String Tension Causing Bending

Paul DiTullio · Post by **Paul DiTullio** » 23 Jun 2014 8:04 am

Hello All,

i have a structure related question, and i am hoping some of you can offer some insight.

i have recently completed a lap steel build, and i am concerned about the amount of tension the strings are exerting on the guitar body.

i have it tuned to open E, and the total tension is about 160lb. when i rest the guitar on a flat surface, the back end (toward the tailpiece) is flush with the surface below it, but the headstock end is raised above the surface by just under 1/8 inch. this bowing is due to the string tension.

i have 3 questions:

1. is this bending hurting the guitar?
2. i would prefer to remain in open E, but should i tune to open D to relieve some tension (i know i can capo, but id prefer not to)
3. if i keep it on open E, will the body continue to bend upward more and more over time, or has (will) the wood reached an equilibrium of sorts?

thank you for your time

Bill Moore (RIP) · Post by **Bill Moore (RIP)** » 23 Jun 2014 8:58 am

It sounds like the body of the guitar is just not stiff enough. It could be because it's too thin or the type of wood it's made from. You could tune down a step, it won't matter unless you want to use a lot of open strings, hammer ons, etc. or go to lighter strings. I once had a lapsteel with a noticeable upward curve to the body, it didn't matter much as far as playability goes.

Alan Brookes · Post by **Alan Brookes** » 23 Jun 2014 9:32 am

You need to show us a picture of the guitar before we can assess the problem. If it's any consolation, the first lap steel that I built back in 1963 bent considerably under the tension of the strings, but it never harmed the tone, and that lap steel was in use for at least fifteen years. In fact, I believe that the way that the body flexed actually improved the tone.

Bill Hatcher · Post by **Bill Hatcher** » 23 Jun 2014 9:32 am

need to see a pic and also know what materials you used.

if you can release the tension on the strings and the gap goes away.....you need to build another one out of maple.

also another easy trick to see how much strength your guitar has is this...lay the guitar on a flat surface and connect it to a tuner. play a note and look at the tuner while you lift the guitar up by the very end of the headstock and watch the tuner. if you see the pitch go flat, you know you have flex in the headstock/body.

after you do this, then lay the guitar back down flat and hit a string and start to detune some other strings. that will give you a good idea of how much flex you have.

also, you can hit a string and push down on the end of the headstock and see how much the string goes sharp. that will give you a measure of the strength of the design of the guitar.

i would not worry about just a little deflection either way, but if you see a lot...well that translates into loss of sustain for sure.

Alan Brookes · Post by **Alan Brookes** » 23 Jun 2014 10:08 am

One thing to remember about string tension is that the crucial measurements, assuming that the body can take the push of the strings, is the angle at which the strings pass over the bridge and nut, and how the strings are anchored at the bridge end. It's what is known in physics as the "triangle of forces". For instance, if the strings pass over the bridge at a right angle and pass through the body, anchoring at the back, then the forces will be at an angle of 45 degrees. If, however, the strings pass over the bridge and only lightly touch them, with the strings anchored to the end of the instument, then the forces on the bridge will be nearer 180 degrees, and the weight on the bridge hardly anything. That's the main difference between a regular jumbo guitar with a bridge glued to the table and one with a tailpiece.
I say the "push" of the strings rather than the "pull" because the tension of the strings is trying to compress the instrument, not tear it apart.

Paul DiTullio · Post by **Paul DiTullio** » 23 Jun 2014 12:32 pm

Thank you for your responses. i feel like i should clarify a bit.... the bending mainly occurs at the headstock. as you can see in the pictures, there is a large pocket in the headstock, making it a bit weaker. i am mainly concerned with the integrity of the wood here. i would hate for the tension to cause any damage in this area.

now, a little background on my build:

It is all my original design, and i constructed everything except the electronics, screws, strings, and tuning pegs (i did make the tuning peg knobs though). i made the body from a piece of curly maple, and all the other wooden parts i made from Burmese rosewood. i made the nut, bridge, ferrules, and inlay dots from brass stock. i hope you like the build... and that you can see why im so concerned. thank you again

Karl Fehrenbach · Post by **Karl Fehrenbach** » 23 Jun 2014 12:39 pm

Simply stunning!

Bill Hatcher · Post by **Bill Hatcher** » 23 Jun 2014 1:05 pm

beautiful!

you have some nice woods there that are plenty strong. have you taken the tension off the instrument and the laid a straight edge the entire length on the back to see if the guitar is flat?

Paul DiTullio · Post by **Paul DiTullio** » 23 Jun 2014 1:33 pm

thank you!

yes, i did that today, actually. i also used a mirror as the surface to ensure the measurements are accurate. the total deflection is between 1/16" and 3/32"

Bill Hatcher · Post by **Bill Hatcher** » 23 Jun 2014 2:42 pm

ok thats not bad. when you string it up to tension, how much to you have?

just noticed....you are left handed

Paul DiTullio · Post by **Paul DiTullio** » 23 Jun 2014 3:07 pm

Good eye!

With no strings, the bow is about 5/64. " the deflection from string tension is about 3/32."
So the total displacement under tension is about 11/64 "

That is under a ~160 lb load.

Alan Brookes · Post by **Alan Brookes** » 23 Jun 2014 3:27 pm

That's a nice-looking lap steel. Is the headstock just glued to the backing sheet or have you put screws through from the back?
I almost always reinforce my headstocks with screws, either in from the back or from the front, countersunk, and then filled and smoothed over, or covered with some sort of laminate. If you're going to rely entirely on glue then what sort of glue did you use and how long did you let it dry?
Most luthiers use hide glue but it takes about two weeks to cure or it will creep under pressure.
Also, what sort of woods did you use?

Paul DiTullio · Post by **Paul DiTullio** » 23 Jun 2014 3:31 pm

Thanks Alan,

The curly maple body is all one piece, so the headstock is not glued to the body, but it is one continuous piece. The rosewood inlayed in the pocket is 1/8" thick. Does that clarify things? Im not sure i answered what you were asking for

Bill Hatcher · Post by **Bill Hatcher** » 23 Jun 2014 3:33 pm

it would be nice not to have the deflection at all when not under tension, but...thats the way building goes sometimes.

did you use titebond glue to glue up the neck? if so, it imparts water into the joint and sometimes will cause a neck structure that is pieced together to warp in either direction either forward bow..like yours or back bow. did you check that after you glued it all up and left it for a while?

i wouldnt worry about what you have there. its not enough to bother anything. if you can start shooting arrows with it....thats when to worry...

Paul DiTullio · Post by **Paul DiTullio** » 23 Jun 2014 3:39 pm

Hey Bill,

I had sort of addressed your question in my response to Alan.

All of the curly maple body is one piece. The only locations where adhesives (West System G Flex epoxy due to the oily nature of rosewoods, as well as my paranoia) were used were at the rosewood/maple interfaces.

Alan Brookes · Post by **Alan Brookes** » 23 Jun 2014 3:56 pm

Would you post a photo of the back, please.

Paul DiTullio · Post by **Paul DiTullio** » 23 Jun 2014 4:03 pm

Sure thing

Rick Barnhart · Post by **Rick Barnhart** » 23 Jun 2014 4:22 pm

Beautiful work, Paul. It reminds me of one of my favorite builders....the forums own Tom Pettingill.

Bill Creller · Post by **Bill Creller** » 23 Jun 2014 6:52 pm

That is a super nice looking instrument !

Hope you can resolve any problem with it.....

Bill Hatcher · Post by **Bill Hatcher** » 23 Jun 2014 6:58 pm

Paul DiTullio wrote:Hey Bill,

I had sort of addressed your question in my response to Alan.

All of the curly maple body is one piece. The only locations where adhesives (West System G Flex epoxy due to the oily nature of rosewoods, as well as my paranoia) were used were at the rosewood/maple interfaces.

sorry i missed that. man with one piece..i vote for the wood just moving on its own. if you jointed it all flat when you made it and no water based glue...it cant be anything else. i just dont see 6 strings exerting that much pull on that thickness of maple. just my opinion....but it sure is nice looking!!!

Chris Templeton · Post by **Chris Templeton** » 23 Jun 2014 7:11 pm

Very beautiful. I don't think it will bow over time, given the track record of the many lap steels built from wood.

Tom Pettingill · Post by **Tom Pettingill** » 23 Jun 2014 9:26 pm

Beautiful work Paul! You have some serious time invested in that beauty!

How thick is the maple through the neck / headstock area? How thick is the maple under the rosewood on the headstock?

Paul DiTullio · Post by **Paul DiTullio** » 24 Jun 2014 5:04 am

Thanks a lot, Tom and Chris!

Bill,
I interpreted "…but it sure is nice looking!!!" as ‘youre out of luck regarding playability.’
The guitar still plays like a beaut and holds tune. My concern is that over time, the high stresses could cause damage to the headstock. im not sure If I read too far into your comment.

Tom,
The maple, at its highest point, is 1.6” through the neck and headstock. Under the rosewood, the maple is 0.6” thick, which I believe is the reason for the deflection.

David Mason · Post by **David Mason** » 24 Jun 2014 9:00 am

That's not a whole lot of wood, but not weak either and those tuners are pretty low - there's not much more pull they can put on it. I'd play it for a while and see what happens. If worst comes to worst (or it bothers you, which can often be the same thing

) you could pretty much lock 'er down with a couple of full length 1" square stringers, screwed and glued and the heads covered with... doodads. As nice as you work & if you can find some more of THAT rosewood it might just keep getting prettier anyway!

(Or just wait a few years, is there anyone who hasn't "gone eight" eventually?)

Eugene Cole · Post by **Eugene Cole** » 24 Jun 2014 9:53 am

Paul DiTullio wrote:Hello All... ,

i have recently completed a lap steel build, and i am concerned about the amount of tension the strings are exerting on the guitar body.

i have 3 questions:

1. is this bending hurting the guitar?

2. i would prefer to remain in open E, but should i tune to open D to relieve some tension (i know i can capo, but id prefer not to)

3. if i keep it on open E, will the body continue to bend upward more and more over time, or has (will) the wood reached an equilibrium of sorts?

thank you for your time

Before I get to the questions: I thank you for the guitar-erotica. Your guitar is a stunningly beautiful site to my eyes.

1. The static load if it is consistent can induce a warp which is not really hurting the guitar but is unnerving. The maple should be plenty strong and plenty robust in this configuration. I would not worry about it unless something substantial changes. I urge you to keep an eye out for delamination and glue-joints failures. These failures are problematic on any instrument and require immediate attention. Alan Brookes already addressed the "triangle of forces" so I will skip over that.

2. Your tuning should correlate to the string gauges you are using so the option of retuning could also be addressed by a change in string gauges. Since you are speaking in terms of total tension I think that you already know this (however a good argument is rooted in stating the premises which the argument is based upon). This guitar appears to have a short-ish scale length so string tensions at a given tuning would be lower than they would be on a longer scale length.

3. I would expect whatever warping which the wood will do to be done within 6 months of when you put strings on it and started playing it.