Bubble gage (was Re: consistent downbearing measurements)

[Ron Nossaman]

>With a dial or stick gage,
> >your overall bearing will have to take into account the vertical offset
> >across the bridge from front and back bearing (which you don't know), as
> >well as deviation of the back scale from the plane of the speaking length
> >where the gage is resting - all of which is dependent on the length of the
> >gage and width of the pin row spacing.
>
>Which limits their utility in communicating information (it's hard to know
>if everyone is on the same page, so to speak), which seems to be one of
>David's concerns.

And mine, which is why I've always advocated discussing bearing in degrees.


>At some points on the bridge the feet of the gage have to be so close
>together that whatever reading you take seems suspect.

I think so.


>I agree.  Although, unless there is a severe mismatch between string angle
>and bridge top angle, I even question whether there would be a termination
>issue.  For slight mismatches (even for some amount of negative bearing) I
>think the bridge pins are going to work well enough to terminate the
>speaking length.

They do, which is why you can measure negative front bearing in a whole lot 
of pianos out there that aren't exhibiting any specific tonal problems 
undeniably due to negative front bearing.


>I think these numbers could perhaps, at least on a new
>board, be an indication of a potential bridge rolling problem.

Not by themselves. A trip underneath with a string and flashlight to check 
crown in the area the suspect bearing readings came from would tell a much 
more complete story.


>   It seems to
>me that if you choose the amount of bearing you want to put on your board,
>then the ideal way to put it on would be with front scale and back scale
>forming the same angle to the bridge, so that forces on the bridge at its
>front face and back face are the same.  If they are not the same, then it
>seems there is the potential over time to roll the bridge.

I don't think that typically has anything to do with bridge roll, which is 
essentially soundboard structural failure.


>Explain to me the
> >process of using the stick.
> >
> >Ron N
>
>
>Well, first you walk softly .....
>
>Phil Ford

Speak softly, maybe?

Ron N