Official Luthiers Forum!

Used to use this app called Bubble Level made by NixGame: https://play.google.com/store/apps/deta ... n_US&gl=US

And I could have SWORN that it used to have a zero out tare feature to it so that you could squish the phone onto the bridge with putty then zero out then reduce the string tension and have your measurement. But it doesn't seem to have that anymore.

What app, if any, are you all using for this measurement?

Not actually answering the question but:

I tried an electronic angle measuring thingy but couldn't make it work at all - this is a pretty small angle we are trying to measure.

I went back to TG's method (stick puttied to bridge and measure vertical change at the ends) but with a longer stick to make measurement easier.

I think it was Alan Caruth mentioned using a infra red pointer pointed at the wall a real distance away.

Cheers Dave m

Level ruler pro, zeros, resolution to 0.1 degrees in both axis.
I just stick it on with Blu Tac and measure 3 times.
Consistent results every time, just be sure to hold the body of the guitar steady.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I don't use an app. I don't see how I would attach my phone to the bridge on my guitars and I'm skeptical about how precisely it would measure the small angle. I attach a small laser pointer to the bridge with it pointing at a ruler hanging vertically about 6 feet away in line with the centerline of the guitar. I note where the laser dot lands on the ruler, release the string tension, and note the new position of the laser dot on the ruler. Then do the math to calculate the rotation angle at the saddle. The size of the triangle created by the setup gives good resolution for the rotation angle (a 1/8" movement of the laser dot on the ruler = 0.1 degrees of saddle rotation).

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I think there’s a native one on iPhones. Look in ‘utilities’, find ‘measure’, then use the level function…

I should have mentioned I have Android. Installing the Level ruler pro thanks for that.

With the Bubble Level when it used to zero out I would do like 20 iterations and they were well within an acceptable range of error so I trusted it. I tried using a ruler and it was an exercise in humility trying to get the dang thing to not fall over. I like the laser idea though.

That's what I use.

I think Trevor says 2°-3° of rotation? Looks like there are finer scale app options available. Know what I'm doing during my lunch break.

M

IIRC, he says 2 degrees is a maximum

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I do believe it's 2deg max too. I tend to want to shoot for under that and let nature takes its course over time to achieve the rest.

Why do you care what the rotation angle is? Can you control that? I really have never thought about it but I think I'd like to understand more.

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Hutch

Get the heck off the couch and go build a guitar!!!!

Just reviewed the videos again. I got my info from the O’Brien/Gore vids where he says “about 2°”, so yes 2°. During the vid he demonstrates using blue tack, a ruler and a formula calculate rotation, follow by him pulling out his smartphone as option #2.

Cool, M

Bridge rotation is due to torque on the bridge from string tension, and is an indication of the "system" of soundboard and bracing stiffness.
2 degrees is the maximum safe rotation which prevents future failure of the top while maintaining maximum responsiveness.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

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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

I guess that it's good that technology has enabled us to do this kind of fine measurement. But at least in my case, getting too wrapped up in the technology takes away from the pure joy of building. I was an IT guy/engineer for 40 years before I retired. Guess I got it out of my system (no pun intended...)

I'll never build that perfect guitar, but I'll enjoy building the ones I do largely by following the old empirical methods of hit and miss, hopefully including lessons learned within that process.

Dave

I like getting away from too much technology too, but the equation in the book is cool because it reminds me of calculating % grade for roads, ditches, etc.

Ah! That angle cube thing looks perfect. Thanks Trevor.

Since Trevor is here he can correct me if I am wrong but basically I think what he and some of his colleagues did was collect a bunch of 'good' sounding guitars and test them in various ways. Good in quotes because that is also part of the testing was defining what is good, what is bassy, what is boomy, what is boxy and so on. Those English words can mean different things to many people. The only way you can do that is to get musicians to play an instrument and ask them what they honestly think. Then you measure those 'good' guitars for their resonant frequencies and other physical properties one of which was bridge rotation. They found that the good guitars all fell within that range. And so the next step is to try and replicate.

So basically the 2deg rotation range has been found in guitars that people generally consider to be good guitars. Excellent may be a better word then good. That, together with all the other measured properties of what makes an excellent guitar, is a recipe now for making guitars.

So it starts off subjective but it's something that people all generally agree with... That this guitar sounds good! Then it turns into something practical.

The technology is a sort of back door to what very talented luthiers have been doing for centuries and some do today. A short cut if you will. But still, even for an experienced and talented luthier, the technology also creates consistency and a method and an approach that narrows down the many variables that can get in the way.

I only started taking the science engineering approach about 5-6 years or so ago when I started doing the simple deflection testing. Interestingly when I started using the Gore/Gillet acoustic testing to determine top thickness it pretty much matched my deflection for any given guitar shape (so far) that I had previously determined was a 'good' guitar.

But prior to that it would have been guess work based on the experience of building only 50 guitars, which is not a lot. After 300 I may be able to flex a piece of wood and thin it out to what would be determined by my deflection test or the acoustic bonk testing. But I don't have the time to wait for that

I do have several sets of top wood that are cut at odd angles that non of these testing methods will work on and of course I still plan on building with them, so there I will have to rely on my own intuition and experience.

Basically the sci/engineering methods are just another tool in the shed.

I don't want to derail the discussion about how to measure bridge rotation. But what do you do with the information after you have it. The instrument is already made at that point. Is this a matter of loosening the bracing with selective carving to get more bridge rotation if it is too stiff, or just trying to apply the experience gained from the measurement of guitar X to the design of guitar Y?

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I'm sort of like ballbanjos and Bryan on this. Using CNC for years as a machinist, I like using hand tools. I like tuning by ear. But I can see where some things can be used as a relative gauge to where the instruments sets as far as "normal". My motto has always been, "why be normal?" Normal says that you are in the ballpark. I want to be in the field playing, not just in the park in the stands.

I got Audacity the other day thanks to the help of bionta here. I checked 4 completely different violins, Maple, walnut, birch, spruce, AY cedar. very high arch, very low arch, big f holes, skinny f holes, fat f holes. All 4 had an A0 of 273. go figure.

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Why be normal?

You could try a lighter gauge of strings if the rotation is too much, or add a bridge doctor, "Musser brace" or some other way to stiffen the top to keep it from failing. Re-topping would also be an option if one really thinks the top will fail.
If there is little or no rotation it might be over braced. If the guitar doesn't sound "good" then heavier strings, carving the braces, or thinning the top (refinish work) might be an option.
For those who dance on the edge of "lightly built" the rotation measurement might be a good reality check.
I believe science and engineering can only take you so far when it come to musical instruments, but it can get you close and sometimes show you when you have crossed the line. I tend to use intuition and empirical knowledge and have had a few train wrecks along the way to get it.

On the guitar I am currently working on the rotation was at 2.3deg so I reset the neck angle to where the rotation is now 1.7deg and it made a radical improvement in the tone. The bridge was not able to rock back and forth so the top was squashed out. Now it's much more responsive with good sustain. It just doesn't have a good bass response, at least not like the other ones I have built of this model guitar. I have my own ideas as to why but that's another story.

Ok, I'm confused. How does resetting the neck angle change the bridge rotation (unless the action was incorrect)?

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Steve Smith
"Music is what feelings sound like"

Changes the saddle height for the same action and hence changes the torque on the bridge.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Thanks Colin, simple but I missed that one.


Sent from my iPhone using Tapatalk Pro

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Steve Smith
"Music is what feelings sound like"

That's correct. And we are talking a VERY small difference too. I tend to like the 1/2in string height over the soundboard rule and after that change I was well within the acceptable +/- range with no need to shave the bridge.

Intuition is very important and, I think, efficient. It would be very distracting to stop and do some type of measurement or test instead of just tapping and squeezing during the building process. Interestingly, jfmckenna's post shows how intertwined science and intuition can become. It seems that his experience was empirical to the extent that he verified the angle of bridge rotation theory with an actual "experiment" - he reduced the torque on the bridge and noted a radical change in the tone of the guitar. It started out as a scientific theory, but that scientific notion is now part of the empirical lessons he's learned or perhaps had previously verified, not simply by reading "the Books," or where ever he learned about the bridge rotation, but by experimentation. And it was very successful. It is great that we have so many voices and materials that give numerous choices on what processes to use to build the best guitars that we can.

It took me almost a year to go through the Gore/Gilet Design volume. The math was way beyond my pay grade. But, I was able to follow the ideas, and how the variables in the equations played out. What interested me most was the applicability and the ability to develop a strong methodology to fine-tune and optimize the guitar after it is built, along the lines of jfmckenna. One can go in and thin braces, only to find that the guitar did not need brace thinning, but brace stiffening, or thin the top when the opposite was required, maybe because the sides needed additional mass. Fortunately, I have only had one guitar that was a clunker, but I have tried to optimize my guitars to some extent after-the-fact. In this regard (not the building of clunker's), but after the fact refining, my approach was more intuition and guesswork. In this and other forums, I've learned a lot from Trevor's, Alan's, and other people's posts on these topics. Alan's posts on wolf notes, which are a preeminent example of after-the-fact tinkering, are but one example, and this is one more.