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Discussion Starter #1
i know this probably has been talked about before but a quick search didnt turn up what i'm looking for. i parted out my rock rig a few years ago when i started diesel sled pulling but i still hang around here a bit. well i am stepping up classes and i'm building a tube chassis this time. here is a quick sketch that i have done.



the suspension on these things is a full 180* from the rockcrawling types since my rear rockwell F106 will be welded soild into the frame and the rockwell FDS75 front axle will have very limited suspension travel.

my question is what are the pros / cons of using square or round tubing in this applicaiton. square would be alot easier to work off of and i'm thinking in this application that it could be very close to as good as round, yet barker and the other pulling chassis builders use round. basicly why should i not use square.
 

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I think that in a perfect world (har har) square would be sufficient assuming you could control what axis the forces are applied. In rock crawling, round is better because it distributes forces around the tube which is necessary when all twisted up.


Two bud lights and a shot says that makes sense.
 

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Gravity Works!
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Besides the obvious, your asking gravity testers what is best for flat ground traction etc etc etc. I buy a lot of my steel from these guys and that's their specialty.

Check out some of the work they do. Now I know a lot of it is tractors, but IMO not much difference really.

http://haslagsteel.com/SourcePack/2-Gallery-with-XML/photos.html

I'm sure "Tex" on here knows these folks quite well and you may even PM for some insight on this matter as well.
 

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Discussion Starter #5
i'm mainly just asking chassis builders their opinions. in my current motorsport the engine is trying to twist the chassis where on rocks its actually the ground thats twisting everything. my main concern is that its going to twist more being square, which means less power to the ground and or loading and unloading of the chassis. square is just soooo much easier to work with
 

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I'm no chassis builder but the math says a square tube x" on a side is stronger, stiffer, and heavier than a round tube of x" in diameter with the same wall thickness. The moment of inertia is higher because there is more material further from the center line (think about an I beam).
 

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Tortional rigidity will be achieved better through design than material selection. I am far from an expert or chassis builder but I did stay at a holliday inn express. You need to place torsion boxes in your frame to prevent the twisting. I cant think of a reason anyone should talk you out of square.
 

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In regards to the added weight of round vs. square....I am assuming that you really dont care? Shit, you guys add tons of weight anyway to make the weight class.

I would go with square (simply because it is so much easier to work with), but like was mentioned above, focus on a rock solid design.

Rob
 

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Round is better when you bend in multiple directions and attach at different angles.

Moment of inertia is also where you get your bending strength. you can search google for equations basically the further the material from the center the stiffer it is (I beam or height of the 4 bar ladder frame you have the picture) basically the distance the material is from the center is about x^4 stiffer, round is the most efficient shape for stiffness vs amount of material (weight) in all direction not saying you cannot add a little extra wall to have a square tube to be just as stiff.
I'm not even saying you need the chassis to be really stiff, whats wrong with a little flex.

You can use the same moment of inertia equation to calculate the chassis stiffness gets hard with multiple shapes. CAD can calculate it instantly.

You cannot just look at moment of inertia or you will be building with 4" exhaust tubing. Then you need to also consider buckling thinner tubing especially dented buckle quicker you need to find a happy medium.

Square is easier for mounts and when building a square chassis mitering the corners. Expect more welds with square and obviously a boxy look.

Just look most truck frames are just 2 bar ladder frame with thin C channel in places like the middle it might get boxed in and they work just fine.

Basically you put enough metal there I don't care what kind of tubing it is round or box it will work just fine.

Hope that helps you make your decision.
~Justin
 

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I'm no chassis builder but the math says a square tube x" on a side is stronger, stiffer, and heavier than a round tube of x" in diameter with the same wall thickness. The moment of inertia is higher because there is more material further from the center line (think about an I beam).
ding ding ding! We have a winner!!!!
 

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Rock Stacker
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For your application square is best. Stronger and easier to fab your design.

Round is popular because it is easy to bend in a bender.
Uniform strength around axis.
You don't need to worry about keeping it from rotating.
A hole saw or grinder replaces a miter band saw.
The rule books use round.
Most common material by chassis builders.
Lots of round designs and pictures to work off of.

All these +'s don't make round better for your use.
 

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Discussion Starter #12
thanks guys, looks like i will be using 2x2 heavy wall for the 4 main rails and 2x2 thin for the bracing and 1/2" plate for the weight box....yes we add 2,000-2,200 lbs 60" forward of the front axle centerline so weight is not a concern. limit for the class is 7500lb and the chassis and weight box comes in somewhere around 1550lb not adding in the drive train, which the engine is around 900lb then turbos are 50ish x 3 = 150lbish....only motor sport in the world that heavier is better.....we even add weight to our cranks, like +20lbs to the counter weights....think about how much sense that makes
 
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