bracing between tail block and neck block

[James Sanford]

I recently built a mandola and used a 7/16" dowell rod as a brace between the neck block and tail block. The rod was located approximately 1/4" below the top of the rib.

I am currently building an A style mandolin with cherry back and ribs and western cedar top and tone bar bracing. I plan to brace this mandolin in a similar manner as noted above. I am doing this to assist in reducing the effect of sting pull.

I do not believe that this small reduction in chamber volume will affect the tone of the mandolin but am curious about any unforeseen disadvantage to using the rod brace from tail to neck block.

Anyone with experience?

[Charles E.]

You may want to use carbon graphite rods, they are very light and stiff.

http://www.lmii.com/CartTwo/Secondproducthead.asp?CategoryName=Truss+Rods%2FNe ck+Parts

The smaller pultruded rod should be perfect.

[devilsbox]

How does the mandola sound?

[Graham McDonald]

If you have a local archery shop, they will have carbon fiber arrow shafts in a variety of sizes. I can't see any downside in using a brace between the neck and tail block, and the Larson brothers (I think) used the idea on some of their instruments a century ago. I made one experimental mandolin using two arrow shafts, one above the other, with the idea of using the upper one as a coordinator rod to adjust the neck angle. It didn't work very well. 8-( But adding that structural rigidity is a good thing.

back to the drawing board...

graham

[James Sanford]

How does the mandola sound?

The mandola sounds fine. I don't have much to compare it to but I do not notice any evidence of a degradation in tone.

[Big Joe]

Unless you have thinned your rims and tops and backs poorly you don't need the extra support. What would the purpose be?

[James Sanford]

Unless you have thinned your rims and tops and backs poorly you don't need the extra support. What would the purpose be?

I guess I am what you call a "belt and suspenders" type. It just seems to me that over time that the force from string pull coupled with environmental changes could be mitigated by the extra support.

I realize that some mandolins, particularly Gibson, have survived for over 100 years without this type of support but I am not a professional luthier and I don't have as much confidence as I should in my methods. I have not gained enough experience to evaluate a piece of wood and determine if it is stiff enough to graduate in accord with the information I have or should I leave it a bit thicker in the main stress areas.

Thanks for your question, it has helped me evaluate this more clearly.

Experience solves a lot of doubt.;)

[Lefty Luthier]

A bit of simple stress analysis indicates that with a nominal string tension of 155 pounds and assuming a rim to soundboard and backboard contact radius of 0.50 inches, the shearing force on the glue joints is less than 5 pounds per square inch. Any sort of stiffener would serve no useful purpose. Hoop deformation would be nil as well.

[Big Joe]

As long as the glue does not let go there should be no issue. Sometimes we see old instruments that the rim/top or rim/ back glue has let go part way and the owner does not get it repaired, or may not realize it is loose for some time. There can be some shrinkage of the top or back or if the top or back has been off for some time not only do you have shrinkage, but may have some rim deformation. However, the rods would not really help in those situations. If they are properly glued and kept in normal conditions they can easily last a very long time. Most deformation damage or shrinkage is a result of not getting the repairs done when they should be. There is a lot of strength in an instrument when it is glued together properly.

[Hans]

I've done it many times with carbon fibre on non-Loar instruments and 6 & 12 strings. What it does is relieve some of the compressive tension that the strings impose on the top. On guitars I also use a tension rod at the bottom to help keep the neck block from rotating.
Does it work? Ask Cody Shuler from Pine Mountain RR, or check out their new album.

[sebastiaan56]

Ive had the same thought for the same reasons James. I have heard of builders laying CF tape over the linings to increase rim stiffness. Or even CF tape as bindings.

[Charles E.]

On the front and back cover of the new GAL journal are photo's of a guitar by Gary Southwell, the back cover show's his use of CF rods. In the interview he talks about his experiance with them.

http://www.luth.org/

I think they would be useful on flat top-back mandolin family instruments.

[Tavy]

Can I throw another thought in here - I'm sure you guys are quite correct when you say there's no structural reason for braces like these - and there's a long history of perfectly sound historic instruments to support that. However, some electric guitar makers use a single full-length block (headstock to tailpiece) to increase rigidity and hense sustain. I realise we're not building electrics here, but I wonder if longitudinal brace would increase rigidity and hense sustain? I have no evidence either way, just thinking out loud here... might also be more appropriate for CBOM's than bluegrass mandolins.

Cheers, John.

[Lefty Luthier]

Can I throw another thought in here - I'm sure you guys are quite correct when you say there's no structural reason for braces like these - and there's a long history of perfectly sound historic instruments to support that. However, some electric guitar makers use a single full-length block (headstock to tailpiece) to increase rigidity and hense sustain. I realise we're not building electrics here, but I wonder if longitudinal brace would increase rigidity and hense sustain? I have no evidence either way, just thinking out loud here... might also be more appropriate for CBOM's than bluegrass mandolins.

Cheers, John.

A more rigid neck certainly increases sustain but I doubt it helps much on a soundboard. My greatest concern about stiffened soundboards is alteration of the resonant modes that make a mandolin what it is.

[James Sanford]

You may want to use carbon graphite rods, they are very light and stiff.

http://www.lmii.com/CartTwo/Secondproducthead.asp?CategoryName=Truss+Rods%2FNe ck+Parts

The smaller pultruded rod should be perfect.

Just a quick note regarding carbon graphite products. I found ACP Composites in California and ordered from them. Both tubes and rectangular CF for truss rod. www.acp-composites.com

Good prices. NFI

[Big Joe]

Unless the brace is attached to the soundboard it won't stiffen the soundboard itself. It may help sustain and it can alter the overall tone in different ways. How you will never know until you try. It could help reduce the chance of body flex under great duress, but I can't imagine it getting that much stress with the back and top on the instrument... again especially on an arched top/back instrument. Just my thoughts.