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X-brace shifted too far forward?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=28633		 Page 1 of 1
Author:  JeremiahB. [ Mon Aug 16, 2010 8:49 am ]
Post subject:  X-brace shifted too far forward?
Just wanted to get some feedback too see if maybe I'm making a mistake before I close up the box. I'm building an OM using bracing dimensions from some vintage 30's Martin OM's. X-braces are 5/16" wide and 5/8" tall at intersection. The angle is 100 degrees and intersection is 7/8" from the soundhole. Do I have the x-braces too far forward shifted? I was intending to go for somewhere between 1.125" and an inch but when I clamped with the go-bars they shifted a little forward and I thought "what the heck - let's see what happens." Am I asking for trouble?
Author:  SteveSmith [ Mon Aug 16, 2010 8:57 am ]
Post subject:  Re: X-brace shifted too far forward?
Got a picture?
Author:  JeremiahB. [ Mon Aug 16, 2010 9:03 am ]
Post subject:  Re: X-brace shifted too far forward?
Sure - give me a second.
Author:  TonyKarol [ Mon Aug 16, 2010 9:23 am ]
Post subject:  Re: X-brace shifted too far forward?
See where the bridge will sit on the X ... you might need to amke a slightly longer bridge so that the wings hit the X enough ... shouldnt take much ....
Author:  JeremiahB. [ Mon Aug 16, 2010 9:26 am ]
Post subject:  Re: X-brace shifted too far forward?
OK - here are some pics. I didn't realize until now that I went so crazy with the scalloping. Does it look like it may be too weak structurally?
Author:  Fred Tellier [ Mon Aug 16, 2010 9:43 am ]
Post subject:  Re: X-brace shifted too far forward?
Like Tony said, check the bridge overlap on the lower legs of the brace, it looks pretty wide at the bridge location from the photo. I like your scallops though if you need to remove the braces if you decide against their location you may break them there. The wide angle and forward location will probably make the bridge plate quite large. I hope you used hide or fish glue if you decide to take the braces off it will be much easier.

Fred
Author:  JeremiahB. [ Mon Aug 16, 2010 1:28 pm ]
Post subject:  Re: X-brace shifted too far forward?
I've attached a pic that shows where the bridge would sit on the x-brace. Is the span too wide?
Author:  bluescreek [ Mon Aug 16, 2010 4:44 pm ]
Post subject:  Re: X-brace shifted too far forward?
I am sure someone here will not agree with this , but you have to look at the scalloping. You did take it down pretty far and there is a good change that the rotation force a the bridge may cause you issues in about a year or so. You will not loose much tonal quality by keeping the bracing in front of the pins unscalloped.
Think of the top forces as they apply to what your need to brace. Between the bridge and the neck block you care in compressive force. The bridge plate and bridge are in a rotational torque and at the neck block, that is getting rotated by the neck and the string tension. You have pulled the braces a bit too far front. I am sure it may sound killer but that top will belly.
Taking too much off the bracing is deadly to the top.
Author:  JeremiahB. [ Mon Aug 16, 2010 5:11 pm ]
Post subject:  Re: X-brace shifted too far forward?
So, you think I should chisel these braces off and start over. I'm really wasting some good red spruce brace stock. This is the second x-brace I've screwed up for this guitar. The other I cut too short. (measure twice, cut once - good advice) So, if I push the x-brace back about a .25" (which would be about 1.125 from the soundhole or 8.875" from the 14th fret) keep the 100 degree angle at the x and take it a little easier on the scalloping, I should still end up with a fairly freed up top, but one that's also structurally sound - correct?
Author:  Mark A Thorpe [ Mon Aug 16, 2010 6:01 pm ]
Post subject:  Re: X-brace shifted too far forward?
What is the height of the braces at the lowest point of where you scalloped?
Author:  bluescreek [ Mon Aug 16, 2010 6:09 pm ]
Post subject:  Re: X-brace shifted too far forward?
yes take the braces out and start over. That would be the best thing to do. Do you have a blue print ? Michael Payne has some decent plans. The Antes plans are just way over built. If you get the bracing wrong you can have problems . The key is to balance the top and braces to match what you want to accomplish.
A hard blue grasser loves the older D18's and 28's . If you are an attack player you need a stiffer top. A finger picker likes the top a bit less tight and you can scallop or use a thinner brace. Most of what I use is 9/16 high and bracing a bit under the 5/16 . Smaller bodies I will use 1/4 inch bracing. I do a scallop of about 1/3 and let 2/3rds of the material on the brace.
Think what the stresses are and how they apply to the top. Adjust your top braces accordingly. There is a science to this but there is also an art.
Good luck.
Author:  Howard Klepper [ Tue Aug 17, 2010 1:55 pm ]
Post subject:  Re: X-brace shifted too far forward?
Set the point where the arms of the X cross the bridge wings first. IMO the distance of the X intersection from the soundhole is just an artifact, not a design consideration.
Author:  Steve Davis [ Tue Aug 17, 2010 5:08 pm ]
Post subject:  Re: X-brace shifted too far forward?
Another possibility if you dont want to go backwards would be to consider a lattice in the lower bout
or an alternative to the the regular tone bars such as those discussed in this thread
viewtopic.php?f=10101&t=28402&p=379620&hilit=double+X+brace#p379620

I think some of the double x braces pictured there there are a bit heavy but that doesnt mean they cant work and Ive seen pictures of them in Somogyi s book. I just went for one X in the lower bout but it was small guitar and it isnt finished yet :roll: A bit of an unknown sound wise but stucturally it could solve your problems.
Author:  Darryl Young [ Tue Aug 17, 2010 7:29 pm ]
Post subject:  Re: X-brace shifted too far forward?
Going with Steve's line of thinking, in case things turn out too loose and there is too much bridge rotation.........you could make a PMTE brace that runs along the rear of the bridge plate, now, before you close the box and while you have good access. If you get too much belly after stringing up for a bit, you could unstring and glue the PMTE brace in place making sure to glue to the bridge plate and both X braces. This may be enough to rescue the to in it's current form..........but I surely don't have the experience to make this call for you. Just a thought.
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