Official Luthiers Forum!

Dear brains trust, some advice please. I am retopping a Martin OM style instrument (one of my earlier builds). 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?

Generally you want the legs of the X brace to cross under the ends of the bridge. This depends on the X angle and how far up or back it is located from the soundhole.

Based on what I've read and heard, I've always thought of the effect of the x-brace angle being a looser top with a wider angle and a tighter top with a narrower angle, all else equal. Some builders talk about using a wider angle to increase bass response and a tighter angle to reduce it. I haven't seen any discussion about how cross-grain stiffness would factor into it though. Interesting question.

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I think the way you think makes sense if what you are going for is your defined sound.The other option, which is what I tend to do is, keep all things bracing equal. That is the bracing is a sort of baseline. Now you are blessed with a super stiff top so you can make it thinner over the same bracing and get a more responsive guitar. It's even more complicated then that but since I started doing deflection testing this is the way I think now.

Thanks Clay and Jay. I agree with both comments. it certainly is a given that the bridge needs to overlap the arms of the X-brace but the narrower angle will increase the extent to which that happens. It should reduce the tendency of the bridge to forward rotate. As Jay says, I have heard that a wider angle (imagine a J-45 with a wide lower bout and a 100 degree angle) will have more of the lower part of the soundboard free. But there are many other variables, such as whether you tuck the braces into the linings or feather them out (I do the latter), and the thickness of the soundboard at the perimeter. At the other extreme the new, much hyped, Taylor V-bracing is very "up the middle".

JF, thanks for reminding me about deflection testing. This would be the logical way to address my question and fine tune the result. I have the Gore/Gilet books but I haven't used that technique yet. It might be time to learn. Trevor lives half an hour down the road from me.

"Thanks Clay and Jay. I agree with both comments. it certainly is a given that the bridge needs to overlap the arms of the X-brace but the narrower angle will increase the extent to which that happens."

Unless you "forward shift" your X brace - move it closer to the soundhole as I heard was done on some of the older Martin guitars.

I guess I tend to do things a bit backward from everybody else. My plates are all pretty much a standard thickness depending upon the size of the guitar and the species of wood, then I work the bracing to suit. Picked this up from Robert Bouchet. Tops and braces get weighted deflection and sorting. Needless to say, I do an immense amount of tap tuning.