How to calculate the downward pressure?

[Paco Jimenez]

How can I calculate the downward pressure exerted by the strings over the top? Or simply the downward force over the bridge?

Thanks.

[Michael Lewis]

This is a physics problem involving string tension and break angle of the strings over the bridge. If you keep the bridge in the vicinity of 1" and the break angle around 12 degrees you will be "in the ball park". Typical down pressure of the strings is around 40# or a bit more. Obviously it will be different for different guitars, so if you want that specific answer you need a little math and the measurements from your guitar.

[Paco Jimenez]

Thanks for your insight, Michael.

Just found it.

The point is about setting up a particular guitar for lower gauge. A lady I know has an old german guitar with an adjustable Stauffer neck join. She loves her guitar but the lady is very small and has very small hands too. She hardly manages to drive it properly with a .013 strings set and a regular action. We tried .012 and .011 sets and she's comfortable with the .011 gauge but the guitar doesn't respond well. My idea is making a taller bridge and increasing the neck angle accordingly in order to get the same downward force as with the regular bridge and a heavy gauge. I hope she will manage to drive the guitar good enough this way.

[Alan Carruth]

I suspect there's an optimum break angle for any given case, but whether that case depends most on the guitar or the strings I can't say. I do know, from experiments with a changable 'hook' tailpiece that increasing the break angle beyonda certain point can kil the sound, rather than giving you more. As usual, there are probably a bunch of different variables that work together in a complicated way on this.

Alan Carruth / Luthier

[Paco Jimenez]

I do know, from experiments with a changable 'hook' tailpiece that increasing the break angle beyonda certain point can kil the sound, rather than giving you more.

Alan, Do you mean using a given fixed gauge of strings and altering the breakover angle while keeping the bridge height constant? (And pitch, I guess) Or just raising the bridge accordingly to the breakover angle and thus increasing the load over the top too?

[Michael Lewis]

You might try Tomastik or La Bella flat wounds in .012. They are supple, smooth, and comfortable.

The less mass in a string the less energy it can deliver when it is picked. Heavier strings have more mass, so, they can move more mass (soundboard structure) when they are in motion. Lighter strings just have less output in them.

[Paco Jimenez]

Yes, this is why I want to reproduce the same load on the top.

Alan, AFAIK, cellos are supposed to have the strings at about the same angles at both sides of bridge vertical axis. Something that doesn't happen in regular archtop guitars. Would this be one of the reasons why the top resonance gets dampened? Altering the breakover angle noticeably more in only one side while keeping it constant at the other side?

[Andrew Mowry]

I have a spreadsheet on my web site that calculates downward force and a variety of other variables for archtop instruments.

[Paco Jimenez]

Great. Thanks, Andrew.