Alternative String Attachment to Top Plate

[OiAcoustics]

Cheers to all you luthiers down under! I’ve been following your inventive streak for years and incorporating aspects such as liberal use of CF. I finally reached the outer frequency limits (hi and lo) in what I’ve been doing with small bodied guitars, and decided it just wasn’t good enough. I contacted Trevor (TG) for insight to The Book, and here I am. Please forgive my ignorance as a couple of weeks isn’t very far to get into his extremely dense and fascinating manuscript, but I am redesigning as I’m reading to allow ideas to gel, and some things are coming up.

This question pertains to an alternative string attachment to the top plate.

My Design Parameters (which may or may not be very relevant):

a. Active resonant top: 394mm (OM) width or 343mm (0) width, equally balanced frequencies across equal tensioned 25” (635mm) scale strings.
b. Lutz/Sitka top (steel string SS) or redwood/red cedar (fluorocarbon/classical CC)
c. String break angle: 35 - 45 degrees.
d. Bridge mass: centered on the active top plate.
e. Neck intersection: driven by the bridge location.
f. Soundholes: moved to upper bouts for maximum unbroken top plate area. Size, location, placement and dispersion are all beneficial in my mind, but I imagine I’ll get pushback on that!
g. Sides: Double laminated quartersawn fiddleback walnut or bigleaf maple with plain sawn maple interior
h. Live Back: quartersawn fiddleback walnut or bigleaf maple.

The total mass/weight of the bridge "assembly" = all of the components (taking more into account than TG):

1. Bridge 7.5mm thick = 18gr in Indian Rosewood, 14gr in walnut, maple or cherry.
2. Saddle 4.75mm thick (for better ramping) in bone = 9gr
3. Pickup: transducers (SS) = 3gr or pressure zone mic (CC) = 6gr
4. String Termination: instead of a standard bridge with bridge pin holes, I am leaning towards allowing the strings to extend to the tail edge of the bridge per the (prototype) photo, whereby they miss the bridge with shallow grooves. (Almost everything is make on a small CNC). The string ferrules hook into a dense phenolic resin (PR) tieblock under the bridgeplate = 5gr. (Temporary pins are pulled after strung up for a net weight gain of 0 compared to Ebony Bridge pins = 5gr)

Total bridge assembly without considering the bridgeplate ~ 35gr.

Positives:

A. This arrangement seems to meet TG’s condition that a pinned string is better than clamped – the string movement beyond the saddle is about the same as at the nut.
B. Grooves remove less mass (from what it a light bridge) than holes and is stiffer.
C. This is more adaptable to large bass ferrules with oversized pins, or hematite beads tied to CC strings (instead of a heavier classical tieblock).
D. The bridgeplate is not calc’d into TG’s weights as it is a different consideration and spread over a larger area, becoming part of the soundboard than part of the bridge assembly. But it's there and they relate. My inclination is to take it through the outer transverse brace so there isn't a hard butt joint (but I may be wrong). I’m guessing he stops it to allow for the outer falcates to do their thing “somewhat” independently.

Bridgeplate weight in rock maple per TG plan is (.75g/cc ave) ~ 14gr. A silver maple (.46g/cc) plate = 9gr is less hard to resist ferrule shear, but that’s resolved by the PR tieblock. And if more across the top flexibility is to be allowed, it can be thinner, and allow for that outer falcate action.

So a net bridgeplate savings of 5-10gr for a total weight of Bridge Assembly + Plate = 25-30gr seems well on target.

The obvious potential negative (leaving the aesthetics aside, but this is a contemporary forum!):

*** The pull point on the bottom of the top plate is pushed back towards the tail so that, assuming a 13mm string height, the intersection to the plate is ~ 15mm from the saddle... will this add to bridge rotation (likely) and is that a good or bad thing, or simply neutral depending on the final design? I have not anticipated or calc’d how it might relate to a 2 degree rotation. My bridge is a bit deeper along the string axis than standard to compensate for rotation and provide more resistance to saddle shear.

Thanks to anyone for thoughts & discussion. I haven’t gotten deep enough into The Book to understand how across soundboard stiffness interplays with falcate bracing, which certainly will influence the final decisions, but for the moment I am assuming that allowing extra influence via flexibility from #1 & 6 strings, per D above, rather than a very stiff bridgeplate based on its shear strength to resist string ferrule blowout, is all in all a good idea… but no doubt that will be a further discussion. Again, forgive my incomplete understanding of all this, as I drink from the well...

[lamanoditrento]

Hi OiAcoustics, welcome to the forum.

You may like to read check on Frank Falbo's work and the detailed discussion in this thread from a few years ago
http://www.anzlf.com/viewtopic.php?f=1& ... dge#p81580

[OiAcoustics]

Greetings! I'm good friends with Frank and we share a lot of ideas... Despite the optics of the through-top string, there is nothing in common between the two systems. And BTW the strings never touch the bridge or the spruce top.

This is simply a question as to whether moving the string pull back 8mm or so from a standard pinned location, radically affects the top's response to the modified torque on the falcate layout. Like anything, there are potential gains and losses. In this case the positives as I laid them out will outweigh a torque change, if that doesn't negatively impact the response.

[kiwigeo]

Welcome to the forum. My first thoughts........that top is going to get banged around behind the bridge every time you do a string change.

[seeaxe]

Vive la difference! Interesting post.

My opinion

The torque on the bridge is solely dependent on the tension in the strings and their distance above the soundboard, assuming the two are more or less parallel. How the the bridge and soundboard respond to that do not change the applied torque. How much rotation you see at the bridge will depend on how much of the string tension is supported elsewhere.

If the strings turn more or less through 90 degrees and are anchored more or less vertically, (conventional pin bridge) or within the bridge itself (classical bridge) then all of the string tension is taken in shear on the bridge/top glue joint and is transferred into longitudinal compression of the sound board. Whether your strings touch the bridge or not, the force is transferred via the saddle, through it to the glue joint.

If the string anchorage point moves toward the heel of the body, some of the string tension will be taken in compression in the soundboard and correspondingly less in shear at the glue joint. For example if the strings run at 45 degrees from the saddle to the soundboard where they anchor, then the upward force at the anchor will be roughly 70 percent of the string tension. Another 70 percent will be applied directly to the soundboard in longitudinal compression. As the anchorage point moves further back away from the bridge the compression component increases and the upward component decreases. The extension of this is a tailpiece anchorage, there is no torque (and thus rotation) at the bridge and no shear force, which is why they can "float". All of the string the tension is carried to the heel.

So I don't agree that the small difference in position (compared to the span of the beam/plate system of the soundboard) in where the strings anchor will lead to more bridge rotation. If anything it will be less.

What benefit do you get from anchoring the strings that way?

Cheers

[OiAcoustics]

HI Martin

Surprisingly, banging the top around while changing strings is not an issue with the change in pin location! It's super quick and simple. Far more damage is done by picks or fingernails! Or for that manner in radical changes in humidity, which is kind of brutal here on the edge of the desert and sea.

[OiAcoustics]

HI Richard

Thanks for your thoughts. Looking at it solely as a vector force, I agree with you - you could think of it as partway towards a tailpiece setup. The concern I have is in looking at it from the perspective of the string pulling directly from a point further towards the tail might cause that area to want to rise more than bridge rotation alone. Would that indicate to adjust the taper of the center of the two interior falcate towards the tail?

For sure we're all after monopole mobility - as TG points out in relation to everything else. But in my mind the idea that a string bending down at a sharp angle causes the top to move cleanly up and down - as if it were weights on a spring - can't be true. The string angle is causing the bridge to rotate or wobble back and forth. The sharper the angle, the more shear is placed on the saddle. I'm attracted to falcate as the light structure intercepts that movement, directing the forces towards the more flexible edges, resulting in top mobility. Yes, that's totally simplified...and I still have hundreds of pages to read, reread, and digest...

The advantages, to me, are in A-D as described in the OP, mostly towards using a variety of strings and substituting the weight of very strong PR tieblocks for bridge pins. I certainly can't get past the fundamental notion that pulling up from the plate bottom is more efficient than, say, a classical tie bridge that simply wants to be ripped off the top as there's no mechanical linkage beyond glue. So my direction is good for basses and classical and tenors and ukes (which started this whole thing). I didn't mention it, but I have fat bass strings that just don't want to make that kind of bend and the high .010 A on the a tenor guitar just wants to snap right off (yep, I've been there, and had to make a nice rounded ramp to prevent that, which couldn't have happened with a sharp return).

So maybe I needn't even worry on it and in the end it all balances out. I'm actually going to pop the back off a guitar I don't like the sound of and convert it to falcate with a live back (the more rigid I made them as a "reflector", the worse it got!) - I'll just find out!

Oh, BTW, I think I found an error in The Book directly related to this. The verbiage calls for a 10mm bridgeplate extension to the front of the bridge and 15mm to the tail, but the plan shows the opposite. Methinks the plan makes more sense to resist showing a bent edge through the top as the bridge is forced into rotation. But if my concern here has any validity, perhaps 15mm on both sides is safer... a think plate that is not designed for ferrule shear hardly weighs anything. Any clarity on that from anybody?

[lamanoditrento]

Do you have any FRC for your guitars with this bridge arrangement OiAcoustics?

[OiAcoustics]

Not yet, as I decided that the series of guitars I've been building with a modified X / bridgeplate are "constricted" in both lows and highs and just not good enough. I didn't need a spectrum analyzer to hear it, but assumed (AKA "make an ass out of me") it was do to the small 0 body size. Funny thing happened, when I build the first OM size based on that plan, it wasn't any better! Hence this full on redesign and deep dive into falcate bracing. The Book did answer a few questions and inclinations right off the bat as to why, and offered a superior solution. But I will test current frequencies, slice off the back and convert them to live and the tops to falcate (albeit without being able to do the full process), and then build an all new one with the full testing, and post the 3 FRC for comparison.

In the meanwhile, I am not gaining any real clarity on the ideal across top flexibility. Any clues as to where in The Book I can find that?

[Dave M]

Not quite the answer as I think you mean stiffness across the fully braced top...?

However Trevor does demonstrate clearly that the cross grain stiffness of the top plate is very unimportant compared to the long grain stiffness. Can't tell you offhand where in the book this is but may have some time later.

[seeaxe]

HI Richard

Thanks for your thoughts. Looking at it solely as a vector force, I agree with you - you could think of it as partway towards a tailpiece setup. The concern I have is in looking at it from the perspective of the string pulling directly from a point further towards the tail might cause that area to want to rise more than bridge rotation alone. Would that indicate to adjust the taper of the center of the two interior falcate towards the tail?

Well I always look at forces as vectors, because they are vectors so thats the only way to look at them i.e. forces always have magnitude and direction. Thats why as the string anchor point moves back wards from the conventional pin bridge, the direction of the tension force in the string at the anchor point changes from close to right angles to the sound board to about 45 degrees in your case. If you draw the vector diagram you can see that to resist that force, you need a vertical component equal to 70% of the string tension. Plus a horizontal component of the same magnitude.

Because the horizontal component is co-linear with the soundboard it doesnt induce any rotation, just compression. The vertical component applies the torque to the soundboard and is smaller, and gets progressively smaller as you move further back and the angle between the strings and the soundboard decreases.

Add to this the fact that the smaller vertical component is being applied nearer and near to the support (the heel of the guitar), the amount of bridge rotation will likewise reduce. Reading your words above it seems as if you may be thinking the top is less structurally capable away from the bridge, however if you consider that the falcate braces are the "beams" resisting the torque then it shouldnt matter, as long as everything is properly glued together and working as it should. Thats a strength issue more than a response issue??

I would strongly plus one the recommendation to do some FRCs to get an objective measure of what's happening. Trevor's book explains in some detail that what you hear (or think you hear) isn't always whats happening.

Good luck anyway and i hope you get the results you are looking for, one way or another.

All the best

[OiAcoustics]

Hey Dave

Yes, I caught that general sense in The Book, and it's probably not significant enough for Trevor to spend more thought or time analyzing the issue.... Onward.

Hi Richard

Yep, the only way to know is to try it out. I'm getting real close to pulling the back off an 0 guitar I'm not totally pleased with after I do the FRC on it as is, then rebrace the top to falcate (with the string position as is since the bridge and tail holes are already there) and convert it to a live back. Then retest without altering much else just to compare those two changes. Should be fun and interesting... If I come across anything worth sharing I'll post.

I appreciate the feedback! Hope you are all doing well down under - it's still a long row ahead of us here in the US, and the live music scene (I host an acoustic series www.sbacoustic.com) has taken a tremendous beating...