[pianotech] String riding up on bridge pins, was Whistlin' Wixey

George F Emerson pianoguru at cox.net
Thu Aug 12 22:35:42 MDT 2010


Ron N wrote:
> I am that some that insist it doesn't happen without some aberrant 
> condition like near vertical bridge pins and negative bearing. A number of 
> folks have claimed to have measured strings up bridge pins, but no one has 
> shared the method other than "I tapped the string, and it moved down". 
> This is accounted for by the rounded down string track observable on old 
> bridge caps. How exactly did you measure and observe this?

I described in the previous post measuring the presumed string diameter with 
the depth gage of digital calipers from the top of the string to the top 
face of the bridge, first, just in front of the back pin, then just behind 
the front pin.  What I did not say is that this was on a new piano on the 
factory floor with positive bearing front and back, with no bridge damage, 
and with good pin angles.  I don't recall the numbers, but the string 
measured significantly higher on the front pin side.  I lightly tapped the 
string down.  The string responded with a click-zing, suggesting some 
movement of the string.  I repeated the previous measurements, which yielded 
an agreement in the measurements front and back.

One could argue that the fact that this was a new piano is the problem.  The 
string was not yet seated to the bridge, and once it was, it would never 
ride up again on the pin.  The problem with this argument is that I and many 
others have observed it in many pianos, old and new.  For decades, if not 
generations, piano tech's have observed this, spoken of it, taught it, and 
written about it.

The only argument that I have seen in opposition to the idea of strings 
riding up on the bridge pins is that it defies logic.  How can a string ride 
up on the pin, if there is positive down bearing, etc.?  There are two 
problems with this.  First, it under estimates the influence of friction 
between the string, under 160 lb. of tension, and the pin.  Secondly, it 
overlooks the fact that side bearing force is many times greater than down 
bearing.

Suppose we built a model with a bridge below the zero-bearing line.  Do you 
suppose that we could force the string up on the bridge pins to reflect a 
reasonable front and back bearing, relying on the surface tension between 
the string and pin to hold it at that position?  I suspect you could.  Do 
you suppose that we could deflect the string further up on the pin, and it 
would be held there by the same surface tension?  Again, I suspect so. I 
confess that I have not built such a model.  Maybe I will, but for now, I 
have "bigger fish to fry."

So, Ron, I turn the same question around on you.  What measurements or tests 
have you done, to prove that strings do not ride up on bridge pins?  I know, 
I know ... you cannot prove a negative.

Frank Emerson 



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