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Scale Length vs Tension
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Author:  Mike OMelia [ Mon Mar 22, 2010 10:48 pm ]
Post subject:  Scale Length vs Tension
Is it not true that as scale lenght increases, so does overall tension (loading on the bridge)?

I want to modify my harp guitar design to be a 14 fret. Original scale length is 25.5". But if I want to maintain bridge location and sound hole location, then I need a longer scale. My supplier suggested 28.63". Now that is long! But as I recall, that means I need more tension given a tuning. Plus, string buzz can increase due to the amplitude of the vibrations meaning I must maintain a higher action (not fond of that).

Help please!

Mike
Author:  DennisK [ Mon Mar 22, 2010 11:30 pm ]
Post subject:  Re: Scale Length vs Tension
Well, the 3 variables are tension, scale length, and string gauge. So if you use lighter strings, then they can be longer with less tension... but with that much added length, they might break before getting up to standard tuning. Baritones have scale lengths like that though, so you could do a baritone harp guitar :)

If I were you, I'd modify the body instead. Probably a full redesign, since I love the designing part. But for a more patch-style fix, you could move the bridge up a bit, shorten the upper bout, maybe add more frets to keep the soundhole from moving so much.
Author:  Mike OMelia [ Tue Mar 23, 2010 8:56 am ]
Post subject:  Re: Scale Length vs Tension
Yes, I agree. I am going to look at subtle changes to allow the 25.5" scale. It will involve bracing mods. Maybe a slight sound hole shift.

There is more than enough tension on the bridge as it is.

Mike
Author:  Fred Tellier [ Tue Mar 23, 2010 5:18 pm ]
Post subject:  Re: Scale Length vs Tension
Why not just reshape the guitar part of the harp and the treble side of the harp neck to clear the 14th fret. A 000 12 fret Martin for example has a body that is 2 frets longer than a 14 fret 000, if you look at the outlines the 14 fret is the same shape except for the front of the body is moved back to give the clearance. The bracing is about the same on each so little body changes will be needed.

Fred
Author:  Mike OMelia [ Tue Mar 23, 2010 7:49 pm ]
Post subject:  Re: Scale Length vs Tension
Fred, I do not know what u mean by "front of body is moved back". Do u mean upper bout is squished along longitudinal axis? Bridge is in same location?
Author:  Fred Tellier [ Tue Mar 23, 2010 8:37 pm ]
Post subject:  Re: Scale Length vs Tension
That is basically it, take a look at the shape of a 12 fret 000 and a 14 fret body, even the Martin D started life as a 12 fret and the body was shortened. The bridge will be in the same place as with the bracing just the length of the upper bout is less. As a thought you could make the body a cutaway even it you only expose the 14th or 15th fret. I don't think I have ever seen a cutaway harp guitar.

Fred
Author:  Jeff Highland [ Tue Mar 23, 2010 9:09 pm ]
Post subject:  Re: Scale Length vs Tension
Actually when Martin went to 14 frets from 12, they shortened the body by about one fret and then moved the bridge forward (towards the neck) by the remaining amount.
The soundhole and bracing retained their position relative to the bridge.
Author:  Mike OMelia [ Wed Mar 24, 2010 8:42 am ]
Post subject:  Re: Scale Length vs Tension
Fred, Jeff, thank you. Fred, there are plenty of hg cutaways. I just do not want to do that on this project. I want to create a 14 fret that looks like an original Dyer.
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