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Bionicdodge.Com  |  Bionic Tech  |  Street Tires, Suspension & Chassis (Moderators: donram360, adamsredlines)  |  Topic: X-Brace Fabrication. 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: X-Brace Fabrication.  (Read 19693 times)
98Dak408
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« on: January 07, 2009, 02:14:59 PM »

I'm thinking of fabricating an X-Brace for my '98 R/T Dakota.

As far as round tubing for the brace, I am contemplating using 1 1/8" or 1 1/4" or 1 5/16" tubing with a wall thickness of .120

The metal plate that will connect to the stock crossmember that the 4 tubes attach to (that form the X) I am thinking will be 3/16" thick.

It will be similar to the Kenny Brown system:  http://www.moparmusclemagazine.com/howto/37249_dodge_dakota_chassis_upgrade/index.html

Darren also has this set up.  See reply #25:  http://64.150.167.247/forum/showthread.php?s=a138c25441e025d0a1c8730fd673b0fb&t=42004&page=2

If anyone has any comments in regard to the tubing size or anything else pertaining to this proposed project I would appreciate it.
« Last Edit: January 08, 2009, 01:46:49 AM by 98Dak408 » Logged

98Dak408
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« Reply #1 on: January 07, 2009, 06:09:04 PM »

Apparently the KB X-Brace used 1 1/4" round tubing.
« Last Edit: January 07, 2009, 06:09:51 PM by 98Dak408 » Logged

Salarguy
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« Reply #2 on: January 08, 2009, 10:36:34 AM »

Square and rectanglar tubing resist flex far better than round and the necessary bends can br indexed far more accurately. It seems to me more thought went into the TVR brace.
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98Dak408
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« Reply #3 on: January 08, 2009, 03:52:52 PM »

Chassis Engineering by Herb Adams, does specify that "Square tubing is much stronger and stiffer in bending than round tubing..."  But I do see a mixture of round and square tubing in their bulkheads, although the main frame rails and much of their bulkheads are square tubing.

How To Build Hot Rod Chassis by Boyd Coddington, I see rectangular tubing for the main frame rails, while a considerable amount of round tubing is used for cross-members and x-braces. 

I also see that a number of items such as upper and lower control arms, ladder bars, panhard rods, caltracks, 4-links, etc. are made with round tubing.

X-Braces or x-members create frame strength in large part due to triangulation.  Triangular shapes are much strong than square and rectangular shapes.  I wonder what forces are actually at play in an x-member?  Are the forces more in compression and tension with these frame members rather than resisting the forces of a bending load?  I also wonder if there is some torsional force at play as the frame flexes?  As one frame rail flexes down and the other flexes up, is this a twisting action rather than bending?

It was also noted that .120 tubing wall thickness was good for frame members.

Just trying to reason this out in order to make a good decision on design and what materials to use.
« Last Edit: January 08, 2009, 04:10:12 PM by 98Dak408 » Logged

Salarguy
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« Reply #4 on: January 08, 2009, 04:35:27 PM »

Torsional strength is paramount in that setup and square/rectangular is far better than round. The 'secret' Ramchargers torsion bars were hex front to rear!
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98Dak408
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« Reply #5 on: January 08, 2009, 04:53:02 PM »

In regard to torsion, torsion bars, axles, and drive shafts are round, so I would think round tubing would be a better application.
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98Dak408
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« Reply #6 on: January 08, 2009, 04:59:00 PM »

I found this tid-bit on the Vette Mod site:  http://www.vettemod.com/forum/showthread.php?t=1409

The question posted by an individual is: 
"I'm trying to do some comparison between the stock squarish sections of the frame, and the equivalent if made with round tubing. Anybody seen any formulas for calculating torsion of a square tube?"

An answer in response was:
A calculation for square tubing will not bring you anything, as the stock frame is 2 u channels welded tgether, and not even all the way through. if you weld it up completely, then you will sort of have a square tubing frame...but still not exactly.

What do you want with torsion? You're not talking about a driveshaft, considering the whole, yes there's torsion but what you want to look at is just stiffness in a single direction.

For a same diameter and wall thickness a square tube will be stiffer than round, however the round will have a significantly less cross sectional area (and weight), if you even out the cross section (larger diameter round tube) the difference will be less.

If weight is no issue and size isn't either, a square and round section, same weight, same wall thickness...the round will be stiffer.


D = OD, d = ID
____________
Area of round tube = PI/4 (D^2 - d^2)
Area of square tube = (D^2 -d^2)
Moment of inertia, round tube = (PI/64)*(D^4 - d^4)
Moment of inertia, square tube = (1/12)*(D^4 - d^4)



This is interesting: 
Quote
If weight is no issue and size isn't either, a square and round section, same weight, same wall thickness...the round will be stiffer.
« Last Edit: January 08, 2009, 05:06:25 PM by 98Dak408 » Logged

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« Reply #7 on: January 08, 2009, 05:00:13 PM »

They are all usually solid round. In any given material there is more flex strength in a tube than the same diameter in solid. Torsion is twisting effect and I'm sure those X braces under load experience more twisting than bending. And wouldn't the square tube be more compact with the same weight and strength?
« Last Edit: January 08, 2009, 05:05:48 PM by Salarguy » Logged

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« Reply #8 on: January 08, 2009, 05:19:11 PM »

Quote
And wouldn't the square tube be more compact with the same weight and strength?
Yes I suppose so.
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98Dak408
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« Reply #9 on: January 08, 2009, 05:21:55 PM »

This is an interesting link:  http://www.factoryfive.com/table/ffrkits/roadster/specs/frame.html

"Round Tube vs. Square Tube
There is ongoing debate about the use of round vs. square tubing in frame design.  Most of it is from people who are selling one or the other. The bottom line is that it costs more to produce an original style round tube frame because it’s more difficult to design and manufacture.  Many companies have taken the easier path of making square tube ladder style frames.  We felt our challenge was to make an original style frame stronger without losing its inherent beauty or period correctness.  Good engineering means form and function.  Once we committed to building a round tube frame we found some pleasant surprises.

• Round tube weighs less than square tube.
• Round tube is available in stronger steel.  There is no rectangular 1020 DOM tubing, it’s all 1010.  1020 DOM is about 30% stronger than 1010.
• In torsion, (exactly the kind of stress that cross members are subject to) round tubing is much stronger than square tubing.  Makes sense why there are no square drive-shafts, right?
• Under vertical bending loads, square tubing is stronger, but since the round tubing is available in higher grade steel, the advantage square stock had in vertical loads is diminished. 
• Round tubing helps make a lightweight ladder frame, with excellent vertical and superior torsional load resistance.
• A well-designed round tube frame is the hallmark of professional chassis engineering.  They are complex, requiring intricate jigs, difficult to design and they are usually made from better materials.  They are also really beautiful."
« Last Edit: January 08, 2009, 05:23:06 PM by 98Dak408 » Logged

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« Reply #10 on: January 08, 2009, 05:57:08 PM »

I just did a little Google and it seems My theory was totally wrong. I found two hits that suggested the only strength benefit for square over round tube would be if it needed both torsional and bending strength at the same time. X-brace?? The PTO driveshaft for my buddy's saw mill is--- you guessed it--- square! The slip joint is square solid stock inside square tubing. It's 50+ years old so it must be an old theory! Laughing
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« Reply #11 on: January 29, 2009, 04:43:57 PM »

Picked up some 1.25 O.D. x .120 wall x 1.01 I.D. round tubing today as well as some 1 O.D. x .250 wall x .510 I.D. for the x-brace.  The 1" tubing with the .250 wall thickness will be used to mount the larger tubing to the frame.  Doing so will also close the ends of the larger tubes off from collecting dirt.  I'm probably going to use grade-8, 7/16 bolts to secure it. 

I’m still trying to decide which “x” shape is better for the best performance.  The TVR brace does not extend as far front-to-back along the frames rails as the Kenny Brown brace does.  Is this better or not quite as good?  The shorter distance requires less work but is it better?

Also, the stock cross-member of the '98 Dakota makes centering the “x” within the frame rails difficult, due to the contour where the drive shaft is, which is offset from center along with the driveshaft.  I believe Kenny Brown designed their brace to conform to that condition.  Is that aspect not as good as the TVR design?  I’m not sure if I’m willing to butcher the stock cross-member to center things, especially if the performance difference is marginal at best.

Aside from the stock cross-member of the ’98 Dakota positioned mid-way along the frame rails, the stock braces from there to the front of the vehicle all seem to be secured at the bottom of the frame rails.  From the center cross-member back, all the stock cross-members are secured at the top of the frame rails.  I assume this was done to make room for necessary components such as engine…and rear differential for instance.

The front driveshaft loop I fabricated secures at the top of the frame rails and I think I’ll fabricate a rear driveshaft loop that secures to the bottom of the frame rails.  Everything combined should increase the rigidity of the frame.  The improved handling and off-the-line performance hopefully will make up for the increased weight of these additional frame members.
« Last Edit: January 29, 2009, 07:14:21 PM by 98Dak408 » Logged

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« Reply #12 on: January 29, 2009, 06:22:39 PM »

if you really wanted to test your theory out, get a hydraulic pump, ie, one used in auto body like my uncle uses, along with a few gauges ie, torsion, PSI, and such.  create two equal x braces, and hook them up to mocked up "applications"  submit both to the same pressure levels, torsion, flex, so on and so forth.  then measure torsion twist, flex, and all that good stuff.  you should be able to see first hand which is better then.  but, when i worked at the selway corporation and made all sorts of cookey metal creations,  square always resisted better.  but, theoritically, a egg, with pressure points on two opposite ends, positioned perfectly, can withstand infinit pressure levels.  but, piping is not egg shapped, its circular, so, more succeptable to crushing than square tubing in the point that it already has a curve to squash, where as box tubing is harder to crush due to it having to have the flat sides warped to crush inboard/outboard.  square should rule all.  now, a half circle, with the sides stationary, and pressure exerted on top, can withstand also infinite pressure levels, hence the old bridge styles, using the "key" at the top, which was the defining part of it.  but, for all stated applications in previous posts, the square should be stronger.  If you have enough HP to need a x brace, a few pound difference between circle and square tubing should not be at the top of your worry list.Smile
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98Dak408
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« Reply #13 on: January 29, 2009, 06:38:03 PM »

From doing a little research, this is my understanding: If you take a 1" diameter square tube and a 1" diameter round tube, the square tube will be stronger in horizontal load than the round tubing, but will weigh more.  If you take the same weight of round tubing, it will be larger in diameter, but will also be stronger than the same weight of square tubing.  Round tubing is better in torsion also.  If space is a consideration, square tubing may be better.  If not so significant an issue, round tubing is better.  Square tubing is easier to mount brackets to and fit.
« Last Edit: January 29, 2009, 06:41:03 PM by 98Dak408 » Logged

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« Reply #14 on: January 30, 2009, 05:12:10 AM »

I do find conflicting information on tensional strength of square vs. round tubing.  Let's consider a round driveshaft.  And for argument, let's say square is stronger in torsion.  Then why would the auto companies make round driveshafts?  Round takes less material than square and is lighter.  But then why wouldn't the auto company make a smaller square drive shaft?  Hmmm?   

Salarguy pointed out that some torsion bars are a hexagon (6-sided) in shape.  That's kind of a cross between round and square.  Hmmm. 

I guess I'm beating a dead horse.  I've already made a decision to use round tubing and have purchased it.  Time to get on with it. 
« Last Edit: January 30, 2009, 05:12:34 AM by 98Dak408 » Logged

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