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Why is it that most acoustic guitars have the bridge pins closer to the saddle on the bass side of the bridge and farther away on the treble side. I know that the saddle usually has to be slanted, but why not run the pins an equal distance and parallel to the saddle to make break angle consistant across the entire bridge?

Some makers drill their pin holes on a radius to avoid having a stress line on the top and as such the break angle is not consistant. Am I wrong here, or is having a even break angle across the saddle for all the strings not really a issue?

When you increase the break angle by closing the distance between the pin and the saddle you are increasing downward force to the top plate and with this usually more commonly applied on the bass strings it seems to me that the downward force is disproportionate.

ideas?

I think it is the simple fact that the saddle angle places the bass strings closer to the pins.
I am one of those guys that lays my pins out in an arc, but I shift the arc a bit to bring the treble pins closer to the saddle. Ramping the string slot forward toward the pins is one way to compensate for the difference.
Break angle sure makes a big difference in the sound of each string. I often "Tune-up" a repair guitar with one or two week sounding strings by working the slot to increase the string angle.
I think of the string slots as individual volume controls.
A few guitars ago, I laid out a bridge with the pins equidistant from the saddle. It looked so funny (on a symmetrical bridge) that I abandoned the idea.

Thanks for the reply. After doing some looking on the internet I see that some Yamahas and Fenders have their pins parallel to the saddle. I think also that this could be used to even out perceived sting tension and help playability.

The basic truth to why the base strings are closer is that most drill their pin holes on a perpendicular plane to the center line of the bridge and the fact the saddle is compensated long on the bass side. This leads by nature to the brake angle being steeper on the bass side. i prefer the brake angle being about a 12-15 degree angle rather than the near 30 you see on Martin style bridges. I accommodate this by pin holes on a radial plane offset a distance from the rear belly of my bridge. This puts more room between the saddle and the pin holes. Now it is still closer on the bass side due to scale compensation.

I suspect the unequal spacing of the two E string holes from the saddle arose when Martin switched from gut to steel strings. Gut (or nylon) don't require much compensation, so they originally made both the slot and the pin hole line perpendicular to the center line. Later they started to angle the saddle slot to get better intonation, but since the jigs were set up to use the pin holes as locators for bridge glueing they didn't change that. This is conjectural, but makes some sense.

I'm in the middle of a rather large project investigating the effects of break angle and string height above the top on sound. I had a nice conversation last week with the fellow who's helping me with the statistical analysis, and he assures me that I have enough data to be able to say something meaningful about the effect of height, but not about break angle. In other words, if break angle does make a difference, it's small and hard to measure, at least on this guitar.

This is not to say that it can't make a difference. A large change in break angle does effect the static loading on the top, and this could change the way it vibrates. It does not, apparently, 'transmit more sound' to the top, though.

One aspect of the static load that bears some thought is the tipping force on the saddle. The actual force that the string exerts is directed along a line that bisects the break angle: of the string drops at a 30 degree angle when it goes over the saddle, the down force is actually directed 15 degrees forward of vertical. This results in a significant force trying to split out the front of the saddle slot, and the more the break angle the larger this force is. You can get around that by tipping the saddle itself back. This is roughly what violin bridges do, and it's why they can get away with that little slip of wood without breaking it. I've been using a 9 degree back angle on the saddle for several years, and it seems to work well.

Changing the break angle certainly does effect the workings of an under saddle transducer. Those things like a large download, so increasing the break angle usually makes them work better. Back angling the saddle slot helps here, too.

I have little guitar building experience, but I don't think the break angle by itself has anything to do with the downward force on the top. The way I see it, the only things that should matter in that respect are total string height above the top and the distance between the saddle and the front of the bridge. But I may be wrong.. Maybe someone more knowledgeable than me has a comment on that..?

Edit:


Maybe I'm just ignorant, but I don't see how the break angle by itself affects the load on the top... Would you mind giving a brief explanation?

Although I can't seem to find a decent one on the web, a force vector or force triangle diagram would most easily explain the situation.
F1 would be the string tension in transit towards the nut. F2 would be the force applied downward upon the saddle from F1 (and is determined in partial from string break angle) F3 would be the resultant force bearing downward at an angle tangent to F1 and F2. This would most likely point at a position in mid-air directly below the soundhole. String break angle is the variable here with the resultant F2 and F3 depending on it.

Briefly, as near as I can figure it:
The string does push down on the saddle top, and it also pulls upward wherever it's anchored, so the saddle is pushed down, and the back edge of the bridge pulls up. Thus there's a rotational moment set up. The lower the break angle, the further back from the saddle the center of rotation seems to be. This makes some intuitive sense: if the string was anchored back at the tail block there would be no upward force directly on the back edge of that bridge, and it would just push down, while if the string folded over the saddle at 90 degrees, the torque would act right at the back of the saddle. The torque depends on the height of the strings off the top, but the point on the top the torque acts through depends on the break angle, or so it seems.

This is one of those cases where I looked at, and drew, enough vector diagrams to get myself thoroughly confused, and just went out and measured the thing.

here we go:

http://images.google.com/imgres?imgurl=http://id.mind.net/~zona/mstm/physics/mechanics/vectors/findingComponents/vect1.gif&imgrefurl=http://id.mind.net/~zona/mstm/physics/mechanics/vectors/findingComponents/findingComponents.htm&usg=__ppp5HjC1FdsG2xlzhpEzhMwW9HA=&h=400&w=400&sz=6&hl=en&start=53&um=1&tbnid=cN9msHwYWwUwcM:&tbnh=124&tbnw=124&prev=/images%3Fq%3Dforce%2Bvector%2Bdiagram%26ndsp%3D18%26hl%3Den%26safe%3Doff%26sa%3DN%26start%3D36%26um%3D1

this might help explain what's happening to the top plate.

Think about it, we as builders are trying to contain a series of gyroscopic motions created by string vibration (a combo of an ocilliation curve and a sine wave), centered around a saddle that is off axis (due to compension) resting upon a symetrical guitar body (think standard dred guitar here) and using braces and tone bars -to make an instrument that sounds and plays good.

I seem to remember seeing sme good vector diagrams in the archives.
They were actually drawn on a guitar which makes it a little easier for a dunce like me to grasp
FWIW Ervin Somogyi runs his pins parallel to the saddle maintaining a constant break angle

Steve, why didn't I think of that! I'm a card carrying mfg eng and a member of sme. The site is a little dense to navigate though. A quick search of force vector diagrams didn't come up with anything guitar related -only pertaining to cutting tools. I'll have to write to the librarian. Thanks for the tip though.

I too drill my pin holes parallel to the saddle. It looks good, kinda of helps with the stress line and it looks good to me. clinton

arie wrote:
"...we as builders are trying to contain a series of gyroscopic motions created by string vibration (a combo of an ocilliation curve and a sine wave)..."

If you want to know how strings really work, I posted a PDF on my web site detailing the series of experiments I did several years ago(http://www.alcarruthluthier.com/Downloa ... Theory.pdf).

There's no math to speak of, but it might not be easy reading either. Strings are a whole lot more complicated than I ever thought.