How do I determine my bridge height before the guitar is strung up ?

[emmsone]

You may or may not have read my most recent posting on my "carboard box" thread where I state I'm about ready to work my bridge to the final dimensions. So far despite hours of searching on the interweb, i'm struggling to find anything that mentions how tall my bridge should be. So far I've come across 2 answers, one here on the AGF from 2009 where someone stated that it depends on the thickness of the top and the neck angle and one on another forum i've now forgotten the name of where someone randomly in the middle of the conversation said 10mm is standard.

I'm pretty sure that both these statements are right, but there must be a way to calculate/determine in advance how tall the bridge is supposed to be. The plans i've been vaguely working from do not include a height dimension for the bridge (i'm also using my own tweaked shape seeing as the outline shape it gave would not fit on my blank) nor a depth dimension for the saddle slot.

Everything i've found on online this subject is mostly related to saddle height, and the adjustment of it. But none of this is really helpful as its all adjusted relative to the already installed bridge and already routed saddle slot, and the other measurement is the string height at the 12/13th fret which is hard to work out when you can't actually string the guitar to check that

I'm concerned if I make my bridge too tall, its going to kill the resonance of the guitar and if I make it to low, even if I route a shallow saddle slot I won't get the strings high enough.
Right now my bridge blank is 12mm tall and my saddle blank is 11mm tall.

Is there some way I can use a straight edge off my fretboard to give me an approximate total height? From there I could probably work out the dimensions needed. Also, what is a "normal" depth for the saddle slot?

I'm going round in circles here and not finding myself coming to a decent conclusion, any and all help is hugely appreciated as I'm hoping to get this task done on early next week!!

thanks in advance

David

[John Arnold]

I make my bridges 0.35" thick. Martin's standard bridge is 11/32" (0.344").
I set the neck angle such that the straightedge on top of the frets is 0.39" to 0.40" above the top at the bridge. In other words, the bridge is 0.04" to 0.05" lower than the straightedge.
This is for a 'standard' action of 6 to 7/64 on the 6th string, and a saddle height between 0.12" and 0.14" above the bridge.
Saddle slots are usually between 0.21" and 0.24" deep. A good rule of thumb is that the saddle slot should always be at least as deep as the amount sticking above the bridge.

HTH.....

[Trevor Gore]

David,

One of the critical design parameters for a guitar is the height of the strings off the top of the soundboard, because that height and the total string tension determines the total torque on the soundboard, which then governs how stiff you have to make the assembled top and bracing to be able to withstand that torque over a long period of time. The height of the strings off the top also has implications on the sound produced.

The normal range of string height off the top for a steel string guitar is 12-14mm. (i.e. about 1/2" typically). If you design so that the bridge rotation under the torque from the string set you want to use is about 2 degrees, it will get you in the right stiffness ball park to produce the typical tonal outcome that people aim for.

The neck angle then has to accommodate that string height off the top whilst producing the right action at fret 12. Adjusting the saddle height to get the right action is really a backwards way around of doing things.

Not only that, it is nice to have the top of the neck shaft co-planar with the upper bout, so you can glue down a flat fretboard whilst also maintaining the right action and the target string height off the top.

What that all means is that it is the tangent to the longitudinal curvature of the upper bout which determines the neck angle, if you want all the other criteria to hold true. So a straight edge on the upper bout (which should be co-planar with the neck when it is attached) aimed over the (future) saddle position should have a target offset. The magnitude of that offset changes depending on the top curvature you're using and whether you're using a 12 or 14 fret neck join. For the top curvatures, fretboard thicknesses, fret heights and 12th fret neck joints I frequently use, the gap between the top and the projection of the tangent to the upper bout over the saddle position should be 2.5mm.

That gives you ~13mm string height off the top and a typical playing action at the 12th fret. You can then choose how thick to make your bridge and how much saddle protrusion you want to make up that 13mm. So 9-10mm of bridge and 3-4mm of saddle is typical.

You need also to be aware that altering the bridge height by a significant amount (1-2mm) will also significantly alter its mass, which will have a consequential significant affect on the sound of the guitar. This is yet another reason for aiming for a target string height off the top initially.

Yes, to do it "properly" is that involved, which is why you won't find much written about it (with a couple of notable exceptions!)

[emmsone]

Originally Posted by John Arnold

I make my bridges 0.35" thick. Martin's standard bridge is 11/32" (0.344").
I set the neck angle such that the straightedge on top of the frets is 0.39" to 0.40" above the top at the bridge. In other words, the bridge is 0.04" to 0.05" lower than the straightedge.
This is for a 'standard' action of 6 to 7/64 on the 6th string, and a saddle height between 0.12" and 0.14" above the bridge.
Saddle slots are usually between 0.21" and 0.24" deep. A good rule of thumb is that the saddle slot should always be at least as deep as the amount sticking above the bridge. HTH....

Thanks John! this gives me a start point and gives me a ball park that I can work into

Originally Posted by Trevor Gore

David,

One of the critical design parameters for a guitar is the height of the strings off the top of the soundboard, because that height and the total string tension determines the total torque on the soundboard, which then governs how stiff you have to make the assembled top and bracing to be able to withstand that torque over a long period of time. The height of the strings off the top also has implications on the sound produced.

The normal range of string height off the top for a steel string guitar is 12-14mm. (i.e. about 1/2" typically). If you design so that the bridge rotation under the torque from the string set you want to use is about 2 degrees, it will get you in the right stiffness ball park to produce the typical tonal outcome that people aim for.

The neck angle then has to accommodate that string height off the top whilst producing the right action at fret 12. Adjusting the saddle height to get the right action is really a backwards way around of doing things.

Not only that, it is nice to have the top of the neck shaft co-planar with the upper bout, so you can glue down a flat fretboard whilst also maintaining the right action and the target string height off the top.

What that all means is that it is the tangent to the longitudinal curvature of the upper bout which determines the neck angle, if you want all the other criteria to hold true. So a straight edge on the upper bout (which should be co-planar with the neck when it is attached) aimed over the (future) saddle position should have a target offset. The magnitude of that offset changes depending on the top curvature you're using and whether you're using a 12 or 14 fret neck join. For the top curvatures, fretboard thicknesses, fret heights and 12th fret neck joints I frequently use, the gap between the top and the projection of the tangent to the upper bout over the saddle position should be 2.5mm.

That gives you ~13mm string height off the top and a typical playing action at the 12th fret. You can then choose how thick to make your bridge and how much saddle protrusion you want to make up that 13mm. So 9-10mm of bridge and 3-4mm of saddle is typical.

You need also to be aware that altering the bridge height by a significant amount (1-2mm) will also significantly alter its mass, which will have a consequential significant affect on the sound of the guitar. This is yet another reason for aiming for a target string height off the top initially.

Yes, to do it "properly" is that involved, which is why you won't find much written about it (with a couple of notable exceptions!)

Thanks Trevor! this will be somewhat helpful on this build but hugely helpful in the future. The reason I say this is that my build (link here http://www.acousticguitarforum.com/f...63#post5124963 ) is my first and I did not design it, I just worked from a set of plans. The plans gave me sizes and dimensions to allow me to build the guitar, but obviously didn't mention any design philosophies, reasons for those dimensions being as they are or corrections if dimensions were not 100% accurate to the plan etc. All that area of detail I had to read up on myself elsewhere and have learnt from the Custom Shop and all the fantastic help of those who've chimed in here in the Build and Repair section on my thread.

As far as my guitar goes relative to what you've said, I've deviated from the original plans slightly in that I do have a neck angle, its approximately 1 degree (the plans had no angle, but i only worked this out well after the neck angle had been included, however i'm not concerned about having included one),
I did sand the neck angle into the upper bout too as you mention,
this guitar has a 12th fret neck join,
the top has no radius, its flat apart from the upper bout being angled slightly to meet the neck angle,
I don't have the guitar with me (its stored at the workshop) but when I ran a straight edge along the neck/upper bout before the fretboard was attached I had a small gap where the bridge will be, i didn't measure it, but it would be approximately somewhere in the 2mm range.
You're also talking about bridge masses, obviously all wood (i'm using rosewood here) is of a slightly different density but are you aiming for a target bridge mass within dimensional limits? or a target dimension within mass limits?

Thanks for the help (its also an interesting topic)
David

[Trevor Gore]

Originally Posted by emmsone

...You're also talking about bridge masses, obviously all wood (i'm using rosewood here) is of a slightly different density but are you aiming for a target bridge mass within dimensional limits? or a target dimension within mass limits?

Thanks for the help (its also an interesting topic)
David

There are three main things to get right on a guitar (which are the summation of many contributing factors). These are structural integrity, playability and musicality. The first two are fairly self explanatory. (Neck angle etc. clearly fit under the playability heading). A possible fourth is aesthetic appeal, but that is such a contentious subject that I generally just subscribe to the "keep it simple" rule. With the first two under control, that leaves musicality; the things distinguishing the sound of that particular guitar. One of the major contributors to musicality is the pitching of the modal resonant frequencies, which I like to have at particular target frequencies. Modal resonances are what you are listening to when you hear a guitar. Bridge mass is one of the variables available that can be used to control modal resonant frequencies and so that is how I use bridge mass, not as a particular objective in its own right. However, I do try to keep my bridges pretty light, so they accelerate better under the strings' vibratory forces. (More acceleration => more sound). So the typical bridges I use range from 15 to 20 grams.