Truss Rod Analysis up on website

crdeppe · Post by **crdeppe** » Mon Dec 01, 2003 6:33 pm

I've now got some simple analysis of how the truss rods work in my 4001. Before I go on I'd really like some opinions on what I came up with. It's all on my repair project website at http://www.tseint.com/4001project

ojobob2 · Post by **ojobob2** » Tue Dec 02, 2003 3:59 am

Chris, its very easy to understand and very well illustrated...good work!!!

johnhall · Post by **johnhall** » Tue Dec 02, 2003 6:25 am

The fifth drawing is misleading. While this is indeed what will happen if the rod is tightened in air, in actual use, the rods are constrained, not allowed to separate from each other. As a result, the entire rod mechanism instead bows into an arc, which is the force that actually bows the neck appropriately. Imagine it as forcing the rod into a strong straw and tightening it up.

Your drawing does illustrate the flaw of this design, the secondary force that's directed toward popping off the fingerboard. However, the glue joint is usually stronger than the surrounding wood.

The angle on the end of the rod is unintentional, resulting from the angle of the blades on the shears that were used to cut this rod material. But it did tend to dig into the aluminum top plate, stabilizing it to some degree.

The upward bend of the threaded portion of the rod came almost always as a result of adjusting the rod with the improper tool, i.e. a wrench or socket. The long handled nut driver recommended for adjustment supported the nut and kept this portion straight. Also, other tools were capable of over-tensioning the rods, bending or snapping the ends, whereas the nut driver tended to slip in your hand before too much pressure is applied.

crdeppe · Post by **crdeppe** » Tue Dec 02, 2003 6:56 am

Thanks to Mr. Hall for filling in some blanks for me. I will say, since I am the original owner of the bass, I am pretty confident that I am the only one who has adjusted the rods (except before it got to the store.) I mention this because even given that I always followed the proper procedure, and used the right tool, the ends of the rod still bent down (especially the rod on the G string side.) So I am inclined to think that there needs to be a some way to keep the bar from bending the rod besides just the angle of the tool when adjusting.

One question on drawing 5: By tightening the nut, the lower bar wants to become shorter than the upper bar. Are you saying that since the rods fit in the channel so tightly and cannot separate that the different forces applied to the bars (tension on the bottom, compression on the top) are what gives the rods the energy to oppose the pulling of the strings?

My next section on the web site will have digital photos of the rods and the bar as they stand now before I do any work on them. I'll also post a drawing of how I envision the new bar working.

Thanks again for all the help.

rhampshire · Post by **rhampshire** » Tue Dec 02, 2003 7:11 am

In my experience, the best way to keep the ends of the rods from bending is to make a new aluminum (or brass) block that is two to three times deeper than the original. You may have to shorten the angled upper section of the rod to gain enough thread. Mark Arnquist has instructed myself and others on how to do this.

The deeper block will lay flat against the bottom of the truss rod cavity and eliminate the curling. I've made two or three new blocks for 4001's with 100% success every time.

riffmeister · Post by **riffmeister** » Tue Dec 02, 2003 8:36 am

here are before and after pics of a brass end bar mod

http://acapella.harmony-central.com/forums/attachment.php?s=55da3fc51cd4d958551e5ce811db72cd&postid=6017235

http://acapella.harmony-central.com/forums/attachment.php?s=55da3fc51cd4d958551e5ce811db72cd&postid=6017249

rhampshire · Post by **rhampshire** » Tue Dec 02, 2003 8:53 am

Yup, that's what I'm talking about... except I make mine about a third deeper than that.

dave4004 · Post by **dave4004** » Tue Dec 02, 2003 9:10 am

Cumpiano's acoustic guitar building book, "Guitarmaking: Tradition and Technology", advocated the same type of truss rod as the older RIC rods. And now Mr. Cumpiano has a website with an update to his original design: http://www.cumpiano.com/Home/Book/textbook.html#Improved%20truss%20rod%20design . And the update is for a deeper brass block to prevent distortion.

There is nothing wrong with the principles of the basic design. There were just problems in practice which are corrected in the current design. Two-piece rods based on the same principle are often used today.

ojobob2 · Post by **ojobob2** » Tue Dec 02, 2003 9:13 am

Peter - that looks like a recent Rick, how the hell did the rods get so damaged?

riffmeister · Post by **riffmeister** » Tue Dec 02, 2003 10:10 am

Owen, I don't know. I got the guitar used a couple of months ago. It is two or three years old. My understanding is that the guitar has had only one previous owner who let it sit in its case most of its life.

johnhall · Post by **johnhall** » Tue Dec 02, 2003 10:21 am

Even fractions of a gram of added weight in the headstock can modify the sound noticeably as well as create unpredictable harmonics which make loud and soft notes. Stick with aluminum if you're making a thicker block (and hope everything is perfect, as a longer block often makes the nuts hit the back of the nameplate).

If you were to X-ray certain years of 4001 bass production, you would see many small lead weights strategically placed internally between the fingerboard and neck, taking advantage of this phenomenon to even out the notes.

Yes, it's compression and tension which make this system work. If you have the rods out, you can see this visually by inserting it in some washers about every 3 inches, tightening it up and seeing how the whole rod arcs. (You can use tape or ty-wraps instead but it has to be loose enough to allow the rods to slide in relation to each other.)

crdeppe · Post by **crdeppe** » Tue Dec 02, 2003 4:18 pm

My proposed design sketches are up, as well as pics of the current state of the rods.

I assume that since I am increasing the size of the bar quite a bit, making it out of brass will change the sound and I should stick to aluminum?

johnhall · Post by **johnhall** » Tue Dec 02, 2003 4:53 pm

Counterbore some holes the diameter of the truss rod nuts, about half the depth of the bar, on the centerline of the truss rods. That way you'll not only gain back some space that may otherwise be a problem but you'll also support the end of the rods even more. Better yet, turn the ends of the nut a bit to take the hex off and inset them into the block at that diameter.

riffmeister · Post by **riffmeister** » Tue Dec 02, 2003 6:26 pm

Chris, make two bars, one out of brass and the other out of aluminum. Try them both and tell us if you can hear any difference.

rictified · Post by **rictified** » Wed Dec 03, 2003 1:11 pm

I had a machinist make me one out of steel (the same size as the original) I didn't hear or feel a difference after it was installed in a 79 4001, I don't have any dead spots. I did before with heavier gauge strings. It's actually pretty even up the neck all the way down to the E.