Re: MM210 ROCKS!!

Your strength numbers are a function of the Section modulous, not the Moment of inertia. (Stiffness is proprotional to the moment of inertia.) Tension strength is proportional to wall (and weight)Compression strength is proportional to wall (and weight) as long as you stay inside Ulhler's constant. For steel this means length MUST be less than 89 times the diameter. (safety factor says no more than 30 to 48 times!)

For the same OD and wall, square is 30% heavier and 60% stronger in bending on the X and Y axis. (Good) I don;t have the numbers in front of me but square does not handle torsion as well as round, it also fails in a buckling moment differently than round. (Bad) But a properly designed space frame (think experimental aircraft) has all members in tension or compression ONLY. Bending monents only occurr if you hang parts (like engines) in the MIDDLE of a tube rather than at an intersection. Or when you have "unscheduled dynamic testing" situations.

As for the weight calc, compare your 1.5" tube with the next size larger diameter and smaller wall. Section modulus goes up, weight goes down. So thinner wall, larger tube is theoretically better. The flaw comes in when the wall thickness gets too thin to resist side impact loads. A dent will cause instant collapse of a compression stressed tube.

For the Mini-Baja that I deal with, the rules committee inspected some "field tested" vehicles. (Consider rolling off a 30' embankment!) The larger diameter/ thinner wall vehicles do not fare as well as "equivilent" tubes of smaller/thicker. One collapses, the other bends gracefully. Think failure mode.

As for the fabrication of square vs round: Square can be bent only on the 2 orthoginal axis, round can be bent however you want. Consider a joint: you can have a perfect multiple member joint in round tube as long as the center lines all intersect. Try the same with square tube with multiple members and multiple planes.

We build a new vehicle from scratch every year. First couple were square tube. Quick and easy to deal with. Next was a small tube heavy wall, easy to weld, weighed less than the square tube version. Next was a larger tube thinner wall that shed more pounds, but was a stinker to weld up.

One thing to watch out for is hanging potential impact load stress points in the middle of any piece of tube. Engine mounts are a great example: lots of vibration stress as well as the possibility of a crash where the engine mount folds up and takes a section of the frame in the wrong direction.

You have the right idea on your original calcs, you just need to work through some of the operational details as well as the unseen "testing" situations. I can tell you that for the Baja buggies that we compete with, less than 50% of the cars finish the 4 hour race. This is a vast improvement from some years back when 50% of the cars did not finish HALF the 4 hour enduro! Touring the pit area was a lesson in failure analysis.

Cheers.


BoyntonStu wrote: 
A comparison of 1.5" square tubing to 1.5" round tubing.

.083 wall   Round has a Moment of inertia of .093
                 Square has a Moment of inertia of .158

                 Round weighs 1.256lb/ft and 50 feet would weigh 62.8
pounds
                  Square weighs 1.6 lb/ft and 50 feet would weigh
79.975 pounds

  If you thinned down the 1.5" square tubing to 0.065 the moment of
inertia 0.128 would still be greater than round at 0.1256  it would
weigh 1.2685 lb/ft and 50 feet would weigh 63.43 pounds compard to 62.8
pounds.  Are the 17 pounds so important?

In other words, for the same weight square tubing is .128/.093 or 37%
stronger in bending moment and much, much stronger with the same wall
thickness.

The round tubing could be of a higher grade steel than 1020 and it
could make up the difference in strength, but at a greater cost.

What are the reasons that square tubing is restrictiver to design?   I
can imagine any design possible with square tubing.


BoyntonStu

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