Square Tube or Round Tube?

There are a couple things I want to clear up about tube. A square tube has the same bending stiffness about any axis through its center of area. This seems weird to some people intuitively because the square is taller across the diagonal than across its flats. However, we know from trivia that if a 2nd order tensor has Iii = Ijj (square has the same bending stiffness across each principal axis and Iij=0 (there is no product of inertia) then the value is invariant under rotation. Physically you could consider that while the height from the neutral axis gets bigger in the diagonal orientation, more of its area (the other diagonal) is now close to the neutral axis and not contributing to stiffness.

Second, a round tube of the same wall thickness and weight as a square tube is significantly stiffer in bending than the square tube. A round tube whose OD is equal to the side of a square tube is less stiff than the square tube in bending (but is lighter). In other words Round tube is more weight efficient than square tube. The thinner the wall, the better round tube does compared to square tube.

In reply to cee'dub:

You make excellent points, however you neglect to bring all the relavent issues to question.. most round tubes are butt jointed.. a weaker connection than the overlap method commonly used in various locations on many racecar (and the Jaguar XK-E) In addition there is more weld surface area on a square tube if a butt joint is required. Finally the skill required to properly cut a round tube so full contact is achieved is monumentally higher than the skill required to cut a square tube to achieve the same thing..

Pick up the front sub-frame of a Jaguar XK-E. It's astonishingly light, one handed light*. This is a car that fully passed all required crash tests with no driver compartment intrusion. (It's made from extremely thin square tube that is brazed together not welded)

rcutclif wrote:

Appleseed wrote: My secret weirdo wish is to narrow the body of a RWD coupe while keeping the track the same. Imaging an open wheel Camaro or FC RX-7?

Or, an open wheel 750iL? hehe Also, I seem to be reading the rules that you can recoup 1008 dollars no matter your purchase price. Therefore, you really could get a lot of budget going with the right car... Can someone confirm or deny?

The 2017 rules specially state you cannot recoup more than purchase price of the car.

So as my car cost $500 I can recoup no more than the $500 purchase price.

Its funny Ya'll mention Trianglation and even Triangle shape tube. but no one mentions How much Stiffer a tringale is compared to a round or Square tube.

Well it is stiffer but when bent must be replaced. a square tube frame is used as a base in a race car only because of rules, a triangle shape or tube has almost NO deflection a round shape allow's deflection in All Directions and a sguare will bend and Stay bent,it can be straightend but the edge's Have been comprmised.

If the OP put's in Diag. bars in all of his box's it will likely be ok, that Vette Kart thing needs a windshield to be drag legal,but I don't know how much of one.

Chrome moly is another animal and must be replaced after being crashed and cost is much higher, But lighter and stronger.

Unfortunately this car has come and gone (2015 thread and challenge), but I still love learning about the strengths and weaknesses of square vs round!

In reply to Robbie: OH!

I Have encountered that before/ Not looking at a Date.

GTXVette wrote: Its funny Ya'll mention Trianglation and even Triangle shape tube. but no one mentions How much Stiffer a tringale is compared to a round or Square tube. Well it is stiffer but when bent must be replaced. a square tube frame is used as a base in a race car only because of rules, a triangle shape or tube has almost NO deflection a round shape allow's deflection in All Directions and a sguare will bend and Stay bent,it can be straightend but the edge's Have been comprmised. If the OP put's in Diag. bars in all of his box's it will likely be ok, that Vette Kart thing needs a windshield to be drag legal,but I don't know how much of one. Chrome moly is another animal and must be replaced after being crashed and cost is much higher, But lighter and stronger.

I'm sorry some extremely sharp racers use square tubing because everything considered it can be made stiffer and stronger, and faster than round tube.. Trans Am cars have been built with both round tube and square tube.. and most of the winner cars were square tube.. Not to mention Jaguar "D" type, and XK-E (well to be fair there is one tube on each side that is round) Now yes they were extremely thin wall and made of the special tubing the British tend to build race cars out of (It's not chrome moly but similar)

"tube frame" race cars like Trans Am use both types of tubing. The chassis which is loaded in predictable ways can be made with square tubing because the loads the chassis sees can be predicted due to the way the suspension and the springs inputs loads into the chassis. The tubing can be oriented in a manner that maximizes the tubes stiffness in the direction required to limit deflection of the tubing and hence the chassis.
The drivers compartment and the requirement to use "round" tubing is because the loads that that part of the car may see and the direction they may come from can't be predicted since most of those loads will come when "things go bad" such as an "off" or contact with another car or guard rail. Round tubing is equally strong in all directions of bending while square is not.

I've seen all kinds of race car chassis, from top fuel dragsters to sprint cars, street stocks, off road buggies and trucks, oval track stock cars, pro mods, sprint cars, and nearly every other thing out there. Nearly all of those fabricators use round tube with a foundation in square or rectangular if required. Then there's this guy. Looks like over 80% of it is all square.

In reply to jimbbski: You are correct in that round tube is equally strong on all directions...... But! Nearly every roll cage I see has bends in it which weakens the tube.. I realize that they make the thickness of the tube thicker (and heavier) to offset that loss of strength but where not required for that purpose. Weight conscience people can use thinner and thus lighter square tube to gain strength where loads are predictable.. Example? the picture in the previous post..

ncjay wrote:

The reason for round tube chassis in the front of dragsters is to replace the suspension they lack.. if you watch a dragster leave, the whole chassis flexes leaving the front wheels on the ground to help steering as long as possible..

In reply to curtis73 :

Your example,

"For instance if you have a 1.5" tube with 1/8" walls that weighs a certain amount per linear foot, and also have some 2" with 3/16" walls that weighs the same, the 2" is stronger." 

went astray. It does not reflect your preceding statement " With tubing (round or square), for the same weight, a larger diameter/thinner wall is stronger than smaller diameter/thick wall. (to a point)."

Perhaps you meant 2" diameter with 3/32" wall weighing the same as 1.5" with 1/8" (4/32") walls. An easy way to calculate the comparative weight is by multiplying the diameter by the wall thickness. In the above case, 2 x 3/32 =6/32 = 1.5 x 4/32. 

Have we a new record for "responds to ancient thread"?