Break angle and downward pressure on guitar top

[Ulf_Lindholm]

Originally Posted by Matt Mustapick

What an interesting arrangement! I wonder what that sounds like?

Yes Batsons are very interesting guitars, but have never played or heard one live. They are using lattice bracing nowadays. There are quite a few videos on YouTube of course and all the usual testamonials from players.

I've been inspired by Batson in my little project now, but with a "tailpiece" construction that produces a higher break angle. I said 15 in the beginning of this thread, but checked a bit more carefully now and it's 10. According to the formula on Mottolas site, that rick-slo kindly posted a link to, this will give a pressure of around 25 lbs on the top with medium light strings and 24.9" scale. Maybe there are more accurate and elaborate ways to calculate it but I have a feeling Mottolas approach is good enough for me. Guitar is half finished, with the top braced and glued to the rib, so I found a 12 kg car rim lying around yesterday and put it on the top and it seemed ok. Top has a 20' radius and CF reinforced bracing. But it still worries me a bit. 25 lbs is quite much.

[Howard Klepper]

Originally Posted by FrankS

So at the nut the string height will be zero (0.013 minus 0.013) and at the 12th fret it will be 0.057" (0.07 minus 0.013). At twice the distance from the 12th fret is the saddle and the height difference from the nut must be 0.057 times 2 or 0.114". Since we know the height of the triangle of 0.114" and the length of 25.5" the angle from the nut to the bridge must be the arctan of 0.114 divided by 25.5 or 0.256 degrees. To find the B angle it is just 90 minus 0.256 degrees or 89.74 degrees. Cosine of this angle is 0.005. Multiply the string tension of 6 strings and you will see that the force is very small.

Frank, the height of the triangle has to be taken from a line between the nut and the tailpiece hinge. This is what you are doing when you use the actual angle formed by the string at the bridge. The string's rise above the fretboard does not determine the relevant angle.

[FrankS]

Howard, I am really not trying to be argumentative since I think we are saying the same thing. Yes, using a string from the nut to the tail piece will get you the two angles. No matter how you do it, the A angle is going to be significantly larger than the B angle and taking the cosine of that will make those larger angles more significant than the smaller angles so the primary force will come from A. The lever arm for the A angle is only on the order of around 4"-5" while for B it is on the order of 25". A change in 0.25 inches on the A side would require a change of 1 to 1.25 inches on the B side to have the same effect. If you have some actual measurements from a flat top or arch top, it might be easier to just do a joint calculation on a typical geometry.

Frank Sanns

[Alan Carruth]

Any time the string attaches at the bridge behind the saddle there is an upward force on the bridge as well as a downward force on the top of the saddle. On a normal fixed bridge the two must be equal. On things like the Batson (which is, indeed, not a new design at all) they could be different, depending on how much downbearing there is from the back of the tieblock toward the tail. The Batson type of design can (but may not) reduce static torque on the top, and it does greatly reduce, if not eliminate, the shear loading between the top and the bridge. To the extent that it does reduce torque it should also reduce the effect of the two longitudinal string signals: the twice per cycle tension change, and the high frequency compression wave that's sometimes called the 'zip' or 'clang' tone.

It has been posited that these could act directly on an arched top, since changes in tension do push downward on it, but unless the neck angle is very much higher than is normal on most arch top guitars or violins I suspect the effect of these is pretty much negligible. As my Indian calculus prof used to say: "As the jangle goes to jero, the sine of the jangle also goes to jero. What I mean I am now explaining" I've looked for those signals on archtops and fiddles, and that's what my data suggests.

[JRB]

The Batson bridge reminds my of the Gelas guitars. Except for the whole double top/reverse neck angle thing.

[Ulf_Lindholm]

Originally Posted by JRB

The Batson bridge reminds my of the Gelas guitars. Except for the whole double top/reverse neck angle thing.

That was an odd guitar! Just proving the point, I suppose, that there are many ways to peel an orange. Never heard of Gelas before. I did some googling now but couldnt find any information about the bracing structure.

[FrankS]

This is not a bridge failure due to upward or downward forces. It is a structural failure of the main body of the guitar. There is not enough structure of the body of the guitar to prevent the tension of the strings from literally folding the instrument in half by pulling on the tail piece. Internal braces like X braces, transfers forces over a wider area preventing this this potential folding area. It is not just the forces on the braces, it is how the braces distribute the loads.

Frank Sanns

[Alan Carruth]

Ulf_Lindholm wrote:
"Guitar is half finished, with the top braced and glued to the rib, so I found a 12 kg car rim lying around yesterday and put it on the top and it seemed ok"

You have to think about the long-term 'stress relaxation' or 'cold creep' of the top. In the world of wooden aircraft structures, I'm told the rule of thumb is that the initial short term deflection under the full design load should be no greater than 1/3 of the maximum allowable deflection. The structure will creep over time: that's why we need truss rods, among other things.

Anyway, if your top deflects, say, 1.5mm under that load in the short term, you can expect it to end up deflecting more like 4.5mm over time with the string tension. What will that do to the action? Remember, too, that mass loading the top is not the same thing as having strings on it: for one thing, the mass doesn't pull directly on the neck. Of course, without the back on it, you can't put on a string load, but you may have gotten to that by now.

[sweiss]

I never should have started reading this thread. Now my head hurts.