Official Luthiers Forum!

For all those folk out there that measure bridge rotation a la Gore/Gillet. Have you ever built an Acoustic Guitar with more than 2 degrees of bridge rotation? Did you leave it or remedy to 2 degrees?

My latest Dred build sounds really good and has a Bass sound I've been looking for but has 2.48 degrees of bridge rotation. T(1/1)1 is 82HZ. T(1,1)2 is 152Hz.

All things point at going back to add stiffness back into the top but thought I'd ask for others opinions / experience first.

Cheers.

"My latest Dred build sounds really good and has a Bass sound I've been looking "

Then leave it alone

You answered your own question.

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"Act your age, not your shoe size" - Prince

That is what I want to do.

The plan is to sell it. I wouldn't want it coming back in a year or Two because it had deformed.

The book says a little more or less is acceptable but doesn't quantify how much.

Cheers.

This may help to "settle down" bridge rotation.
You might want to try it.
It's easily fitted immediately behind the bridge plate, easily removed, and doesn't need the CF lamination.
From a master of the dreadnought/bluegrass guitar, Mario Proulx - "the PMTE. The little brace behind the bridge plate was nicknamed the "Proulx Magic Tone Enhancer" by Bryan Kimsey, who was the first person who noticed it. It's "magic" because, in theory, it should ruin the guitar, but instead, it really does liven things up.
For two reasons: first, it stiffens the bridge plate area, but most importantly, it adds some mass where it can do the most good. Yes, mass is good, in the right place......"

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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 think 2.5° should be Ok. If you still have the guitar in a month recheck it for stability; see if under string tension it is rotating more. I had an 8 string classical that initially had a bit more rotation than yours. Two weeks later it was still moving so I retopped it.

I like Colin's idea.

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http://www.Harvestmoonguitars.com

I find that taking the scallop too deep is a recipe for disaster I don't scallop lower than 5/16 in
if making the top flexible was the secret for guitar tops we would be bracing paper there is a balance and you can tip it too far

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

It would be interesting to see a full image of that because it looks like the X-brace angle is wide open and also almost looks like possibly 3 lower tone bars? That one that joins up to the bridge plate looks very high up on the X.

2 tone bars but almost Lowdonesque? finger braces -
http://www.proulxguitars.com/buildup/images/rosettes%20and%20purflings/DCP03892.jpg

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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.

Ah ok that's different for sure.

The worst problem with underbracing is when you get a sharp bend at the back edge of the bridge, resulting in it peeling off after a few years and not having a flat surface to glue it back on. A bridge plate extending behind the bridge will offset the bend point so it doesn't stress the glue joint. A thick soundboard may spread out the bend enough even if the total deflection is over 2 degrees. The PMTE brace should also work, stiffening the portion of soundboard under the back edge of the bridge.

With the bridge peeling problem dealt with, 2.5 degrees is probably ok. Especially if the saddle slot is angled a few degrees back to start with so it doesn't tilt forward under tension (higher chance of splitting at the ends of the slot). Intonation can also go bad as the bridge rotates forward over the years, but can be remedied with nut compensation. If you can't move the saddle breakpoint any further away from the 12th fret, move the nut breakpoint toward it.

It' fine, enjoy it. The best sounding guitar that I ever built has too much bridge rotation (for some) , a nice belly, braces telegraphing though the top and she sounds like heaven.....

Have you ever worked on a Somogyi? Same thing, telegraphing, lots of rotation and killer sound especially the bass.

A guitar should be viewed as a system and a whole lot of other things too as Mario used to offer a bellows is a great analogy too. As such it's not about isolating one thing such as bridge rotation and sitting in judgement of same. It's all about how the entire structure, the system works together to produce the desired outcome.

You will see a whole lot of speculation and over thinking here, no insult intended to anyone. But at the end of the day you cannot apply a strict scientific method to what something sounds like. Many have tried, all have failed. Tone is subjective and guitars are tone systems. The variables are endless including the climate where the dang wood grew.

Specs offer some guidance but that's all I would ever use them for because it's very possible that what you have produced is superior to what you were shooting for and if you only heed someone's specs you may overlook that you did a dang fine job!

Enjoy it.

Norm Blake was quoted as no belly no tone Martins also develop a belly
without seeing the final brace configuration I would assume that some over scallop may have occurred
most likely not the end of the world
it sounds like a guitar play it

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Thanks for all the input everyone. It's much appreciated and given me food for thought.

I'm going to leave it alone and see what happens. Agree with the comments that it's not all about one aspect. In my case it's probably a little knowledge is a dangerous thing.

I made the Soundboard a little thinner compared to my previous measurements and thinned the edges. Bracing was kept the same. I do scallop the x brace. Just checked the measurements I took at the time and the lowest point funny enough is as John mentioned 8mm (5/16).


I'll keep an eye on it for the next few months.

Cheers.

Just one more thing to add, if you see like a creasing in the area around the bridge wings then you will need to be sure that the braces are glued in place properly. I had this excessive rotation issue once and it drove me crazy trying to figure it out. Under string tension I could slide a very thin spatula under the brace.

Bridge rotation is partly a function of the torque, which depends on the string tension and the height of the strings above the top. If you can lower the string height a bit that would reduce the rotation, but it would probably call for a neck re-set.

I often wonder how general Gore's 2 degree criterion is.

I always interpreted his comment in two ways. As a target when doing an analysis of a guitar design, and on a guitar there should be some rotation of the bridge but not too much. If there is no rotation who knows how stiff the top is.

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http://www.Harvestmoonguitars.com

Wood 'cold creeps'; keeps moving under a sustained load and takes on a 'set', even when the load is well below the nominal yield point for short-term loading. The creep tends to be fairly fast at first and slows down over time. In guidelines for wooden aircraft construction the FAA says that initial deflections under design loads should not exceed 1/3 of the value that can be tolerated over the long term. Presumably this reflects an expectation for long-term creep being about twice again as much as the initial deflection. That implies that a two degree initial bridge rotation could result in six degrees of creep over time. That would certainly serve to concentrate a large peeling load along the back edge of the bridge, if nothing else, so it makes sense. But I do wonder how that all relates to things like brace profiles. Also, CF may not creep as much as wood, so composite braces might have different long-term deflections. Again, there are probably differences in that. The epoxy binder in the vertical webs that North shows in his photo may well creep in shear under load, where the brace caps that builders like Smallman use would be less likely to, since they're under tension and compression. Nothing's ever simple with guitars...

Maybe someone should warn Mario Proulx about that.
I'm keeping my head down!

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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.

If I do go back to remedy I think this will be the first thing I try. I hit my target number of 12mm (1/2) string height above the soundboard, some wiggle room at least.

What's given me second thoughts is the calculation was done with 0.52 to 0.11 gauge strings. I'm going to test again with some 0.12's and check the difference.

Never a dull moment.

Changing the string height at the bridge seems to change the sound of the guitar more than many other adjustments will. One reason resetting a neck is preferable to shaving a bridge. If you like the way the guitar sounds you may want to leave it alone. If it sounds good with the 11's you may want to stick with them. 12's will just add more tension.
As Alan said- over time you may get some additional rotation. It may not be as significant as 3 times the initial amount - I'm sure the FAA is being very conservative in what it allows in an airframe and accounts for the variability of wood products. Generally when we crash and burn with our guitars it's from our playing and not the guitars fault.

Haha! I'm pretty brave at times too when I know Grumpy isn't watching.

Darn right!

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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.

Cay S wrote:
"Changing the string height at the bridge seems to change the sound of the guitar more than many other adjustments will."

That's what I found in a fairly careful experiment several years ago. When I changed the break angle without changing the string height off the top nobody could hear any difference in the sound; in a 'blind' test they were guessing 'same' or 'different' each half the time. When the string height was raised on the same guitar, but all other conditions were the same, the only people who had any trouble at all telling the difference were a couple of old rockers with blown hearing, out of more than a hundred trials. Raising the strings off the top also made measurable differences in the output of the guitar, all else equal, while changing the break angle didn't. You do not get more power from the strings by raising them off the top, or by altering the break angle. What changes is the mix of overtones, but that's all.