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Originally Posted by GaultierRedon14
I've been quietly following along and learning from your posts, Tim. Thanks! One question that came to mind after reading your last post was, how does scale length affect your thicknessing and bracing, and/or voicing?
For example, a client orders two guitars, same model, same tone woods, same playing style and string gauge, etc., but wants one to a scale length of, say, 24.9 or 24.75, and the other to be 25.4 or 25.5.
So, how big of a difference might there be in the thickness of the top, or size of the braces between the two?
Is this essentially the same question you have to answer for a client who wants to build two identical guitars but one designed for medium gauge strings and the other for light gauge strings?
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That's a great question GaultierRedon14. Scale length isn't as critical of a factor to apply to my voicing process as is string gauge. For example there is ~7 lbs of string tension difference between a 24.75" scale length and a 25.5" scale length. That ~7 lb difference is more likely to be felt by the player because fretting will be easier felt on the shorter scale since it takes less force to fret each string. Theoretically that ~7 lbs less string tension could make the guitar have slightly less volume but I think it would be difficult to prove since there has never been two EXACTLY identical guitars built, that I am aware of, to compare equally.
An 11/52 "light" set of strings has roughly 145 lbs of string tension verses a set of "medium" 13/56 strings that have 190 lbs of tension to apply to the top and neck. That 45 lbs of string tension difference is huge and should be taken into account in a lightly built and responsive guitar.
Brace placement is another critical factor to consider when laying out brace locations on 2 tops of different scale lengths. Since the bridge location is .75" lower on a 25.5" scale than on a 24.75" scale one must shift the sound hole, bridge plate, X brace intersection and all other braces to accommodate for the differences in scale length.
For example: If I mounted a "short scale" 24.75" braced top on a guitar body then glued the bridge at a “long scale” 25.5" location then there would be way too much torque on the top bracing since the X intersection is .75" further away from the bridge then it should be. This top will likely not respond appropriately or would soon fail due to insufficient support for the torque applied to the bridge from the longer scale length. In this example it’s not a matter of the scale length being the problem but rather mounting the bridge to an insufficiently supported area of the top.
Adjusting the top thickness for the differences in string gauges is a mathematical (cube root rule) process if using a deflection board, as we do. The stiffness of the top is measured by placing a heavy weight on the top and then measuring the amount of deflection or bend it has under that same applied weight. As we remove material from the top, by making it thinner, the top will bend or deflect more as it gets thinner. As the cube root rule defines stiffness is proportional to the height or thickness of the top and it only takes removal of a small amount of material to make a significant difference in the top’s or braces stiffness. We have target deflection numbers that we use for different body sizes and string tensions. This assures me that each top will be sufficiently stiff enough for the string gauge the customer will be using. This is only a starting point because we then have to brace the top and voice the top after that. We continue to use the deflection board during each step of the process to monitor the changes in deflection and how this correlates to the tone and responsiveness we are targeting for each unique client.