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So I bought a bunch of fiberglass go bars off a kite building supply website. Someone on the AGF recommended them as well as a certain thickness he uses. Well I got the bars today in the mail and they looked a little bit small to me, so I measured the pressure on a scale and it was only 800 grams (or 1.7 lbs). I thought go bars were supposed to be around 8 lbs of pressure? Did I just waste 60 bucks?

What thickness are they?

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.187 or 3/16"

how much bend when set? there should be about 1" less space between the top of the go-bar deck and the piece being clamped versus the length of the go-bar. So if the go-bar is 24" long, you want 23" between the piece and the top of the go-bar deck.

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I thought the size of the bend does not affect the pressure?

Don't have a go bar deck but, if I remember from my readings, there's very definitely an "optimal" amount of bend. (eg it'll be back down close to zero, just before it breaks!) Test different openings on your scale & see what you get.

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Dave
Milton, ON

The most pressure I could get was just over 800 grams, from all ranges of bend.

the go bars tend to be hardest just before they bend , but what you are looking for is about 8 to 10 lbs of force per rod. Not all fiberglass rod is the same.

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John Hall
blues creek guitars
Authorized CF Martin Repair
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Well I guess I am going to have to get some thicker ones, I guess I could keep these for more delicate operations...

Seems strange. Most folks who use fiber glass rods use 3/16" rods. I use wood, it's cheaper and more readily available

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That is strange, could the length affect it? They are 48" long right now.

Most definitely. Cut those rods in half and you'll be golden.

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Jim Womack

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DON'T CUT THE RODS, till you decide the final size of your go-bar deck.

What is the distance between the top of the go-bar deck and the piece being clamped? With thinner rods, the longer they are is seems (I don't have the scientific proof on this) they don't want to produce as much downward force versus shorter rods.

Many folks say the optimum go-bar deck height is 24-26" with 24" go-bars. With a radius dish, the plate and braces, that should bring you in the 1-2" bending range for the go-bar deck.

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"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

Well.... I am building my go bar deck right into my bench, I have a 4 foot section of it with a melamine surface and will be building a shelf over top, so the shelf 24" above the bench would probably be awkward.

Going 24" will help a lot (and will make efficient use of the stock 48" rods). I did 1/4" diameter my first time around and they were way too stiff so I bought the 3/16" and they've been great.

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Jonathan Kendall, Siloam Springs AR

Well, I cut one down to 36" and the pressure now weighs in at 1350 grams or 3 pounds. 36" is about what the height I want to make the shelf above my bench, so I guess I could make them work if I used lots? Like one an inch when glueing bracing?

if you can, make your go bar deck height adjustable. i used lengths of 1/2" all thread as the four posts and i can adjust the height from 0 to around 36". that covers everything from braces to closing the box, etc.

plus when you arent using it you can just collapse it and stuff it under your bench. folds up real flat like. thats where it is now or id take a picture for you.

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sweat the small stuff.

Length will have little to do with overall pressure . Instead of 8lb the 48 inch may be 7ish but expect something close to the 8 to 10 lb ranges for best results. Now if you are looking at 20 feet that may be a bit different . Wood will work short term but they can break and weaken but if you have an endless supply and you don't mind making them , go ahead , they will do the job.
I found hickory and elm to be the best for wood . but it is hard to find them without runout.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I do the same as Heath. It's nice in an annoying way to use the same deck for gluing braces and gluing the plates to the rims. Annoying only because it takes a while to thread the knobs up and down. I'm probably going to fix this by getting pipe and cutting it to length. One set for gluing braces and the other for gluing the plates. I'll use them as quick stops.

+1 on making the height adjustable. I started out with a fixed height and wood bars. That worked but I soon found myself using a lot of different lengths and finally went to the all-thread and 24" fiberglass go bars. Put some vinyl tubing around the area of all-thread where you'll be moving parts in and out so if you hit the wood on it you won't damage it.

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Steve Smith
"Music is what feelings sound like"

It's simple column buckling theory.
the load will be inversely proportional to the length squared
So yes the length matters significantly.
If the shelf height for 24" bars is too low for you, make some riser boxes you can place on the benchtop.

I've used different dia. size dowels for gluing way before I starting building a guitar. I used maple 5/16 X 36" dowels for my deck and requires a bit more perfection with distance...but they're cheap enough that I cut them down if necessary and use them up with my business anyway. I find they have more than ample pressure to do what is necessary. I contemplated fiberglass bars...but OH...Man they are expensive.
Kent ( the Cheapskate ) Bailey

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Exactly right 24" at 3/16 diameter will produce between 7.5#-8.5# of dow force once it starts bending. It dows not mater how much you bend them. Once you reach the bucking point (pretty much moment the rod starts deflecting or bending) that is as much pressure you will see from any given length. IF you want 8# at 48" long you are pretty much going to have to buy 1/4" rods to get there.

3/16" x 48" are $1.90 each. Get two 24" go bars from each one - not expensive at all. Mine measure 7 lbs force each with about 2" deflection.
http://www.intothewind.com/shop/Repair_and_Kitemaking/Fiberglass_Rods_for_kites/3-16_FG_Rod

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Steve Smith
"Music is what feelings sound like"

I want to say here for the sake of those that may not know. You gain no extra clamping force by bending more than 3/4" to 1" you only add restrained spring loading. I never bend my rods more than 1". The added spring load does nothing for clamping pressure but can cause the rod to come loose and fly across the room or poke through your top if bumped . Keep the spring loading to a min and you will never have a rod come lose and punch a hole or bad dent in your tops.