sizing carbon fiber neck reinforcements

[Jon Whitney]

I'm not into adjustable truss rods - I don't like their disadvantages, and I think a neck should be rigid enough and stable enough that adjustment should never be needed. But, I don't want to get into an argument about it either, so please don't send me in that direction.

I've built a number of banjos using a single 1/2 x 1/8" steel bar let and epoxied into the neck as neck reinforcement. I've also built about three instruments using .070 x .437" carbon fiber strips in a similar manner (this is the largest available at my local hobby shop). To tell the truth, I haven't had any problems with any of these necks, but they've mostly been kept in my home in Utah, where the humidity is always fairly low, and I haven't heard from the owners of the few that have been sent away.

Now, however, I'm trying to upgrade my designs and building to near-professional quality or better. So I'm wondering if I have reinforced the necks adequately. Should I go with a larger CF strip such as 1/8 x 3/8" or 1/8 x 1/2"? Should I go with two strips per neck? The next two I build are going to be maple standard length 5-string banjo necks which I would like strong enough for steel strings, even though I'll probably string them up with Nylgut for tone reasons.

[Christ Kacoyannakis]

I don't make banjos, but Greek bouzoukis, which have long thin necks, and 8 steel strings. I make up the neck with maple, and laminate a strip of ebony in the center (either one 1/4 inch wide strip or two 1/8 inch wide strips). Then I inlay 2 1/8 inch wide by 3/8 inch thick carbon fiber bars on either side of the ebony. I don't have any problems with my necks. Now, I have seen one bouzouki maker who puts in three of these same sized carbon fiber bars in one channel at the center of the neck. He doesn't glue them in. He pretensions the neck by machining a slight parabolic curve in the top of the neck, and then glues the fretboard on, using a perfectly flat caul. The parabolic curve of the top of the neck pulls against the tension of the strings. Then he places the whole neck/fingerboard assembly in a vacuum chamber and inserts tubes from a container of epoxy to the chamber containing the carbon fiber bars, so that the area around the carbon fiber bars is completely filled with epoxy. He guarantees these necks for life (including to subsequent owners of his instruments). Here is a link to his YouTube video. It is all in Greek, but you can get an idea for what he is doing. There are also English subtitles. His name is Christos Spourdalakis, and is a very fine instrument maker. http://www.youtube.com/watch?v=ask-6esJboM

[Barry Daniels]

I use two bars in guitar necks that are .090" thick and 1/4" high. These add considerable strength to the neck even though they are much smaller than most carbon reinforcements. I think having two bars would be better for you than going to a larger single bar because you get a lot of sideways and twisting resistance from this configuration.

[Mark Swanson]

I agree with that- if you want your trussrod to work, you do not need as much stiffness as many folks might think.

[Jon Whitney]

Thanks for your help. I've ordered some CF bars and I'll see what I can make work. I'm leaning towards a T-bar configuration made up of a 1/2 x 1/8 vertical and two smaller horizontal pieces at the top.

Does anyone have a good suggestion on cleaning or roughening the surfaces of the bars to ensure good adhesion with epoxy? The last one I tried, I epoxied on some veneer on both sides to shim it to the thickness of my saw kerf, and the veneer tended to peel off a little.

[Bob Menzel]

Clean the CF with Naptha and use a Polyurethane adhesive.

[Craig Bumgarner]

I use two 1/8" x 3/8" CF bars on steel string guitar necks. 1/8" table saw blade cuts the slot, epoxy to glue in. No other truss rod. 30 minute job start to finish. Stiff enough that I need to build a little relief into the fretboard as the strings can't pull it all in. Necks are very stable over a variety of climates, seasons, years. I don't use ebony fingerboards as I think they are a major contributor to instability.

[Michael Lewis]

I have found that using .020" or .030" X 3/16" strips of CF in the neck from nut to neck joint help stabilize it. I put one on each side under the fingerboard just in from the edge so it won't show, and a wider one in a trough near the of the back of the center of the neck, and covered with a filler stick. This triangulates the strips in the cross section of the neck. Since the CF is strongest in tension this configuration takes advantage of that strength.

I scuff sand with 180 or so, enough to get rid of any shiny surfaces, before installation. I use well mixed good quality epoxy (West Systems) for installation.

Use of a truss rod is not excluded with this set up as a rod will flex the neck a bit, though it will require more adjustment to make it move. For nylon strings you probably won't need a rod.

[Mark Swanson]

In the two mandolins that I posted a couple of weeks back, I used 1/4" square CF tubing. One piece down the center of the neck is all I used, no trussrods. They seem to be very stable so far. A mandolin neck is shorter anyway, I don't expect having any issues.