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Question about truss rod design.

Discussion in 'Luthier's Corner' started by Human Bass, Sep 15, 2008.

  1. Human Bass

    Human Bass

    Aug 26, 2005
    I was reading the Warmoth page on truss rods


    There they mention the single vintage truss rod, the double truss rod and a 3rd design I've never seen and left me quite curious:

    "A third design now enjoying some popularity is a variation of the vintage single rod which is adjustable in two directions. While this design is touted as superior, it is our opinion that well designed necks built from carefully selected and cured woods exhibit none of the problems this design is proported to solve."

    So, do you guys have input on those?
  2. Human Bass

    Human Bass

    Aug 26, 2005
    Bump :(
  3. SDB Guitars

    SDB Guitars Commercial User

    Jul 2, 2007
    Coeur d'Alene, ID
    Shawn Ball - Owner, SDB Guitars
  4. dave251

    dave251 Wendler Instruments

    Feb 5, 2004
    Lawrence, KS
    There are definite advantages/flaws in each of the designs.

    The "traditional" rod is light, easy to make(re: cheap), and difficult to install correctly. Special fixturing is necessary and the need for absolutely accurate slotting of the neck material, and the correct machining of the wood filler on the top of the rod is imperitive. In otherwords, it's pretty easy to screw up the install unless you have very repeatable fixturing and machining operations. Once installed correctly it works very well, but is MUCH more susceptible to climatic seasonal changes than the double rod. The reason for this is the bearing loads at each end of the rod are born by the lumber....so humidity changes will have more effect.

    The double rod( whether it is single acting...bending the neck backwards...or double acting...allowing the introduction of forward bow) has it's engineering compromises also. The rod itself is more expensive to make, although MUCH easier to install. A good router table and fence is all that is necessary for accurate installation, and there is no need for the wood filler strip that is required for the single rod. The original Rickenbacker design uses this rod, and is the first example that I'd seen....from the sixties....

    The double rod is also HEAVIER, and can cause neck balance problems, especially on instruments with ultra light weight bodies, such as the ones I make from Western Red Cedar. This is easily overcome with a longer top horn though, and compact headstocks using lightweight tuning machines.

    The single biggest advantage, in my opinion, with the double rod is that it is much less susceptible to humidity influences. Since the end to end bearing loads just the rod itself, and does not compress the neck lumber along it's length. So the loads the rod makes are either above or below the rod....exactly where the stress needs to be.

    Another minor disadvantage of the double rod is that sometimes they can vibrate obnoxiously in the neck....but good accurate, simple fixturing, and building a slight underbow in the neck can correct this...as long as it's tight, it doesn't rattle.

    I use the double, double acting rod in my necks. I think the long term advantages(ie, STABILITY) outweigh the other factors.

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