Bridge/Saddle/Break Angle geometry

[donnyb]

Hi and thankyou for reading.

I have reset the neck on a 50 year old, large bodied 12 string. It also needed the bridge replaced as it was lifting badly and had been shaved to bare minimum by previous owners, so that came off first.

I made a jig to apply string tension to check the reset angle as I went. Only with 6 strings, as I decided I wouldn't fit a new bridge until the reset was completed. To do so, I secured the NEW 12 string bridge through the rear 6 pin holes with screws , oversize washers, and nuts many times during neck heel sanding, and applied 6 string tension 2 steps up from normal pitch.
Not entirely 12 string tension accurate, but an indication of how the neck and the old body was responding to higher tension.

With the current 'bolt -on bridge' situation, and neck projection to bridge top good, I can now experiment with the action.

Im seeking some opinions please on best mix of bridge and saddle height adjustments while its in this condition. Here's some current measurements below, with 6 strings fitted at normal pitch:

New bridge height at D-G station : .415"
New Saddle Height : .360" (total height, 'on the bench')
Exposed saddle height: .180"
Slot depth: .180" (3/16")
Slot width : .108" (7/64"")
Break Angle : 30 degrees
Body top to bottom of D string : .575"

Currently , to assess action, I used an old saddle that has an exposed height of .125" (1/8") at the D/G station. This gave a nice action over the 12th fret ( no capo on 1st fret) of 1/16th " on the treble E string, and 7/64th" on the bass E, with 6 strings at normal pitch. But a 1/8" exposed saddle is too little I think.

I believe that when the other 6 strings are fitted later, the action will increase by 1/32", more or less. Probably more based on the reset journey experience.

My inclination with the above measurements is to reduce the height of the bridge by sanding 1/16" (.060") off the flat bottom, to compensate for the 6 extra strings, and to get the bodytop to bottom of D string down to .515" . That would make the bridge a bit over 5/16" thick.

However, thats going to affect the current 30degree break angle some. By how much is for a maths person ! And maybe the tonal difference wont be great ?

All further action adjustment via the saddle height, which is currently exposed .180" above the bridge top.

Am I on the right track ?

Regards and thanks,

[Alan Carruth]

I did a rather long experiment a few years ago on the relative effect of break angle and string height off the top on tone. Often folks will swap in a taller saddle and remark on the change in sound, attributing it to the increase in break angle, but the height off the top changed too. I decided I'd isolate variables.

I set up a classical guitar in such that changing the way the strings were tied behind the saddle would alter the break angle using the stock saddle. With the strings 11 mm off the top, I had break angles of either 6 or 25 degrees. I then made a new saddle that restored the 'high' break angle using the 'low' tie: the strings were then 18 mm off the top (don't try this at home!). That gave two setups with the same break angle but different string heights, and two with the same string height but different break angles.

I used a mechanical plucker to drive the strings, and recorded six plucks on each open string for each of the three setups. The plucks were carefully done so that they were always in the same place along the string, so the energy in each string partial would be the same for each string and case. The sounds were recorded using a standard setup on my computer. Plucks on each string in each setup were strung together to make a 'synthetic strum' for each condition. These were played back through ear phones in random pairs to see if people could hear any difference. The sets of six plucks for each string in each setup were compared to make sure the plucker was consistent. It was. Each pluck was analyzed for it's rise and fall time, maximum amplitude, and harmonic content. The objective results were compared with the listening tests.

For any one string changing either the break angle or the height off the top made no difference in the total power output. The rise and fall times were not changed significantly, nor was the maximum amplitude. The plucker put a certain amount of energy into the string, and the guitar turned some of that into sound with no measurable change in the efficiency due to the setup change.

There were measurable changes in the spectrum of the sound, however. Most of the sound in each case was produced by the strings moving 'up and down' with respect to the plane of the soundboard, and pulling the top along with them.

The string tension does rise twice per cycle, pulling the top of the bridge toward the nut, and the rocking motion thus produced does make some sound. This is doubled in frequency compared with the string fundamental, and would sound an octave higher if you could isolate it. But the actual force is much less than the 'up and down' pull, some of the top motion cancels out, since part of it is moving 'up' while part move 'down', and we make tops to resist this sort of motion, so even if the force was the same it would produce less sound. However, raising the strings higher off the top does put a bit more energy into the second partial (12th fret 'chime' tone) and also the 4th in some cases (5th fret 'chime').

There is also another tone, the high frequency 'zip tone', which is a compression wave within the string. It's usually up around the 7th or 8th partial in guitar strings, but is not tied to the pitch the string is tuned to, and it generally dissonant. It seldom develops much power, but since its high pitched and dissonant it cuts through. It drives the bridge similarly to the way the string tension change does, so you get more power in the 'zip tone' when the strings are higher off the top.

These changes produce an audible difference in the sound. In the listening tests nobody could reliably tell the difference between a 6 degree break angle and a 25 degree break angle when the string height was the same at 11 mm off the top. However, everybody could pick out the difference when the string height was raised from 11 mm to 18 mm, with the break angle in both cases being the same 25 degrees.

Again, there was no measurable difference in the power output between cases. A bit more energy in the complex signal at one or two frequencies was balanced by slightly less at other frequencies, as near as I could tell. So long as the string stays in contact with the top of the saddle throughout it's vibration cycle it will transfer all of the energy it can to the top.

In this experiment the strings were carefully driven perpendicular to the top; there was no sideways force on them to cause the string to slide or roll on the saddle. Six degrees of break angle was 'enough'. In a normal pluck it would not be, most likely. I have not done the measurements to see what the actual minimum acceptable break angle is, but my experience suggests that it's on the order of 15 degrees or so, more or less.

The break angle over the top of the saddle produces a static tipping force pushing the top of the saddle toward the nut. This does not help to produce sound, but it does tend to break out the front of the saddle slot. Minimizing this can be helpful. One way to do that is to cut the slot so that the saddle tips back away from the neck. If the saddle angle bisects the break angle there is no net tipping force. This is how tall, skinny violin bridges are kept from tipping over or warping over time.

Making the strings higher off the top also increases the leverage trying to peel the bridge up along the back edge. Once that stress exceeds the peel strength of the glue line the bridge will come up. If you're going to go with a tall saddle and bridge you'll need to make sure the bridge footprint is large enough to take the force. A stiff top helps, too. Those are the thins that determine how high you can go.

Trevor Gore, in his book, sets out a rule of thumb for this: the bridge should rotate forward about 2 degrees under string tension. If it's much less than that the top is probably to stiff and heavy to move well, and if it's more it may not be stiff enough. We're talking 'Goldilocks' here: not a law of nature....

That's my story and I'm sticking with it.

[donnyb]

Brilliant Alan.

Thankyou for sharing those precision results with me and others here.
I need to read this several times, so its going hard copy on my printer, and then to the coffee table !

One question while my heads still spinning : in layman's terms, does the tone of the guitar improve the higher the strings are off the guitar top ? If "yes", does the height difference need to be big, eg 5mm to be able to easily hear a substantial improvement ? Or is an improvement in tone perceptible between say, 11mm off the soundboard and 13mm off (the latter being oft quoted standard height) ?

[Alan Carruth]

'Improve' is subjective. When I did the listening tests I just asked people if the sound was 'the same' or 'different'. Some did volunteer that the high saddle gave a little more 'edge' to the sound; which I'd attribute to the added power in the 'zip tone'. Keep in mind that I went from having the strings a more or less 'normal' 11 mm off the top to 18 mm, a 60% + rise, which is a lot, and on a classical guitar. I would not even consider that sort of rise on a 12-string, carrying three times as much tension to begin with, lest the bridge fly off and kill the cat. Nor should you need it. With the usual sort of compensation the sound should be 'crunchy' enough even with the strings low off the top, which is certainly the structurally smart way to go.

[donnyb]

Thanks Alan.

Being able to experiment easily with the bridge height at present, your article has prompted me to ask you and others reading this, if there is a " pecking order" of bridge area parameters derived from science and
/or experience ? I mean is one factor of greater importance than others ?

Yesterday, I shimmed up the old 1/4" thick bridge and a used shaved saddle again as follows,

!. Soundboard to bottom of D - G string station : .500" approximately

2. Bridge height .345 " (11/32")

3. Saddle exposed height .150" (9/64")

This gave a clearance over the bass G, no capo on 1st, of a tad under 1/8", with just 6 strings to pitch.

Because its yet to get the octave up further 6 strings , Im tempted to make the new bridge height lower to 5/16" by taking 1/32" off the bottom of it, as I dont want to lower the saddle height anymore. This will make the soundboard to underside of D string . 470 ".

How do these numbers look ?

[Alan Carruth]

The main 'job' of the bridge is to tell the string how long it is, so that it will know what note to make. To do that it has to stay where you put it, and also needs to have a certain amount of mass and stiffness, so it doesn't jump around too much as the strings pull it. All of this depends on it staying glued down.

The static string loads on the bridge glue line are a shearing load, equal to the string tension, and the torque load produced by the height of the saddle above the top, which pulls it up at the back edge and pushes it down in front. The shear and torque loads add up to a peeling stress on the back edge of the bridge, and when that exceeds the strength of the glue line the bridge peels up.

Increasing the distance from the front to the back edge along the line of pull reduces the shear load along the back edge disproportionately to the increase in 'footprint', which is why 'belly' bridges work.

The bridge is also the heaviest single 'brace' on the top, and one of the stiffest. A light weight bridge tends to favor high frequencies, more output power (which may not equate to 'loudness'), and a faster response. It can also produce more 'wolf' notes.

The more the saddle protrudes from the bridge the higher to pressure along the front edge of the slot. The tipping force is more concentrated along that edge without more wood to support it. This can distort the slot over time, and lead to splitting out the front of the slot. Many people feel it can also cost some sound: I don't have data on that. A 1/4" thick bridge on a 12-string, with the strings 1/2" off the top, sounds a bit low to me, unless you have a wide bone saddle that will resist distortion, and it fits really well. It would be even better if it could be angled back.

It's hard to know how much neck lift you'll see when you get the strings on, and thus how much you'll want to take the saddle down once it's settled in. The amount of saddle above the bridge is only a factor in the sense that it allows you to drop the action if you want to, IMO. There are folks who claim it's a variable in tone in itself, but it's hard to say how it could be, and the experiment to find out one way or the other would be very difficult to do.

Rather than use a shim, I'd make a new bridge the right thickness. I don't know why the bridge is off, but if it peeled up by itself, rather than being removed, I'd say that points to the use of something with a deeper 'belly', at least. You're stuck with the pin holes as they are, and thus the relationship between those and the saddle location.You may find you'll want to 'ramp' the back row of holes to get a 12-15 degree break angle. I'd tend to go for a bridge mass of 30 grams or so, and maybe even more. A wide saddle, 1/4" or so, will allow you to compensate every string at the bridge, which will help the intonation up the neck a lot. If you take the opportunity to compensate the nut you can do even better. As long as you're at it...

[donnyb]

Yes, thanks Alan, I do have a new 3/8 " thick bridge for the job .

Im using the old bridge just for now. It was shaved down by previous owners to 1/4", (too low) so I'm shimming that to find an acceptable finished height for the new bridge to marry well with acceptable heights of the saddle and of the bodytop to underside of D/G string.

I removed the old bridge at the outset of the neck reset job as it was lifting at the rear (as you refer to) and during the reset used the new bridge bolted down (with large diameter washers !) through 4 of the rear 6 holes, to allow me to apply 6 string tension to the front row, 2 steps up from pitch, to check heel sanding angle progress.

I was bemused during this to find I had to have a lot of neck to bridgetop projection to achieve a good action under the 12th fret. Put it down to a soft neck , as body bracing had looked fine.

Last night , as I was starting to think Id underset the neck, so I had another look at the body bracing. It all looked good , but when I pulled on one brace at a time, one behind the bridge ...... you guessed it.

It was floating along 1/3rd of its length from where it was originally butt glued to another brace. I'll repair it and do another neck projection test under 6 string tension (plus 2 steps up) after a couple of days.

Maybe this might reduce the amount Im currently thinking I need to take of the saddle and bridge to get a lower action at the 12th.

[Alan Carruth]

"It was floating along 1/3rd of its length from where it was originally butt glued to another brace."

Ah yes; the less-than-half lapped joint with no cap. You'll need to reinforce that once you get the brace back down or it will just peel up again. A cloth patch might work, but with the torque on a 12 I doubt it. It might help to put corner blocks in where the braces meet on all four sides and then put a patch over that.


Hope the ASCII art comes through....

Edit: it didn't.

[donnyb]

I'll try to attach a photo of the brace Alan. Its the one that I've pushed a black zip tie under. From there to the left to where it butt joints with another brace is separated

Ive not used the cloth before. Is it special or insect netting ?

[donnyb]

https://imgur.com/HxXqSzI

Try this .

[Alan Carruth]

That's the 'lower tone bar'. I thought it might have been the X brace up at the joint, which is a pretty common one when it's just butted like that.

Actually, in the picture you posted you can see that there's a cloth patch over the X-brace crossing, which has probably held that joint together. It looks like either cotton or linen, in a coarse weave. Martin traditionally used round linen 'gun patches' for this. The patch is wet with hot hide glue and formed over the brace crossing before the glue gels. You can also use Titebond, but it's best to dampen the cloth with water before working in the glue.

In your case it's hard to see whether the brace peeled up from the end or in the center, but I actually suspect the latter. The bridge torque acts to lift the top up there, and the brace didn't want to go along. It looks as though the original glue job might have been done with hide glue, in which case all you should have to do is work some more of the same glue into the loose joint and clamp it. A block of the right size, or a turnbuckle, can be used to bridge between the brace on top and the back (with luck, at a brace), and you can then clamp over the outside (carefully!) while the glue dries. Check the brace in front of it as well; a thin feeler gauge or a piece of shim stock .002" thick will give you a better reading.

[Victory Pete]

Always an interesting topic. Here is a simple experiment I did with a 12 string.

https://www.acousticguitarforum.com/...d.php?t=448701

[Alan Carruth]

If you look at Pete's pictures, you'll see that with the low saddle some of the strings are barely breaking over the saddle at all. There may not have been sufficient down force to keep them from rolling or sliding across the saddle top when picked normally. That may be what affected the pick feel, as he mentioned. I should have flagged that at the time, but I must have had other things on my mind. So, yes, break angle can matter if you don't have enough, but once you get there, it doesn't. String height off the top then becomes a variable in determining tone. As Pete says, the power doesn't change, but the timbre does.

[Talldad]

The break angle is a funny one. Mathematically it should be relevant, but the maths assumes that there is no frictional forces on the saddle and that the string is free to slide. In reality, there is friction and it means that from the point where the string meets the saddle and tails off inside the system can be considered as one fixed unit, saddle, bridge, pins and ball ends.

To Alan Carruth's point, when there is no break angle friction tends to zero and this makes a difference mathematically.

Well that's how I understand it anyway.

[donnyb]

OP here.
Thanks for extending the discussion as its very relevant to where Im heading with this post reset journey, especially seeing that both bridge and bridgeplate on this aged 12 string are to be changed by either replacement or repair.

I noted the comments re break angle increase and to do so "in moderation" (thanks Alan), and also the peghole slotting especially for the rear 6 strings to match as much as possible (I assume) the break angle of the front row.