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Fretboard/Bridge alignment
http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=6947		 Page 1 of 2
Author:  Rod True [ Fri Jun 02, 2006 12:26 am ]
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Hesh, I think you could be too high on this one, you want about 1/32"-1/16" maximum gap after the frets are on. That's what I have mostly read around the forums. Since you have about double the guitars built that I do, you still have much more experience than me.

The down side of a high saddle is the torque that it will put on the front of the bridge. The high saddle can put enought torque on the front of the bridge to crack it, you don't want that.

One other thing that comes to mind with a high bridge is that the angle of attack that the stings make between the pin holes and the top of the saddle could be too steep, I think that a too steep angle can produce a more muffled sound, or at least not produce the true sound potential of the guitar.

I'm sure others with mucho more experience and wisdom than I will correct me and offer more insight.Rod True38870.3942361111
Author:  Michael Dale Payne [ Fri Jun 02, 2006 1:05 am ]
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As the saddle rises from the slot the greater the load on the slot and the greater chance the saddle will pull forward. I like my freted plane to be 1/16" to 3/32" max above the bridge this leaves enough room to work the action and intonation.

Every high saddle I have ever seen has cracked the slot at one end of the saddle or the other.
Author:  Pwoolson [ Fri Jun 02, 2006 1:20 am ]
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With the exception of the torque on the front of the bridge, I think a higher saddle is a very good thing. More break angle produces more volume. To counter the torque, I angle my saddle slot 8? back so that the saddle is pushing down more than twisting out. this allows me to have bit higher saddle.
Now back to the original question. I lay a straight edge on the board without frets and want it to sit about 1/16" above the top of the bridge. This will allow the action to be set with a higher saddle.
Author:  John How [ Fri Jun 02, 2006 1:33 am ]
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I put the fingerboard plane (unfretted) at 1/32-1/16 above the bridge and will end up with 3/16" of exposed saddle which will allow for action adjustment in the near term. I think you want no more than a third of the saddle exposed for good support.
Author:  Michael McBroom [ Fri Jun 02, 2006 2:16 am ]
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I agree with Paul regarding break angle. Having enough is essential.

To set the initial saddle height, here's how I do it (a modification of the method Cumpiano shows in his book):

First, the fingerboard needs to be fretted and the neck needs to be mounted on the guitar. By this time I have also installed the bridge and cut a saddle, but I haven't cut its height yet.

I have a 3 ft. straight edge that I lay down atop the frets and I have two shims, one for the 1st and one for the 6th strings, that I have cut to give me the exact distance I want the strings to be at above the 12th fret. I place the two shims at the 12th fret, and rest the straight edge against the 1st fret and the shims, one at a time, then mark the saddle on each side with a pencil. I then draw a line between the two marks on the saddle and that gives me the necessary ramp (for classicals with no fb radius). I use a stewmac radius block for establishing the radius on my steel string necks. I lay this edgewise against the saddle and then scribe an arc, connecting the two marks I previously made, then file the saddle down to the line.

This gets me very close to optimum saddle height. I'll usually err slightly on the high side so I can file down to optimum height after checking the action once the guitar is strung up to pitch.

Best,

Michael
Author:  Todd Rose [ Fri Jun 02, 2006 2:44 am ]
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The other effect of the increased torque is increased distortion of the top - bellying behind the bridge, dipping in front. The amount of torque on the guitar's top is determined by the height of the strings off the top, not just by the saddle height per se. In other words, a thick bridge with a shorter saddle will have more or less the same effect as a thinner bridge with a taller saddle, if the total height is the same. To me, it seems wise to think in terms of total height, size the bridge and saddle accordingly, then get the neck angle right.

I'm also a proponent of adjustable necks, so that the neck angle can be easily and repeatedly corrected as the guitar does its slow motion dance in response to string tension and other forces of nature.
Author:  TonyKarol [ Fri Jun 02, 2006 3:00 am ]
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Todd you are on the right track for sure - the more or less part is this - if the strings are a given height, say 1/2 inch off the top, a taller bridge/shorter saddle combo will produce less twisting torque that that of a low bridge high saddle - the break angle on the saddle creates most of the torque.
Author:  Kevin Gallagher [ Fri Jun 02, 2006 3:34 am ]
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   A steeper break angle across the top of th saddle allows the leverage to direct its force more toward the front edge of the saddle slot while a more gentle break angle across the saddle to the pin holes or anchor points allows it to be distributed in both forward and downward directions more thus, relieving some of the effect on the front of the saddle sot and bridge and directing it into the top through the front edge of the bridge itself.

   I always set my neck angle to give me 1/16" clearance as my straight edge rides the unfretted fingerboard toward the top of the bridge. This is with the neck set perfectly flat with no relief and provides an environment where my saddle height above the bridge will be about 3/16" for comfortable action....not too high, but not as low as is possible.

   As string tension is applied and the proper amount of relief is allowed to be introduced by it into the neck's length, good height of the strings off of the top as well as properly proportioned amounts of saddle above and below the top of the bridge are presented. I'll usually end up with 3/16" above the bridge and a little over 1/4" in the slot which I cut to .265" in all of my bridges and at a 6 degree angle toward the back of the bridge to allow more controlled down force.

   A high bridge with little exposed saddle and a low bridge with more saddle exposed providing the same string height off of the top will present different amounts of torque on the top since the angle formed by the approach of the strings from the neck and their interface with the pin holes or anchor points as they're pulled over the top edge of the saddle. The effects of the leverage will have to vary practically.....that is, unless, you have a very wide footprint for the lower bridge to be able to duplicate the angle presented by the high bridge/low saddle combination.

     This is only the case, though, if all other factors and dimensions are the same with the exception of the actual bridge height.

   Leverage seems like such an easy science, but when you have the equivalent of the weight of a grown man hanging from a glued patch the size of a bridge on a guitar, alot is to be considered. The physics of leverage and torque when applied to guitar construction are infinitely variable due to the organic and unpredictable nature of the materials we use and the unique desires and needs of each player that we serve as we build them.

   How many of us would trust a glue joint of that size to support our weight...175 to 225 pounds or even more....and for decades without failure?...Not me. It's an amazing thing that we can take for granted any time.

Regards,
Kevin Gallagher/Omega Guitars

   Kevin Gallagher38870.5282291667
Author:  Todd Rose [ Fri Jun 02, 2006 3:37 am ]
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[QUOTE=TonyKarol] Todd you are on the right track for sure - the more or less part is this - if the strings are a given height, say 1/2 inch off the top, a taller bridge/shorter saddle combo will produce less twisting torque that that of a low bridge high saddle - the break angle on the saddle creates most of the torque.[/QUOTE]

I'm not an engineer, Tony, and there are others here who could speak more authoritatively, but I'm not sure you're exactly right about this. It seems to me that whether the strings continue almost straight back (as with an extremely low saddle) or dive sharply down behind the saddle doesn't matter much, if at all, with regard to how much torque is put on the top, if the height of the strings off the top is constant. If the strings could just be glued somehow to the saddle and end right there, I think the torque would be the same. If I'm dead wrong, please correct me.
Author:  John How [ Fri Jun 02, 2006 4:11 am ]
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I think I agree with you Todd as far as torque is concerned. Break angle in my mind only applies downward pressure.
Author:  Rod True [ Fri Jun 02, 2006 4:19 am ]
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Todd, I will have to disagree with you.

Take a look at this



As you can see, there are forces pulling at the saddle in all three senarios.

The first, (optimal for guitars, I think) transfers the force at "equal" amount down to the saddle and out toward the back of the guitar, thus the torque on the top is also equal as there is pull from the back of the string and push on the saddle.

The second exerts a lot less force down on the saddle. Most of the force is applied back where the strings end (some where in the middle of the lower bout or at the tail I guess), since there is not as much downward force on the saddle, the torque on the top is less.

The third exerts most of the force down on the saddle, so now the reacting force pulls the top up hard and the torque under the saddle will be very high.

There is as Kevin has stated alot to consider with such a force on such a small area. Now if the bridge were say 4" x 8" (very large right) than what you have done is esentially made that top that much thicker for a larger footprint, and the torque would be less (the strength of the top is relivant to the thickness and stiffness). If the bridge were 1"x3" (very small) the torque on the top would also be more as the surface area is less.

Now a taller bridge will made the top that much more stiff in the bridge area, less torque seen by the top as it is effectivly stronger. But a taller saddle on a lower bridge with have to opposite effect.

The top will experience the torque differently as the strenght of the top (thickness, stiffness) goes up and down.

I'm sure that I havn't explained this very well, but I hope you see that there will be a difference in torque based on the angle of the downward force exerted by the string on the saddle.Rod True38870.5603935185
Author:  Michael McBroom [ Fri Jun 02, 2006 4:22 am ]
Post subject: 
Todd,

I'm not an engineer either. But it's actually a rather basic calculation that a 1st year physics text will have examples of. The greater the "break angle, the more force is projected downward both at the saddle and at the terminal point on the bridge.

Here's a pic of some numbers I just crunched illustrating this. WARNING: trignometry in use!



This is actually a simplification, since the angle of F with respect to x is slightly less than 90 degrees.

Best,

Michael
Michael McBroom38870.5659259259
Author:  Rod True [ Fri Jun 02, 2006 4:25 am ]
Post subject: 
Nice Michael, with numbers too.
Author:  Alan Carruth [ Fri Jun 02, 2006 7:31 am ]
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Think of it this way:

Suppose you're talking about a string with 10# of tension on it, and it can slide perfectly freely over the saddle. If the string is anchored just behind the saddle, rides right up the back side of it, and then rolls over the top edge, then there is 10# of force downward on the saddle, and 10# of force on the top of the saddle pulling it forward. The resultant vector is at 45 degrees from the top of the saddle, down and forward, and has a magnitude of about 14#.

Assume the saddle is pivoted at the bottom edge, and held vertical only by the surface of the front of the slot. If the saddle is 10mm tall, and the slot is 5mm deep, then the leverage is 2:1, and the maximum force on the front of the saddle slot is 20#. If the saddle is 10mm tall, and protrudes 3.33mm, then the force is less; about 13#. No saddle showing would give 10# of force, and a slot of 'no' depth would make the force infinite.

However, if the saddle is tilted back so that it bisects the breakover angle, then there is no force on the front of the slot, eveen though there is still a torque on the bridge/top system. This is the way fiddle bridges work, more or less. There's a small amount of friction from the strings that helps keep them from falling, but mostly they're effectively angled back a tad.

The _dynamic_ forces that the string exerts as it vibrates are a lot less than the _static_ tension load. There is a transverse force, generated as a consequence of the fact that you pull the string aside to pluck it, and a tension change, because the length of the string changes a little as it vibrates. You know that the transverse force is much smaller than the tension: when was the last time you pulled a string up to pitch with your bare fingertip? Would you want to try? The tension change is generally a lot less than the transveres force: I've measured it.

If the transverse force of vibration were equal to the static tension then you'd need to have a break angle of 90 degrees at each end of the string to keep it fromhopping off the top of the bridge or saddle. Since it's smaller, you don't need that much angle. In fact, the break angle you use at the head end, of about 12 or 15 degrees, is enough to keep the string from hopping off the top of the saddle, no matter how hard you play. Note that Bennedatto used about a 12 dgree break angle on his archtop bridges. I can't see how more break angle than that buys you anything you want: all it does is try harder to break out the saddle slot.

Because of the tension change as the string vibrates the height of the strings off the top can change the tone. The tension changes twice per cycle, and torques the bridge and top. Because guitar tops are made stiff to resist the static load of the strings this tension signal is ineffective below about 350 Hz on most guitars. Thus one would expect a bit more high frequency from a taller bridge that has more leverage.

It's very difficult to change the break angle and the saddle height independantly on a completed guitar, so it's hard to sort out the effects of string height off the top and break angle on the tone. We need to do some experiments on this, but it might take a while, given the ample funding available. Not.
Author:  Billy T [ Fri Jun 02, 2006 8:23 am ]
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[QUOTE=Alan Carruth]but it might take a while, given the ample funding available. Not. [/QUOTE]

NOT
OK!!! Now it makes sense! I thought, somebody has "ample funding", I've never heard of that before!!!

I've always wanted to experiment with dropping the saddle right done to the SB height, as much as possible. The mechanics of the string torque on the bridge, by the vibrational shortening of the string would be accentuated by the process.

Vis-a-vis, In opposite, if the bridge was 12" tall the same string length variation would allow a very small amount of energy to be transfered into the SB.
Billy T38871.0358564815
Author:  Pwoolson [ Fri Jun 02, 2006 9:53 am ]
Post subject: 
I think we might be talking about two different types of torque here. One being that on the saddle trying to break the bridge and the other on the top in general. Todd mentioned that it didn't much matter to the top if the break angle changes, and I agree. The top is affected by the overall height of the system. If you have a 1/2" bridge with 1/4" saddle sticking out, the torque on the to is the same as if you have a 3/8" bridge with 3/8" saddle sticking out. BUT the torque on the saddle will be vastly different per Rod's diagram.
Author:  TonyKarol [ Fri Jun 02, 2006 11:17 am ]
Post subject: 
You like low action right ?? Leave it.
Author:  Todd Rose [ Fri Jun 02, 2006 12:06 pm ]
Post subject: 
It may be just the limits of my brain or of my level of comprehension of physics and trigonometry, but I am still seeing it the way Paul is. I see that the way the force is acting on the saddle and saddle slot (both the front wall and the floor of the slot) will vary a great deal depending on saddle height and saddle back-tilt, but I don't see how the total torqueing force on the bridge area could be different if the total height of the strings off the top is the same.

Let's suppose that instead of a saddle we had a low bar fixed to the bridge that the strings passed UNDER (think some sort of wide staple). Then, instead of the strings going down into bridge pin holes, they went UP and were tied to tall screw eyes screwed down into the back part of the bridge. (Again the height of the strings off the top is the same.) Have we then reversed the torque on the top? No. Have we changed it at all? I don't see how. But, like I said, maybe it's just that my brain, like the Grinch's heart, is two sizes too small.
Author:  TonyKarol [ Fri Jun 02, 2006 11:59 pm ]
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I thought about this alot last night about how to explain it - I see where Paul and Todd are coming from, thinking that if the string height is 1/2 inch, no matter what the bridge saddle config is, then the torque is the same. But, do you agree that the down pressure on the saddle changes with break angle, given the string height stays the same ??? Then the torques has to change also. Why is that - because the force on the saddle is not directly down on the top - for that to be true the string would have to break down at the top at the same angle it approaches the saddle. But it doesnt do that ever. The string creates forces on the saddle as shown way back in those force vector drawings. The greater the angle the more force you get on the saddle. Rotational torque is a function of force and distance from the rotational centre, which is essentially the physical centre of the bridge. If you move the force away from the centre, the same force produces more torque - think of a wrench or socket drive - same force further away creates more torque. So, by increasing the break angle, we create more force on the saddle which is closer to the leading edge of the bridge, and thus the rotational torque will have to increase.

Paul mentioned that there are two torques, one of the saddle trying to break the slot, the other on the top - well if the saddle slot doesnt break, than that force is being applied to the top - it has to go somewhere.

There are actually two torques applied - one where the string breaks off the bridge pin hole, the other at the saddle. The two forces (not torques) may be the same total for two different saddle/bridge configs (again assuming the string height stays constant), but the position and value of each force can change, and since they occur at different distances form the rotational centre, the torque applied will be different in each case.

Todd - lets take your example of the staple type saddle with some sort of eye bolts holding the ball ends. You are correct, the torque has not reversed - why - because the direction of pull is still the same about the centre of rotation, and thus the direction of rotational torque is the same. Now want to change the torque value WITHOUT changing the tension or saddle config - thats too easy - make the eye bolts longer and the torque will go up - its a simple lever at at work - same force, longer distance - if this werent true longer wrenches wouldnt work at all.

The bottom line in my mind is if you beleive that the downforce on the saddle can be changed by changing break angle, then the torque has to change with it - you are moving the position of the force, and torque is forcexdistance.
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