Truss rod and Spanish foot/neck-top join

Construction and repair of Classical Guitar and related instruments
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Andy Culpepper
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Truss rod and Spanish foot/neck-top join

Post by Andy Culpepper » Sun Nov 05, 2017 10:57 pm

Hi all,
I've been asked to build a classical guitar with a truss rod. I only build with a solera and Spanish foot.
Preferably the truss rod adjustment nut should be at the sound hole so I will have to drill through the two harmonic bars above the sound hole.
I have the low-profile 2-way truss rod from Stewmac, the longer one.
My question is, if anyone has experience with how to place the truss rod in the neck, and then join the neck to the top.
The flat face of the truss rod will not be able to contact the bottom of the fingerboard directly, it will have to be deeper in the neck so that the ~2.5 mm can be shaved off the top of the foot for the soundboard to be glued on. Is it OK to make a perfectly fitting cap over the rod and then glue the fingerboard on top of that?

Any advice would be appreciated.

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Trevor Gore
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Re: Truss rod and Spanish foot/neck-top join

Post by Trevor Gore » Mon Nov 06, 2017 12:10 am

A more usual approach on steel string guitars is to slot the soundboard, with the top of the truss rod up against the bottom of the fret board for its full length.

However, what you are proposing can work, but it puts the truss rod 2-3mm deeper in the neck if you use a even depth slot. This has potential problems. The first is that at the nut end of the neck you need enough coverage over the truss rod on the back of the neck so that under full load you don't blow out the back of the neck. The number I use is 4mm coverage minimum. This should also be enough to avoid any accidental carve-throughs when shaping the neck, which would be a total disaster on a solera/spanish foot build. The second issue is the access hole(s) through any upper transverse bracing. The deeper the truss rod in the neck, the closer the access hole comes to the open edge of the upper transverse bracing. You don't want that hole to turn into a notch.

A bit of measuring might be in order.
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Andy Culpepper
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Re: Truss rod and Spanish foot/neck-top join

Post by Andy Culpepper » Mon Nov 06, 2017 3:50 pm

Trevor, thank you for the reply. I did some careful measuring and it looks like everything will work out just right with 4-5 mm of neck wood behind the rod at the thinnest point. The harmonic bars on my design are pretty high at 16mm so it looks like no problems there.

SteveL123
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Re: Truss rod and Spanish foot/neck-top join

Post by SteveL123 » Mon Nov 06, 2017 4:24 pm

Speaking of truss rod, I have a 1977 Takemine C132S which has a truss rod but there is no way to adjust it since the harmonic bar has no hole in it to allow a socket / extension/ ratchet to pass through. I know there is a truss rod since a magnet sticks to the entire length of the neck center and I also see a hole in the heel block. Did Takemine forget to drill a truss rod access hole in the harmonic bar or was it standard practice to leave the hole out and drill one when a neck adjustment is needed?

Marshall Dixon
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Re: Truss rod and Spanish foot/neck-top join

Post by Marshall Dixon » Mon Nov 06, 2017 4:33 pm

Andy Culpepper wrote:
Sun Nov 05, 2017 10:57 pm

Is it OK to make a perfectly fitting cap over the rod and then glue the fingerboard on top of that?

Any advice would be appreciated.

This is the way I learned to do it on steel string guitars. The one time I didn't use a filler strip the rod rattled at a resonant frequency and had to be repaired, even though I gobbed some silicone caulking in the channel as recommended. Guess I didn't use enough.

I've used the double action truss rods from LMI which have a "step off" at the adjustment end of the rod. Seems like that would give better clearance for the soundboard and that a shorter rod could be made to end where the neck meets body. The other end of the rod doesn't need to extend all the way to the nut and you have some leeway to adjust the fit at the body.

A tapered slot to minimize the chances of "blow out" at the nut end would be easy enough to work out. I've never done that and never had a problem (yet).

The transverse braces can be made a little higher in the middle to acommodate the adjustment hole if needed.

Jon Gillard
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Re: Truss rod and Spanish foot/neck-top join

Post by Jon Gillard » Tue Nov 14, 2017 7:35 pm

I think it’s a good idea not to install directly beneath the fingerboard, so that when you fit the filler strip you can push the rod down in the middle, which may reduce the chance of rattle and pushing the fingerboard off when under tension. I allow a 2mm recess on steel string necks, I think this is explained in one of Robert O’Brien’s luthier tips videos.
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Jacek A. Rochacki
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Re: Truss rod and Spanish foot/neck-top join

Post by Jacek A. Rochacki » Tue Nov 14, 2017 10:06 pm

Some guitars by Kenny Hill are fitted with truss rod; for a couple of years I was playing Kenny Hill model Madrid and truss rod was there. Here is what Kenny says on the truss rod in his guitars (text and illustrations)
http://hillguitar.com/website/news/tech ... s_rod.html
and I thought that it may be of help.
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Andy Culpepper
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Re: Truss rod and Spanish foot/neck-top join

Post by Andy Culpepper » Tue Nov 14, 2017 11:27 pm

Thanks all for the tips. So I ended up having to modify (shorten) the rod a little to fit and extend from just past the nut to the second harmonic bar, and re-welded the end. Now it's securely installed with plenty of silicone caulk to prevent rattling in the neck. The top (flat) side runs right under the top so about 2.5 mm deep in the neck. I'll get a shot of the neck a bit later. Never done this before but it seems like it should work...

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Chris Sobel
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Re: Truss rod and Spanish foot/neck-top join

Post by Chris Sobel » Wed Nov 15, 2017 3:52 am

Looks nice Andy!

I really like truss rods... a lightweight and low profile rod doesn't add much weight to the guitar overall and the advantages for dialing in the relief and long term stability are a huge plus. I use one plus two titanium or CF outriggers on my Steel Strings but haven't put them in any classical's yet.

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Dave M
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Re: Truss rod and Spanish foot/neck-top join

Post by Dave M » Thu Nov 16, 2017 9:24 pm

Andy I may be misinterpreting here but the truss rod adjusting nut does not need to be near the soundhole. A reasonably long allen key can be used to reach the truss rod adjustment nut that is only a bit further forward than the neck body join...?

I too always put a truss rod in classical builds, though I don't use the Spanish neck body joint. (I do tend to follow Trevor Gore's design ideas.)
Dave

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Chris Sobel
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Re: Truss rod and Spanish foot/neck-top join

Post by Chris Sobel » Thu Nov 16, 2017 10:15 pm

Dave M wrote:
Thu Nov 16, 2017 9:24 pm
Andy I may be misinterpreting here but the truss rod adjusting nut does not need to be near the soundhole. A reasonably long allen key can be used to reach the truss rod adjustment nut that is only a bit further forward than the neck body join...?

I too always put a truss rod in classical builds, though I don't use the Spanish neck body joint. (I do tend to follow Trevor Gore's design ideas.)
You’re right it does not... more commonly the adjusting nut is left at the end of the neck either at the nut or the neck body joint. I think this way should work—I’ve seen it in at least one factory Takamine.

Chris
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Andy Culpepper
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Re: Truss rod and Spanish foot/neck-top join

Post by Andy Culpepper » Fri Nov 17, 2017 12:33 am

Hmm yeah I probably could have put the end at the first harmonic bar. I don't know about all the way recessed into the heel block because it seems like the harmonic bar would be blocking it there. It's pretty close to the foot. Might have worked though. Maybe next time. I did want to make it pretty user-friendly and accessible although I have a feeling he'll probably never need to adjust it. Most of the pictures I found online had it this way, I think because you can adjust it under string tension.
We did not want the adjustment at the nut end, that would have been much easier :D

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Chris Sobel
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Re: Truss rod and Spanish foot/neck-top join

Post by Chris Sobel » Fri Nov 17, 2017 8:59 pm

Andy Culpepper wrote:
Fri Nov 17, 2017 12:33 am
Hmm yeah I probably could have put the end at the first harmonic bar. I don't know about all the way recessed into the heel block because it seems like the harmonic bar would be blocking it there. It's pretty close to the foot. Might have worked though. Maybe next time. I did want to make it pretty user-friendly and accessible although I have a feeling he'll probably never need to adjust it. Most of the pictures I found online had it this way, I think because you can adjust it under string tension.
We did not want the adjustment at the nut end, that would have been much easier :D
You drill small holes through the harmonic bars to give the Allen key a path to reach the truss rod adjusting but... pretty much what you have done but the rod doesn’t come out as much. This way works well especially if you are constructing the guitar as an integral unit and not with a neck joint of some sort. I do like the easy access..
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SteveL123
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Re: Truss rod and Spanish foot/neck-top join

Post by SteveL123 » Sun Nov 19, 2017 12:11 am

Easy access is nice, but is it necessary? How often will access be needed? That extension looks to be about 2 to 3" long. Is the end of the extension a snug fit in the harmonic bar or "hanging"? What is its natural frequency and will it make undesired vibrations?

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