square vs. round (1 Viewer)

[CrackedCranium]

can anyone tell me if there is any difference in the strength of round versus square tubing? Meaning, does one bend easier than the other? I'd like to build my roll cage out of square tubing, but want to make sure I build the strongest one possible.

 

[RockDoc]

I thought this was going to be another 60 vs. 62 front clip debate.

Sorry, I can't help you on the tubing, but am interested to hear.

 

[snowboardonsnow]

I thought this was going to be another 60 vs. 62 front clip debate.

Ditto

I know some people do it, more often for the beams and such. I don't know how you would bend it? I also know square is heavier than round, like 2" Sq verses 2" round. Both have their strength qualities.

I am no expert here, I have just made a few exoskeleton cages.

I am looking forward to seeing any reason why from someone with more expertise

 

[snowboardonsnow]

Oh sorry, I can tell you what I know of the strength differences. A Sq. tube will hold up better this way (horizontal)
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Where round will be stronger this way (vertical)

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And I am not sure but I think round holds up better to being dented but don't quote me on that...or any of this. Someone with more knowledge will speak up I am sure.

 

[StaleAle]

Round wins in the case of roll cages because...

Assuming materials, wall thickness and external dimensions are similar, round tubing has a better stiffness to weight ratio than square tubing. That is why it is used for most experimental aircraft, bicycles, etc...things that need to be strong but lightweight. Round is also better for structures when you do not know the direction of force that will be applied, such as for a roll cage. Square tubing can be stiffer if the forces are aligned with the flat portions of the tubing, assuming the wall thickness of the materials are comparable.

From a fabrication perspective, round is usually easier to bend, but fabricating compound joints where you are cutting and welding round tubing at multiple angles in three dimensions can quickly get pretty complex for the DIY-er. The square tubing structure will weigh more but might be easier for you to to fabricate if you are not an accomplished pipefitter.

You will probably have adequate protection with 2" stock in either cross-sectional shape as long as the you use appropriate stock and design braces to add stiffness in all three dimensions. Both shapes are strongest in compression and in tension longitudinally so try to ensure bracing points towards the expected forces, side-to-side and fore-n-aft.

Also, be sure to pay attention to the hardness of the steel. In general, the harder the steel the stiffer the steel, but the harder it will be to fabricate.



Good luck!

 

[NocalFJ60]

I think there is a reason why all the roll cages I see use round tubing. Start a roll cage build thread in the hardcore room and you will get plenty of help while you build it.

HardCore Corner - IH8MUD Forums

 

[2mbb]

In some instances square is stronger than round, when comparing the same nominal size (ie. size and wall thickness). The square tubing has a greater perimeter which spreads the forces out more. The square will be much stronger if you try to bend it along the diagonal. However, most of the roll cages are built from round because of what is stated above: strength to weight, ease of fabrication, etc. Also the round tubing doesn't have a "diagonal" orientation, so it will have the same strength regardless of the direction of the applied force. This makes it much easier to design the lightest cage with the required minimum strength.

July 2009 4x4 Truck Repair & Tech Questions - Chevy Blazer Driveshaft, Toyota Motor Mounts, Rollcage - 4-Wheel & Off-Road Magazine

 

[ntsqd]

Somewhere I ran across the math that showed that bending square on the diagonal was not appreciably stronger than bending round tube so long as both tubes were of roughly the same sectional modulus and, obviously, the same metal alloy. An analysis of the results was that the near extreme fibers were offset by the near near neutral axis fibers, creating an average that was essentially the same as the round tube bending modulus.


EDIT:
Who ever that author contacted for the answer in the magazine was either HUA or not a structural/mechanical. This an early in the course "Statics & Strengths of Materials" question. The bending strength of a tube is due to the distance that the extreme outer fibers are apart. Witness an "I" beam. The top & bottom flanges take all of the load. All that the web does is keep the flanges the same distance apart for the full length of the beam. With a square tube the sides do the same job as the "I" beam's flange, and not a lot more. Recall how turning a rectangular tube on edge (loading direction parallel to the long side of the tube) holds a lot more load than laying it on flat (loading direction parallel to the short side of the tube).

 

[fjdemon]

Ditto

I know some people do it, more often for the beams and such. I don't know how you would bend it? I also know square is heavier than round, like 2" Sq verses 2" round. Both have their strength qualities.

I am no expert here, I have just made a few exoskeleton cages.

I am looking forward to seeing any reason why from someone with more expertise

LOL me too!