View Full Version : When square and when round (tubing of course)


carguy123
02-05-2008, 12:09 AM
If you were fabricating a kit car chassis when would you want to use round vs. square or rectangular tubing.

Square tubing just seems so much easier to work with.

brianpgriset
02-05-2008, 07:33 AM
Round tube is used often because it can handle torsional (twisting) stresses far better then square tube.

Defender Chassis
02-05-2008, 08:07 AM
Square tubing just seems so much easier to work with.

Why?











(Why does a reply have to be 10 characters?)

carguy123
02-05-2008, 02:20 PM
Well, if for no other reason than the accuracy required to notch round tube. Butt joints are much easier to make accurately and hold tolerances.

In the round vs. rectangle torsional stresses, it would seem that rectangle would let you put the most resistance to movement in the plane that your are most likely to get the stress.

I have seen and known a couple of kit car builders. They all started with square and rectangle tubing but eventually switched to round mostly due to how it looks &/or public perception. They thought it looked more professional.

But round tubing gave them a much higher misteak rate with cuts and fittin which means assembly time increased.

Attachement points have to be more precise on round - there's a much larger margin for error on a wide flat surface. It also took longer to weld. One thing I think might be better with round would be twisting when tack welding.

But if there is a really, really good reason that round is better I will make the effort to work with it. The chassis mock up is already in round plastic plumbing pipe although it has slip fittings for joints.

brianpgriset
02-05-2008, 04:17 PM
For cases of simple pure bending moments or simple tension or compression square tubing is better at handling stresses. If every member of the chassis was subject to only those forces, you could easily make it out of square tube and not sweat it. Unfortuantly torsion is going to be present in the chassis no matter what. Round tube simply more efficient in torsion becasue of the location of the material relative to its center.

Also another thing to consider is if you use equal sized round and sqaure tubing the square will weigh more. But honestly I'd bet you'd be A-OK using sqaure tube for the chassis.

Each has its own pros/cons

Vicegrip
02-05-2008, 10:04 PM
It is hard to find ERW square or rectangle tubing made from high quality metal but there are many grades of round tubing from ERW to drawn on mandrel in a selection of alloys and specs (DOM).

carguy123
02-05-2008, 10:25 PM
It is hard to find ERW square or rectangle tubing made from high quality metal but there are many grades of round tubing from ERW to drawn on mandrel in a selection of alloys and specs (DOM).

Ok, I didn't know that.

What I had gotten to thinking was to make the "base" out of square/rectangle &/or some of the more basic mounting points and finishing out the middle with the round.

I know the round is lighter due to the missing corners, but does anyone know how much lighter? I mean is it 10% lighter, 20% or just 2-3%?

d110pickup
02-06-2008, 07:02 AM
Carguy,
Take a look at one of the steel suppliers reference books and you'll find a ton of information.
For example:
2" diameter steel tube with .125 wall weighs 2.50 lbs. per foot.
2"x2" square steel tube with .125 wall weighs 3.05 per foot.
As you can see if you build an entire frame & roll cage from square tube it'll weigh quite a bit more. Also do a google search for square tube roll cages and you'll see they generally look horrible. Also square tube is bendable only in two planes while round doesn't have that limitation. If you look at what others have done you'll see that square is used quite often for frame rails and round is used for the remainder.
Just my .02 cents.
Mike

Jononon
02-06-2008, 07:48 AM
I'd like to recommend: http://www.carrollsmith.com/books/ngner2win.html as an excellent primer.

Vicegrip is right, 1020 DOM, for example, ideal for chassis tubing, is only available in round section.

d110pickup
02-06-2008, 10:52 AM
I'd like to recommend: http://www.carrollsmith.com/books/ngner2win.html as an excellent primer.

Vicegrip is right, 1020 DOM, for example, ideal for chassis tubing, is only available in round section.

I agree, the Carrol Smith book is excellent, in fact anything I've read from him is worth having.

Mike

carguy123
02-06-2008, 11:57 AM
Got my reading glasses on.