Reducing x-brace angle?

Talk about musical instrument construction, setup and repair.

Moderators: kiwigeo, Jeremy D

Post Reply
User avatar
Mark McLean
Blackwood
Posts: 660
Joined: Thu Apr 10, 2008 2:03 pm
Location: Sydney

Reducing x-brace angle?

Post by Mark McLean » Sun May 27, 2018 11:19 am

I am retopping a Martin OM style instrument. This is an Engelman spruce top and at 2.5mm it is still fairly stiff across the grain, compared with the last few spruce tops that I have worked with (which were sitka, lutz and adi). I am bracing it with sitka and I have just made up the main x-brace. My question is about varying the angle of the x-brace joint.

I have three different sets of plans modeled on Martin OM models and the angle of the joint varies between 90 degrees and 95 degrees. I am talking here about the angle between the lower arms of the x-braces (the area where the bridge plate sits). I have also looked at other makers. Larivee is more narrow - a touch under 90 degrees. Gibson x-braces are really wide, around 100 to 103 degrees. The OLF SJ plans (similar size across the lower bout to a OM) have an angle of 87 degrees. I made one of these with Engelman and was pretty pleased with it.

So I glued an X which is 5 degrees off a right angle. I could use it with a 95 degree spread across the lower bout, or turn it the other way and make it 85 degrees. Because this is a fairly stiff top across the grain I am more inclined to use the narrower angle - figuring that it doesn't need so much cross-grain reinforcement. Is this the right way to be thinking about this? What are the likely consequences of a narrower angle?

User avatar
kiwigeo
Admin
Posts: 9424
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Reducing x-brace angle?

Post by kiwigeo » Sun May 27, 2018 2:15 pm

I don't usually pay too much attention to the actual angle the braces intersect at....my main driver is making sure the braces pass under the end of the bridge wings.
Martin

blackalex1952
Blackwood
Posts: 552
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Reducing x-brace angle?

Post by blackalex1952 » Sun May 27, 2018 4:56 pm

If the brace angle varies it either closes or opens the area where the braces can sit on the tail end side of the soundhole. This has an effect on the area of the soundboard that is available for low frequency response. It also can allow for forward shifting of the X brace junction which also effects this area and can allow for the soundhole to move in position lengthways, either forward towards the neck or back towards the bridge. Because of other variables in the design due to brace stiffness, top thickness and the amount of soundboard arching, it is a very complicated thing which a single answer to a question like this is difficult because all these things by themselves effect the sound of an instrument individually but, like a resonant system with multiple resonances changing one effects the other. Trevor Gore, hopefully, will chime in here with his opinion-do you have the Gore/Gillet books? I recall hearing a talk with Gerard Gilet and I think I recall he mentioned that one of the properties of falcate bracing is that it allows for more flexibility in tuning the cross grain modes and long grain modes of a soundboard as the braces act more independently of one another than other bracing methods. But that was a passing comment in the discussion at the time. I would like to know more from Gerard and Trevor on this matter as well. Bob Benedetto has said that with the bracing on X braced archtop guitars, opening or closing the angle of the braces changes the brightness and projection of the guitar, but the plates need to be carved thicker down centre of the guitar than parallel braced archtops. Parallel bracing, he says, is more able to support string tension, and the plate graduation is thickness carved more evenly than X braced archtop soundboards. I haven't heard him talk about changing the amount of arching. He does mention in his book "Making an Archtop Guitar" that the braces, however they are done, need to be directly underneath the bridge height adjustment poles. Martin's statement re making the braces sit under the bridge is correct. It is important to understand how the bridge transfers string energy to the soundboard, and also the effect on admittance which to me effects sustain and this is also dependent on frequency as admittance is basically impedance, which is frequency dependent. When talking about admittance my understanding is that, basically, the ratio of string energy transmitted to the soundboard from the string to the amount of string energy reflected back into the string to keep it vibrating affects sustain. Selmer Maccaferris with high arching and ladder bracing have less sustain and a shorter faster note, giving projection and clarity for soloing. Acoustic guitars which have note clarity and brightness are best for lead playing and clear chordal voicing. Guitars where the notes tend to blend into each other are best for singer songwriter accompaniment. In pianos, there is a great difference between bass strings and treble strings frequency wise as the piano has the highest dynamic range and frequency range of any acoustic instrument. Along with the size of the piano which effects string length tension and the windings of the bass strings, there are two bridges to allow for the different impedences of these two groups of strings to be controlled and adjusted. There is a bit of info in Somogyi's books, which I have on temporary loan, worth a look but I think that the Gore/Gilet books are of more use and better bang for the buck. Somogyi's book is more qualitative than quantitative compared to the Gore/Gilet, which contains great info re technical understanding of guitar design parameters. Even though I have researched all this, I don't know all the answers! I hope this thread evokes a very informed discussion and becomes the hottest topic!!!! Looking forward to all the answers of our incredible online community members-Cheers, Ross
"Everything I say on the topic is based solely upon inexperience and assumption!"

blackalex1952
Blackwood
Posts: 552
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Reducing x-brace angle?

Post by blackalex1952 » Sun May 27, 2018 5:26 pm

I forgot to mention above that arching a soundboard effects sustain, top resonance and projection. Confused? So am I. That is why, as well as knowing a bit of "left brain" luthierie, experience is also important, and so is tradition. Look at violins...they evolved over generations on a largely experiential level, the learning and development passed on master craftsman to master craftsman. I jump at the chance to interact with a more experienced luthier in real time if I can afford it and they have the time to share. In that situation I will ask some questions...but with someone like that, verbose as I may seem here, I SHUT UP WATCH AND LISTEN! "But I learn, Mr Fawlty"-Ross
"Everything I say on the topic is based solely upon inexperience and assumption!"

User avatar
Mark McLean
Blackwood
Posts: 660
Joined: Thu Apr 10, 2008 2:03 pm
Location: Sydney

Re: Reducing x-brace angle?

Post by Mark McLean » Sun May 27, 2018 10:02 pm

Ross
I do have the G&G books and, like you, I am intrigued by the interface between the qualitative and quantative aspects of this interesting passtime that we have got ourselves involved with. I actually had a good chat with Gerard Gilet, while buying wood and gear from him one day a few years ago, about this very question. Gerard is often inclined towards chatting more than might be good for his business interests. But, as you say, there is a great quality in discussion between a master of the craft and a willing pupil, which hopefully gives them both some satisfaction. Apropos of nothing at the time, Gerard raised the notion of closing down the x-brace angle a bit to stiffen up the middle of the soundboard and open up the boundaries. It didn't have much application for me at the time but I was jotting down some notes as he talked. Now it has relevance - so I suppose I had better put it into effect.

If I was as quantitative as Trevor I would be working this out by use of deflection testing, or some such. Shamefully, I am not quite into that degree of data-driven practice. But I think it is good, at the very least, to try to apply well-informed instinct. This forum is invaluable to help develop that.

So, regarding my decision about which way to rotate my X-brace........ I might go with the narrow option and see how it works out. As you say, there are many other variables which will influence the final outcome. This is a guitar for my younger son. I am sure he will forgive me for any imperfections.

simso
Blackwood
Posts: 1626
Joined: Mon Apr 25, 2011 10:36 pm
Location: Perth WA

Re: Reducing x-brace angle?

Post by simso » Mon May 28, 2018 9:41 pm

Just to add more confusion if you will.

I get guitars in that are stuffy in sound and not vibrant, on these guitars I measure bridge rotation and find they are in the .2-.3 degree range from no strings to strung up. This is using my jig setup and digital angle display.

Guitars that are resonant / full sounding I find measure in the .5-.9 degree range.

Guitars that have top issues typically have over 1.5 degrees of rotation,

Now here is the spanner in the works, I have read and own trevs books and read his responses always with interest, Trev advocates around 2 degrees of rotation. So possibly Trev might come along and clarify how he measures his rotation.

To this end back on topic, with all things being equal, top thickness, side stiffness and so forth a guitar with a narrower x brace will have less bridge rotation than a guitar with the wider x brace, as the centre area of the guitar has added stiffness.. Something to think about.

Steve
Steve
Master of nothing,

Do your own repairs - http://www.mirwa.com.au/How_to_Series.html

simso
Blackwood
Posts: 1626
Joined: Mon Apr 25, 2011 10:36 pm
Location: Perth WA

Re: Reducing x-brace angle?

Post by simso » Tue May 29, 2018 1:22 pm

I did a tute this morning showing how I measure bridge rotation, may be of interest to this conversation

http://www.mirwa.com.au/HTS_Assess_Bridge_Rotation.html

Steve
Steve
Master of nothing,

Do your own repairs - http://www.mirwa.com.au/How_to_Series.html

User avatar
kiwigeo
Admin
Posts: 9424
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Reducing x-brace angle?

Post by kiwigeo » Tue May 29, 2018 1:40 pm

Love your KISS approach to jigs....I'd have never thought of using the Carbatec inclinometer I use to set uo my table saw blade angle to measure bridge rotation. Just brilliant. How long tills Stewmac get a hold of this one?? :mrgreen:
simso wrote:
Tue May 29, 2018 1:22 pm
I did a tute this morning showing how I measure bridge rotation, may be of interest to this conversation

http://www.mirwa.com.au/HTS_Assess_Bridge_Rotation.html

Steve
Martin

User avatar
Trevor Gore
Blackwood
Posts: 1352
Joined: Mon Jun 20, 2011 8:11 pm

Re: Reducing x-brace angle?

Post by Trevor Gore » Tue May 29, 2018 5:52 pm

simso wrote:
Mon May 28, 2018 9:41 pm
...So possibly Trev might come along and clarify how he measures his rotation...
Like all simple methods of measurement (i.e. can be done in the average workshop/does not need a metrology lab), they're never quite as simple as they might look. The bridge rotation measurement method I wrote up in the book has some limitations, but gives comparative results if done the way it’s illustrated in the book: chock the guitar, set up the measurement references, dump the string tension, measure the offsets, do the calc. and obtain the bridge rotation. The full details are in Design Section 4.4.2.

The limitation is that because the frame of reference for measurement is outside the guitar (frame of reference is the bench) as well as measuring bridge rotation, bending of the whole guitar neck and body as a beam, which appears as a rotation, especially when measured close to the ends, is measured as well. The measurement method is designed to cancel out any pure rise and fall of the soundboard, but not the rotation due to body bending effects.

On guitars like I build, with large neck blocks, plenty of bracing around the sound hole, stiff necks, etc., the bending of the whole instrument is quite small and the measured bending at the bridge location due to this source is less than ~0.2 degrees (I’ll explain how I measured that in a minute). However, many older guitar designs have wimpy neck blocks, no filler piece between the neck block and upper transverse brace, tiddly sound hole braces, no carbon fibre, etc. etc., and so bend a lot more. Possibly more than 0.5 degrees, but I’ve not done much testing on this.

Measuring the angle using a smartphone app by Blutacking the phone to the bridge works OK,

Image


as again the reference is external to the guitar and should give comparable results to the book method (and it does). Steve’s measurement system is similar to the smartphone method, so that should be more-or-less comparable too. However, Steve’s method differs from mine as i) the effect of the plastic frame will likely be to reduce the amount of measured rotation (ideally the frame should sit fully on the bridge itself) and ii) he measures rotation due to rising tension, I measure due to falling tension. The difference is that the soundboard carries on creeping under load for a while after tension is applied so the measurement depends somewhat on how long you wait. This effect seems to be less if rotation is measured by dumping the string tension. It's always important to compare apples to apples, but comparing oranges to oranges is fine, too.

To get a more accurate measurement of bridge rotation, you need more exotic gear, more time and a lot more care. I modified my new monopole mobility rig so it had two gauges and measured the front and back of the bridge relative to the rims, using the same basic equation as before. It always gives a smaller rotation than the book method, due to the whole instrument bending component not being included. If you want accuracy, that is the way to achieve it, but it takes more gear and quite a lot of time and care to set up.

Whilst the bridge rotation test is a simple rough and ready structural deflection test, at the end of the day, as guitar designers and builders, we are more interested in the responsiveness of the instrument. Measuring monopole mobility is a much better method for assessing that than the bridge rotation test.

simso
Blackwood
Posts: 1626
Joined: Mon Apr 25, 2011 10:36 pm
Location: Perth WA

Re: Reducing x-brace angle?

Post by simso » Tue May 29, 2018 6:05 pm

Thanks Trevor,

That makes sense, I am not actually measuring the rotation of the bridge but the area the bridge is attached to, had not thought of it like that.

Steve
Steve
Master of nothing,

Do your own repairs - http://www.mirwa.com.au/How_to_Series.html

User avatar
Mark McLean
Blackwood
Posts: 660
Joined: Thu Apr 10, 2008 2:03 pm
Location: Sydney

Re: Reducing x-brace angle?

Post by Mark McLean » Wed May 30, 2018 5:40 am

Great ideas for measuring this variable. I will definitely do this when the instrument is finished - to see if I stuffed it up! Might then need Steve's planned tutorial about how to increase the rotation (carve the bracing through the soundhole I am guessing?). Would deflection testing of the free plate also give me some indication of how it is performing?

simso
Blackwood
Posts: 1626
Joined: Mon Apr 25, 2011 10:36 pm
Location: Perth WA

Re: Reducing x-brace angle?

Post by simso » Wed May 30, 2018 11:48 pm

kiwigeo wrote:
Tue May 29, 2018 1:40 pm
Love your KISS approach to jigs....
Got to be simple otherwise I couldn’t do it :D
Mark McLean wrote:
Wed May 30, 2018 5:40 am
Might then need Steve's planned tutorial about how to increase the rotation (carve the bracing through the soundhole I am guessing?).
I love tweaking inner x braces to add more rotation, does not happen that often maybe once a year.

But

I always check a guitar that I have refitted a bridge too after it’s torn off, some tops are very very loose, contact the owner and usually I can get permission to line the x braces to add strength, rareoccassions I get to pull the back and replace the x braces

Steve
Steve
Master of nothing,

Do your own repairs - http://www.mirwa.com.au/How_to_Series.html

Post Reply

Who is online

Users browsing this forum: No registered users and 31 guests