Official Luthiers Forum!

Howdy all, I`ve been poking around looking for any reference to the downbearing angle, or some may refer to it as string break-over angle on bridges. I just finished giving my piano a good tuning and regulation and found out that I had overlooked that when I restored the ole gal. Made alot of difference in the overall tone and sustain (to me, anyway). Has anyone looked into this detail on guitars? I know that piano intonation is a horse of a different color, but couldn`t it come into play on guitars, too?

_________________
Give me 50 cents worth of regular.
Check my oil too, if you don`t mind,,,

Coe,

I think there has been a lot of discussion on this. Instead of the string break in relation to the bridge, it is usually expressed in string break in relation to the saddle since this is the string bearing surface. Frank Ford covers this on his site in his saddle making articles.

From what I've read, it can affect tone and response tremendously.

_________________
Joe Breault
Merrimack, NH
Perpetual novice

I checked out his site,too. It just seems that there`s only a brief mention of it just about anywhere I look. Lots of info on reshaping and materials, though.

_________________
Give me 50 cents worth of regular.
Check my oil too, if you don`t mind,,,

Cocephus,

Yes, I've looked at this. I was interested in seeing how break angle affects the downward force directed onto a guitar's top.

Got a highschool or college physics book? Measuring the force acting on a saddle as well as the tie block or bridge pins is usually covered from the point of view of a wire or cable being supported by a pole and then being anchored at the ground. The math is fairly simple, and one can calculate the force being exerted at the saddle and/or bridge pins or tie block if the string tension and break angle are known.

Best,

Michael
Michael McBroom38644.4583796296

_________________
Live to Play, Play to Live

Did you see this article? http://www.frets.com/FRETSPages/Musician/Guitar/Setup/Saddle /saddle01.html
Near the middle of the page, Frank starts talking about string break.

_________________
Joe Breault
Merrimack, NH
Perpetual novice

The math is fairly simple, and one can calculate the force being exerted at the saddle and/or bridge pins or tie block if the string tension and break angle are known.

This is covered in the section I wrote for David Hurd's "LeftBrain Lutherie". And if I remember Al Carruth's thoughts correctly, he found that a certain amount of downward force generated by the break angle is needed, and above that it doesn't add much benefit. I think the amount of break needed turned out to be less than I had speculated.

to understand the effect of break angle on tone about all one has to do is work on one guitar where someone has attempted to lower the action of a bad neckset by cutting down the saddle. just putting a normal height saddle back in will restore the volume and tone, but unfortunated doesn't do much for the action, i.e., it is neckset time.

A few comments and observations on downbearing in pianos:
Caveat: I haven't hought about this stuff for about 2 decades and I'm at an age where CRS can be an issue.   

1. The string & bridge arrangement of a piano is more similar to an arch top guitar than a flat top. The string crosses the bridge but is terminated on a hitch pin in the cast iron plate, which is not connected to the soundboard.

2. The "correct" amount of downbearing is necessary for "proper" tone. To little and the sound will be thin and tubby in the bass, too much and the sound won't sustain or project. (These are generalizations. There are exceptions that crop up occasionally)

3.   The bearing is set at the factory. The board is crowned. There are two methods to do this.

a. Heat the soundboard assembly to dry it out and glue it to the ribs. It will then swell as it cools down and reabsorbs atmospheric moisture. Steinway (my former employer) uses this method.

b. The ribs and bridges are cut with an arch (similar to curved braces on a guitar) and the crown is forced by gluing the board to them. IIRC Yamaha uses this method

Mason & Hamlin uses a combination of both methods. There is no one way that is "best". Lots of opinions (mostly ignorant) but the chosen method is only one of a large number of factors that effect the overall result.

3. The bearing is then carefully set by setting the height of the bridge using a set of specialized guages. The string tension (aprox 180 lbs per) will press the board into the proper shape with the proper downbearing.

4. Since the string is also terminated on the bridge with 2 pins (thereby adding sidebearing) the bridge knotching is of critical importance. Each string must be terminated in both planes identically, otherwise it is impossible to tune the string accurately. (false beats)

5. The downbearing (and the pitch of the piano!) fluctuates with the relative humidity over the seasons.

6. Pianos lose crown (and thereby downbearing) as they age. This in not a uniform process. It can take anywhere from 50 - 100 years. In addition some pianos develop pleasant (but less substantial) tone as the crown goes.

I'm going to have to buy and read David Hurd's book. Maybe I can then make some intelligent comparisons.

Regards,
Chris

That was an interesting post on piano bridges. I wouldn't have thought of that. I'm not sure how much bearing (so to speak) it has on flat top guitars. I know that archtops are very sensitive to the break angle, because it does put a compression load on the top. That changes the way the top vibrates. I suspect the same is true of pianos, and that the folks who make them have figured out ways to take that into account. I've looked at a number of flat top guitars, and never seen a big change in the top frequencies when they were strung up, so I don't think it makes a lot of difference in the tone. I could be wrong, but it is hard to play guitars with no strings on them to compare.

This is one of those cases where it's really difficult to sort things out. You can't usually change the break angle without also changing the height of the strings above the top. Since we know that the tension in the strings varies as they vibrate, and that there is thus a twice-per-cycle torque load on the bridge, changing the saddle height (and thus the length of the lever) should make a difference in the sound, irrespective of the break angle.

As far as I can tell, you only _need_ about a fifteen degree break angle over the bridge top to keep the string in firm contact. That's about the break angle we use at the nut end, and it works there, right? Sure the string sits in a notch on most nuts, but zero frets work, too, so long as the angle is sufficient. Besides, if the notches were tight enough to hold the stirngs down, they'd pinch, and make tuning difficult. Resos use tight notches to make up for the lack of downbearing.

It's also sort of hard to figure out what the forces on the bridge are. For one thing, there's a lot of friction on the top of the saddle, so the tension of the little bit of string between the saddle and the pin could be quite a lot different from (and lower then)the tension on the vibrating part. The tension on the vibrating part of the string _has_ to be the amount that causes it to go at the right frequency, and, as far as I can see, all of that tension _has_ to act on the top of the saddle to pull it toward the nut.

Now, if the top of the saddle were a pulley, so that you could ignore the friction, you could figure out how much down force there is on the saddle top from the break angle. If the string ran straight across, so that it made no angle, there would be no down force, and the string would probably not work well, as it would 'hop' off the saddle top when you plucked it. If the string ran straight down behind the saddle, then the down force would have to equal the tension in the vibrating part. There would be a 'resultant' downward on the saddle top at a 45 degree angle, with .71 times the tension (if I got my vector analysis right). Any angle between zero and 45 degrees would result in a down force on the saddle top in between those two numbers.

The fact that the resultant points forward (toward the nut) means that there's a tipping force on the saddle whenever there is a break angle, and the more angle the more tip. If you angle the saddle back so that it bisects the break angle the force tipping it back is equal to the force tipping it forward, and you get the whole resultant acting to push the saddle down into the slot. This should help keep from splitting off the front of the bridge. Back angled saddles seem to work better with USTs as well. Violin bridges are set up this way, more or less, and those thin little slips of wood can hold up for a long time without bending if you get the angle right.

So, with any break over the saddle, there's some force on the strings behind it pulling up on the bridge. But, there's a force doing that anyway: the torque from the saddle height! Is the upward force on the back edge of the bridge greater or less or the same with more break angle? That's what I can't figure out. I'm of the opinion that the break angle doesn't make any difference in this respect, but I have to admit that this is more a gut reaction than anything else.

Thanks for the input, guys.

_________________
Give me 50 cents worth of regular.
Check my oil too, if you don`t mind,,,

"Is the upward force on the back edge of the bridge greater or less or the
same with more break angle? "


Alan,

Seems to me that the total force on the top would remain constant, that
being determined by the height of the saddle with respect to the line
running from the top of the nut to approximately the base of the saddle.
That vertical component, which is what most concerns us here, I believe
will remain constant as the anchor point of the string on the top moves
closer or further away from the back of the saddle. BUT the effect of the
vertical component upon the horizontal component, (as dictated by the
force pulling the string toward the nut) will, however, be somewhat
diluted as the string's anchor point moves further away from the back of
the saddle. Imagine if you will, having a line of bridge pin holes 1/4"
apart, from the back of the saddle to the tailblock. If you put a string in a
bridge pin hole nearest the back of the saddle, there would be more
vertical force at work but very little horizontal force. As you move from
the string from hole to hole towards the tail block, there is less and less
vertical force at work, and more horizontal force. So, I think there IS less
vertical force pulling up on the top as you move the bridge pin hole away
from the back of the saddle as intuition would tell us, but the combined
forces as defined by vertical and horizontal force vectors must remain
constant, assuming that nothing is moving, (which of course it is, but I
think we can overlook that in this exercise). No matter where the bridge
pin holes are located with respect to the saddle (defining the break
angle), the work done by the saddle, bridge and top in resisting the pull
of the strings from the nut would be the same, but the effects on
torquing the bridge would definitely change. As we move the bridge pin
holes further away from the saddle, we have less to worry about since the
vertical component becomes weaker and the horizontal, which is resisted
in a line more parallel with the top, becomes stronger. The TOTAL stress
on the top from resisting string tension, would however be constant.

Whew! I'd better quit here, for fear of beating this into the ground. BTW,
Alan thx for the input about wolf notes on mimf the other day. Been
reading up on the web, most of it from you.burbank38646.7885763889

_________________
now known around here as Pat Foster
_________________
http://www.patfosterguitars.com

I think you're right; that the total force remains constant, but that the location within the bridge 'footprint' itself of the max upward force changes. Maybe. I've got to think about this some more, when I get a chance...