Bridge pin angles

[Ron Nossaman]

>>Circumstantial evidence is probably the best you'll do here, depending on 
>>whether the monitored intentionally levitated string stays up the pins, 
>>and if it is found naturally occurring in the wild.
> 
> 
> The first step is to actually find one occurring in the wild.  With a few 
> people armed with feeler gages we should quickly be able to build up a 
> large number of checked pianos.  The more pianos that are checked that have 
> no strings levitated above the bridge cap, the harder it's going to be to 
> believe that this happens.  If we do find some (and the strings are checked 
> to see that there is positive downbearing) then I'll have to devise Plan B 
> for determining if they can levitate.

Exactly.


>>Not exactly. It's enough to damage the cap, but not nearly to the extent 
>>and angular deformation of notch edges we see in older bridges.
> 
> 
> If it is, then the bearing areas I'm assuming are too large or there's 
> something else going on that I don't understand, perhaps involving the 
> bridge pins.  Using the bearing area that I used in the early days of this 
> thread (.005 Sq. In.) for getting a feel for whether the string could 
> indent a rising bridge cap, the bearing stress from downbearing is far 
> below the allowable of the maple.

I think the figure you quoted was for immediate and permanent 
damage. Time under load and cumulative compression set effectively 
lowers it, but that's a secondary detail. The point of my comment 
was that, while the figures show that the downbearing load is a 
relatively very small and insignificant  contributor to cap 
compression, it does contribute something.


> Without bridge pins would be instructive I think.  But more importantly 
> without the side bearing and the clamping effect at the pin that goes with 
> it.  

Perhaps, if only to validate the math indicating the minor roll 
downbearing plays here.


>Another comment here - I originally proposed doing this and humidity 
> cycling it.  As I think about it I don't see why.  Humidity cycling 
> shouldn't really have any significant effect on what's happening due to 
> downbearing.  If the bridge height increases, and the soundboard underneath 
> it rises, the downbearing shouldn't change significantly.  It probably 
> would be easier and just as instructive to choose a downbearing angle at 
> the high end of the range and forget the humidity cycling for the 
> downbearing part of the test.

I agree.


> Also you mentioned the clamping effect from the angled bridge pins and the 
> sidebearing.  I hadn't really been thinking about that.  But because of the 
> pinching effect at the pins, the sidebearing is also going to be pushing 
> the string down into the cap at the pin.  I don't know how significant that 
> load is.  Time to run another number.

Wasn't that the 5.4 lb figure?


> Also, I'll note again here, that I don't think we'll ever get to the point 
> where there's zero cap indentation.  Because the contact between a cylinder 
> (the string) and a plane (the bridge cap) is a line, the theoretical 
> contact area, or bearing area, is zero, and the bearing stress is 
> infinite.  With real world strings and caps I think there's going to be 
> some tiny bearing area with very high stress.  The cap will indent slightly 
> until the contact area is great enough to lower the stress down below the 
> bearing stress allowable of the cap material.  So, even for a very hard cap 
> material such as you're making, you might still see a little 
> indentation.  What I don't know is what an acceptable upper limit is.  What 
> would be the deepest indentation at which we wouldn't hear string anomalies 
> from the string not being perfectly seated?

The indentation can be quite deep with no noticeable tonal problems 
if the pin is solid. The screwdriver test, or CAing the pin of a 
false beater illustrates that, I think. The pianos with bridges 
notched well back from the pin, with no termination related tonal 
problems are also a good indicator. That's the whole point of all 
this, and the fact that seating the string doesn't fix the real 
problem because the string isn't un seated in the first place. I 
don't expect zero indentation with my caps. The hardness is just 
half of the intent. I'm also looking for something that doesn't 
change dimension with humidity swings, crush cap edges, and loosen 
pins like solid maple caps do.


>>>So, perhaps I'll descope this.  I would be happy to accomplish numbers 1 
>>>and 2 strictly in a static situation.  I think this would be educational 
>>>and I don't think it would take that long to do, if you have some way of 
>>>accomplishing humidity cycling....
>>
>>I'd want one additional measurement of pin height above the cap surface 
>>through the fluctuations.
>>...
>>Ron N
> 
> 
> Why?

To determine an affect, you need to detect a cause. If you're trying 
to determine whether bridge caps rising and pushing strings up pins 
crushed the cap, there ought to be a measurement of how much 
relative movement between pin and cap occurs at the cap surface.
Ron N