FB radius vs Bridge radius

[garybaldy]

I posted this on an old thread. Either people haven't seen it or maybe aren't interested, anyway:

I've been trying to make the best of an old Hofner Congress and attempted my first fret dress. (the neck is surprisingly straight!). It's worked out OK so far but I still have a bit of choking on the D string around the 14th fret. Just by looking, I can tell the fb radius is less than the bridge, ie the fb is more curved. I was going to match the bridge radius to the fb as in the linked Stewmac video (at 1.08). That may be just enough to stop the choking and maintain the current good action on the two E strings. The consensus here seems to suggest that that is not what should be done. Some years ago, without having done any research, I did the same to a Chinese 'ES125 copy' and it work out very well. Is the Stewmac video wrong?

[sgosnell]

In theory, the saddle radius should be very slightly flatter than the fretboard, because the strings fan out from the nut to the bridge. And in theory, a compound radius of the fretboard is better than a simple radius. IME, in practice, it's more trouble than it's worth to do all this, because the playability isn't improved enough to make it worthwhile. Making the saddle radius match the fretboard radius is almost always close enough. If the saddle radius is too flat, the center strings will be lower (closer to the frets) than the outer strings, and vice versa. This can be approached from a couple of directions. One can reradius the entire saddle, keeping all the slots the same depth, or just cut the slots a little deeper to lower the high strings. Either will work, and IME it's somewhat easier to deepen slots as necessary, resulting in some slots being deeper than necessary, and then reradius the saddle to make the slots all about the same depth. The slots only need to be deep enough to hold the strings in place. How far one goes with this is controlled by one's anality. My first concern is playability, getting all the strings as low as possible without buzzing, and then equalizing slot depths and radiusing the saddle as a secondary concern, to be taken care of at my leisure. But that's just me, and my own priorities.

[garybaldy]

In theory, the saddle radius should be very slightly flatter than the fretboard, because the strings fan out from the nut to the bridge. And in theory, a compound radius of the fretboard is better than a simple radius. IME, in practice, it's more trouble than it's worth to do all this, because the playability isn't improved enough to make it worthwhile. Making the saddle radius match the fretboard radius is almost always close enough. If the saddle radius is too flat, the center strings will be lower (closer to the frets) than the outer strings, and vice versa. This can be approached from a couple of directions. One can reradius the entire saddle, keeping all the slots the same depth, or just cut the slots a little deeper to lower the high strings. Either will work, and IME it's somewhat easier to deepen slots as necessary, resulting in some slots being deeper than necessary, and then reradius the saddle to make the slots all about the same depth. The slots only need to be deep enough to hold the strings in place. How far one goes with this is controlled by one's anality. My first concern is playability, getting all the strings as low as possible without buzzing, and then equalizing slot depths and radiusing the saddle as a secondary concern, to be taken care of at my leisure. But that's just me, and my own priorities.

Currently the fb radius on the hofner is 10" and the saddle (plastic insert) is 13". I'd like to sand the saddle to 10" which would reduce the height of the ends. I can then raise the whole saddle to achieve the same action as I now have on the E strings. If that then doesn't stop the choking on the D string I'll have to file the frets a bit more.

[sgosnell]

That should work. A 3" difference in radius is usually noticeable. Most saddles seem to come much flatter than most fingerboards, for whatever reasons. I doubt your choking problem is due to uneven frets, with that much radius difference. It's just that the D and G string actions are too low when the outer strings are adjusted to comfortable action. Sanding the saddle to the proper radius should help with that. It might be helpful to increase the slots slightly as you go along, so that you don't remove them entirely and then have to recreate the string spacing. I would probably just watch, and lightly increase the outer slots as needed. You can always reslot if needed, but keeping what you have is easier, if the spacing is indeed correct now.

[garybaldy]

That should work. A 3" difference in radius is usually noticeable. Most saddles seem to come much flatter than most fingerboards, for whatever reasons. I doubt your choking problem is due to uneven frets, with that much radius difference. It's just that the D and G string actions are too low when the outer strings are adjusted to comfortable action. Sanding the saddle to the proper radius should help with that. It might be helpful to increase the slots slightly as you go along, so that you don't remove them entirely and then have to recreate the string spacing. I would probably just watch, and lightly increase the outer slots as needed. You can always reslot if needed, but keeping what you have is easier, if the spacing is indeed correct now.

I have a few crude radius templates made out of business cards. 10 12 13 and 14". There's quite a big difference between 10 and 13 so it should be an improvement. I made a 13 radius block for my last project which was sucessful. It's a chore but I think I'll make a 10". The string spacing is good.

[garybaldy]

Before I could do the fret level I had to tighten the truss rod to get the neck as straight as I could. As the TR nut was quite hard to turn and I was afraid something may snap, I induced a slight back bow with a homemade clamp. It was all guesswork but I had about a 24 thou back bow and left it clamped for a couple of days before tightening the rod. It's the straightest budget European archtop neck I've seen!

[strumcat]

One of the nice things about a Strat or Strat clone is that the saddle heights (as well as positions) are individually adjustable for each string. You can give the strings any bridge radius you like with a few turns of a hex wrench. And if you don't like the neck radius, shape, or feel, you can just bolt on another one without much trouble, other than changing the strings and doing a setup. The "through the body" stringing with springs goes a long way toward keeping it in tune even if you do a lot of bends, unlike a Gibson SG or similar. And all of the hardware is really robust. One thing I don't like about Strats is the lack of individually adjustable pole pieces on the pickups, though.

[garybaldy]

One of the nice things about a Strat or Strat clone is that the saddle heights (as well as positions) are individually adjustable for each string. You can give the strings any bridge radius you like with a few turns of a hex wrench. And if you don't like the neck radius, shape, or feel, you can just bolt on another one without much trouble, other than changing the strings and doing a setup. The "through the body" stringing with springs goes a long way toward keeping it in tune even if you do a lot of bends, unlike a Gibson SG or similar. And all of the hardware is really robust. One thing I don't like about Strats is the lack of individually adjustable pole pieces on the pickups, though.

Yes, the strat saddle system is very convenient and well thought out. With all the individual pieces that make it up it's funny to think that once you have your preferred neck and have made your preferred saddle adjustments it never has to be adjusted again. I'm ok with the fixed pole pieces. What I dislike about my 1997 Inca Silver standard is the bridge pickup sharing the middle pickup's tone. (It has a no load notch) I like a tone for the bridge but with the current wiring if the bridge tone is in use it affects 4 of the 5 switch positions. May be after 28 years I might re-wire it! But it's still in mint condition.

[Strat-itis]

Yes, you want the bridge and fretboard radius to be the same, why would you not want that? If you can get the bridge radius the same then do that. If not, you can actually level the frets to a closer radius, but you have to be wary about taking down too much material. Side note is that fret levels can come out ineffective in the upper range if there is no fallaway put in. That's where you angle them down further at the heel to compensate for where the neck ceases to bow.

[garybaldy]

Yes, you want the bridge and fretboard radius to be the same, why would you not want that?

I do want it and I know how to achieve it.

[Strat-itis]

Haha then do it to it.

[garybaldy]

The Hofner is now playable! I increased the curvature of the saddle to match the fb but still had to work on the frets to get rid of the choking at the 12th. The guitar clearly hadn't been played for many years judging by the original strings and the intonation must've been dreadful. You can see where the bridge has been for years.
It's not the greatest of guitars but a fun project.

[JCat]

If you would measure string height for each individual string, you would find that for equal string height the bridge will have to be flatter than the fretboard. Reason being that the nut string spacing is narrow compared to the bridge, that strings are flared, spreading outwards.

The bridge radius is a function of nut radius, nut width and bridge width.
The greater the ratio bridge width/nut width, the greater the ratio bridge radius/fretboard radius.

This means that in the event bridge radius would equal fretboard radius, the middle strings (D and G) would end up higher over the frets compared to the outer strings, which in turn would make the middle strings feel stiff. This is indeed a real life scenario (quite common too), not just theory.

So why are there on-line setup instructions advocating that the bridge radius should equal fretboard radius?
I can think of a few plausible reasons:

The target audience may not be able to tell a good setup from a poor setup (in which case none of this really matters). We won't be able to assess an on-line demo guitar, we would just have to take the guy's words for it...

There are reasons to separate nominal bridge radius from effective bridge radius (i.e when the bridge is under tension from strings tuned to pitch and intonated). In real life the effective bridge radius is often flatter than the nominal radius due to compression, tilt and sag.

There's more than one tuning system and there's more than one string set configuration of gauges and tension. Some string sets got higher than average tension of the middle strings in standard tuning, some string sets come with middle strings that feel a bit loose and therefore tend to buzz and then some people may want more clearance of the middle strings.

It's true, that when you got a bridge system with individually height adjustable saddles (e.g Fender) you could just set the height for each string so that it feels good. The resulting bridge radius would be the result of the string set chosen, your personal preferences and a few other things. You don't have to measure, just go by feel.

The problem arises when you got a fixed radius bridge. You would then first have to verify that its nominal radius matches the fretboard...then you have to slot the bridge, tune and intonate and check the effective radius...chances are that when you're done, when the guitar plays nice, you'll find that the bridge radius is flatter than the fretboard.

One more thing: Don't assume that the nut radius equals the fretboard radius. You've got to measure each string. It's easy to measure string height for the outer strings, but much harder to accurately measure the middle strings...

In conclusion: bridge system setup is a major differentiator when comparing guitars.

[sgosnell]

The bridge saddle radius should ideally be slightly flatter than the fingerboard radius. But the actual difference is so small as to be negligible. It's much easier to just make the radius the same as the fretboard, and very slightly deepen the center slots, just a fraction of a millimeter, if necessary. Making the radius of both the same is an acceptable starting point. The nut radius should be exactly the same as the fretboard radius unless the fretboard has a compound radius. Getting the nut slots to the proper height above the frets is part of a setup. I've never seen a guitar come from any factory with a perfectly cut nut. It's too expensive (and time is money in a factory) to take the time to do it right, so it's done quickly with the slots too high, but close enough for factory work. Getting the nut slots exactly right is a tedious, time-consuming process, but well worth the effort to get playability and intonation just right. Same for the saddle slots, but since the saddle is adjustable for height, there is a little more leeway.

[garybaldy]

If you would measure string height for each individual string, you would find that for equal string height the bridge will have to be flatter than the fretboard. Reason being that the nut string spacing is narrow compared to the bridge, that strings are flared, spreading outwards.

The bridge radius is a function of nut radius, nut width and bridge width.
The greater the ratio bridge width/nut width, the greater the ratio bridge radius/fretboard radius.

This means that in the event bridge radius would equal fretboard radius, the middle strings (D and G) would end up higher over the frets compared to the outer strings, which in turn would make the middle strings feel stiff. This is indeed a real life scenario (quite common too), not just theory.

So why are there on-line setup instructions advocating that the bridge radius should equal fretboard radius?
I can think of a few plausible reasons:

The target audience may not be able to tell a good setup from a poor setup (in which case none of this really matters). We won't be able to assess an on-line demo guitar, we would just have to take the guy's words for it...

There are reasons to separate nominal bridge radius from effective bridge radius (i.e when the bridge is under tension from strings tuned to pitch and intonated). In real life the effective bridge radius is often flatter than the nominal radius due to compression, tilt and sag.

There's more than one tuning system and there's more than one string set configuration of gauges and tension. Some string sets got higher than average tension of the middle strings in standard tuning, some string sets come with middle strings that feel a bit loose and therefore tend to buzz and then some people may want more clearance of the middle strings.

It's true, that when you got a bridge system with individually height adjustable saddles (e.g Fender) you could just set the height for each string so that it feels good. The resulting bridge radius would be the result of the string set chosen, your personal preferences and a few other things. You don't have to measure, just go by feel.

The problem arises when you got a fixed radius bridge. You would then first have to verify that its nominal radius matches the fretboard...then you have to slot the bridge, tune and intonate and check the effective radius...chances are that when you're done, when the guitar plays nice, you'll find that the bridge radius is flatter than the fretboard.

One more thing: Don't assume that the nut radius equals the fretboard radius. You've got to measure each string. It's easy to measure string height for the outer strings, but much harder to accurately measure the middle strings...

In conclusion: bridge system setup is a major differentiator when comparing guitars.

So what do you make of the Stewmac video in my post #1?

[JCat]

So what do you make of the Stewmac video in my post #1?

Regarding string geometry:

Dan: "I like the string spacing a little bit wider"

Then ideally the bridge is going to end up even flatter. Rb = Rn x Wb/Wn

Rb = effective bridge radius; Rn = effective nut radius;
Wb = Bridge width, Wn = Nut width (width = string spacing outer strings, c-t-c)

That is provided you want equal string height (ref to my previous post)

Regarding use of radius sanding block to pre shape the bridge saddle:

Dan: "Fretboard is 10", we want to make the bridge saddle the same"

That's nominal bridge radius, as the bridge has to be re-slotted. (ref. to my previous post)

Regarding use of understring radius gauges:

These are great (not to say mandatory) tools when working on a fixed radius bridge (becasue it's very difficult to meassure string height with precision). The radius gauge validates that the slotted radius is even, that no individual string is higher or lower than the others, and it tells the effective radius when slotted and under tension, strings tuned to pitch.

Concern: You have to have a particular reason to go with an effective bridge radius that equals the fretboard radius. (ref. to my previous post). One such reason could be that the luthier foresees that the bridge is going to get flatter over time...Another reason could be that that the luthier knows that guitar setup is an iterative process. By leaving the middle strings high, he could later file the D-G slots lower if needed without having to touch the outer slots.

Dan: "The owner of this guitar just started taking guitar lessons".

Then the owner won't mind.

Regarding proportional string spacing

This is something we always do when slotting a nut, but something most people don't do when slotting a bridge. I wouldn't recommend it, unless there's a particular reason that I don't see in this case, as the guitar got a floating bridge.

Note: String to fretboard alignment is one of the most important criteria for the guitar to play well. This means that parameters like bridge alignment and neck alignment are critical parameters. If the guitar got fixed bridge posts (e.g Les Paul), the only way to adjust string alignment would be to slot the saddles off-center. In this case you may find that some degree of proportinal spacing would be required.

Regarding use of abrasive chord

Is a useful tool for some jobs, but not for string slots. You'll likely ruin the slots. Proper nut files, used with the right technique, is the way to go.

Regarding Dan:

He's a great source of wisdom that has done a lot for the community. He's written books on the subject and made hundreds of instruction videos. Most of them are great. That's not to say that I agree with him on every subject, but that's not important. What matters is that I learn and reflect on what's important to make my guitars play great.

[JCat]

The bridge saddle radius should ideally be slightly flatter than the fingerboard radius. But the actual difference is so small as to be negligible. It's much easier to just make the radius the same as the fretboard, and very slightly deepen the center slots, just a fraction of a millimeter, if necessary. Making the radius of both the same is an acceptable starting point. The nut radius should be exactly the same as the fretboard radius unless the fretboard has a compound radius. Getting the nut slots to the proper height above the frets is part of a setup. I've never seen a guitar come from any factory with a perfectly cut nut. It's too expensive (and time is money in a factory) to take the time to do it right, so it's done quickly with the slots too high, but close enough for factory work. Getting the nut slots exactly right is a tedious, time-consuming process, but well worth the effort to get playability and intonation just right. Same for the saddle slots, but since the saddle is adjustable for height, there is a little more leeway.

I agree with almost everything you say here, except for the part "But the actual difference is so small as to be negligible".
When it comes to guitar setup, many times things that truly matters are invisible. Tiny little details that make or break the setup, but that we can't see with a naked eye. Therefore we need tools, techniques, processes and know how to get it right.
Like you say; getting the string slots exactly right is a tedious, time-consuming process.

In particular when it comes to the bridge saddle radius:
If I put a 12" radius gauge next to a 16" radius gauge in front of me on the table, I can't see the difference. But when used understring to meassure the bridge, the difference is huge. To me.

[garybaldy]

If a fb has been shaped to a particular radius then the radius at the nut will be the same as the radius at, say, the 12th fret and at the end of the fingerboard regardless of the width at those respective locations. So the projected radius of the fb at the bridge will remain the same also. So width across strings or fb doesn't affect radius unless it's a compound radius. The width of the arc is changing but not the radius.

Now, the radius of 'strings' at the nut will be greater than the 'fb' radius at the nut. (By action plus fret height)
The radius of the strings at the 12th fret is greater than the radius of the strings at the nut because the action at the 12th fret is greater than the action at the nut.
It follows that , because of the increase of the action, there will be a projected increase in string radius at the bridge.
So say take a 12" radius fb, the string at the nut will be (action plus height of fret) around 12.040". At 12th fret, string radius around 12.100". Projected at bridge 12.180". So, a barely noticeable change in radius from fb to strings at bridge. About 1.5%.