6-String Bass Neck - 1 or 2 Truss Rods?

Discussion in 'Luthier's Corner' started by StarbardGuitar, Dec 20, 2014.

  1. StarbardGuitar


    Feb 18, 2013
    Hey everyone,

    I have a quick question. So, I have plans for a 6-string build next after my current build http://www.talkbass.com/threads/ash-flame-maple-rosewood-4-string-pj-build.1112943/. For the 6-String, it will either be a 34" or 35" scale, most likely a 3 or 5 piece laminate neck. I'm wondering, should I use one truss rod, one truss rod with carbon fiber rods, or two truss rods? What are advantages/disadvantages of each one? My gut tells me that one truss rod alone just couldn't handle all of that string pull, but I don't know for sure.

  2. JustForSport


    Nov 17, 2011
    You might want to read thru this:

    Due to the added cumulative tension of 6 strings, and the fact that most typical string sets have more tension on the small string side,
    plus the fact that once the neck is together, it's too late to change one's mind: 2 trussrods, definitely. Most manufacturers make them that way for a reason.
    CF rods cannot adjust for differing string tension, and 2 CF rods (one on each side) mostly work to stabilize any side-to-side stresses: push-pull against each other,
    altho there is some resistance to fore-aft bending (that may even counter the trussrod action).
  3. One truss rod can handle the tension without any problems, but you run the risk the wider neck might develop a twist at some point in its life, I would (and have) not cut down on the costs for the few bucks they cost.

    For the price you pay for these things nowadays it is stupid not to use two!
    Last edited: Dec 21, 2014
  4. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    One truss rod is enough to handle the structural loads. That's not the problem.

    The wider the neck is, more likely it is to develop twist, if there's an imbalance in movement of the wood from one side to the other. The way to keep a neck from twisting is to build it carefully, paying close attention to the alignment and symmetry of the growth rings in the wood, and the dryness and stiffness of the wood. If you do that well, you won't have an issue with twisting.

    The advantage to having two truss rods is that you have a way to adjust to counteract any twist that develops.

    The disadvantage to having two truss rods is that they themselves can cause the neck to twist, and be more sensitive to short-term movement of the wood. With two rods, it's hard to keep them exactly equally loaded. So, when the wood tries to move, one rod will apply more restraint than the other, pushing the neck to twist, when it might not have otherwise. You keep adjusting them, to keep them balanced.

    Personally, I only go to two rods on the crazy wide necks, like 3" and wider. For a 6-string, I build a carefully matched laminate and a good single truss rod down the middle.
  5. StarbardGuitar


    Feb 18, 2013
    Thank you both for the responses. That was why I figured I should ask this question. I figured that with two truss rods, you run the risk of twisting the neck by uneven adjustment of the rods. So, would the best option (in theory) be one truss rod with two CF stiffening rods on each side of the rod? The neck for this bass will be fairly wide. I am building it for a customer, and his current 6 string bass has a neck that is as wide as a typical 5 string, leaving closer spacing since it has 6 strings, but he wants this one to have a bit wider spacing.
  6. Two rods give some you control over the twist. With only one you don't have any control over the twist, and if it twists you're screwed.
  7. JustForSport


    Nov 17, 2011
    If they need to be adjusted unevenly to keep the neck straight due to uneven string tension or internal wood stresses, adjust them accordingly. You can't do that with a single trussrod.
    If someone adjusts them unevenly and it induces twist, they're 'not doing it right'.
  8. Whatever Bruce says... is pretty much golden as far as I'm concerned....

    The only things I'd add are these;
    1. The highest tension strings on electric bass are the D and then the A in most normal sets. The tension on the other strings reduces evenly as you get to the edges of the neck. The order of tension goes like this; D, A, G, E, C, B. So in many ways it balances itself out in the centre of the neck.
    2. The thickness and density of the neck will make a huge difference. If you have a super thin mahogany neck, then yes 2 TRs would be good. For a chunky maple/bubinga style neck (like I tend to build) your TR will work a lot less hard. Scale also effects this, the longer the scale, the harder the TR will work too.

    If you're worried, you can always add a few steel/graphite rods. That will help too.

    P.s. there are ways of "untwisting" necks. Course we'd all prefer not to go there. :rollno:
  9. HaMMerHeD


    May 20, 2005
    I go with two rods on a 6-string neck.


    That neck is about 3.5" wide at the 24th fret. It's 5 piece of maple, set with opposing grain. It also has a pultruded carbon fiber rod in the center laminate, under the fretboard, which is a pretty thick bloodwood slab.
    tfernandez likes this.
  10. Dojix

    Dojix Guest

    May 24, 2014
    Brisbane, Australia
    Either two rods, or one rod with carbon fiber rods.

    This guy, who looks ridiculously professional with it does that.

    Though, a guy above has a point, might want different amounts of tension on different sides of the neck to balance the strings. I'm gonna guess that the guy in the video uses the single method for simplicity with adjusting.
    GMC likes this.
  11. Gilmourisgod


    Jun 23, 2014
    Cape Cod MA
    Is it possible to make a standard tuning 4 string neck TOO stiff with one double action truss rod and two carbon stiffeners? It has to be possible to introduce some minimal relief, it would be a disaster if the the neck could neither front or backbow, wouldn't it?
  12. Trussrods don't actually stiffen the neck, they stop "creep". Creep is a sagging effect that wood suffers over a long period when continuously loaded. It's why a big, long beam that looks more than strong enough to hold a building up bends over time. The metal in truss rod just supports the neck a little so the creep doesn't occur. It also allows you to adjust the amount of bend in the neck... or in some situations, force it to bend more.

    I guess too much carbon or metal stiffening could overdo though? Bruce? What say you?
  13. HaMMerHeD


    May 20, 2005
    I have made a couple of 4-string necks with two carbon rods and one double-action truss rod. Adjustment isn't as easy as it is without carbon fiber, but it's still quite adjustable.
  14. StarbardGuitar


    Feb 18, 2013
    I always use dual-action truss rods (or whatever you'd like to call them) so I can force bow into the neck if neccessary. So that isn't too much of a concern for me.
  15. GMC

    GMC Supporting Member

    Jan 1, 2006
    Wiltshire, UK
    Not really...but you might need to stand on the neck to get it to bow initially and then you use the truss rod to "set" that bow while standing on it. If you just turn the rod expecting it to be able to apply enough pressure to bow the neck where the neck has a pair of carbon rods is most likely going to cause the rod to fail. Dual action rods are weaker than the old aluminium box one way rods.
  16. catboyzee


    Jul 20, 2007
    Question: What if the bass neck had two truss rods and one stiffening rod mounted under the truss rod closest to the low B string? Has any builder tried that and if so, what was the result? Thanks.
  17. HaMMerHeD


    May 20, 2005
    Seems to me that a stiffening rod directly under the truss rod would force the neck to be a lot thicker than it needs to be, and players wouldn't like it. Even given a slim truss rod (3/8" deep) and a small stiffener (1/4" thick), you've got 5/8" of depth to deal with, if the truss rod directly touches the stiffener So the neck (not including fretboard and frets), at that point, would be a minimum of about 3/4" thick. Add the fret board and frets to that equation, and you're pushing a 1-1/8" thick neck in the center (between the A and D strings on a standard 6). And the further away from the center that structure is, the worse it will be for the player.
  18. Geoff St. Germaine

    Geoff St. Germaine Commercial User

    First off, I agree Hammerhed that this will likely cause you to have to build a neck with a thicker profile. Secondly, with a typical string set the B string is going to be at the lowest tension with the D string at the highest, so building a neck to be stiffer towards the bass side is going to be counter productive as far as neck relief goes.
  19. Brad Johnson

    Brad Johnson Inactive

    Mar 8, 2000
    Gaithersburg, Md
    DR Strings
    Kevin Brubaker has been building sixes with single trussrods for about as long as he's been building. But he has his rods machined locally and they are oversized compared to typical OTS rods. I have four sixes that I gig with on a regular basses, so far so good.
  20. tjclem

    tjclem Commercial User

    Jun 6, 2004
    Central Florida
    Owner and builder Clementbass
    I have always used one rod 4,5,6 and 7 string no problems as of yet. I didn't want to have to make the necks thicker and that would be required using 2.