Bridge Rotation at 2.3deg

[jfmckenna]

This is a very small 12-fret parlor guitar. I am happy with the tone of it however there is quite a visible telegraphing or ghosting, if you will, of the bracing in front of he bridge. The distortion in the top is apparent in the right lighting and is seen to make a triangular depression with the x-bracing in front of the bridge. I see this in pretty much all my guitars but this one is more noticeable.

The neck angle is spot on according to my design and the action is already as low as it can go.

I guess my question is, what would I expect tonally if I refretted the guitar to make a minor adjustment in the fretboard plane to lower the action enough to get 2deg rotation and is it worth it? Compared to other's I've built this one is lacking a certain punch to it. The other guitars sort of jump first when you pluck a string while this one is more relaxed. It's so hard to explain this stuff. It still sounds good, just different and I am wondering if the weaker top with more rotation is harder to move. Like when you tune up and apply tension to all the strings the bridge ideally would rotate to a perfect teetering point such that when you pluck the string the bridge rotates back and forth on it's access with ease. But when you go past that point it doesn't rotate backwards as easily as it does forward thus removing that dynamic from the system and the tones that come with it... Something like that.

I've built a few of these guitars and have experimented with very light building. My fear is that in the months to come the rotation might increase and squash the tone right out of the top.

My method for measuring the rotation is to hold the middle of the neck of the guitar in a vice such that the body is suspended on the other side of the vice. Using a phone and some poster putty I mount it to the bridge so the phone is not touching the strings. Then I zero out the bubble level and decrease the string tension, take note, and increase the string tension and take note again. The values match close enough so I am fairly confident it works.

The thought just occurred to me that I could make another saddle for it that barely sticks out, the current saddle protrudes about 3mm now. Then measure the rotation again and see if it even moves...

Regards.

[Trevor Gore]

John,

I've seen some seriously "roller-coaster" tops that have sounded fine, so that in and of itself doesn't appear to be sufficient criteria for less-than-best sound quality. Similarly, "print through" of the bracing pattern is pretty normal on highly responsive guitars. One of the things that I've found is that if the edge area is too thin and lacks resilience, that can muddy up the tone a bit. Reducing the saddle height to reduce the bridge torque will tend to take some more "punch" out of the sound, which is likely what you want to avoid. So I don't think the sound quality will improve by reducing the saddle height by changing the fretboard angle.

However, to me, 2.3 degrees of rotation on a X-braced (presumably no carbon) guitar is pushing the envelope in terms of long term structural integrity. To reduce the rotation to less than 2.0 degrees will need at least a 15% reduction in torque on the top. Assuming you stick with the same strings and that the string height above the sound board is ~12mm, you'll need to lower the saddle by ~2.00mm and compensate with the fretboard angle.

As I see it, if you're going to invest serious time into the guitar the best things would be to do a drop-in re-top. Otherwise, do nothing and enjoy it whilst it lasts (which may be quite a long time!)

[jfmckenna]

Appreciate the detailed response Trevor, many thanks. Yes this is traditional x-brace, no carbon, and the string height is 1/2in which is more or less 12-13mm so I could probably get away with lowering it a hair. I may just reset the neck angle instead of refretting too since it's so slight I probably can re-intonate the saddle...


Anyway thanks again.

[AKEric]

If you don't mind, I'd like to jump in here as I've just measured the rotation on my current build last night. The subject is a medium body, 14 fret, falcate (sitka spruce) with carbon fiber per the Books. My 3rd guitar and first using the Gore/Gilet process. The tops (sitka again) thickness is good per the target thickness spread sheet. I set the maximum brace height at 6.5 mm, and the string height is at 13.5 mm. My rotation ended up at 1.3 degrees. Obviously a bit over built. There's probably not much that can be done at this point (carbon fiber and all). I have yet to level the frets and do the final set-up, so I can't speak to the tonal response yet. For future builds, though, and assuming similar material properties for the braces used, I would consider going down to 5.5 mm max brace height. Can anyone give me input on this?

[jfmckenna]

Can you not sand or file somehow the carbon fiber? If anything what I learned, or should say learned again because I used to always do this, is to aim high that way you can always remove material to get the tone you want. In fact I just did that on an 000 I built about a year ago, shaved the braces, and the tonal improvement is astounding.

By the math it looks like you took 15% of your original brace height but don't forget that in the case of bracing there is the cubed rule.

[Trevor Gore]

My rotation ended up at 1.3 degrees...

Let me refer you to this post from a while back:

viewtopic.php?f=33&t=6321&p=71613#p71613

Don't get too hung up on the bridge rotation if the modal frequencies and the monopole mobility are right. However, if the modal frequecnies are too high and the monopole mobility too low, you know what to do!

[AKEric]

Thank you both for your replies. I think trying to manipulate the braces in a no-go. And Trevor, you have put my mind at ease. I'll put bridge rotation lower on the priority list. Thank you so much for your books. Such a pleasure to have great and relatively easy to follow guidance in this passion.

I very much wanted to attend the O'Brien class you taught last summer, but was unable. The videos you produced for him are a good substitute though. Thank you for them.

Eric

[Trevor Gore]

Eric, there's the possibility of another class at Robbie's 10/11/12 Jul 2020. Please contact Robbie if you're interested.

[AKEric]

Thanks for the heads up Trevor. I'll look into it. Coming from Alaska makes it difficult, but knowing your travel itinerary, I guess I shouldn't complain. With your connection to Charles Fox, have you approached him to sponsor an symposium? I very much enjoyed the time I spent learning his process.

Eric

[Trevor Gore]

I haven't spoken to Charles about a future class. Maybe an idea for another occasion!

[jfmckenna]

Just thought I would follow up with my results on this for anyone reading in the future. One thing for sure if you want to build guitars on the edge of responsiveness you have to be willing to "fix" them when they are brand new. And using bolt on necks systems and reversible glues help a lot in that.

So I reset the neck angle and the string height at the bridge went down 1/16th in which is roughly 1.6mm. I also made a new bridge for two reasons one was because resetting the neck forward would shorten the length and make the intonation off. That could probably have been ok by reshaping the saddle because it was so little but I also wanted to make a stiffer bridge thinking that might stiffen up the patch area and help with rotation too.

This is a parlor guitar based on a guitar built in 1895 so it's got a very small pyramid bridge where the trough in the wings is very thin and flexible. I stiffened that up and made the foot print very slightly larger, a common bridge replacement technique to cover the existing foot print.

So that did add more variables into this equation and as such it's not easy to make any determination but... The guitar sounds so much better now it's astounding. I was ok with the way it sounded before but now it's full of overtone and resonance and has considerable more sustain. The sustain might come from the slightly heavier stiffer bridge.

The bridge rotation is now measures at 1.7deg.

By the way after doing the acoustic testing to determine the target thickness the results for this torrified Red Spruce top were .8in thick. That kind of scared me so with deflection testing based on my previous results with this guitar model I stopped at .85. Still pretty thin but this top did feel stiff too. This is a very small guitar though with an Upper bout = 8.4in Waist = 7.0in Lower bout = 12.4in.

In the end I'm quite pleased with the result. Moral of the story again is be prepared and willing to make a guitar perfect.

[Trevor Gore]

John, do the before and after tap responses tell you any more about what changes happened?

[jfmckenna]

Well silly me I didn't make a frequency response chart before but I did just make one after. Method for doing so is to hold the guitar in the playing position but keeping the back free. Guitar is strung up to tension with felt under the strings to mute them. Using an App called Spectrum Analyzer on my Android phone I hit the saddle of the guitar with the special tapping hammer in several places while the phone is approximately a half meter away or closer.

[Jim watts]

JF, What's the green plot? Sorry I'm just trying to understand.

[jfmckenna]

The green is the live value and the red is the peak value since the last refresh. So once you start the app the green line is dynamically moving with every sound that it hears then you punch the top of the guitar and it records the peak red value. So when you take the screenshot of it the live green value is still present but you can just ignore it it's just background noise.

[Jim watts]

Got it. I run VA on a PC so the live view is in a separate window.
Thanks.