Bridge pin angles

[Ron Nossaman]

> There is also downbearing.  If the bridge cap is 
> going to move up relative to the pin, and the string is going to go with 
> it, then these forces have to be overcome.  By my calculation in a previous 
> post the stress induced in the cap was 4080 PSI.  This is considerably 
> higher than the 1500 PSI allowable.

And is considerably higher than downbearing induced loads, which 
would make this the primary mechanism for crushed notch edges. That, 
and the fact that the angle of the crush track at the notch edge far 
exceeds any downbearing angle.


> Also, you bring up another good point which I don't think I've seen 
> mentioned.  As the bridge moves relative to the pin, the pin hole is 
> probably not going to want to stay parallel to the pin.  So, the bridge is 
> probably trying to tweak the pin, as you say.  This may put considerable 
> stress on the edge of the hole at the cap surface, which would tend to 
> enlarge the hole, which would tend to make the pin loose, which would tend 
> to exacerbate pin flagpoling, which some think is the source of false 
> beats.  The more you look at it, the more this pin termination seems to be 
> a bad idea.

Friction going up also will put more side bearing stress on the pin 
by leverage from the pin angle, crushing wood in the hole behind the 
pin.


> I don't want to put words in his mouth, but I believe 
> that Ron is saying that strings don't climb pins - period.  The notch edge 
> is receding from the string (at least in dry weather) because it has been 
> crushed.  This gives the appearance that the string has climbed the pin 
> because you can tap it down at the pin into the crushed portion of the 
> notch.  But the string hasn't lifted itself completely clear of the cap 
> across the entire width of the bridge.  

I am.


>>Pictures... grin... yes this would do a lot of good.  I'd get accused
>>of faking them by those who have already decided whats going on.
> 
> 
> Perhaps.  But it might convince others on the fence, or it might convince 
> the skeptics to get out their feeler gages and see for themselves.

Pictures are much easier to fake than math and logic, and so are 
going to be of no use. Something this supposedly common and obvious 
should be reproducible. If strings get up and stay up pins against 
positive downbearing, offset angle, and pin angle while people are 
playing the piano continually until the tuner can come and tap them 
back down, it ought to be dead easy to pull them up the pin and make 
them stay there any time you like. I don't see how they could be 
kept down. The feeler gage can prove it happened, and playing the 
thing for a while and trying the gage again will prove that strings 
stay up pins. I'll most definitely attend the convention class where 
that is demonstrated.


>>One final note.
>>
>>The whole line of reasoning Ron N lays out lives and dies upon the 
>>existance of
>>negative bearing when the string is off the cap.  Alls one has to do is find a
>>case of strings off the surface of the bridge while at the same time 
>>finding plenty
>>of positive bearing.
> 
> 
> I believe he did say that he doesn't believe any piano that has positive 
> bearing could have strings off the bridge.  He also seemed receptive to 
> being proved wrong.  Anyone with a feeler gage (and a downbearing gage, I 
> might add) can do so.  No one has spoken up yet.

As I continue to point out, even under positive bearing, the notch 
edge will still be below the string after it is sufficiently crushed 
by cyclic bridge movement.


> I also don't think he said that string seating was useless.  I think he 
> said that it was temporary.

I said it was temporary, and I said it didn't fix the problem, 
because the resulting tonal problems are almost entirely from loose 
bridge pins..


> 3.  To investigate the effects of string vibration alone is a little 
> trickier.  Perhaps the setup in number 2 but with no downbearing.  Subject 
> this to string vibration.  One potential problem here - is the 
> arrangement of two bridge pins having typical angles, but no side to side 
> offset, clamping the string down in a realistic enough way for this test to 
> be meaningful.  Thoughts on this?
> 
> Phil Ford

It isn't conclusive, since I can't know the piano's entire service 
history, but de-stringing a bridge, I typically see more pin and 
notch damage on the speaking side. I have no way to determine 
whether this is from play, front bearing angles, or seating of strings.

Ron N