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More on bridge location
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=22449		 Page 1 of 1
Author:  Alan Carruth [ Thu May 21, 2009 12:38 pm ]
Post subject:  More on bridge location
The existing thread has turned into a 'poison' one for me: every time I try to reply I get bumped. So here's the reply I've logged on five or six times trying to post.

Parser wrote:
"I think the resistance in bending and in conforming to the "ideal string" is primarily a function of the tension of the string, the material, and the diameter. The breake angle might affect it some, but I wouldn't expect it to be significant."

All of those variables, and at least one more I can think of, define the radius of curvature you'll get on the string as it goes over the saddle. If the string has to bend through a greater angle, and it has a given radius, then it will be bending further in front of the break point on the bridge, and have a longer 'dead' length at the end. Thus, it seems to me, greater break angle should require more compensation. If you don't see that, it argues that the ends of the strings are not dead in fact.

The other variable you left out, or, at least, skimmed over, is whether the string is wound or not. It's pretty common on Martins to see the end of the low E string arcing upward infront of the saddle, as the windings on the bottom of the string jam together and keep it from making as sharp a bend as it should. It would not surprise me if that arced end was really dead.

I'm not sure if the same holds for plain strings, though. Obviously we need to cook up some experiments on this to figure it out one way or the other.
Author:  Carey [ Thu May 21, 2009 1:24 pm ]
Post subject:  Re: More on bridge location
-Greg Byers has done some thorough work on this subject, which can be found at byersguitars.com/research/intonation.html CW
Author:  Parser [ Fri May 22, 2009 6:25 pm ]
Post subject:  Re: More on bridge location
Sorry for the delay Al, I have been having some problems logging in for some reason..!

Here's a screenshot from a quick FEA run that I did to demonstrate this phenomenon in the case of a string that comes straight out of a solid. You can see where the string starts to knee over right near where it terminates. The deformed shape is the first mode of vibration.
Image

I do agree that some goofy stuff happens on the low E and probably the other overwound strings as well...!
Author:  bluescreek [ Sat May 23, 2009 6:32 am ]
Post subject:  Re: More on bridge location
To add to this discussion , what effects do work hardening of the string infer? It seems that as the strings properties change from work hardening the intonation suffers. I think this is why sometimes you have one string that goes sour. Again thanks for the brain fodder
Author:  Alan Carruth [ Sat May 23, 2009 2:19 pm ]
Post subject:  Re: More on bridge location
Parser:
Yup, that's the normal shape of a vibrating beam or rod with a clamped end. Strings coming over the saddle are not clamped. Most FEA programs will show what happens with a 'hinged' end. Maybe strings aren't exactly hinged, either; most likely they're somewhere between hinged and clamped.

If that's the case, then there is some 'stiffness shortening' effect going on, and the question is, how much of the added compensation of bass strings is due to that effect, and how much to all the other things? We know that strings change tension and pitch when they're displaced, which requires some compensation. Anharmonicity due to string stiffness (which is a seperate thing from the clamped end effect) might also account for some of it. When you add in the effects of a moving bridge, which vary from note to note, things get hard to sort out. With all that's going on, I'm skeptical that the clamped end effect is the main one.

bluescreek:
I'm not sure if the alloys they use in strings work harden or not. As far as I know, the only steels that do to any great extent are stainless: I used to 'love' that stuff when I worked in the machine shop. As soon as your drill bit gets a little dull, the work is harder than the tool, and you just burned another bit.

All sorts of things can throw out the vibration of a string. I gather that many of the problems with wound strings are from crud in the windings, which makes the string both stiffer and heavier locally, and adds to the damping too. Wear can alter the flexibilty and mass distribution of plain strings, and corrosion is another problem.

I saw a paper once in which they used paired optical sensors at right angles to look at the motion of a plucked string. If the string had a scratch on it the path it followed could be nearly chaotic, owing to small differences in the stiffness in different directions. Wild stuff.

When I set out to do my string measurements I figured that it would take, like, three weeks. It was three years before I finally got the paper out. Strings are far from simple, and they're the simplest part of the system!
Author:  bluescreek [ Sat May 23, 2009 2:38 pm ]
Post subject:  Re: More on bridge location
As a machinist I understand the influence that the materials strength properties can bring to the table. I know we may be way overthinking on this but lets face it , the variables that are involved are many and complex . As you said strings are the easy factor , just look at the way a saddle may deflect upon loading of the strings. We often think of the saddle as this stiff piece of bone but even that has micro vibrations from the strings as they stress the saddle in movement.
All this and no alcohol .
Author:  Jeff Highland [ Sat May 23, 2009 5:05 pm ]
Post subject:  Re: More on bridge location
John,
As i recall, work hardening occurs when you bend or pull a metal past its yeild point or elastic limit.
I would expect that a guitar string would already be work hardened from the process of drawing it into wire.
In service there would also be some effect where it is wrapped on the pegs and bent at the saddle contact.
Unless accidentaly kinked. there would not be any additional work hardening in the main length of string between nut and saddle as all movement is with that elastic limit.
regards
Jeff
Author:  bluescreek [ Sat May 23, 2009 6:13 pm ]
Post subject:  Re: More on bridge location
work hardening can be a wide term. In essence the metal will change as it ages from the stress and the corrosion. This all plays into this. Yes bending it as you describe is a prime example . It has to do with changes in alignment of the molecular structure.
This all changes the physical properties albeit slight they changes do have an affect. If this didn't happen you would never need to change a string unless it broke or became to corroded . Maybe fatigue may be a better choice of words here .
Author:  Jeff Highland [ Sat May 23, 2009 7:36 pm ]
Post subject:  Re: More on bridge location
What's that John, you have to change strings?
Is that covered under the warranty?

Yeah fatigue is probably a better term.

Work hardening is a useful technique in automotive parts where a forged (stamped out) conrod is stronger than if it had been machined out of the solid.
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