>>As I understood it, listening to Tom Lowell describe the proverbial >>drawing on the napkin, the purpose of the gauge was as much to be able to >>isolate the front and rear components constituting net bearing. Maybe I >>misunderstood. > >No, you didn't, but that sidesteps my point. Measuring overall bearing >with a bubble gage, you get a reasonably accurate picture of overall >bearing, be the strings wrapped, or plain wire. 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 what do you do with wrapped strings? The bubble gage eliminates all > that and gives you a direct reading of the difference in plane between > speaking length and back scale regardless of front and rear bearing, or > whether or not the string is wrapped. It also allows you to take > measurements of front and back bearing by adding the third measurement of > the segment across the bridge. I know, Tom says to start on the bridge > segment, but I disagree. So do I. > I say zero on the speaking length, get a good accurate reading of the > back scale, and then fit what you can get from the bridge top in between > the two. It's easier to measure that way, less error prone, and you > aren't working from the least reliable measurement of the three as you > are when you start with the bridge top. At some points on the bridge the feet of the gage have to be so close together that whatever reading you take seems suspect. >> Whether or not it makes a difference whether we achieve a given net >> bearing with a front and rear positive or with a mixture of positive and >> negative, is a sub-question of soundboard loading, apart from the issues >> relating to termination. This, apropos your statement above "... (the) >> load the strings are putting straight down on the soundboard". Do you >> have any thoughts regarding possible acoustic differences (effect on SB >> impedance) between a (basically) neutral load (positive front and rear >> bearing) and one which introduces distortive potential ( positive / >> negative)? This would be another reason for measuring the individual >> components. > >Yes, I do. Front and rear bearing are termination issues, not soundboard >loading issues. 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. I think these numbers could perhaps, at least on a new board, be an indication of a potential bridge rolling problem. 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. >>>>Still, the challenge of accurately measuring and conveying the nature >>>>of a particular piano's string deflection, including the bridge, is, to >>>>me, a separate question from how much or little there should be or how >>>>it does or doesn't affect the instrument's qualities. >>> >>>You'll never get an absolute answer to that one. >> >>This wasn't meant as a question. > >You said it was a question, but then what I gave you wasn't an answer either. You guys are confusing me. >>In other words, I should be able to relate the net downbearing I measure >>with a bubble gauge with what I would get using a Hartman stick. > >Ok, how? How do you measure overall bearing with the stick? How do you >take pin row spacing, and vertical offset from the bridge top angle into >account, and how do you do wrapped strings? With the bubble gage, I zero >on one segment, and read the angle from the other. Explain to me the >process of using the stick. > >Ron N Well, first you walk softly ..... Phil Ford
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