round vs square tubing

each has its place. square has a flat side, can be very handy. Square is stronger for a given size, i.e. 1" dia, vs. 1" sq. but round gives more strength for a given weight. If you do odd angles its easier to notch round to fit. Depends on what you're making. A race car or airplane with lots of odd angles and need for low weight always uses round. If you're building a box-like unit use square.
Karl

Square is stiffer and stronger in bending. Round is stiffer and stronger in twisting. If you're making a space frame, chassis torsional stiffness usually is the limiting parameter, so round has a slight edge, on paper, at least.
However, there are practical issues, as Karl has pointed out. That's why professionally designed space frames have been made of both kinds of tube.
Do you have a chassis design yet?
-- Ed Huntress

With square tubing it's easier to keep holes and angled end cuts properly aligned. You can clamp a piece of angle to round tubing but the clamps either dent the tubing or slip off.
Jim Wilkins

This and other questions that you have posted indicate that you may not have the skills to properly execute saddle weld on round tubing. Go with the square tubing. It's much easier to weld correctly.

wrote: (clip) Which one would you use for a tubular chassis? ^^^^^^^^^^^^^^^^^ Then, you could complicate your thinking by using some of each. Even mix in some angle iron where appropriate. It has a lot to do with strength and weight, but also the complexity of the joints, and what you have to fasten to it.

The first thing to do is get the rulebook for whatever racing you want to do. Most organizations have pretty specific requirements for tube shape, diameter, wall thickness, and material for roll bars/cages. Then call the nearest chassis inspector for that organization and talk to him/her. They are the ones that will have to sign off on it before you go racing, so you might as well get their input at the start.
-- Regards, Carl Ijames carl dott ijames aat verizon dott net (remove nospm or make the obvious changes before replying)

A quick look at the Mechanical engineers handbook shows that for a cantilevered beam with a load on the free end will cause a certain deflection or bending of the beam. A little calculation shows that the deflection of a round tube will be a factor of 1.69 times that that will be experienced by a square tube of the same dimensions and wall thickness and material. Ergo the square tube is stiffer in this particular application. Very little additional effort could compare them in torsional loading.

Just be aware that there are two different things that one may want to compare here. When you say "same dimensions," you're probably comparing a round tube of, say, 1" dia. with a square tube that measures 1" across the flats, with the same wall thickness in each. In that case the square tube will be stiffer but it will also weigh more, by about 27%.
If you thicken the walls of the round tube to equal the weight of the square tube, the square still has a stiffness advantage. But if you make the weights equal by increasing the diameter of the round tube (by around 25.06%) while leaving the wall thickness the same for both square and round, the round tube comes out stiffer in bending by around 21%.
In a race car it may make more sense to compare weights for different shapes of equal wall thickness. In that case the round tube wins on all counts.
-- Ed Huntress

Don't forget that you need to specify the tubing make-up also. Each of the types has different specs....Seamless, Seamed and DOM all can be used if the sizes are corrected to get the required design numbers. If you are going to be building to meet a set of rules, make sure you read and understand them thouroghly before you get your materials. You don't want to get something built and it not be within the rules and specs.

Ed you are dead on. The deflection is inversely proportional to the fourth power of the diameter while the weight only goes up with the second power, assuming the thickness stays the same. Another factor for consideration in tube structure design is the resonant frequency of that element which is a factor in cracking, and is a function of the square root of the stiffness/mass. The deflection being inversely related to the stiffness. At least in my helicopter that resonant frequency can be more important than the weight.