Bridge Pin Offset

[Overs Pianos]

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Terry,

>Hello Ron Overs - or anyone else that cares to chime in - in 
>your.....not-so-recent post below, you indicate that you use 10 
>degrees of string offset (between front and back rows of bridge 
>pins), and a 20 degree pin inclination.
>
>I was laying in bed this morning trying to work through the process 
>of punching some holes in a bridge top (well, actually, the process 
>of locating the holes).

Yes, its amazing how much design occasionally goes on when one is in 
the horizontal position.

>Now I can do a pretty good job of copying a pattern off an old 
>bridge and transferring it to a new one - I can even straighten 
>things out a little bit. But one should be able to start from 
>scratch and get a straight line from the tuning pin, under the capo, 
>down the speaking length, get the targeted offset angles across the 
>bridge and come out with a wire oriented parrallel to the speaking 
>length on its way to the hitch pin.
>
>It sounds simple enough, but when you actually get down to it, it 
>appears difficult to do with precision. Many take the course of 
>marking the bridge top with two dots along a straight line from the 
>capo/agraffe to the hitch pin. While that may be "good enough" in 
>many cases, I would feel more comfortable having complete control 
>over this process.

Yes I agree with you. Some designers deliberately angle the speaking 
lengths towards the plate struts as they get closer to them (ie. at 
the bridge). This allows for the dog-leg under the strut to be 
reduced without compromising the speaking lengths. Therefore, in 
these cases it is not advisable to make the speaking lengths 
nominally aligned with the hitch pins. Furthermore, as Ron N also 
mentioned in his post, the hitches are rarely placed accurately. 
Nevertheless, it is amazing how good you can make it all look with a 
bit of care.

>There are several factors that will conspire to make this process a 
>less-than-straightforward one: varying string diameter, bridge pin 
>angle (and resultant string position away from bridge pin hole in 
>bridge top), distance between forward and aft bridge pins, hitch pin 
>position, and softness of bridge cap (I suppose we are using 
>maple/boxwood/whatever hard enough to not worry about this?).
>
>Do you have a methodical approach for locating new bridge pin holes 
>that result in the desired 10 degrees of string offset? Thanks.
>
>Terry Farrell

Yes we do. The problem of establishing the correct bridge pin 
position to produce the desired string offset angle is one which has 
plagued most of us who do bridge work. As you mentioned, pin diameter 
and inclination, wire diameter and front to back pin group distance 
all influence the resultant string offset angle.

For some years now we have been using a formula (which I wrote) to 
calculate the required rear bridge-pin-hole offset from the 
straight-through point. This formula is also incorporated into our 
piano design spreadsheets to take the guess work out of calculating 
string offsets (this is an area of advice in Wolfenden's wonderful 
Treatise on Pianoforte Construction which I found to be wanting). 
When designing a new piano on a spreadsheet, the desired string lines 
and termination points are calculated first, then the bridge-pin-hole 
positions are calculated from the theoretical location of the string 
'kink' (in terms of its X and Y values). The smaller spreadsheets 
which we use for small workshop repairs give us information expressed 
as millimeters of offset required from the straight-through position 
(ie. offset of the rear pin relative the front pin). These are 
wonderfully handy sheets, since they help to limit errors to the hand 
skills department.

Ron O.
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        OVERS PIANOS
Grand piano manufacturers
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Web:   http://www.overspianos.com.au
Email: mailto:ron@overspianos.com.au
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