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Double Bass back braces

#21
basswraith said:
I know a modern bass maker who puts a cross bar in a round back, under the post area. I wonder if the back will experience any of the complications discussed earlier....Im sure minimaly if at all as these basses are very well made. I have enjoyed each one I have tried The sound is mature , powerful and of high quality.
I wonder if the maker could throw in his 2 cents on this concept..cross bar in a round back.
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I think it's a good idea as long as the brace only extends part-way across the back. It allows the back to be carved thinner for more sound and easy response. To me nothing sounds worse than a bass with a really thick back. ;)
 
#26
Matthew Tucker said:
Is it the shrinkage of the cross-brace or the shrinkage of the back itself that causes things to come unstuck?

What would be the implications of a crossbrace made of a non-shrinking material?
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Robbie MacIntosh finally got me to understand this. It's the shrinkage of the back. It's normal to have some movement in the back across the way and sometimes alot[not seasoned properly or drastic changes in climate] The cross bars want to keep it from moving. What they actually do is keep the inner [glued] surface in line while the outer[varnished] surface contracts. Robbie explains this as the "bi-metal strip" phenomenon, a reference to our high school physics days.
 
#27
Jeff Bollbach said:
Robbie MacIntosh finally got me to understand this. It's the shrinkage of the back. It's normal to have some movement in the back across the way and sometimes alot[not seasoned properly or drastic changes in climate] The cross bars want to keep it from moving. What they actually do is keep the inner [glued] surface in line while the outer[varnished] surface contracts. Robbie explains this as the "bi-metal strip" phenomenon, a reference to our high school physics days.
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Yes, I can see this is why many Basses have partially opened back center seams in a wedge like a 'V'. The top of the 'V' being the outside varnished area and the inner part of the 'V' being the raw wood under the Crossbars. My Mystery Bass has an Ebony strip inlaid less than half way in the center of the Back. This was a repair to cover up and correct the outer Back Shrinkage. My Loveri has a Purfled strip down the center of the Back. This too was a repair as well. Seasoned wood is the best prevention I guess but the nature of the beast will prevail.
 
#28
Jeff Bollbach said:
The cross bars ... keep the inner [glued] surface in line while the outer[varnished] surface contracts. Robbie explains this as the "bi-metal strip" phenomenon, a reference to our high school physics days.
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So following this line of logic, a thinner flatback may suffer less differential "V" shrinkage than a thick one, and a flatback made of three or more pieces of wood may suffer less because the glue line "weak spot" is distributed across two or more points??

And perhaps slab-cut wood - quite often seen in old flatbacks, though not as pretty - may be less prone to cracking in that vertical grain direction?

Does your experience support this line of thought?
 
#29
Matthew Tucker said:
So following this line of logic, a thinner flatback may suffer less differential "V" shrinkage than a thick one, and a flatback made of three or more pieces of wood may suffer less because the glue line "weak spot" is distributed across two or more points??

And perhaps slab-cut wood - quite often seen in old flatbacks, though not as pretty - may be less prone to cracking in that vertical grain direction?

Does your experience support this line of thought?
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Innerstin' thoughts. I like your dubbage of this phenomenon as "v" shrinkage. Yes it would be less with a thinner back but then you are approaching another risk factor in the strength of the back. Two different glue lines just makes me think it'll be twice the problems. However I am intrigued by the slab cut idea, never thought of it in this regard. I'll have to ask Robbie.
 
#30
Jeff Bollbach said:
Innerstin' thoughts.
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I don't see why a glue line is necessarily a weak point, for arched backs have a single glue line and don't suffer the same issues.

I'm wondering whether the braces play any part in resisting cracking from shrinkage, maybe they are even the sole cause?

I figure the centre brace's main function is to support the pressure of the soundpost.

The other braces are there mainly to strengthen the flat plate and protect it from external knocks and physical movement that the curve of an arch-back would look after. The bend or curve at the break also stiffens this part of the back.

But in terms of "v" shrinkage, the braces seem counter productive.

The slight arching in the "flat" back by shaping the ends of the braces gives strength as well as looks. So I'm wondering whether building a very slight curve into the back of the ribs would be possible to do, giving the "flat" back a slight strengthening arch. You see it in basses that have no defined break in the upper bout sometimes. But you'd have to find a way to stop the back wanting to spring straight again! A monster bending iron, perhaps??? :hmm:
 
#34
Matthew Tucker said:
And perhaps slab-cut wood - quite often seen in old flatbacks, though not as pretty - may be less prone to cracking in that vertical grain direction?

Does your experience support this line of thought?
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No. Studies show that wood cut on the slab swells and shrinks about twice as much as quartered wood. This movement occurs mainly across the grain.
 
#35
Matthew Tucker said:
Bob's bass has a 7mm back and 4mm ribs . That's quite thick isn't it?

I was actually thinking of a way to curve the *whole* back very slightly, not just the break.

And who is Robbie Macintosh?
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Robbie is a great bass luthier in upstate NY. Before he got into basses Robbie was an expert cabinet maker and is very knowledgable about woodworking. He is also a charter member of B.L.A.B.L.A.
 
#36
Robbie is also the man that did the fantastic restoration on the JB Allen on display at Arnolds place. http://www.aesbass.com/bassgallery.htm

The job is so clean it's almost hard to believe the Bass is 165 years old. He is also the one that first noticed my Prescott was cut down. After Arnold started the restoration all kinds of new surprises popped up...

Oh, and by the way, he's also a real nice guy. I met him with Arnold as the VSA convention recently.
 
#37
Matthew Tucker said:
Bob's bass has a 7mm back and 4mm ribs . That's quite thick isn't it?

I was actually thinking of a way to curve the *whole* back very slightly, not just the break.

And who is Robbie Macintosh?
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If you want to curve the whole back you need to cut a curve into the ribs the way flat-top acoustic guitars have, which usually use a curved dish covered with sandpaper. The curve probably wouldn't need to be very much, so you may not need to steam bend the wood....however I haven't yet built a flat-back upright bass. This is just speculation.

4mm ribs does seem kind of thick to me, too.
 

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