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square or round tubing

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12K views 30 replies 16 participants last post by  JEEP_TJ_FREAK  
#1 ·
I went to my friends house today with 30' of square tubing ready to build my long arms and he refused to believe that square tubing is stronger that round tubing. Well who is right?
 
#3 ·
In an application where there is a bending force round is stronger. With round the force is applied evenly around the entire surface. With square you have areas which are weaker than others. Round is the way to go on longarms.
 
#6 ·
Let me rephrase that. In an application where the forces are directed in one direction square is stronger. That's why a crane or engine hoist, or truck frame is square. All of the force is in one predictable direction. In an application where the bending force can come from any direction round is stronger.

Hit a square tube on the diagonal between the two planes and it will bend easier than hitting it directly on one of the flats.

As someone else stated it's a helluva lot easier to do threaded inserts on round than square anyhow.
 
#9 ·
As I see it, this is a fundamental strength of materials question. IMO, the fair way to compare round tube to square is pick a similar wall thickness between the two and a similar amount of material so that the weight per linear foot between the two is constant.

That is, comparing the length of both tubes of the same length with the same wall thickness each weighing the same. For calculating bending on a simple cantilever beam, one equation is:

WL^3/3EI

where W is load applied (uniform distributed), L is length of beam (raised to the third power), E is elastic modulus of the material and I is the moment of inertia of the beam. Every term between the two beams is constant except the moment of inertia. Since the moment of inertia term is in the denominator, it is logical to deduce that the higher the value of moment of inertia, the lower the bending, the stronger the beam.

For a round tube, the moment of inertia, I, is calculated as:

pi(d2^4 - D1^4)/64

where D1 (raised to the fourth power) is the inner diameter of the tube and D2 is the OD.

For a square tube, the moment of inertia is calculated as:

(L2^4 - L1^4)/12

where L2 is the outside thickness of the square section and L1 is the inner wall distance.

- Square tube sample calc: 1 x 1 x 0.125 wall.

I = (1^4 - 0.75^4)/12 = 5.697e-2 in^4

the median length of material around the square section is 3.5 in.

- Round tube sample calc: the equvalent case round tube would have a circumference with the same value as the median length of the square tube (3.5 in). Mean diameter therefore is 1.114 in. So, OD is 1.24in, ID is 0.99in.

I = pi(1.24^4 - .99^4)/64 = 6.899e-2 in^4

Please check my work you math-heads, but it looks like the round tube is stronger than the square tube.

that is how I get it.
 
#10 ·
Math equations look good on paper and work for rocket science, but building a rock crawler isn't rocket science!
Real world ******* booty fab is not as exacting, you only need to worry about anti-squat and dive. 1/4" square tubing will withstand a lot of abuse, as will 1-1/2" cromo or DOM. The major lift kit manufacturers use 1-1/4" for their long arms, so anything bigger is that much better.
Besides, if you got a bunch of square tubing, if you bend something, it's cheap enough to replace it.
 
#11 ·
Geesh said:
As I see it, this is a fundamental strength of materials question. IMO, the fair way to compare round tube to square is pick a similar wall thickness between the two and a similar amount of material so that the weight per linear foot between the two is constant.

That is, comparing the length of both tubes of the same length with the same wall thickness each weighing the same. For calculating bending on a simple cantilever beam, one equation is:

WL^3/3EI

where W is load applied (uniform distributed), L is length of beam (raised to the third power), E is elastic modulus of the material and I is the moment of inertia of the beam. Every term between the two beams is constant except the moment of inertia. Since the moment of inertia term is in the denominator, it is logical to deduce that the higher the value of moment of inertia, the lower the bending, the stronger the beam.

For a round tube, the moment of inertia, I, is calculated as:

pi(d2^4 - D1^4)/64

where D1 (raised to the fourth power) is the inner diameter of the tube and D2 is the OD.

For a square tube, the moment of inertia is calculated as:

(L2^4 - L1^4)/12

where L2 is the outside thickness of the square section and L1 is the inner wall distance.

- Square tube sample calc: 1 x 1 x 0.125 wall.

I = (1^4 - 0.75^4)/12 = 5.697e-2 in^4

the median length of material around the square section is 3.5 in.

- Round tube sample calc: the equvalent case round tube would have a circumference with the same value as the median length of the square tube (3.5 in). Mean diameter therefore is 1.114 in. So, OD is 1.24in, ID is 0.99in.

I = pi(1.24^4 - .99^4)/64 = 6.899e-2 in^4

Please check my work you math-heads, but it looks like the round tube is stronger than the square tube.

that is how I get it.
Basis of this argument is theoretical and has no real world application. The comparisons between square and round tube of different O.D. is ALWAYS going to give advantage to the tube with the larger O.D. given similar wall thickness. Try comparing sections with the same O.D. and mass per unit length, as well as a comparison between sections of the same O.D. and wall thickness. It seems like a good idea but you should always take the availability of commonly available materiel from both a design and cost standpoint.

nathon001 said:
I went to my friends house today with 30' of square tubing ready to build my long arms and he refused to believe that square tubing is stronger that round tubing. Well who is right?
If you are comparing square tubing with round tubing given that it is the same material, same O.D, and the same wall thickness square is going to be stronger. Look in the library in the AISC Design Guides or a CISC “Handbook of Steel Construction” to see where my reference material comes from. For an example we will use 4”x4” square tubing compared to 4” round tubing as well as 3.5”x3.5” square tubing compared to 3.5” round tubing. All material strengths are based upon use of CSA G40.21 Gr. 350W material (ASTM A572 Gr. 50 Similar)

4”x4” x 0.188” wall has a Mr (kN-m) of 20.3 and weighs 9.46 #/foot. 4” x 0.188” wall has a Mr (kN-m) of 14.1 and weighs 7.67 #/foot. Advantage to square because it is 1.43 times stronger yet only weighs 1.23 times as much per foot.

We can also compare 4”x4” x 0.188” wall which has a Mr (kN-m) of 20.3 and weighs 9.46 #/foot with 4” x 0.250” wall which has a Mr (kN-m) of 18.2 and weighs 10.0 #/foot. Advantage to square because it is 1.12 times stronger yet weighs less per foot.


3.5”x3.5” x 0.188” wall has a Mr (kN-m) of 15.2 and weighs 8.17 #/foot. 3.5” x 0.188” wall has a Mr (kN-m) of 10.7 and weighs 6.67 #/foot. Advantage to square because it is 1.43 times stronger yet only weighs 1.23 times as much per foot.

We can also compare 3.5”x3.5” x 0.188” wall has a Mr (kN-m) of 15.2 and weighs 8.17 #/foot with 3.5” x 0.250” wall which has a Mr (kN-m) of 13.7 and weighs 8.69 #/foot. Advantage to square because it is 1.11 times stronger yet weighs less per foot.
 
#12 ·
Note: I checked my math above and it looks to be right, but the square is stronger because it has the lower moment of inertia. On that I was incorrect. My bad. However . . . . .

jasonmt said:


Basis of this argument is theoretical and has no real world application. The comparisons between square and round tube of different O.D. is ALWAYS going to give advantage to the tube with the larger O.D. given similar wall thickness. Try comparing sections with the same O.D. and mass per unit length, as well as a comparison between sections of the same O.D. and wall thickness. It seems like a good idea but you should always take the availability of commonly available materiel from both a design and cost standpoint.
Theoretical?? Hello, where do think AISC and CISC get there numbers from? Strength of Materials. It is the basis for all structural engineering. You're slamming the very source of your argument.

:rolleyes: but thanks for showing me my conclusions were wrong. :rolleyes:

donjr5 said:
Math equations look good on paper and work for rocket science, but building a rock crawler isn't rocket science!
Real world ******* booty fab is not as exacting, you only need to worry about anti-squat and dive. 1/4" square tubing will withstand a lot of abuse, as will 1-1/2" cromo or DOM. The major lift kit manufacturers use 1-1/4" for their long arms, so anything bigger is that much better.
Besides, if you got a bunch of square tubing, if you bend something, it's cheap enough to replace it.
Don't know where this is coming from at all. The original post asked for which is stronger round tube or square. JasonMT and I took different approaches. He compared table date of cross-sections, and I compared same mass/length sections with identical wall thickness. Both took math to get to the answer: square is stronger.

But maybe ******* booty fab from Summer Tooth Billy Bob is the better way to get an answer:flipoff2:
 
#13 · (Edited)
Geesh said:
As I see it, this is a fundamental strength of materials question.
IMO, the fair way to compare round tube to square is pick a similar wall thickness between the two and a similar amount of material so that the weight per linear foot between the two is constant.

That is, comparing the length of both tubes of the same length with the same wall thickness each weighing the same. For calculating bending on a simple cantilever beam, one equation is:

Square tube sample calc: 1 x 1 x 0.125 wall.......

Round tube sample calc: the equvalent case round tube would have a circumference with the same value as the median length of the square tube (3.5 in). Mean diameter therefore is 1.114 in. So, OD is 1.24in, ID is 0.99in.

Geesh said:
Note: I checked my math above and it looks to be right, but the square is stronger because it has the lower moment of inertia. On that I was incorrect. My bad. However . . . . .



Theoretical?? Hello, where do think AISC and CISC get there numbers from? Strength of Materials. It is the basis for all structural engineering. You're slamming the very source of your argument.

Perhaps I expressed my thoughts in a way that didn't get my point across: I wasn't slamming them, just you. AISC/CISC at least checks their work.

:rolleyes: but thanks for showing me my conclusions were wrong. :rolleyes:

:It is more than your conclusions that are wrong.

But maybe ******* booty fab from Summer Tooth Billy Bob is the better way to get an answer:flipoff2:
[/QUOTE

So you set out your argument that both the square tube and round tube should be of similar wall thickness and the same unit mass, but after you get lost in the math the square tube is 1"x1"x0.125" wall which weighs 1.497#/foot and 1.24"x0.250 wall round tube which weighs 2.67#/foot? You didn't even prove your argument, you just spewed out meaningless #'s out of thin air. Then you cut me down because I proved your initial argument using a reference book. NICE :zzz
 
#15 ·
TNToy said:
The most frustrating thing on this BB is wading through link suspesion threads full of posts like this. :rolleyes:

NO!
Sure, you can booty-fab your suspension, but you're liable to have a hard time staying on the road or getting up the hill.
As far as the mathematical equations, etc., I've been in the real world for over 20 years, and in most situations, the real world is not very similar to the theoretical world. For a mall cruiser, do your math and make it pretty. For the rocks, make it functional and strong. Square tubing or round, if it's got 1/4" walls and your welds are good, it's gonna handle about anything you can dish out. It ain't rocket science, it's Jeep engineering!
 
#16 ·
donjr5 said:

I've been in the real world for over 20 years, and in most situations, the real world is not very similar to the theoretical world. For a mall cruiser, do your math and make it pretty. For the rocks, make it functional and strong. Square tubing or round, if it's got 1/4" walls and your welds are good, it's gonna handle about anything you can dish out. It ain't rocket science, it's Jeep engineering!
:rolleyes: Ok.. so rockcrawling defys all the laws of physics?
The only reason mathamatical equations, if you using the proper ones, are not representing how things are actually working is becuase you are overlooking factors. Sure, if you overbuild it strong enought, it will hold up fine, but at the cost of weight and money. It's always a good idea to overengineer somewhat so that the part should last, but overkill is not always a good thing. The more you overkill parts, the heavier it's gonna get, and the more stuff your going to end up breaking.
 
#17 ·
jasonmt said:
Geesh said:

Theoretical?? Hello, where do think AISC and CISC get there numbers from? Strength of Materials. It is the basis for all structural engineering. You're slamming the very source of your argument.

Perhaps I expressed my thoughts in a way that didn't get my point across: I wasn't slamming them, just you. AISC/CISC at least checks their work.

:It is more than your conclusions that are wrong.

So you set out your argument that both the square tube and round tube should be of similar wall thickness and the same unit mass, but after you get lost in the math the square tube is 1"x1"x0.125" wall which weighs 1.497#/foot and 1.24"x0.250 wall round tube which weighs 2.67#/foot? You didn't even prove your argument, you just spewed out meaningless #'s out of thin air. Then you cut me down because I proved your initial argument using a reference book. NICE :zzz
Put the verbal darts down. Did you even read my post?
  • 1.24" OD - 0.99" ID = 0.25" Diametral,
  • that is 0.125" wall, not 0.25" wall (draw two concentric circles, complete an OD then an ID, see the diff?)
  • almost the exact same mass per unit length
  • D'oh
At what part in this fray are you going to comprehend what you read before you assail me. And read my response, Mr sensitive, when was I slamming you? Even close to the level you have slammed me, twice now.

Again with "Meaningless Numbers" rant. . . I won't even reiterate my earlier argument. There is nothing wrong with using fundamental math to solve and argument. It wasn't like splitting the atom, just comaring two similar tubes with the same mass/length, one square one round.

But if the argument was asking "Please look in your AISC manual and tell me which off-the-shelf tube has the best 'Mr'", then by all means let's take your approach. And what is "Mr"? You show units of torque, but you don't even take time to define your terms. How does "Mr" make one tube stronger over the other? Could you at least explain that? And this "1.43 times stronger yet weighs 1.23 times as much", show the relevency. Complete your work, God forbid, DO THE MATH.

Why is it there are often those who shun math? It may be a PITA to do (and often I do it wrong, see above), but it is the one way to factualy solve the type of argument presented in this post.
 
#18 ·
Geesh said:


Put the verbal darts down. Did you even read my post?
  • 1.24" OD - 0.99" ID = 0.25" Diametral,
  • that is 0.125" wall, not 0.25" wall (draw two concentric circles, complete an OD then an ID, see the diff?)
  • almost the exact same mass per unit length
  • D'oh
At what part in this fray are you going to comprehend what you read before you assail me. And read my response, Mr sensitive, when was I slamming you? Even close to the level you have slammed me, twice now.

Again with "Meaningless Numbers" rant. . . I won't even reiterate my earlier argument. There is nothing wrong with using fundamental math to solve and argument. It wasn't like splitting the atom, just comaring two similar tubes with the same mass/length, one square one round.

But if the argument was asking "Please look in your AISC manual and tell me which off-the-shelf tube has the best 'Mr'", then by all means let's take your approach. And what is "Mr"? You show units of torque, but you don't even take time to define your terms. How does "Mr" make one tube stronger over the other? Could you at least explain that? And this "1.43 times stronger yet weighs 1.23 times as much", show the relevency. Complete your work, God forbid, DO THE MATH.

Why is it there are often those who shun math? It may be a PITA to do (and often I do it wrong, see above), but it is the one way to factualy solve the type of argument presented in this post.
I agree. I think that the formula's are more useful personally, because it helps to explain why something is, not just simply that it is. When it comes time for me to actually verify the strength of a particualr material, that's when I will resort to the tables, just to make sure I don't mess up the math.

Coming from a electronics engineering degree, I don't know as much of the mechanical engineering as I would like. Do either the tables or the formulas take into consideration the angle the force is applied to the square tube, as mentioned above. By this I mean on two axis. The one would probably affect both types equally (I am guessing): For example if the force is pulling straight down on a horizontal tube, as opposed to pulling down at an angle. If the force is pulling in a plane parrallel with one of the sides, or if it is an angle. For example if this was the tube cross-section [] and the force was pulling towards 2:00 (on a clock face). How would the formula factor in these sort of conditions, and what effect does it have on square tube? I don't see how this should be a factor for round tube though, because the circular nature fo the pipe should handle the force equally in all radial directions.
 
#19 · (Edited)
OverThrottle said:


I agree. I think that the formula's are more useful personally, because it helps to explain why something is, not just simply that it is. When it comes time for me to actually verify the strength of a particualr material, that's when I will resort to the tables, just to make sure I don't mess up the math.

Coming from a electronics engineering degree, I don't know as much of the mechanical engineering as I would like. Do either the tables or the formulas take into consideration the angle the force is applied to the square tube, as mentioned above. By this I mean on two axis. The one would probably affect both types equally (I am guessing): For example if the force is pulling straight down on a horizontal tube, as opposed to pulling down at an angle. If the force is pulling in a plane parrallel with one of the sides, or if it is an angle. For example if this was the tube cross-section [] and the force was pulling towards 2:00 (on a clock face). How would the formula factor in these sort of conditions, and what effect does it have on square tube? I don't see how this should be a factor for round tube though, because the circular nature fo the pipe should handle the force equally in all radial directions.
JMO, here, don't wanna flame war again

In my example, I sited a simple uniformly distributed load for both sections. No matter how you rotate the round section, the moment of inertia is the same. For the sqaure section it would seem to vary as you go around.

Due to symmetry, the applied load need only clock a range of 45 degrees (from the top flat to the square rotated like a diamond) to cover every case. I seem to recall checking on this once and it turned out not to change much (maybe not even at all, I cannot remember), showing the square tube the stronger-in-bending candidate over the round tube.

But you bring up another point. Tubing used for our applications rarely sees pure bending or evenly distributed loading. It is typically a case of a complex loading environment that changes with time (very dynamic). There are multiple moments combined with buckling, tension, etc.

Unless someone takes the time to generate a dynamic FE model (which is a huge time consumer), the best judgement engineering approach is to side on the conservative
  • Pick static loads and apply factors that multiply forces beyond you think the rig will see. the vehicle has a known mass, gravity is a constant, but impact and drop-off loads can be many times that constant.
  • Apply the simpler SOM formulas
  • Verify that the material or section won't fail
  • Size up or down accordingly
  • There ya go!

That is how I would do it. We had to put a big structure on a plane and had to meet safety of flight with hand calcs. Similar scenario. Just went conservative, satyed withing the wieght budget and the hardware was proven safe to fly.

Good luck
 
#20 ·
Geesh said:
But you bring up another point. Tubing used for our applications rarely sees pure bending or evenly distributed loading. It is typically a case of a complex loading environment that changes with time (very dynamic). There are multiple moments combined with buckling, tension, etc.
Which is what I mentioned earlier in much simpler, non-engineer terms so us laymen could actually understand it.:flipoff2:
 
#21 ·
OverThrottle said:


:rolleyes: Ok.. so rockcrawling defys all the laws of physics?
The only reason mathamatical equations, if you using the proper ones, are not representing how things are actually working is becuase you are overlooking factors. Sure, if you overbuild it strong enought, it will hold up fine, but at the cost of weight and money. It's always a good idea to overengineer somewhat so that the part should last, but overkill is not always a good thing. The more you overkill parts, the heavier it's gonna get, and the more stuff your going to end up breaking.
Of course rock crawling isn't straight line, but you do have a point about the overbuilding/weight issue. I'm soaking all this up before I start building...
 
#22 ·
To answer the original question at hand, if you compare round vs. square with the same diameter or width, wall thickness, and material the square would be stronger for a suspension link. That is considering bending, tension and buckling strengths, all of which a suspension link is subjected to. The loading on a suspension link is really very simple, I'm an engineer, I know the equations, I don't need to try and dazzle you with them, it took me 5 minutes to look up moments of inertia in a handbook and compare them, also some experience and common sense doesn't hurt. It doesnt matter if the square is flat or on edge. Yes the square will be heavier and maybe you should compare strength vs. weight but I don't have time for that right now. Another thing to cosider is material selection, square is pretty much only available one grade, but round is available in DOM and 4130, both of which are stronger than square tubing grades, and 4130 has the potential to be heat treated to increase it's strength drastically.
 
#23 ·
donjr5 said:

Sure, you can booty-fab your suspension, but you're liable to have a hard time staying on the road or getting up the hill.
As far as the mathematical equations, etc., I've been in the real world for over 20 years, and in most situations, the real world is not very similar to the theoretical world. For a mall cruiser, do your math and make it pretty. For the rocks, make it functional and strong. Square tubing or round, if it's got 1/4" walls and your welds are good, it's gonna handle about anything you can dish out. It ain't rocket science, it's Jeep engineering!
Great. So what are you going to do when, since you only worried about anti-squat... you end up with a rear suspension that won't let you stick your line during articulation, because it has 4 inches of rear steer? Or, as usually is found along with that... it has a roll center so low the body practically pulls the rig over the moment you look at a side-hill the wrong way?

Dude, it's not terribly complicated to build a suspension, but it's not quite *THAT* easy to build one that really works well.
 
#24 ·
SeaBass44 said:

STFU........when you find out why round is stronger, also see how hollow is stronger then solid;)
Really? Now I am really confused. I had always assumed hollow was used more often simply for weight and cost savings. I know that a thick guage is stronger then a thinner one of the same OD. If hollow truly is stronger then solid, than at some point, the my statement would have to be false. At what point would a thinner wall be stronger than a thicker?

I guess I am going to have to invest some time into reading up on mechanical engineering. The more I hear, the less sense it seems to make! :confused:
 
#25 ·
OverThrottle said:


Really? Now I am really confused. I had always assumed hollow was used more often simply for weight and cost savings. I know that a thick guage is stronger then a thinner one of the same OD. If hollow truly is stronger then solid, than at some point, the my statement would have to be false. At what point would a thinner wall be stronger than a thicker?

I guess I am going to have to invest some time into reading up on mechanical engineering. The more I hear, the less sense it seems to make! :confused:
Dont waste your time, hollow IS NOT stronger that solid. This is a misconception I have been hearing lately, I don't know where it came from but it is total BS.
 
#26 ·
OverThrottle said:


Really? Now I am really confused. I had always assumed hollow was used more often simply for weight and cost savings. I know that a thick guage is stronger then a thinner one of the same OD. If hollow truly is stronger then solid, than at some point, the my statement would have to be false. At what point would a thinner wall be stronger than a thicker?

I guess I am going to have to invest some time into reading up on mechanical engineering. The more I hear, the less sense it seems to make! :confused:
"Stronger" just gets tossed around too much. Rarely will the force in which a certian part is "stronger", will be mentioned.

Roughly for solid vs tube, tube is "stronger" per unit of weight;)

As for getting your ME, there are lots of E's on this board, but only a handful that can properly apply what they learned in the text book, to real world apps. I got a ME sitting 10 feet from me now, that looked at a ball check-valve for 45 minutes before he figured out how it works. He could check his text, and run some equations, but not really have any fawking idea what it means..besides being a number on the paper.:D

Guess I am saying, take everything you see on here with a grain of salt.;)