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this is probably the worst idea ever but...

BadDog said:
Now further imagine that your cage built from triangles, connected together at nodes so it has the shape a cage should have, gets hit hard right above the driver’s door by a boulder in a hard roll. That bar will undoubtedly deform, probably flatten and bend inward a bit, BUT (and here is the key), for it to bend very far the ends of the bar must get closer together! Shortest distance between 2 points and all that. If the bar is not straight, and the length of metal remains roughly constant, the end points MUST get closer together.
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So the metal would have to tear/crack/split for the pressure on it to go any further into the occupant area because the points trying to come together are limited by the strength of the other nodes, thereby strengthening the local impact area as well as diverting the pressure of the impact to the other nodes.
Right?


EDIT*** I have quite a bit of work to do to modify my cage. Thanks BadDog.
 
Remember when they said in Billy Maddison that everybody was dumber from listening to Billy ramble on about nothing? Well listening to Baddog is the exact opposite.

You make everybody that takes the time to read your input smarter and I would invite you to please write a book for an answer to any of my questions.

Thanks for giving me a brush up on physics and great info. on triangulation.:bow: :bow:
 
Thanks for the kind words guys, I'm just repeating what I have learned here and over the last several years.

Sounds like you got it under control Leper, good luck!
 
SUPER'BURB said:
Remember when they said in Billy Maddison that everybody was dumber from listening to Billy ramble on about nothing? Well listening to Baddog is the exact opposite.
:bow: :bow:
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I beg to difffer. I FEEL really stupid. However, you are actually correct in that I am now actually MORE intelligent than when this all began.

So yes, Baddogg. Thanks for the book. Full explanations help us all know why in the hell you are supposed to do something, as oppossed to just saying "'cause you're supposed to".

I understood everything you said other than the whole strap stock thing. Now I know that too! I'm so damn smart now!!!
 
well written explanation... which I thought was easy to understand.... even in redneck!!!

basically.. do what dog said
no square tube... save it for something else
 
Great write up! This really is a great thread. :bow: :bow:
 
i definatly (sp?) had to read that a few times but it makes sense now. im glad i asked:bow:
 
BadDog said:
That's actually pretty accurate. :D

Ever notice how you can create a triangle with ball joints at all 4 points, then grab the points, and you can't distort the shape?
Not to bash at all, as i think this is a really informative thread with great input, but i am scratching my head at the triangle with 4 points. was this a typo or am i missing something?:confused:
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I was speaking 4th dimensionally...
;)

Yeah, it's a typo... :laugh:
 
For a full size rig, typically 1.75 x 0.120 is use for most. People sometimes use smaller and/or thinner for inner structure where impact is not a prolem and all loads are tensile/compressive, but I stick to 1.75 x 0.120 and 1.5 x 0.120 in HREW or DOM.
 
How do you know where to put the triangles and where are they most important?
 
Any place you expect to handle large loads needs to be triangulated to transfer the load into tensile/compressive forces immediately before it starts distorting the rest of the structure.

Figuring out how that fits around occupant access, drive train, seating, storage, and all the rest is what separates the pros from the armatures.

My initial truggy design (most of which is still in use) was my first attempt and could have been better in hind site, but it's held up pretty darn well considering the beating it's taken and it's only needed a few tweaks and replaced tubes (has a couple now that could use replacing for that matter). I originally figured out about what it needed to look like to fit my chassis and required seating, then I built a 1:16 scale model using bailing wire (mechanic's wire) and hot glue for welds. Next I pressed on various points I expect to take major hits such as barrel rolls and end-os. If it felt "soft", I knew it needed triangulation and I could see what moved and how. The wire bends relatively easily with even light “bending loads” (sort of like a 1.75 x 0.120 HREW tube with 5k lbs of weight bearing on it!) but it’s fairly resilient when loaded tensile/compressive. An engineer would do the same with computer modeling, but lacking that training as well as the rather expensive software, I had to go the primitive route. But, unlike some of the other home-brew cages on rigs I’ve wheeled with out here (and seen first hand folding in a relatively light roll), mine has taken several hard hits from the side (no end-os yet thank goodness) without any significant deformation other than bent tubes at the point of impact.
 
Square tube VS Round Tube:

Ok, here are how the numbers fall out for the 2x2 square VS. 1.75 Round I assumed that the wall thickness for the square tube was 3/16, wich is the smallest you can usually get in 2x2:

1.75x0.120 wall tube
Cross sectional area = 0.61 (inxin)
Area moment of inertial = 0.21 (in*4)
Section modulus = 0.526 (in*3)
weight per foot = 2.05 (lb/ft)

2x2x0.1875 wall square tube
Cross sectional area = 1.25 (inxin)
Area moment of inertial = 0.668 (in*4)
Section modulus = 0.668 (in*3)
weight per foot = 4.32 (lb/ft)

So what does this mean? The 2x2 square tube is significantly stronger than the 1.75 round tube in tension (cross sectional area), has higher bending rigidity and strength (area moment of inertia and section modulus) and has a higher buckling resitance in compression (area moment of inertia, again)

The down side is that it weighs 2x as much, and if 1.75x0.120 wall is considered strong enough for a cage, than the 2x2 square is just way overkill.

So the answer to the origional question is yes, you can use the 2x2 square tube. Post up if the wall thickness that you have if it is not 3/16, and I'll post up the mumbers for whatever you have.

I think the round tube would look better though:D
 
I have several sticks of 2x2x0.120 1010 alloy tube in my shop for gate posts and such. It's also available as thin as 16 gauge with no problems, maybe even thinner. Still, 2x2x0.120 will be heavier and stronger based on your calcs, but the numbers will be closer.

And I believe your numbers apply to structurally oriented force, where material deformation is not taken into account. Where bending strength is important, real resistance across the diagonal is WAY lower than force oriented normal to the tube walls due structural deformation. Your area moment of inertia and section modulus only apply to forces normal to the tube walls. If you have access to software (or skill to do by hand), try modeling it in FEA to see the differences.

Of course, if you have it properly triangulated and the force is oriented so that those triangle can work, then you only need to be concerned with the compression/tension strength, so cross section is pretty much everything in the typical models. But what happens when you take a hard roll, catch a point load (shelf lip?) above the passenger door, the a-hoop typically has little to know triangulation and the b-hoop is often not properly triangulated forward/back (typically trapezoidal due to passenger access requirements and such). If the load is normal to the impact span, square is stronger. But if it hits off normal (i.e. off perpendicular) then bending resistance drops dramatically as the angle off normal increases as it rapidly falls below the strength of round.

Not to mention it’s hard to bend square smoothly, so if you use square for a cage, you wind up with a tinker toy or erector set looking affair. :D
 
loafer said:
The down side is that it weighs 2x as much, and if 1.75x0.120 wall is considered strong enough for a cage, than the 2x2 square is just way overkill.
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given enough thickness you could use flat bar for a cage. It isn't going to look good and your cage will weigh 1000 lbs or better. Most homebuilt cages are as has been said far less than the ideal design due to the fact we have to get in and out of them. So the way I see it square tube would be a less than ideal material in a less than ideal design. Interesting idea though
 
I have the biggest head ache known to man:eek1: j/k

very informative I have been wondering about this for quite some time and it all makes sense finally.

Good thread!
 
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