side  of a couple of strings until they lifted off the=
 bridge, then let go and watched what happened. The strings that had no side=
 bearing went right back down to the bridge.  The strings that had decent=
 side bearing stayed up! I did several in the sixth octave of the piano=
 (this is where the misplaced pins are) and the strings rode up above the=
 bridge anywhere from 10 to 15 thousandths according to my feeler gauge.
>
>This little test, while not very controlled, would seem to point to side=
 bearing, and its attendant friction as one of the major contributing=
 factors in keeping those suckers up in the air.
>
>Comments anyone?
>
>John McKone, RPT
>St. Louis Park, Minnesota
>(612) 280-8375
>
>-----------------------------------------------
>>From   : Ron Nossaman <nossaman@southwind.net>
>Sent   : 04/11/97
>To     : pianotech@byu.edu
>Subject: Re: bridges/seating
>
>At 08:58 PM 4/10/97 -0600, you wrote:
>>Ron,
>>
>>If you're asking why the string can creep up against the pressure of=
 downbearing and friction, you need look no further than the impact of the=
 hammer (on a grand anyway) Especially on a hard use piano.
>>
>>John  McKone, RPT
>>St. Louis Park, Minnesota
>>(612) 280-8375
>>
>
>
>Nope, nope, not at all. It's quite obvious that a string can be knocked up=
 hill against tension, friction, or anything else you care to name. You can=
 here it happening if you work at it. It's quite clear to me how a string=
 GETS up a bridge pin. I'm saying that it's not possible for it to STAY up=
 off the bridge, AT REST, against tension and side/down bearing and pin=
 angle, unless there is something physically wrong with the bridge pin. What=
 else would hold it up there magically???? David Copperfield plating? There=
 has to be a real reason for an observable physical phenomenon. I get enough=
 mysticism and "have faith, trust me" metaphysics from Government.=
 Demonstrations can illustrate that it's possible (I'm still working on that=
 one), but I don't think anyone can realistically champion a fix without a=
 clear understanding of what's broke. Let's talk "mechanism". I'm willing to=
 accept the notion that this happens only in the context of a rational=
 explanation as to WHY it is possible.  Wouldn't you like to know too?
>
>
> Ron Nossaman
>
>
> Ron Nossaman