Live back arching

[Dave M]

Trevor

I am making a falcate braced steel string using your plans for a medium body SS. I braced the back yesterday into a 3 m dished work board and was a bit surprised at just how steep that arching is. I wondered at one point if the back would flex into it but it was fine.

I was wondering why you use such a small radius for this live back?

My thought was that to achieve the right resonant frequency you need a fairly thin back and therefore use the high arching to recover the strength needed for a sound instrument...

Interested in your thoughts.

Cheers Dave

[kiwigeo]

Dave, Im using a 15' radius dish for the back on my current Gore bulld. The body is based on the plan in The Book.

[Trevor Gore]

I was wondering why you use such a small radius for this live back?

My thought was that to achieve the right resonant frequency you need a fairly thin back and therefore use the high arching to recover the strength needed for a sound instrument...

You pretty much answered the question, Dave. I came to that number more by luck than judgment, but it worked so well, straight off the bat, that I stuck with it. The arching supplies stiffness rather than strength, but without the weight (mass). So an arched plate intrinsically has a higher monopole mobility for the same stiffness. I didn't really want multiple braces to tune, so that meant three braces and with the 3m arch I can get from a non-live back to a live back quite easily over a frequency range of around 40 Hz scope. I've also never had a back crack or turn inside out, which is another benefit of high curvature.

Dave, Im using a 15' radius dish for the back on my current Gore bulld. The body is based on the plan in The Book.

Ah ha! That would explain why you needed such a high back brace on that guitar to get the T(1,1)3 up. Check out Fig 1.6-10 and Section 1.6.5 in Design.

Also with a low T(1,1)3, the T(1,1)2 frequency gets repelled down more due to the usual coupling, so more bracing is required to get the T(1,1)2 up. Turns into a viscous circle pretty quickly!

[kiwigeo]

Ah ha! That would explain why you needed such a high back brace on that guitar to get the T(1,1)3 up. Check out Fig 1.6-10 and Section 1.6.5 in Design.

Also with a low T(1,1)3, the T(1,1)2 frequency gets repelled down more due to the usual coupling, so more bracing is required to get the T(1,1)2 up. Turns into a viscous circle pretty quickly!

I was just sitting down at my local cafe thinking about Dave's question and the influence of radius of the plate on stiffness crossed my mind.

[Dave M]

Trevor thanks for the clarification.

I am noticing that the shaping of the sides + lining for the back joint is a little trickier with the high arching. The head block in particular needs plenty of height left to accommodate the arch. I actually glued a little shim to it but I suspect it will be all removed in the final fitting.

This will also demonstrate the value of the binding ledge cutting jig when we get to that stage.

And Martin good luck with that build.

Cheers Dave