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I have a hard time understanding this concept since the braces can slip some before the glue grabs.

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Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

Yup. 7-8 lbs. This is for 1/4" rods. Seems to be pretty consistent.

Yes, this is hard to understand. The normal force is the force required to buckle the rod. It does not increase if you increase the amount of flex. There is a tangential force component as well, and is proportional to the amount of flex. It's like a spring force. And it increases with flexure. And it is what causes side slip and can be the source of great danger. Never over flex these things. You simply do not need to, nor does any good come from it.

See this: http://en.wikipedia.org/wiki/Buckling

First off this is NOT how you should do it. I recently decided to go with a go-bar set up that uses my ceiling and work bench. I still hadn’t gotten new bars and decided what the optimal length would be for various operations, So I went with what I had handy. I post this to demonstrate that as long as you are careful to put the ends of the bars directly above the point of contact there is no lateral load. You can put in a fair amount of flex and be okay, but it is not the safest thing to do, yes I was wearing safety glasses! Also keep in mind that the height is much greater than 24” in this picture. That makes excessive flex a bit easier because the bars are so long, but remember it also reduces the clamping force generated.

Also of note, the scale may be confusing, this is a mandolin top (about 2/3 the size of an OOO).


Edit, let's try this again with it rotated properly.

Second edit: I did not mean to imply that the added flex in a long bar reduces the force, it is the taller gobar deck (requireing long bars) that reduces the force. For a given diameter, the longer the bars, the less force they will produce. . .

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I have made some out of ash; results were mixed.
Allow me to clarify by stating that they were not for a guitar project, but for clamping inlay into the center of doors and panels (cabinet project).

1/2" x 5/16" with a length of 32".
I did not measure clamping force; simply wanted something to hold the pieces down and provide a little pressure.
So you may have to do some experimenting to determine the proper amount of flex in relation to size to obtain the desired pressure.

For me, a large hindrance to making some out of wood is the fact that wood can be all over the place in regards to providing the desired pressure. It's wood, and, just like selecting and preparing the material for your bracing, the stiffness (and thus the amount of pressure provided by a given cross section) can vary.
I suspect the amount of runout in the wood will have a huge impact on the final results.

So yes, making your own from wood can work, but you're gonna have to experiment to determine the dimensions that will best accomodate your needs, then resign yourself to the inconsistency of using wood.
If you simply do not have the funds for fiberglass rod, it's an option.
If the concern is availability, then places like Goodwinds ship internationally.

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Always have plenty of sandpaper; it's rough out there!

Amazing input from everybody on this thread, thanks so much ...I have changed my views through 180 degrees ....

I did a bit of surfing and came up with this company in the UK who are selling 4mm fiberglass rods (which would be the equivalent of the 3/16") for what seems to be to be a staggeringly low price ...I figure I can get a single gobar for like 20 pence, if I buy the 4 metre lengths. Is there a catch? Is it the case that "fiberglass is fiberglass is fiberglass", as I think Gertrude Stein once said, or is there a huge difference in quality between what kite suppliers sell and what window blind suppliers sell?

I have no problem buying internationally from Goodwinds if the product is a quantum leap ahead of what I can get from the company above.

Also, how many gobars would you normally use to close the box? I am thinking one every inch along the rim would be about the mark, is that too many, or too few ? ( I am using fish glue throughout the build btw, for everything).

I don't mean to start an argument, here, but I wanted to see for myself whether the force exerted by a go-bar increases the more it is flexed. I did a simple test using a scale to measure the force with different amounts of bar flex. The more I flexed the bar (i.e. the greater the deflection), the higher the scale read.

Take it or leave it.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

From the previous posts in this thread, it seems that any diameter can be suitable provided it is the correct length. So if we're aiming for approximately 8-10 pounds of pressure, we have already established that 3/16" rod with a length of 24" works well for our application.
The 4mm diameter is slightly smaller than 3/16"; actually closer to 5/32". So will it work? Yes, but with the length of 24", will provide slightly less pressure than actual 3/16".
So what to do? If you use 4mm diameter rod, make it slightly shorter in length. Instead of using 24", try 22" (hopefully someone good with the formulas can verify this).
With your stock coming in lengths of 4 meters, making the rods 24" each can give you 6 rods. But if you cut them to 22", then you can get 7 rods.

Here is how I would approach it: Take your 4 meter length and cut off two rods- one at 24" and one at 22". Determine which length works best for you. If you decide that the shorter length (22") works better, you still have enough stock left to get 7 rods.

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Always have plenty of sandpaper; it's rough out there!

Thanks again, gentlemen, advice taken regarding the variability of wooden bars.

I have ordered 14 rods @ 4metres and will probably cut them to 22", but will do the experiment which Robert suggested. 22" (or just under) will give plenty of working space, I would have thought.

The cost works out slightly more than I had anticipated, what with tax and carriage, ( plus 200 vinyl caps) but still very reasonable IMO.

Just FYI, for anyone who finds this to be useful or interesting information... My go-bars are 1/4" D fiberglass, 36" long. With slight deflection, the scale I used (just an ordinary, non-digital kitchen scale) read about 5.5 lbs. At 1" deflection, it's 7.75 lbs. 1 1/2", 8.25 lbs. 2", 8.6 lbs. 2 1/2", 8.8 lbs. 3", 9 lbs. 3 1/2", 9.15 lbs. 4", 9.25 lbs. (These readings are obviously not super-precise, but pretty close.) It keeps going up from there, all the way up past 11 lbs if I really bend the heck out of it, but, as you can see, the pressure went up more than a pound between 1" and 2 1/2", then after that the rate of change slowed down a lot. Beyond 4" of deflection, the pressure only changes about .15 lbs per inch of deflection.

I'd rather get about 9 lbs per bar than 7.75 lbs, so I'm happy having my bars flex about 3" rather than 1". YMMV.

In case you're wondering, I did these measurements really quickly by putting the scale on the floor under my drill press table and lowering the table to increase the flex of the bar.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

I really like having the extra working space of a tall go-bar deck.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

all this talk made me go and check the load of my go bars. they´re the usual 3/16" x 24" fiberglass rods (from goodwinds), and at 1" deflection they produce around 7 libs of pressure - should they punch more weight? or maybe my scale is off...

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member of the guild of professional dilettantes

The Euler buckling formula is not really about how much load a bent bar carries.

It is only about finding the maximum load that a slender column will carry BEFORE buckling occurs.

At this point, in most structures, you would consider that failure has occurred and apply suitable load factors to ensure that you do not approach this situation,

Once the bar is buckled or bent,you will get some increase in force with increased bending but for the range of deformations we typically use in Go bars it can effectively be considered uniform

Small note. If you use too much deflection and the go-bar pops off the brace then there's a chance it can crack a top or back. Please don't ask me how I know.

I now make sure that when there's no deflection the go-bar just reaches, or is just slightly deflected at the top or back material.

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Thank You and Best To All

When I tested my go bars a while ago I was concerned about the accuracy of my measurements and the digital bathroom scale I was using. So I loaded the scale up with 10 go bars and divided the results by 10.

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Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

Just in case any UK builder is tempted to follow up my link above to these 4mm rods ...DON'T !!!

They arrived very promptly, but upon experimentation, I discovered that A: they were only 3.82 mm diameter, and B: that to give a pressure of 8lb they needed to be cut to 12.75", which obviously renders them impractical for use in a go bar deck. Luckily, I was able to unload them onto a friend of mine, who runs a business making curtains and blinds, and she uses vast quantities of these 4mm rods (it is the market they are intended for, after all)

I am negotiating to get a couple of samples of 5mm and 6mm rod from another firm, and experiment further before ordering the required quantity.

I had the same problem sourcing bars recently and considered getting the roman blind rods, however I found an aerial manufacturer called sandpiper I think. A quick email to check and then placed an order for 5mm pultruded fibreglass rods which they cut to 24" lengths - I got 30 for about £22 delivered.
Just googled here is the link - http://www.sandpiperaerials.co.uk/index.php?main_page=index&cPath=7_28