Truss Rods - How They Work - Including The Bi-flex.

[Jack Wells]

Fender hasn't been very descriptive with their Bi-flex truss rod. It's not your typical double-action truss rod that we are familiar with. The Bi-flex uses Fender's single action truss rod concept with some modifications.

A single-action truss rod sits in a curved channel. It is anchored at one end and has an adjusting nut at the other end. Tightening the nut on the rod would shorten the distance between the anchor and the nut if the rod were not installed in the neck. With it installed in the neck this shortening cannot occur so the rod tends to straighten out. This results in an upward force being applied to the center portion of the neck. This corrects excessive relief.

......

Most double action truss rods sit in a straight channel and consist of two rods connected together at their ends ......... one fixed and the other adjustable in length. Turning the adjustment nut one way or the other will either shorten or lengthen the effective length of the adjustable rod thus curving the assembly in one direction or the other. Here's a double-action rod I purchased from Grizzly. It's covered with heat shrink tubing so you can't see the rods but this shows how it curves in either direction.

......

The Fender Bi-flex is a different animal. It uses the concept of the single action truss rod with some modifications. The Bi-flex consists of a single rod sitting in a curved channel. It is anchored at one end and has an adjusting nut at the other end. It corrects upward bow just like a single action truss rod.

Fender necks with the Bi-flex truss rod have a walnut plug with a small hole drilled in it at the headstock adjustment hole. Correction of convex bow is accomplished by turning the adjustment nut counter-clockwise until it contacts the back side of this glued-in walnut plug. Further turning of the nut would increase the effective length of the rod (distance between anchor and the plug) and increase the curvature of the rod resulting in a force being applied to the opposite side (back side) of the channel .......... the skunk stripe. The maximun force would be applied at the thinnest part of the skunk stripe and would probably result in wood failure. To prevent this the Bi-flex neck has an additional anchor for the truss rod installed under the 7th fret marker.

The link below is a blueprint of a Fender Stratocaster neck with Bi-flex truss rod.

http://img.photobucket.com/albums/v2...kBlueprint.jpg

[skydawg]

Thanks, Jack. I've wondered about the action of a bi-flex truss rod for awhile now. What year were these used? Is the walnut plug a sure sign that it's a bi-flex rod?

[Tom S]

Hey Jack, good info!

A question, though...because I always seem to have a problem getting these things right in my head...

Originally Posted by jwells393

Tightening the nut on the rod would shorten the distance between the anchor and the nut if the rod were not installed in the neck.

That makes perfect sense...

Originally Posted by jwells393

With it installed in the neck this shortening cannot occur so the rod tends to straighten out. This results in an upward force being applied to the center portion of the neck. This corrects excessive relief.

...but this seems wrong to me. If you attempt to shorten the installed rod as described, wouldn't that create more tension towards the body, consequently causing the rod to increase its bow, and thereby increasing the relief? It would seem that lengthening the rod (via counterclockwise turning of the adjustment nut) would cause it to straighten more, thereby inducing less bow.

Am I simply getting this wrong?

[ehawley]

Originally Posted by Tom S

Hey Jack, good info!

A question, though...because I always seem to have a problem getting these things right in my head...



That makes perfect sense...



...but this seems wrong to me. If you attempt to shorten the installed rod as described, wouldn't that create more tension towards the body, consequently causing the rod to increase its bow, and thereby increasing the relief? It would seem that lengthening the rod (via counterclockwise turning of the adjustment nut) would cause it to straighten more, thereby inducing less bow.

Am I simply getting this wrong?

Hi Tom...you have to remember that the rod is "captured" at the front and back end and as you tighten the end it wants to PULL the rod straight, thus causing upward force at the low part of the radius on the rod and neck. Hope this helps !
ED

[Jack Wells]

What year were these used?

The date of the neck blueprint is 1987. The Bi-flex patent date is 1983. I don't know when it was first incorporated.

Is the walnut plug a sure sign that it's a bi-flex rod?

You would think so. However mellecaster has said he has seen guitars with the plug that came with a pink slip saying it did not have the Bi-flex truss rod. That sounds like some sort of factory SNAFU to me. Sounds like the plug was put in a non-Bi-flex neck by mistake.

If you attempt to shorten the installed rod as described, wouldn't that create more tension towards the body, consequently causing the rod to increase its bow, and thereby increasing the relief?

Let's see if I can explain it another way............We're talking single action here. The neck keeps the distance between the anchor and the nut constant for all practical purposes. Tightening the nut shortens the portion of the rod between the anchor and the nut. Since the rod is curved, this shortened portion results is less curve or straightening of the rod which applies upward force on the neck.

Good explanation Ed.

[Tom S]

I understand now, guys, thanks. Brain cramp I guess.

[blue metalflake]

Great piece of information - thanks.

[kb1rcefn]

Jack, thanks for the info!! The "cutaway" picture is interesting, in that I did not realize there was as much curvature as there is, within the neck itself. It gives a good understanding of what happens (tightening & loosening)

[Vizcaster]

My assumption was that the hollow walnut plug showed up at the same time as the bi-flex feature. It's used on the 1983 Precision Bass Elite, so the biflex predates the American Standard.

Before the bi-flexes showed up with the walnut plug and a hole in it for the wrench, weren't they all bullet-adjusting rods? In other words are all Fender headstock-adjustable trussrods either bullet or biflex?

[TeleTed]

my tele neck has a broken truss rod. It's a one piece maple neck, vintage style with the adjustment at the heel. It's on my 80's japanese 62 RI. I was told it can't be fixed, but I'd still like another opinion from here. I ordered a replacement from Warmoth.

[mellecaster]

Originally Posted by jwells393

You would think so. However mellecaster has said he has seen guitars with the plug that came with a pink slip saying it did not have the Bi-flex truss rod. That sounds like some sort of factory SNAFU to me. Sounds like the plug was put in a non-Bi-flex neck by mistake.

What would really help...referring to the Pink slip, would be a HWY1 owner that bought his New...and saved the Case Candy....They are the only guitars I have seen the slip with ?...somebody around here has gotta have one ???

[Jack Wells]

TeleTed.............. It can probably be fixed. It just might not be cost effective. However it might be possible without removing the skunk stripe. It probably broke near the adjusting nut. If you removed the walnut plug from the headstock, and used the proper drift, you might be able to knock the rod back toward the headstock enough that you could grab the anchor and extract the broken rod from through the headstock hole. Then you might be able to insert a new truss rod like the Warmoth vintage truss rod. If this didn't work because of something in the channel preventing the new rod from going in, you could rout out the skunk stripe without the problem of hitting metal. Of course this would require a proper jig.

Some luthiers might have the StewMac truss rod repair kit which drills the adjustment hole deeper and adds threads to the rod without removing the rod.

[muddytailgate]

Originally Posted by jwells393

Fender hasn't been very descriptive with their Bi-flex truss rod. It's not your typical double-action truss rod that we are familiar with. The Bi-flex uses Fender's single action truss rod concept with some modifications.

A single-action truss rod sits in a curved channel. It is anchored at one end and has an adjusting nut at the other end. Tightening the nut on the rod would shorten the distance between the anchor and the nut if the rod were not installed in the neck. With it installed in the neck this shortening cannot occur so the rod tends to straighten out. This results in an upward force being applied to the center portion of the neck. This corrects excessive relief.

......

Most double action truss rods sit in a straight channel and consist of two rods connected together at their ends ......... one fixed and the other adjustable in length. Turning the adjustment nut one way or the other will either shorten or lengthen the effective length of the adjustable rod thus curving the assembly in one direction or the other. Here's a double-action rod I purchased from Grizzly. It's covered with heat shrink tubing so you can't see the rods but this shows how it curves in either direction.

......

The Fender Bi-flex is a different animal. It uses the concept of the single action truss rod with some modifications. The Bi-flex consists of a single rod sitting in a curved channel. It is anchored at one end and has an adjusting nut at the other end. It corrects upward bow just like a single action truss rod.

Fender necks with the Bi-flex truss rod have a walnut plug with a small hole drilled in it at the headstock adjustment hole. Correction of convex bow is accomplished by turning the adjustment nut counter-clockwise until it contacts the back side of this glued-in walnut plug. Further turning of the nut would increase the effective length of the rod (distance between anchor and the plug) and increase the curvature of the rod resulting in a force being applied to the opposite side (back side) of the channel .......... the skunk stripe. The maximun force would be applied at the thinnest part of the skunk stripe and would probably result in wood failure. To prevent this the Bi-flex neck has an additional anchor for the truss rod installed under the 7th fret marker.

The link below is a blueprint of a Fender Stratocaster neck with Bi-flex truss rod.

http://img.photobucket.com/albums/v2...kBlueprint.jpg

I am kinda thick:

So a single-action truss rod, will only correct a curvature towards the fretboard side? Is this correct?

I purchased a hotrod dual action truss, but since I ordered it, I decided that I cannot live without the skunk stripe. I tried to fight it, but it's no use.

If I send it back and make one like the one in Jacks super-awesome thread, do I need to ensure that any curvature is not towards the back of the guitar?

[jaydawg]

Jack, great info. I remember us talking about that fender blueprint and what the middle anchor was all about few years ago. Thats sooooooo cool you figured it out.

[Tom S]

Originally Posted by muddytailgate

I am kinda thick:

So a single-action truss rod, will only correct a curvature towards the fretboard side? Is this correct?

It is designed to allow the straightening of a neck (lessening of relief). The string tension does the job of adding relief (bow).

Originally Posted by muddytailgate

I purchased a hotrod dual action truss, but since I ordered it, I decided that I cannot live without the skunk stripe. I tried to fight it, but it's no use.

You actually can install it with a skunk stripe, but it requires a little more finesse.





More info that may (or may not) be helpful can be found on page 2 of this thread:

http://www.tdpri.com/forum/telecaste...different.html

[muddytailgate]

I guess the adjustment would need to be from the butt end then, provided there is a headstock or do you think a 2-way rod could make the radius?

BTW, the ferraricaster is really neat!

[Jack Wells]

A double action truss rod wouldn't have to have the adjustment at the butt end. You would just have to figure the required depth and rout a constant depth channel and drill to the channel from the headstock. ....... Hmmm......... come to think of it, making it a butt end adjustment would be easier because you could drill the access hole parallel to the channel whereas at the headstock you drill the hole at a slight angle. Double rod truss rods are usually used on guitars with angled headstocks.

[Tom S]

I think these are primarily designed to be installed with the adjustment nut at the heel end.

Stew-mac says the acoustic version is designed to be installed at either end, but they don't say that the "electric" version can't work that way. Frankly, I found the Stew-mac instructions to be a little vague and confusing, but that may be because I'm not that smart.

I'm not sure what you mean about making the radius.

Keep in mind, I'm no expert on these things, I'm just sharing what I learned in making a few necks.

Incidentally, back to your original issue, there's really nothing to prevent you from installing a skunk stripe on a neck (for looks), then adding a fretboard. Obviously, that doesn't apply if you're doing a one-piece neck.

[muddytailgate]

That's what I was thinking (the stripe for looks part), I guess it wouldn't need to be very deep and install the 2way rod under the fretboard.

Thanks again for letting bounce things around!

[Tom S]

It would only need to be as deep as a little more than the difference between the bottom of the heel (where the screws go in) and the lowest point in the neck contour. You understand, right? So it's do-able.

Seems like a lot of effort just to see a skunk stripe, though.

[muddytailgate]

Yep, I got it. Some things just need to be done though.

[UnionAve]

Originally Posted by jwells393

The link below is a blueprint of a Fender Stratocaster neck with Bi-flex truss rod.

http://img.photobucket.com/albums/v2...kBlueprint.jpg

Do you have any other blueprints of this guitar? I have an '87 American Standard strat and I've been looking for a detailed diagram of the tremolo opening on the back that will help me undo some modifications done by someone else.

[Jack Wells]

Here's the only Strat blueprint I have.

http://img.photobucket.com/albums/v2...P2642x1757.jpg

[mlp-mx6]

Thanks for those files.

[Terjesen]

Hi! I have a 1991 Strat Ultra and it's a brilliant guitar. Last week I was trying to adjust the neck to a more concave shape because some strings started buzzing.

When I did so, the walnut plug at the headstock loosened with a snap and it is now "coming out" of the headstock.

The neck did not get a concave shape, and I'm wondering if I have destroyed the lovely ebony neck?

I guess it's a Bi-Flex rod.

Pardon my english, I'm a simple norwegian...