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Andy Birko
PostPosted: Sun Mar 06, 2011 3:10 pm 
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Laurent Brondel wrote:
you all tune your plates, right?


I was actually going to write that in my post but decided not to at the last moment. Here's the thing though - if you pick a tighter rad to increase strength, and then shave the braces down to get you where you were before pitch wise, you've gained nothing in terms of stiffness. You may have gained more resistance from cracking due to low humidity though.

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Last edited by Andy Birko on Sun Mar 06, 2011 3:10 pm, edited 1 time in total.

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Ian Cunningham
PostPosted: Sun Mar 06, 2011 3:10 pm 
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Laurent what are your GO sticks made of? Any specs to them at all?
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Laurent Brondel
PostPosted: Sun Mar 06, 2011 3:27 pm 
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Andy Birko wrote:
Here's the thing though - if you pick a tighter rad to increase strength, and then shave the braces down to get you where you were before pitch wise, you've gained nothing in terms of stiffness.
You wrongly assume that the radius, or arch, is for strength. It is not necessarily. Stiffer, as well as lighter, does not equal better all the time.

Ian Cunningham wrote:
Laurent what are your GO sticks made of? Any specs to them at all?
Ash, from the tree. 3/4" x 7/32".

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Mark Groza
PostPosted: Sun Mar 06, 2011 3:47 pm 
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Laurent Brondel wrote:
Haans wrote:
I had not considered a cylindrical radius on the back (my tops are 15' cylinder), but that may be an interesting experiment. However, I wouldn't expect much difference as I mostly operate on an intuitive level and stop working a back when it sounds good and then work the whole box when it's together.
I use a mostly cylindrical back plate and add a tiny longitudinal arch to prevent the back from caving in when it's too dry. I agree that it does not make much difference. There is a lot of over-thinking about back or top radius: no matter what the radius is, you all tune your plates, right? So any difference a tighter radius will make is compensated (intuitively I should add) by either thinning the plate further and/or bracing.

Mark Groza wrote:
Laurent Brondel wrote:
However, for maximum strength, the back should be arched laterally and left flat longitudinally, like the section of a tube.
So a regular radius dish shouldn't be used when building a longitudinally flat back? What do you use?
I don't use dishes, but a long board with a 5' (or so) lateral arch.
I didn't think so.
I haven't used them for the back either.
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Ti-Roux
PostPosted: Sun Mar 06, 2011 4:08 pm 
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I did my back radius sanding block with 3 pieces of 1'' MDF, carved a 16'' radius on the top of them, then screwed down a 3/16'' plexi on it. Double-side tape and a good 100 grit sandpaper. That simple. The 3/16 plexi gives the perfect rigidity for the job.

And by the way, to get an accurate radius, I took a 16'' chain, attached a pencil at the end, then went up a stairwell to draw the arch on a masonite. (i'm not joking [:Y:] )

I have photo of all that if somebody want.

Francis

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Andy Birko
PostPosted: Sun Mar 06, 2011 4:09 pm 
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Laurent Brondel wrote:
You wrongly assume that the radius, or arch, is for strength. It is not necessarily. Stiffer, as well as lighter, does not equal better all the time.


I assumed nothing - I was just stating a fact for people to think about. In a prior post though, you did write this:

Laurent Brondel wrote:
Because it is under tension it is theoretically possible to use a thinner back for equal strength (to a flat one).

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Laurent Brondel
PostPosted: Sun Mar 06, 2011 4:28 pm 
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Seriously Andy, note the "theoretically".
Secondly, even if a shallow radius adds marginal strength to a plate, it is not its primary purpose. Its primary purpose is to allow the back to become flatter when exposed to lower RHs, instead of caving in and crack.

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Andy Birko
PostPosted: Sun Mar 06, 2011 4:40 pm 
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Laurent, I'm not disagreeing with you! :P

You're not the only one posting on this thread though - My first response was to John who stated:

bluescreek wrote:
Not one ever said the back radius was anything more than a strength provider.

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bluescreek
PostPosted: Sun Mar 06, 2011 7:44 pm 
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The radius back will have more strength ( stiffness ) than a flat back. That is a fact. It also does help with RH changes and allows cross grain movement to a degree.

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Haans
PostPosted: Sun Mar 06, 2011 7:50 pm 
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The easiest way to make a kinda cylindrical (I forget the name...Howard?) sander for ribs and back or top is to get some 1/4" ply and nail it down in the middle. Jack up the ends with a couple of sticks and you have it.
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alan stassforth
PostPosted: Sun Mar 06, 2011 8:29 pm 
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You can stab a potato with a freakin drinking straw, and it will penetrate.
If you flattened out the straw,
it would bend.
So, cylindrical is stronger, end to end.
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Andy Birko
PostPosted: Sun Mar 06, 2011 10:13 pm 
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bluescreek wrote:
The radius back will have more strength ( stiffness ) than a flat back. That is a fact. It also does help with RH changes and allows cross grain movement to a degree.


That's only true (the first part) when all other things are equal. If you shave down the braces because you put a tighter radius in the back to bring the tap tone back down...well then you're not that much stronger now are you?

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Laurent Brondel
PostPosted: Mon Mar 07, 2011 10:00 am 
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Haans wrote:
The easiest way to make a kinda cylindrical (I forget the name...Howard?) sander for ribs and back or top is to get some 1/4" ply and nail it down in the middle. Jack up the ends with a couple of sticks and you have it.
That's the way I do it, except I use hardboard. That would result in a spline curve, and not a perfect radius (to answer for Howard).
Andy Birko wrote:
You're not the only one posting on this thread though
I responded because you quoted me twice in your post.
Andy Birko wrote:
bluescreek wrote:
The radius back will have more strength ( stiffness ) than a flat back. That is a fact. It also does help with RH changes and allows cross grain movement to a degree.
That's only true (the first part) when all other things are equal. If you shave down the braces because you put a tighter radius in the back to bring the tap tone back down...well then you're not that much stronger now are you?
Except trying to stir up controversy, I have no idea what your point is. John, I and others have responded and seem in rough agreement.

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Andy Birko
PostPosted: Mon Mar 07, 2011 10:25 am 
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Laurent Brondel wrote:
Except trying to stir up controversy, I have no idea what your point is. John, I and others have responded and seem in rough agreement.


Actually,I just went back and re-read this thread and I see that I mis-read one of your posts and basically re-stated what you wrote in that post. Sorry about that. What I don't understand is why you disagreed with it when I re-stated the same thing you did.

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Billy T
PostPosted: Wed Mar 09, 2011 5:21 am 
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The differences in the radii of 15' and 20' are about 3/64th(.0467) over that distance of an 18" back. Hardly discernible, either tone wise or mechanically.

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