Bridge pin angles

[Ron Nossaman]

>>Now, the obvious question. Why weren't any up the pins when you
>>checked them all before starting the experiment?
> 
> 
> Because strings don't climb pins? <G>  

Yup.


>As far as freshly strung rebuilds, my shop isn't 
> high volume enough to collect much data, and my next restringing isn't 
> coming up soon.  Perhaps some higher volume rebuilder would take on the 
> task of checking under strings with a feeler gage on freshly strung pianos 
> that have been pulled up to pitch but not had the strings tapped down.  Any 
> volunteers care to check and share the information with the list?

I'm nowhere like a high volume rebuilder, but I'll check the one in 
progress when I get the strings on. The board and bridges aren't 
even started yet, so it will be a while.



> Also, this indicates that I may have to rethink what I was saying earlier 
> about decreasing pin angles to attempt to lower the load from the string on 
> a rising bridge cap.  I don't think you'd want to get the angles low enough 
> so that the strings wouldn't tend to seat themselves on the cap should they 
> somehow get above it.

This is why I've been playing with capping material. I figure if I 
can make caps more crush resistant, the string won't indent them as 
much, and the pins won't get loose as quickly. If they prove to be 
dimensionally stable enough that they aren't pushing the string up 
and down the pin, I win both ways.


>> If you haven't
>>already knocked down the string that stayed up under playing, could
>>you check it daily for the next several days and see if it stays up
>>there? I'm curious to see how permanently it's un-seated, or if
>>overnight temperature changes will overcome the friction enough to
>>let it slip back down.
> 
> 
> I was curious about the same thing, but the piano was going out right away, 
> so I reseated the string.  I'll have to look for another example situation 
> to monitor.

Thank you. I think this is an important point.


> And if a string is found above the bridge cap I'd like some 
> assurance that it hasn't been like that all along, but was in fact seated 
> on the cap at some time and with the passage of time levitated.

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.


  > 1.  Isolating the string's resistance to a rising bridge cap to 
attempt to
> demonstrate that this mechanism does in fact exist and can damage a cap of 
> traditional materials.

That shouldn't be too tough.


> 2.  Isolating the downbearing.  I think we both believe that this load by 
> itself is not enough to damage a hard maple cap.  

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.


>But I wonder if there's 
> some mechanism involving the notch edge and angled pin that would cause the 
> downbearing to indent the cap.  Or if I'm assuming too large a bearing area 
> and that the effective bearing area resisting downbearing is small enough 
> that the cap would be indented.  A simple experiment might demonstrate 
> whether my belief is true or false.

Not that I fell asleep and broke a string pulling it up to pitch, or 
anything like that, but I have had occasion to look at a bridge cap 
that has been under bearing and tension for mere moments before 
de-stringing. The (quartersawn maple) cap was visibly dented, more 
at the speaking side where the front bearing was slightly higher. 
But that isn't entirely downbearing. The clamping effect of the pin 
and offset angles was there too. Bearing alone would have to be 
without bridge pins.


> 3.  Calling everything else 'string vibration' and seeing if the cap could 
> be indented because of it.  Based on comments on this thread I can see that 
> this may be opening up a can of worms.  I didn't consider the right kind of 
> vibration, I didn't properly consider the interaction of string vibration 
> with the other mechanisms, etc. which resulted in less than definitive results.

If you start talking about vibration, you'd better be ready to show 
them your molecules and explaining quantum effects in detail.


> 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.  Make a small section of bridge, put a 
> frame around it on which you can stretch a string and subject this setup to 
> humidity cycling.  If you have some way of cycling I would think that you 
> could quickly put on enough cycles to see cap indentation if some was going 
> to occur.  As a result of this you would hopefully determine how hard the 
> cap material needed to be to prevent this damage.  If you can make a cap 
> that hard, then put that material on a piano and put it into service and 
> see if cap indentation occurs.  If it doesn't then you're done.  If it 
> does, then I suppose that you assume that indentation is occurring because 
> of some sort of string vibration, or interaction of string vibration with 
> the bearing forces, and your experiments have to get much more elaborate if 
> you want to get a better definition of what's going on.

I'd want one additional measurement of pin height above the cap 
surface through the fluctuations.


> On the subject of humidity cycling - you mentioned cycling a bridge and 
> seeing an .011 inch differential movement between cap and pin.  Did you 
> just wait for humidity cycles to naturally occur or did you have some sort 
> of humidity cycling setup.  If so, would you mind describing it?
> 
> Phil Ford

I forced them. I made a bridge sample with a vertically laminated 
root, and a solid quarter sawn cap, notched and pinned. After the 
glue dried for a couple of weeks, I cooked it in a tube with a 
heater bar and guessed (by periodic dimensional measurement)  at the 
low of about 4%MC, then left it on the front porch with my 
soundboard dryer MC gage for about 4 days until it got to 12%. This 
is a pretty extreme range, so that 0.011" represents a worst case 
scenario. This might be a good time to mention something pertinent. 
For the expanding bridge to push a string far enough up a pin in the 
damp cycle to leave the string entirely above the cap (assuming it 
would stay there) in the dry cycle, the movement differential has to 
be greater than the difference in bridge height  at bridge center, 
and at crushed notch edge. If the notch edges aren't pushed higher 
than the bridge center recedes, the bridge can't leave the string up 
the pin even if it would stay there.

Ron N