Bridge pin angles

[Phillip Ford]

>Ron commenting on Phils reply to Rics Post:  My comments in red
>
>/ 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.
>
>--It is also based on the erroneous assumption that sidebearing and 
>hence friction between pin and string increases as the bridge takes 
>on humidity. Since this is not the case, the numbers
>need to be re-run. Downbearing will increase, sidebearing will 
>decrease and so its effect on said
>friction.

I've tried to point out a couple of times that the calculation does 
not assume increased sidebearing and friction because the string is 
moving up the pin.  There is an initial sidebearing.  I've assumed it 
is 8 degrees.  This results in a friction force between the string 
and pin.  This has to be overcome for the string to move, even a tiny 
amount.  Also, as indicated in a previous post, my checks on two 
sample pianos indicated 8 degrees as a lower limit on side bearing. 
I measured sidebearing as high as 13.5 degrees, which is going to 
cause an even higher friction load needed to move the string relative 
to the pin.  Whatever is happening between bridge and pin as a result 
of humidity changes, it's hard to imagine the sidebearing angle 
changing more than a fraction of a degree.  Real life sidebearing 
angles make this slight change seem insignificant.  Anyway, I'll be 
happy to recalculate the friction number if you'll give me what you 
think is a realistic number for sidebearing angle after the bridge 
takes on humidity.

Also, my contention is (and I think Ron is in agreement with this) 
that downbearing is an insignificant factor in this.  It is the 
string's resistance to the upward movement of the cap that is causing 
the damage.  So, I would propose assuming no downbearing.  I think 
the calc would show that the bridge cap bearing stress still exceeds 
the allowable.

>...
>/ 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.
>
>--I think most of us know what Ron is saying, and he is wrong. That 
>is to say it is wrong to say this is the one
>and only, or even primary reason for the strings finding themselves 
>detached from any portion of the bridge.
>Indeed, the condition Ron describes can occur. But there are other 
>conditions that can occur simultanously with strings
>not fully in contact with the bridge.

What conditions would those be?

>...
>
>>>/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.
>
>--no one has ever contested this. What is contested is that this and 
>negative bearing are the only conditions which can see the string up 
>the pins and away from the bridge. Clearly many many many 
>technicians have found unmarked bridges on brand new instruments 
>with plenty of downbearing whilst still finding strings in need of 
>being tapped down.

OK.  Let's say that they have.

>  And just as clearly many many many technicians can find this exact 
>same situation in pianos that have been used for
>a few years.

Are these pianos ones that the techs mentioned above found with 
strings needing to be tapped down that didn't ever get tapped down 
and so are still above the bridge?  Or are they pianos on which the 
strings were tapped down and with time have climbed the 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
>
>--As I have said time and time again.  Of course notch damage can 
>develope over time to fit the bill Ron draws out. And just of course 
>is the likelihood that assuring good string contact with the bridge 
>is a near impossible task once this developes.  But you simply can 
>not turn that around and state that the only reason
>strings ever find themselves off some portion or all of the bridge 
>cap is because of this kind of damage.  No amount of manipulating 
>numbers and numbers perspectives to attempt to show this is the only 
>thing possible can change what is clearly observable. Namely that 
>positive bearing over the entire surface of the bridge surface can 
>exist at the same time as strings are in need of seating.
>
>Cheers
>RicB

I'm not sure what you mean by positive bearing over the entire 
surface of the bridge surface.  Do you think there could be positive 
bearing on some points of the bridge surface and negative on other 
points?  I look on downbearing as a yes/no thing.  Either this string 
has positive downbearing on the bridge or it doesn't.

Phil F