Bridge pin angle

[Phillip Ford]

Phil F writes:
>>I agree.  I think that's the crux of this discussion.  If the cap 
>>material is strong enough (hard enough) then with a humidity increase it 
>>will push the string up and the cap won't indent.


Ron N writes:
>Or the string will be pushed up the pin, and the cap will indent anyway.

It could happen that way.  I suppose it depends on the strength of the cap 
material and the size of the string.  With a pristine bridge cap made of 
material that is not strong enough to keep the string from indenting it, as 
the cap rises the string will crush itself into the cap, increasing the 
contact area between string and cap and lowering the bearing stress.  If 
the bearing stress drops below the allowable before the cap has risen all 
the way, then for the rest of the cap's rise the string would go with 
it.  So, the cap would be indented and the string would be pushed up a 
bit.  One thing to note with this situation - the force required to push 
the string up and the bearing force (the bearing area times the bearing 
stress), have reached an equilibrium.  This means that for the cap to push 
the string up there should be no further indentation.  So, if this string 
was tapped down in the dry cycle, on the next wet cycle it seems to me that 
it would be pushed up again without further indentation of the cap.

However, if the cap material is of sufficiently low strength that as it 
rises the string is able to indent itself to it's full diameter, then the 
bearing area would have increased as much as it can.  If the stress is not 
below the allowable at this point then I think the string will just sit 
there and make a deeper indentation as the cap continues to rise.  The 
thing to note here is that the bearing area never increased to the point 
where the force required to push the string up resulted in a bearing stress 
below the allowable.  So pushing the string up is going to cause further 
indentation.  If you tap this string down in the dry cycle, on the next wet 
cycle it seems to me that there will be further indentation.

Another factor, of course, would be the wood getting stronger as it is 
indented (work hardens, so to speak) - the phenomenon described by Ric 
B.  I've never seen any data describing this phenomenon.

>....
>>I think what is causing the severe indentations is the process of 
>>repeatedly tapping the strings down over the years.  The bridge goes 
>>through some number of humidity cycles which causes an indentation.  The 
>>tech comes along in the dry cycle and taps the strings down.  The next 
>>wet cycle the cap crushes some more.  And on it goes.  So, not only is 
>>tapping the strings down a temporary fix, it's making matters worse in 
>>the long run.

Or could be making matters worse.  It some cases it might not, as described 
above.


>I agree, and tapping in bridge pins would be even worse.

Why?

Phil F