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
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.
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
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?
wrote: (clip) Which one would you use for a
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
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.
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.
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
don't want to get something built and it not be within the rules and
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