Shifted bracing?

[Francis]

I'm building a multi-scaled J-185 style jumbo with Falcate style bracing with a null fret at #9 giving angled nut and saddle.

My question relates to the position of the curved braces. With a conventional single scale length instrument the bracing is symmetric under the bridge. Rotation of the bridge with a multi-scale instrument looses that symmetry - do I shift the bracing towards the nut on the treble side and the tail on the bass side to keep the bracing positioned under the bridge?
Please ignore ends of braces where they are shown as either short of the sides or overlapping

Hopefully this will help?


Thanks

[kiwigeo]

I shifted bracing as per your right hand drawing. The important thing is to have the tertiary braces under the ends of the bridge and the areas between the braces as equal as possible. On my current build I ended up having to make separate bending forms for the right and left tertiary and secondary braces......a total of four forms instead of the two forms you'd make for a single scale instrument.

Before you start your build draw up a full scale drawing to check that everything will fit together. Especially important for things like headstocks if you're planning a straight string pull design.

FYI I made the 7th fret the bias fret and the scale lengths are 658 and 638mm on Bass and Treble side.

IMG_1419.jpg (108.41 KiB) Viewed 8194 times

[Francis]

Thanks Martin - that's confirmed my thoughts on the shifted bracing for a multi scale. My scale lengths are a little different: 654 and 628.6 mm bass and treble respectively. I'm not using bridge pins on this one preferring to drill the bridge from the back:



This is from a Triple O I've just finished (although the image is at an earlier stage). I put a section of thin walled brass tube along the back of the bridge to take the string's ball ends and prevent them cutting into the bridge.

Thanks again - very helpful

Francis

[blackalex1952]

Great question and a succinct answer, thanks guys for another good thread! Ross

[Francis]

Following on from the original question:
I now have the bracing in place and going through the tapping procedure to get a good ring and sustain and I've noticed that the process has produced a 'dome' to the bracing. Not just along the length of the individual braces but across the braces as a whole. Being highest under the bridge and arching outwards in all directions - is this typical for the Falcate brace style or just how mine has evolved during the tuning process?



Francis

[kiwigeo]

Did you glue the braces in with the top sitting in a radius dish?

I notice there's no bridge patch? On my guitar this goes on first and the primary braces are notched to fit over same. The secondary braces butt against the curved ends of the patch. Between the patch and the top is a layer of carbon fibre matting to add stiffness to the bridge area.

I assume the ends of the aft end of the primary braces and both ends of the secondary braces are going to be shortened so there's a gap between the end of the braces and the linings? if you don't do this the top will end up way too stiff and you won't have the option to free up the top by thinning around the periphery of the lower bout once the box is closed up. I end my braces 25-30mm from the linings.

[Francis]

Hi Martin,
Yes the braces were glued using a radius dish (20 foot rad), the ends on the braces have been shortened and taken much thinner at the ends - the image on mine was at an earlier stage. I'm only adding CF tow to the brace tops and my bridge plate will be fitted between the primary and secondary braces as I did with the last multi-scale. The bridge plate is somewhat different in shape to yours and won't have a CF pad underneath either. This is from the previous m/s guitar:
I'm intending something similar on this one but may add a tie brace between the primary's by the soundhole and shorten the bridge plate.

Continuing to tap....

Francis

[kiwigeo]

Back to your original question...Im not quite clear on what you're talking about when you say the braces are domed? are you talking about the top being convex which is normal when youve laid the braces in while the top is sitting in a radiused dish?? Maybe Im missing something here?

[Francis]

Back to your original question...Im not quite clear on what you're talking about when you say the braces are domed? are you talking about the top being convex which is normal when youve laid the braces in while the top is sitting in a radiused dish?? Maybe Im missing something here?

Sorry Martin - it's me. The little sketches under the image were/are supposed to represent schematic sections down the centre and across the bridge line of the soundboard. The double line represents the soundboard the upper shaded area the generalised bracing profile as if you were looking at the bracing from the plane of the soundboard in those two directions.

Having now parred away at the braces the doming is not quite so apparent but still there. Just started applying the CF tow/resin.