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round vs. square tubing strengths?

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  • round vs. square tubing strengths?

    as part of this project of mine i need to build a frame. i was wondering if there is a difference in the strength of square tubing vs. round tubing for the same dimensions. for example, if i have 1"x1" square tubing and 1" diameter round tubing, both with 18 gauge wall thickness, which is stronger? and by what percentage would you estimate? if anyone knows of a web page with such info, it would be greatly appreciated. i haven't hit upon the correct search phrase to find such a place.

    i think the square tubing would be easier for me to work with, but if i lose strength with it, i may go with the round.

    thanks,

    andy b.
    The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining

  • #2
    Round is pretty much the same strength with bending loads from any direction. Square of comparable size and wall is much stronger in bending across the flats, but weaker across the points. If you know the direction of the load, and it's relatively constant (direction), then square is better, if properly oriented.

    Tension is similar for both, but I seem to recall that for column compression loads round is stronger, but not sure...
    Russ
    Master Floor Sweeper

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    • #3
      Tension and compression the square will win out, simply because it has more cross sectional area. Buckling I don't remember which would be better, don't have my Strengths book with me at the moment, but as long as you are more worried about a "crushing" failure of the material rather than a buckling failure go with the steel. This might be expensive, but you could order a "Strengths of Materials" textbook from Amazon if you can find one for cheap (I don't know how much they are). Note not a materials engineering, but one that deals with loading, etc.

      Then again if you are just doing it for one project, you might not want to do that kind of effort
      You never learn anything by doing it right.

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      • #4
        In simple terms, Square tube with the same wall thickness will be 33% stronger than round tube.
        It also weighs about 30% more per foot
        Rich

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        • #5
          Certainly makes sense based on cross sectional area for tension/compression.

          So, I wonder how the force is oriented on the project. If the force is square to the flat sides in bending, or in compression/tension, square wins. Looks like round only wins for arbitrarily oriented bending loads.
          Russ
          Master Floor Sweeper

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          • #6
            Machinerys Handbook has great info on the modulus of different cross sections.

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            • #7
              Square appears to be substantially more resistant to bending no matter what the orientation of the square.

              I'd say in any type of comparison square has it over round.

              Makes you wonder why the race car guys always use round.....maybe because it looks better and can be bent easily in any direction.

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              • #8
                Exactly what i was wondering ...

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                • #9
                  thanks for the advice. so for a frame for this creation, would you think the square tubing would suffice? this is the frame that will use the home-built steering components, and as i mentioned in that post it is for low-speed off-road use. the frame will probably be about 6' long (with about a 50" wheelbase) and won't have any suspension, other than that afforded by the low-pressure tires. my main goal is to keep the weight down (and use steel, not aluminum or something exotic).

                  since my cost for the steel tubing is basically calculated by weight, if i need to use a thicker-walled round tubing to match the strength of the square tubing, the costs will be comparable. i just don't have a tubing notcher at present and figured square would be easier to cut and weld.

                  andy b.
                  The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining

                  Comment


                  • #10


                    Building one of these?

                    WE had plans on a hydraulic motor on each wheel, all 4 steer. A big motorcycle engine running a pump. A small rock crawler. Of course it sits and rusts over the motorcycle shop.
                    Excuse me, I farted.

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                    • #11
                      Has any one here ever seen square tubing used in air frame construction ?

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                      • #12
                        Some good points to ponder in the "round vs. square" debate by someone who has built a few frames and the merits of round vs. square.
                        Have a look.
                        Home, down in the valley behind the Red Angus
                        Bad Decisions Make Good Stories​

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                        • #13
                          Race cars are all about weight vs strength. Round tube weighs less than square tube, round tube is stronger both in torsion and deflection than square tube of the same weight. 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, 4130 chrome moly even stronger again. 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. Some people think that square is easier to work with but having worked extensively with both I find there is not much difference. A holesaw type notcher does not cost much to buy or can be made quite easily, and makes notching, tight fitting joints a breeze. I made my own notcher, from bits and pieces I had around the shop, in a couple of hours. I have built many race car and motorcycle frames with it over the years.

                          Ed

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                          • #14
                            IIRC round has only one advantage over square and that is in torsion,other than that it's square hands down.
                            If square means it's easier to fabricate then your chances of sucess are improved by that.
                            I just need one more tool,just one!

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                            • #15
                              Originally posted by ecortech
                              Race cars are all about weight vs strength. Round tube weighs less than square tube, round tube is stronger both in torsion and deflection than square tube of the same weight. 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, 4130 chrome moly even stronger again. 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. Some people think that square is easier to work with but having worked extensively with both I find there is not much difference. A holesaw type notcher does not cost much to buy or can be made quite easily, and makes notching, tight fitting joints a breeze. I made my own notcher, from bits and pieces I had around the shop, in a couple of hours. I have built many race car and motorcycle frames with it over the years.

                              Ed
                              Back the truck up,square tubing is most certainly availible in 1020,we stock it at work,it's not DOM,but it is electricaly welded just like 90% of the 1020 DOM round tubing on the planet.

                              There is a lot of good information out there in some of the car/motorcycle/truck crowd,but there is also a lot of BS.

                              ASTM-513(SAE 1020-1026) square tubing is common,even MD has it-

                              http://www.metalsdepot.com/products/hrsteel2.phtml?page=sqtube&LimAcc=$LimAcc

                              And for rectangular-

                              http://www.metalspecialty.com/SquareTubing.html

                              I laugh whenever one of the "race/performance/speed shops" sells "DOM" tubing claiming it's seamless,even thou it has "1020 ERW" all down the side of it and costs less than 1/3 of what seamless does.It falls into the nebulous realm of "billet"

                              Round tubing has only one advantage for an application like Andy is considering-it's easier to bend,nuff said.
                              Last edited by wierdscience; 09-05-2007, 01:15 AM.
                              I just need one more tool,just one!

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