Bridge height, how low can you go?

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Tommaso Poggi
Posts: 5
Joined: Fri May 01, 2020 6:03 am

Bridge height, how low can you go?

Post by Tommaso Poggi »

I need to do the final fitting between the neck and body of my benedetto. If i place a straightedge over the frets i now have a 1" gap at the bridge, but i still have a lot of wood under my neck extension, and it looks a bit weird. I don't know why since i followed the measures from the plans, my top should be the correct height and i also used 5 degrees instead of 4.5 for the neck. I'd like to go down a bit more to make it look better, but i'm afaid of messing up the sound. I know from this forum that the break angle can be a bit lower than the 13-14 degrees suggested by benedetto, maybe it's also desirable, but i'm not sure if the bridge height is important only as a part of the break angle equation or if it matters by itself. The break angle can also be tweaked from the tailpiece saddle, so that doesn't worry me too much. I heard of people that found their action rising over time and i'm afraid that if i start with a lower bridge i'll have to lower it even more and end up with something that can't transmit vibration very well. Any advice?

Alan Carruth
Posts: 959
Joined: Sun Jan 15, 2012 1:11 pm

Re: Bridge height, how low can you go?

Post by Alan Carruth »

I suspect that the two things that matter most are the mass of the bridge and the break angle, but that's not engraved on stone tablets from the mount. These things can get complicated.

Experiments I've done on flat top guitars show that when it comes to break angle, more is not better. Once you've got 'enough' all of the force of the vibrating string is transmitted to the saddle and the guitar top. The forces the string puts on the saddle are fixed, so increasing the break angle doesn't produce more sound. From what I can tell something on the order of 15 degrees of break angle should be enough.

On a flat top the height of the strings above the top matters. That's because the twice -per-cycle tension change in the string can rock the top of the bridge toward the neck. On an arch top the tension is normally taken up by the tail piece, so the octave doubled tension signal is not there. Bridge height, per se, should not make any difference.

One experiment I did varying the down bearing on an archtop by altering the pivot point of the tail piece showed that too much down force hurts the sound. Although it would have been nice to have a lot more data points, it does seem to me that there was a threshold involved; up to a certain point the sound was OK as the down bearing was increased, but after that it just died. This is par for the course: one take away from every experiment I've ever done was that I really needed to do a bigger experiment. How all of this relates to simple geometry, and how much is determined by things like the top thickness and arch height I can't say.

Brian Evans
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Joined: Sat Aug 30, 2014 8:26 am
Location: Tatamagouche, Nova Scotia

Re: Bridge height, how low can you go?

Post by Brian Evans »

I did the experiment that Allan alludes to, varying the break angle over an archtop bridge. I went from around 22 degrees, which developed between 50 and 60 lbs of pressure on the top and really killed the sound, down to 12 degrees, which develops around 30 - 35 lbs of down pressure, and freed up the sound considerably, and I have two guitars with around 4 degrees of break angle. My conclusions are that pre-load on the top with a lot of down pressure simply masks the comparatively tiny inputs from vibration of the strings, so high tension, massive download, big break angles are bad. Once you get to a place where the break angle is reasonable, around 12 degrees (which is customarily developed with a traditional archtop geometry) the sound frees up. My experience, and Ken Parker's writing about his instruments, suggests that 4 degrees is more than enough to keep the strings on the saddle and the bridge in place, and Ken certainly advocates low break angles for both headstock and bridge. If you make a tailpiece with the Benedetto pattern Sacconi cable and fulcrum/pivot point, what you'll find is that break angle is ver easy to change - simply vary the height of the fulcrum. The string load runs through the point where the cable flows through the tailpiece, so if that cable is raised off the table of the top, the break angle reduces. When you think about it, do you really want the equivalent of a large dog or a 7 year old child sitting on your guitar top, all the time, night and day, for years? Get rid of that preload!

Brian Evans
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Joined: Sat Aug 30, 2014 8:26 am
Location: Tatamagouche, Nova Scotia

Re: Bridge height, how low can you go?

Post by Brian Evans »

With respect to the oft-quoted "rules" that downward pressure and large surface areas in a bridge footprint are required to "drive the top" and "transfer all the string energy", I say bunk. Physics does not agree. Simply take the example of a tuning fork. A tuning fork terminates with a round sphere, a ball, that you press onto the bridge of your guitar to get the tone to tune up to. The surface area between an arbitrarily hard ball and an arbitrarily hard plane surface (not quite true, the ball is chromed steel and the bridge is usually pretty hard ebony or rosewood) is theoretically zero - a sphere touching a plane is a "point" - a dimensionless theoretical geometrical construct. So if you touch a tuning fork to a bridge or guitar top, you basically have really close to zero footprint. And - the sound transfers fine and is amplified by the guitar regardless of how hard you press down. Just barely touching the top, you get a loud, clear tone, you don't need to press down with 30 lbs of force. And consider how tiny the interface between a .012" string is as it rests on the extremely narrow bridge saddle. You tend to make those quite narrow, not big and wide and sloppy. So, I submit that both high downforce to "drive the top" and creating huge bridges so the foot print is long and wide are not required for great tone. Also why minimizing bridge mass is good, and why those tiny screws with thumb-wheels to adjust bridge height work far better than purists will admit. String vibration transfers through those tiny (albeit far larger than a theoretical "point") screws just fine...

My two or three cents for a Sunday morning... :)

Tommaso Poggi
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Joined: Fri May 01, 2020 6:03 am

Re: Bridge height, how low can you go?

Post by Tommaso Poggi »

Thanks a lot! It seems that i should be fine lowering my neck a bit more then, i myself can't think of too many reasons why a higher bridge should be better if the break angle stays the same, in theory a lower bridge is also lighter. The only thing that comes to mind is that Ken Parker in a conference said that archtop bridges moves primarily back and forward instead than side to side, like violins do, so maybe a higher bridge has more lever to do so and drive the top, but this is rookie speculation in its purest form

Alan Carruth
Posts: 959
Joined: Sun Jan 15, 2012 1:11 pm

Re: Bridge height, how low can you go?

Post by Alan Carruth »

Archtop guitar bridges move 'up and down' to drive the top relative to the plane of the top of the rim. All stringed instrument bridges do most of their work that way. Flat top bridges that are glued to the top can also rock forward and back in response to string tension changes, but that does not contribute much to power, only timbre. Violin bridges rotate around the fixed top of the sound post in response to the crosswise motion of the strings, and this causes the bass foot of the bridge to push the top 'vertically' over the bass bar, which then carries the vibration to the rest of the top.

In my break angle and string height experiment on a Classical guitar I used a 'low' break angle of 6 degrees. It was enough to keep the string in contact with the top of the saddle all through it's vibration cycle, but maybe only just enough. The mechanical 'plucker' I used started the string off moving exactly perpendicular to the top, so there was no sideways force on top of the saddle to cause the string to slide or roll, as it might in normal use. That's why I'd suggest using something on the order of 12 degrees of break. Of course, you can use a lower break angle if you capture the strings in notches to prevent sideways motion.

I like to tilt the bridge back slightly, so that the center line of the bridge (or the line of the adjusting screw shafts, if you use those) bisects the break angle, at least ideally. This is the line of the resultant force over the bridge top, and having the bridge at this angle minimizes the static tipping force on it. If nothing else, it eliminates the chance that the screw shafts will work loose in the bridge base.

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