Bracing question

[mac007]

Hi

I am building a Gore/Gilet type SS (no cutaway but the more modern shoulder as in Drawing 4)

I made the back braces as per the text 20mmx10mm gabled.

And glued them in place.

Now I notice on the drawings for a live back (Drawing 6), the back braces are 6.3x16.

Since I can't reduce the sides readily as they are in place, should I reduce the height to take some stiffness out?

Second question- the GG live back has a 3m curvature. I only had a 15' (~4.6m) dish so used that.

Does this adversly impact the aim of a live back?

Many thanks in advance

MK

[mac007]

I think I have an answer to my own question so I'll post it for info.

GG Design p49-50 discusses cross section dimension vs Youngs modulus. Since I'm using King Billy pine for the braces (Youngs Modulus 1/2 that of spruce) it theoretically needs twice the cross sectional width for an equivalent spruce brace.
Mine are 10mm vs the 6.3 spruce in the design so I'll leave it as is (it is actually a little under the suggested 6.3x2=12.6mm equivalent)

Please correct me if I have this completely wrong.

cheers

MK

[kiwigeo]

If youre using King Billy Pine for the braces I'd measure the stiffness of the bracewood before using it. KBP is highly variable in terms of stiffness.....I ended up ripping the bracing of a top and replacing with spruce after top frequencies turned out way too low

[Trevor Gore]

I am building a Gore/Gilet type SS (no cutaway but the more modern shoulder as in Drawing 4)

I made the back braces as per the text 20mmx10mm gabled.

And glued them in place.

Now I notice on the drawings for a live back (Drawing 6), the back braces are 6.3x16.

Drawing 6 is a composite drawing showing the back bracing for all the models in the books. If you're building the medium body SS (albeit without the cutaway) the drawing/dimensions you need are on the right hand side of Drawing 6, and all start life 10mm wide by 20 high. So no need to panic.

Second question- the GG live back has a 3m curvature. I only had a 15' (~4.6m) dish so used that.

Does this adversly impact the aim of a live back?

The effect of the doming on the resonant frequency of panels is shown in Design Fig 1.6-10. You should be able to compensate by less scalloping of the lower back brace.

[mac007]

thanks for both great replies
mk