Archtop bridge break angle
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Greg Martin
- Posts: 308
- Joined: Thu Feb 23, 2012 7:18 pm
Archtop bridge break angle
What is the optumum bridge break angle for a bridge height of 1-1/16" my tail piece is wood and hinged. What is the best method so i dont over stress the top.im trying to lighten up my next build some what.
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Alan Carruth
- Posts: 1266
- Joined: Sun Jan 15, 2012 1:11 pm
Re: Archtop bridge break angle
I don't know what the optimum is: it could well vary for different guitars, depending on how high the top arch is, how thick it is, and how it's braced. There is a minimum, however: around 5-6 degrees. That's not much, but from testing that I've done it seems to be 'enough', if only just so.
I think of the 'job' of the bridge as being to tell the strings how long they are, so they'll know what pitch to make. So long as the string remains in contact with the top of the bridge throughout it's whole vibration cycle, the vibrating length will be defined, and all of the force that the string vibration produces will be transmitted to the top. So all you need to do is satisfy that for the most forceful pluck you're ever going to use.
Suppose you start out to pluck the string by pushing it down toward the soundboard, and then letting it go. As long as the string is pointing down it's pushing the top downward. At some point, depending on where you plucked the string, the angle it makes with the bridge will suddenly flip to an up angle. This will be approximately the same relative to the resting position of the string as the original down angle. If that up angle exceeds the break angle, then the string will hop off the top of the bridge, so that's the case you're trying to avoid. The question then is that since the 'up' angle is about the same as the initial 'down' angle, what's the maximum down angle you'll ever see? If you fool around with strings and pick for a while, you'll see that you're going to have a hard time pushing the string down far enough to get that angle to exceed five degrees.
You can also push the string sideways, of course. Most arch tops use some sort of string notch on the saddle top to limit sideways motion. So long as the depth of the notch is at least 1/2 the string diameter, it can't move sideways. Usually it doesn't need to be even that deep.
Keep in mind that, unlike on a flat top, sideways string motion on an arch top has no way to drive the top. On a flat top, since the strings are tied to the bridge, changes in the string tension as it vibrates can rock the bridge forward and aft. This is not effective or efficient enough to produce much power, but it does affect the timbre of the guitar a bit: changes in the string height off the top alter the way a flat top sounds. On a modern arch top the tailpiece takes the string tension, so that method of driving the top isn't there.
So if five degrees or so of break angle is 'enough', how much is too much? That's harder to answer. I used to use 'hook' type wooden tailpieces. There was a vertical extension on the lower end of the tail piece that went over the edge of the guitar to bear on the side. I used a tail gut of sorts, but only to keep from accidentally knocking the tailpiece off; it didn't take any string load. If you look at that arrangement you'll see that the line of the strings behind the bridge runs from the saddle top to the pivot point. When the pivot point is below the edge of the top the strings virtually run through the top. Thus, with a hook tailpiece you're not limited as to the break angle, and the down bearing force you put on the bridge can be almost equal to the string tension if you want.
I made one guitar with a very deep extension, and experimented with altering the pivot point to see what happened. When it got too far down on the side the tone just died. This was unexpected: we're usually told that more down bearing gives more power. Clearly that was not the case. What the case is is harder to say. It's possible to come up with a couple of different models of the forces acting on the top, and at least plausible that there is some sort of optimal balance depending on some of the things I mentioned earlier. It would probably require a fairly major research program to figure it all out.
I made a few arch top Classical guitars several years ago. Given the lower tension of nylon strings I wanted to make the tops as light and responsive as I could. In the end I used the lowest break angle I thought I could get away with, about 8-10 degrees (always have a safety factor!), and made the tops as thin as I thought I could get away with as well. Once I got the arch height and graduation stuff sorted out they worked out pretty well. I've also kept the bridge break angle in that range on my steel string arch tops, and had no complaints so far.
I think of the 'job' of the bridge as being to tell the strings how long they are, so they'll know what pitch to make. So long as the string remains in contact with the top of the bridge throughout it's whole vibration cycle, the vibrating length will be defined, and all of the force that the string vibration produces will be transmitted to the top. So all you need to do is satisfy that for the most forceful pluck you're ever going to use.
Suppose you start out to pluck the string by pushing it down toward the soundboard, and then letting it go. As long as the string is pointing down it's pushing the top downward. At some point, depending on where you plucked the string, the angle it makes with the bridge will suddenly flip to an up angle. This will be approximately the same relative to the resting position of the string as the original down angle. If that up angle exceeds the break angle, then the string will hop off the top of the bridge, so that's the case you're trying to avoid. The question then is that since the 'up' angle is about the same as the initial 'down' angle, what's the maximum down angle you'll ever see? If you fool around with strings and pick for a while, you'll see that you're going to have a hard time pushing the string down far enough to get that angle to exceed five degrees.
You can also push the string sideways, of course. Most arch tops use some sort of string notch on the saddle top to limit sideways motion. So long as the depth of the notch is at least 1/2 the string diameter, it can't move sideways. Usually it doesn't need to be even that deep.
Keep in mind that, unlike on a flat top, sideways string motion on an arch top has no way to drive the top. On a flat top, since the strings are tied to the bridge, changes in the string tension as it vibrates can rock the bridge forward and aft. This is not effective or efficient enough to produce much power, but it does affect the timbre of the guitar a bit: changes in the string height off the top alter the way a flat top sounds. On a modern arch top the tailpiece takes the string tension, so that method of driving the top isn't there.
So if five degrees or so of break angle is 'enough', how much is too much? That's harder to answer. I used to use 'hook' type wooden tailpieces. There was a vertical extension on the lower end of the tail piece that went over the edge of the guitar to bear on the side. I used a tail gut of sorts, but only to keep from accidentally knocking the tailpiece off; it didn't take any string load. If you look at that arrangement you'll see that the line of the strings behind the bridge runs from the saddle top to the pivot point. When the pivot point is below the edge of the top the strings virtually run through the top. Thus, with a hook tailpiece you're not limited as to the break angle, and the down bearing force you put on the bridge can be almost equal to the string tension if you want.
I made one guitar with a very deep extension, and experimented with altering the pivot point to see what happened. When it got too far down on the side the tone just died. This was unexpected: we're usually told that more down bearing gives more power. Clearly that was not the case. What the case is is harder to say. It's possible to come up with a couple of different models of the forces acting on the top, and at least plausible that there is some sort of optimal balance depending on some of the things I mentioned earlier. It would probably require a fairly major research program to figure it all out.
I made a few arch top Classical guitars several years ago. Given the lower tension of nylon strings I wanted to make the tops as light and responsive as I could. In the end I used the lowest break angle I thought I could get away with, about 8-10 degrees (always have a safety factor!), and made the tops as thin as I thought I could get away with as well. Once I got the arch height and graduation stuff sorted out they worked out pretty well. I've also kept the bridge break angle in that range on my steel string arch tops, and had no complaints so far.
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Greg Martin
- Posts: 308
- Joined: Thu Feb 23, 2012 7:18 pm
Re: Archtop bridge break angle
Thankyou for the input.loads of things to think thru here. When i asked the question i was thinking about 8-10 degrees,as on my other archtop with a hinged tail piece thats what i had. That archtop had a deep carve.noww if the arch is flatter how would that affect break angle? Lower break angle ?
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Alan Carruth
- Posts: 1266
- Joined: Sun Jan 15, 2012 1:11 pm
Re: Archtop bridge break angle
All else equal, a lower arch would yield a lower break angle. You can alter things like the overstand of the neck and it's back angle to change the break angle as well. The bridge height too. Lots of knobs to twiddle on an archtop.
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Brian Evans
- Posts: 922
- Joined: Sat Aug 30, 2014 8:26 am
- Location: Tatamagouche, Nova Scotia
Re: Archtop bridge break angle
FWIW, I have a resonator guitar with an almost infinitesimal break angle, I bet it's not 3 degrees. It has just as much tone and volume as it had before I lowered the action height to a reasonable level. I don't think that break angle, within reason, is all that important. I suspect that all you have to do is ensure reasonable contact with the bridge, and no more.
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Brian Evans
- Posts: 922
- Joined: Sat Aug 30, 2014 8:26 am
- Location: Tatamagouche, Nova Scotia
Re: Archtop bridge break angle
I just measured the break angle on my red guitar, and it's around 19 degrees. The main reason is that I designed it so that the bridge was in the geometrical center of the lower bout, it has a long scale length (25.6") and the main body to neck joint is at the 12th fret - all this makes for a short distance of around 7.5" between the bridge and the tail compared to most archtop guitars. Since my tail piece design is easily adjusted for attachment height, and hence string break angle, I think I will take Alan's advice and reduce the angle to around 10 degrees, just to see what happens. If anything does, I will let you all know!