[pianotech] Drilled capo bars

[Nicholas Gravagne]

Agreed, JD, but to a point, and depending on many individual factors.

Calculations show that given:

1) Grey Cast Iron (typical @4% carbon) lowest grade 20 with MOE of 10
to 11.6 x 10^6
2) 15 to 18 degrees upward string deflection (under capo)
3) Trapezoidal shape of capo ½" top, 1” base, 2” tall
4) Length 12”, uniform loading
5) moi units^4 = 0.48
6) Number of strings = 20 unisons x 3 = 60
7) Tension avg. = 160 lb. pull

As measured in highest capo of Steinway B. A nearby M&H BB, though
fewer unisons, carries an almost identically dimensioned capo.

Thus, the aggregate upward load on the capo bar would be 2,485 lbs at
15 degrees, and 2,967 at 18 degrees. Using MOE of 11.6 the deflection
of the bar at its center would be 0.020” when strings are pulled up.
Higher grades iron (such as 30) yield higher MOEs of 14.5 and as such
help some to control deflection, but not much.

Even a steel bar of same dimensions (30 MOE) would still deflect by 0.010”

Compromising these bars with through-holes would be risky, especially
for those unisons directly under the holes.

FWIW – and still having fun.

Nick Gravagne, RPT
AST Mechanical Engineering


On Thu, Mar 10, 2011 at 12:17 PM, John Delacour <JD at pianomaker.co.uk> wrote:
> At 11:24 -0600 10/03/2011, Daniel Carlton wrote:
>
>
>> Anyone care to say anything about drilled capo bars? Has anyone besides
>> manufacturers tried this, and had success or not? How exactly is it supposed
>> to contribute to "exceptionally long sustain?" By removing mass from the
>> bar/giving the bar more freedom to vibrate? I'm not an engineer...
>
> The whole idea of a capo bar is that it should be massive and immovable.
>  This mass and immovability is critical in the extreme treble, where many
> makers do use a cast-in front bridge (capo). Makers who use agraffes almost
> always used a special more massive agraffe for the top two sections and good
> results can be had with those.  The bar has several advantages over the
> agraffe but in all cases is far more massive than it needs to be to perform
> its function, simply because it's convenient to cast it that way. Drilling
> holes in it ( and I've never seen or hear of such a thing) is not going to
>  reduce the mass to anywhere near the critical level and is not going to
> make a scrap of difference to the function of the bridge as a massive
> immovable termination that absorbs no energy from the string.   Perhaps they
> think it looks pretty.
>
> JD
>
>



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