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I wonder who has experimented with measuring bridge rotation as described in the Gore books. Am interested in all results but am particularly interested in dread( hand or commercial) results,and if anyone tried it with different gauge strings.
Tom

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A person who has never made a mistake has never made anything!!!

I wouldn't say I've experimented per se, but I have started measuring completed guitars to see if I exceed that target or not...

An off-list communication from another member got me thinking about this recently. It occurred to me to wonder how general that rule is.

An old rule of thumb in wooden aircraft construction says that the initial deflection under load should be no more than 1/3 of the deflection you could live with in the long term. Trevor uses CF in his 'falcate' bracing system, and one of the things that does is limit the long-term cold creep of the top. This could mean that you could stand more initial deflection in a structure that incorporates CF than you would want to see in an all-wood one. I may be worrying about something that's not an issue: I have not gone back to the books to see what he has to say about any differences, if anything. Still, it does point up the notion that, ultimately, any rule of thumb is just that, and needs to be confirmed in your own system. Any data anybody wants to post would help us all.

Should be as easy as measuring the bridge rotation of a series of guitars that sound good and are holding up well over time and getting an average...?

And the results are? And how do you think they sound?

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Jim Watts
http://jameswattsguitars.com

I did this with 2 of my dreads that are less then a year old. They both are decent sounding guitars used mainly for flat picking. Both with light strings. While they were relatively close in amount of rotation,one a bit over 1 degree and one just under 1 degree,the thing that suprised me a bit is I thought the results would be reversed. Had a few problems through the process. Attaching the beam to the bridge went OK with the Bluetak but trying to measure with out hitting and moving the beam was a problem. It occured to me that perhaps one could make a better anchor for the beam by removing the d and g strings(slotted bridge) and use that as a more solid attachment point for the beam. The amount the bridge goes directly up and down may be a factor to consider, but I assume it is irrelevant with the formula just considering the differences in heights at the beam ends. Wiser heads may be able to help me understand that. Looked at D'addairo strings and there is approximately 15% more tension in the medium compared to the light set. Assume this would translate into 15 % more rotation so that puts me closer to Trevors figure. Am going to try to get a better set up and get more confident readings.
As a seperate consideration I wonder if anyone has made up and down measurements of the top using a dial indicator and a given force. IE Five lbs force down on the bridge and measure the resulting downward movement.
Tom

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A person who has never made a mistake has never made anything!!!

And the results are excellent sounding guitars where the bridge rotation approaches but does not exceed 2 degrees. I made a jig with a beam that sit across the waist and another at the lower bout (30cm apart from each other). Across those two beams goes a stick inline with the ruler. The stick is the reference part instead of a lamp arm.
I believe Trevor uses a weight and dial caliper across the bout at the bridge to get his 'monopole mobility' comparisons.

Thanks, both of you! I have not made this measure and have gone off of deflections and frequency response myself, but Trevors idea of bridge rotation made sense to me so thought I might check a few guitars out. I have looked at it an FEA model using some deflection data and came in a little under 2 degrees, that guitar sounds really good in my opinion, it's on the brighter side and it's a classical.
Maybe we could brainstorm up some quick cleaver tool to measure this a little less kludg'y. I'm wondering abut a vertical attachment with a pendulum hanging from it and you just read the degrees off of a scale. What'ya think?

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Jim Watts
http://jameswattsguitars.com

The two degree rule of thumb comes about for a number of reasons:

1) It sets a guideline for the top's longitudinal stiffness and hence deflection under string tension
2) As longitudinal stiffness has a major impact on the mode resonant frequencies, it puts you in the right ballpark to get to the normal range of resonant frequencies, provided you don't use a huge, massive bridge
3) It is about the maximum deflection in a conventionally domed "flat top" that doesn't cause a hollow in front of the bridge (at least on my CF bracing designs!)

The "good" guitars (OK, guitars of a variety of types that we liked) that we tested when writing the book were close to two degrees when tested. Some of the older ones had fairly distorted tops, but ~2 degrees was the elastic recovery when the string load was dumped.

Having said all of that, it is not the intention to have the guitar "tuned" exactly to the 2 degrees of rotation. What needs to be tuned are the mode resonances. The two degrees just helps you get in the right ball park so that can be achieved.


Taking a measurement off both ends of the ruler (as per the notes) nulls out that problem.

Straight deflection measurements like that give you some of the "story", i.e. the static stiffness response. What we're mostly interested in is the dynamic response which adds the inertia effect to the stiffness response to give the dynamic response i.e. the monopole mobility. It's not a hard test to do and it gives you the deflection data you discuss as part of the process. Instructions are in the usual place with monopole mobility charted for a whole bunch of "good" guitars, both SS and classical.

There's some pics from a workshop I did in this thread. I'm using my "travelling kit" to get bridge rotation, but it shows that you can get reasonable data from some pretty basic gear. Of course, once you've got a bracing design "dialled in" you don't need to do rotation tests and monopole mobility tests on every guitar. They should come out pretty similar. (But you still need to do the modal tuning...)

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Trevor And Meddling Fool: Guess I'm not sucking up enough from the books. Went back and looked and there it was for the monopole mobility. Even had it hi-lited..............!! Brain not working like it should. Will have to give that a shot also.
Jim: I can see no reason why your idea would not work as long as one had a constant worked out set up.Might give that a try.
Tom

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A person who has never made a mistake has never made anything!!!

Meddling Fool: If you are into photos and posting here, I would appreciate a pic of your set -up for the beam. If not ,no problem.
Tom

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A person who has never made a mistake has never made anything!!!

If you check out the link TG posted above, it's basically the same as the photo with the yellow ruler a bunch of shots in...

here is a simple jig I made...avoids "math" as much as possible!!!!

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MK5acoustics.com

Wes,Thanks for posting those pics, That's along the lines I was thinking ( see my post above if you haven't). Great!

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Jim Watts
http://jameswattsguitars.com

Wes: That is slick...............!!! Thanks.
Tom

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A person who has never made a mistake has never made anything!!!

Back to Al's point, do you think it's likely that there initial rotation back when they were built in the '30s was likely 2deg (maybe less) and that creep over time has allowed them to rotate to where they are now?

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Formerly known as Adaboy.......

It might be interesting to see how much rotation from strung to unstrung in some of these guitars that have quite obviously distorted over time.
Tom

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A person who has never made a mistake has never made anything!!!

Todd:
The point is not how much total rotation they show now, as a result of cold creep, but rather the short term rotation as between the loaded and unloaded condition. It's unclear to me which you're speaking to. An old guitar could have some serious deformation of the top, but only rotate back a couple of degrees when the strings are slacked off.

When I was doing my saddle height and break angle experiments, one way I looked at bridge rotation was to stick a laser pointer to the bridge and see how the spot moved. You can get get very precise results due to the long base line, and it's minimally invasive.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Todd Stock wrote:
"Which again raises the question as to whether small rotations now presage larger rotations later - and the answer is likely 'sometimes'."

I thought I'd covered that with:
"An old rule of thumb in wooden aircraft construction says that the initial deflection under load should be no more than 1/3 of the deflection you could live with in the long term."

Cold creep seems to start out at a fairly high rate, and slows down over time. your new guitar will probably 'take a set' over time, and if the initial deflection is on the order of two degrees, then you might expect six degrees total in the long term. An old guitar that already shows five or six degrees of rotation in unlikely to see more over time. However, if you slack the strings off on either guitar, you're likely to see something like 2 degrees of relaxation right away.