Question: floating bridge for nylon strings, and strut height/thickness

Construction and repair of Classical Guitar and related instruments
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mayrluthier
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Question: floating bridge for nylon strings, and strut height/thickness

Post by mayrluthier » Mon May 15, 2017 4:27 am

Good day, fellow forum members,

I have been considering a floating bridge for a nylon string crossover jazz guitar (looking for percussive yet mellow sound), and I've been wondering about fan strut height/thickness variations: since the top won't be subjected to torque anymore, but only to vertical compression, I suppose that the struts should be shaved down a bit, however I'm not sure by which amount.

Has any of you experimented with a floating bridge (plus tailpiece) on a nylon-strung guitar?
Could you describe the experiment as successful, and how?

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by Alan Carruth » Mon May 15, 2017 6:21 pm

I've seen several different implementations of this. There are a number of things to think about.

The basic issue is always getting enough break angle on the strings as they pass over the saddle. They have to press against it hard enough so that they don't hop off in some direction as they vibrate. One way to see this is to think of how you would make a very loud pluck by pushing the string down just in front of the bridge as hard as you could and letting it go. Obviously, if you push it down too hard it will come in contact with the frets so that sets a limit as to how far down it can go. As you play closer to the bridge the string will make more of an angle downward for a given displacement. The way the sting moves (which is not much like the way it's often visualized) it will make about that same angle upward a half cycle of vibration after you release it. if that upward angle exceeds the angle that the strings make downward over the top of the saddle it will hop off the top of the saddle, and the fixed point at the end of the string will be back at the tie block. At very least you'd get a sitar-like buzz.

Given that we can define the minimum break over angle. Note that the string doesn't have to break downward to be properly stopped, it just has to make some sort of angle in some direction. There is one steel string maker who fans the strings out sideways behind the bridge, and that works. On pianos they use a couple of little pins in a line and thread the string between them so that the length is defined. Most often, though, on fretted instruments we use a down angle.

For the string to bend downward something has to be pushing it up, and that's the bridge and top. To get enough down bearing each string puts a couple of pounds of down force on the top of the saddle. That sort of load on a guitar top will collapse it over the long run. Very early fiddles used flat tops, but they tended to collapse, so they started carving them to an arched shape to help resist that load. I have actually made a few arch top Classical guitars that worked out pretty well, but it's tricky to get everything into the right balance. The stiffness of the arch also changes the sound, of course, and you need to work out the best way to accommodate that.

On the usual tie bridge the down load of the strings on the top of the saddle is countered by an upward load on the tie block. The actual torque on the top is a function of how high the strings are of the top surface, while the break angle, which is a function of how far the tie block is behind the saddle, dictates the 'centroid' of the rotation: the point the bridge seems to rotate around. Normally there is also an upward force on the top due to the neck angle. At any rate, it's easy to make the top stiff enough to resist this torque over the long term without too much distortion, and without adding too much mass or stiffness that would impede the tone.

The main issue, as it turns out, is keeping the bridge from peeling up. Along with the torque there is a shear load on the glue line that is equal to the total tension on the strings. Because of the way those loads are taken in a glue line the maximum stress is along the back and front edges of the bridge. The torque, of course, pushes the front edge downward, reducing the stress there, but it pulls the back up, and that adds to the peeling stress along the back edge of the bridge.

It's fairly easy to take the shear load off the bridge. You just run the strings over the top of the saddle and down through the holes in the tie block, but then, instead of tying them there you tie them to a tailpiece that attaches back at the lower edge of the top. You get the same break angle at the saddle, and the same torque, but much less peeling stress on the glue line.

On Flamenco guitars they reduce the torque by making the saddle lower. Instead of the usual 11-12mm string height off the top they might use 7mm or so. This allows them to get away with a thinner top and lighter bracing. In theory you could have the strings simply emerge from the top at the correct point, or use a tiny saddle, and reduce the torque to practically nothing. The guitar might be hard to play unless you used a lot of 'over stand' on the neck and set it at a very high up angle, like a Humphrey only more so, and that increases the upward force on the top.

In the end, nothing about the guitar is ever as simple as you'd like it to be. The standard designs have been worked out to get decent compromises among all the competing requirements that work pretty well, but they are compromises. There's some wiggle room, but big changes can be difficult to make work right.

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mayrluthier
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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by mayrluthier » Mon May 15, 2017 7:52 pm

Thanks for taking the time to elaborate such a comprehensive response, Alan!

I think that I'll first try running the strings through the tie block holes down to a tailpiece.

Image

It looks like the most practical approach to what I intend, without compromising the current design.

I'll post pictures when it's done!

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by Peter_T » Wed May 17, 2017 7:15 am

What's the anticipated benefit of this added component vs. tying the strings off at the bridge?

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by Alan Carruth » Wed May 17, 2017 5:55 pm

Peter T asked:
"What's the anticipated benefit of this added component vs. tying the strings off at the bridge?"

Measurements I've made indicate that the tailpiece reduces the load on the bridge somewhat. The bridge still rotates forward, but not as much. This would be consistent with the tailpiece taking up the tension load acting on top of the saddle, but not the up and down forces on the tie block and saddle top. Taking the tension load that way should reduce the chance of the bridge peeling up. There might be some reduction in the sound produced by the tension change and longitudinal compression wave signals in the string, but those don't normally make a lot of sound anyway. You might notice a slight change in timbre.

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by Peter_T » Wed May 17, 2017 11:52 pm

While I can see how those changes are different from the normal spanish-style bridge, do those difference impart any sonic or practical benefit?

Understood that there are cases where a bridge flies off, and that's no fun (almost had a heart attack the first time it happened to me, on an Hill guitar) but it's pretty rare and easily fixed. Does the timbre still reside within the "classical guitar" range?

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by Alan Carruth » Thu May 18, 2017 7:54 pm

The differences in the way the strings drive the top between the two different setups are so small that it's difficult to imagine they'd alter the timbre that much. The problem is that it's really impossible to test this out rigorously. It takes too much time to switch from one setup to the other to reliably remember the tone, and there are lots of issues with recordings. You might get somewhere if you could build to identical guitars that actually sound the same, since you could then switch one and leave the other original, but I'm not sure that's even possible.

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by mayrluthier » Sun Oct 15, 2017 2:29 pm

Hello Alan,

after closing shop and getting back on the horse, I'm about to continue the experiment.

Now, thinking in theoretical terms, according to you experience:

Would this setup channel the string's vibrating energy towards more monopole oscillation of the top, rather than the mixed torque/oscillation, thus increasing volume at the expense of sustain?

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by Alan Carruth » Sun Oct 15, 2017 11:13 pm

I can't give you a definitive answer because I have not done that experiment. The work I have done suggests that the bridge rocking motion doesn't contribute a lot of power in any case, so losing it should not affect volume or sustain. Raising the strings on a standard guitar from 11mm off the top to 18mm, which nearly doubles the torque (don't try this at home!) yielded a signal that had somewhat more energy in the second partial, which is driven by the rocking motion, but no measureable change in either the rise or fall time, or the maximum amplitude. The overall energy delivered was the same as well as I could determine. Keep in mind that there's only a limited amount of energy in a plucked string, and if it comes out at one frequency it's not there to come out at another. Again, this was in a different system than you're contemplating, so your mileage may vary.

It's possible that one sort of bridge would increase the overall efficiency by some amount, and give more power that way, but that's something of a long shot IMO. The guitar is, believe it or not, already one of the more efficient instruments in turning input energy into sound, and making it more efficient risks producing things like 'wolf' notes.

One thing that determines sustain is the relative impedance of the bridge/top system and the strings. Generally speaking the bridge and top have higher impedance than the strings. This is good because the impedance mismatch at the bridge reflects energy back into the string, and thus 'tells' the string how long it is and what note to make. If the impedances of the string and the bridge match all of the energy goes across the boundary, and you get a 'thud' because the string doesn't keep vibrating. That's an extreme example of a 'wolf' note. If you make your bridge lighter to take advantage of the lower stress on it you may find that the sustain does suffer: you can get a sound that is twice as powerful for half as long a time. Doubling the power doesn't sound twice as loud, but you sure notice the lack of sustain. Because impedance is frequency dependent this may happen only on one note; often down around the G on the low E, where the 'main air' resonance lurks. Note that this is not a function of the way the strings drive the bridge; it's a matter of the way the mass and stiffness of the bridge/top system work with the tension and mass of the string at a particular frequency.

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by Contreras » Mon Oct 16, 2017 6:28 am

Intuitively, one might think that the downward pressure on the bridge from the strings passing through would inhibit the vibration of the top to an extent.
Another thought ... is the floating bridge still glued in position? Otherwise it might move under say a well-struck chord.

Just a couple of completely uninformed thoughts.

And a tail-piece on a classical guitar is strange to see.
Put down the bagpipes ...
... and no one gets hurt.

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by mayrluthier » Mon Oct 16, 2017 7:08 pm

Alan,

that was one comprehensive answer!
In essence, there seems to be a narrow tolerance margin for the setup, if I understood things correctly.

I'll go with the traditional glued bridge, perhaps reducing some weight wherever possible, without compromising contact surface with the top.


Contreras,

the bridge was supposed to stay in position because of string downpressure, but you're right: it may have slipped under classical strings, and gluing it to the top makes sense.

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by Alan Carruth » Tue Oct 17, 2017 5:39 pm

One of the issues with floating bridges is they you need to get enough break angle over the saddle to keep the strings in contact, and prevent them from rolling sideways when played. This requires an angle of something like ten to twelve degrees, so the down pressure on the top will be something like 10% of the total static pressure. That amount of static down force will collapse a 'normal' top fairly quickly, so you need to deal with that in a way that doesn't cut down on the sound. Using a normal tieblock bridge but with a tailpiece to take the shear load off the glue line works, but you still have at least most of the torque on the bridge to deal with.

If you don't use the tieblock you need to figure out where the strings are going to go behind the bridge; they can't just pass through the top to get the angle. You can, of course, simply set the neck back and use a tall bridge, and then deal with the down force somehow. Many instruments, such as Neapolitan mandolins, use a 'cranked' top; a crease that runs across at or just behind the bridge to allow for the string break angle. This is reinforced with a fairly hefty brace in most (successful) cases, and breaks the vibrating area up into two sections. The most elegant solution in some respects is the carved (or bent) arched top, such as you find on the violin or viol families. The vaulted top resists the down pressure reasonably well, if it's done right, and allows for the proper break angle. It is somewhat tricky to get this to work on a Classical guitar is a way that produces a tone that's suitable for the standard repertoire. It is also, of course, a completely different beast to build than a Torres style instrument.

The point, I guess, is that all of these issues have been worked out in the past, usually more than once. Every design is a system unto itself; changing one element leads to other changes, and you can end up pretty far afield before you're done.

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by mayrluthier » Tue Oct 24, 2017 1:21 pm

Yesterday, I came across a guitar model made by german luthier Florian Vorreiter, who seems to have thought about the same concept much earlier than me (5-6 years ago, perhaps?); the nylon-string guitar has a floating bridge, the strings are anchored on the internal side of the top through reinforced holes (like a steel string acoustic minus the bridge pins). I assume the strings' ends are either tied into a dense knot to prevent them from slipping through the holes, or tied to tie-block beads.

Image

http://www.vorreiterguitars.com/?page_id=878

This configuration seems to take off some mass from the whole bridge piece, while retaining both downpressure and torque forces.

As for the sound, it does sound a bit different from a typical classical guitar; louder, yet still pleasing. A tad dark.


Youtube
https://www.youtube.com/watch?v=MhoQGGlwGjk[/YouTube]
(it must be opened in YouTube, embedded video doesn't work)

Since there is at least one working example out there, I feel encouraged to give it a try, on an experimental top, just for fun.
The whole idea is to build a nylon-string gypsy jazz guitar suitable for both flamenco, bossa nova and jazz.

I'll get back to this thread once the top has been fitted onto a test guitar.
Last edited by mayrluthier on Wed Oct 25, 2017 2:35 am, edited 1 time in total.

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by printer2 » Wed Oct 25, 2017 12:43 am

The youtube link doesn't seem to be working. The tieblock to tailpiece is an interesting idea. I have a body I made but I have been afraid to put a bridge on it as it is has a cedar top that has been baked a little. This might stop the bridge from being pealed off.
Fred

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Re: Question: floating bridge for nylon strings, and strut height/thickness

Post by mayrluthier » Wed Oct 25, 2017 2:37 am

Link must be opened in YouTube, embedded video doesn't work in the forum, apparently.

Yes, the tailpiece variant was tried by Favino for Georges Brassens, to allow a modified classical guitar to use gypsy jazz Savarez Argentine strings

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