Torsional Rigidity

[killer_datsun]

what has more torsional rigidity a piece of 2x4 steel square tube or a piece of steel round tubing that has the same diameter and wall thickness of the 2x4?

[silver69camaro]

You didn't specify if that was a 2" tube or a 4" tube....

[vanzuuk1]

I dont know the answer but my bet is that round is stiffer.

[David Pozzi]

I've allways read that round is stronger for a given size and weight of tube.
Lots of chassis get built for IMSA type cars out of square tube because it's easier to rivet aluminum panels to and they have to meet a high weight limit anyway.

[killer_datsun]

the tubing would have a diameter of 4 inches

[nancejd]

Also, there is no such thing as a square 2x4 tube, it would be rectangular.

[RobM]

....a roll cage on top of that tube is what gives is the torsional strength, the rec. tube gives you the beam strength you need

[DeepBlue68]

I'm not sure which one you're talking about:
1. Which material has better torsion resistance when you're just looking at a simple fixed beam undergoing torsion, or
2. Which one will give you higher overall rigidity from your rollcage as a whole

If you're talking about which cross section resists torsion better (topic number 1), that can be found using the polar moment of inertia of each of their cross sections. The higher the polar moment of inertia, the more resistant it is to torsional stress.

You didn't say whether you were referring to 2" diameter of 4" diameter, so I calculated both. For the circular cross section, the polar moment of inertia is given by:

2" Diameter
J = 1/32 * pi * diam^4
= 1/32 * pi * (2 in)^4
J = 1.57 in^4

4" Diameter
J = 1/32 * pi * diam^4
= 1/32 * pi * (4 in)^4
J = 25.13 in^4

Since I wasn't sure which kind of cross section you were talking about, I did it for all 3. For the rectangular cross section:

2" x 4"
J = 1/12 * b * h * (b^2 + h^2)
= 1/12 * (2) * (4) * [ (2)^2 + (4)^2 ]
J = 13.33 in^4

2" x 2"
J = 1/12 * b * h * (b^2 + h^2)
= 1/12 * (2) * (2) * [ (2)^2 + (2)^2 ]
J = 2.67 in^4

4" x 4"
J = 1/12 * b * h * (b^2 + h^2)
= 1/12 * (4) * (4) * [ (4)^2 + (4)^2 ]
J = 42.67 in^4

As for which one will give you a higher overall rigidity of your rollcage as a whole (topic number 2), I know very little about rollcage design and have never done any of it myself. Having said that, my guess would be that it will depend more on the design of the rollcage than the type of material used to construct it. If I had to guess, I would say that most of the stress seen by the individual beams in a rollcage is going to be of either an axial (tension/compression) or bending nature rather than strict torsion. But again, I don't know that, that's just what my instinct would tell me. Anyways, I hope that helps. And someone please correct me if I screwed up those calcs somewhere...it's been a while since I was in Mechanics of Materials.

[redss86]

I am not an engineer by any means, but here is my opinion. How much of a cage are you going with? If you are going to put a full cage (8 or more points) I would go w/ 2x3x0.125 wall rect. for the frame material. Otherwise, I would go w/ 2x4 rect. over 4 round any day of the week. In my opinion, 2x4 would be easier to work w/ than 4 round, especially if you have to fit it between the rocker and floor pan. I used 2x4x0.125 wall on my frame, and am planning on putting around 550 hp through 6 spd and 9" w/o a cage.

Again, this is just my .02,
Joe

[killer_datsun]

well this is for a long term project where I am going to try and build almost everything myself. so I am not really starting with any car as a basis

[Matt@RFR]

Why are you stuck on 4" round tube?? I've never heard of that being done, and see no reason for it either. Look at the attached picture. Every single one-off chassis I've ever been around has been built using the same basic materials (meaning 1.375" - 1.75" round tube for everything), and with a few tweaks, would make an excellent road race chassis.

I'm not about to spend the time to do the math, but I'm willing to bet you'd end up lighter using round tube versus RECTANGULAR tube.

(2x2 would be square, 2x4 would be rectangular)

Me thinks you've got a long way to go before you're ready to start designing your own chassis.

[killer_datsun]

I am not stuck on 4 inch round tube it is just something I threw out there to compare to rectangular tubing of the same heigth

[v7guy]

when building something like this you need to take in to account alot more than just the hieght of the material. The width is going to be just as important both from the standpoint of ergonomics and strength. Thought I would bring it up in case it wasn't just a typo in your last post.

If you haven't I would consider doing some reading. There are some really good books out there.

[ramz69ss]

i'm guessing he was looking at some cobra kit car frames,and thought he would ask what people think,,,i'm sure, he is not thinking about using it for a complete frame,,just the main rails

[wendell]

disclaimer: I'm not a mechanical engineer.

I've always been under the impression that EVERY purpose built road race chassis is built from square/rectangular tubing because it yeilds a more rigid chassis.

It may be due to construction techniques that flat faces allow e.g. sheer plates. Either way, you won't find a T/A, Datona, AGT, GTS... car built out of round tubing.

[killer_datsun]

it was close to what ramz69ss said, just something that was fairly close to being the same physical size so a comparison could be more easily made. However there is going to be a cage on the chassis at least 12 points probably more though. I have Herb Adams book "Chassis Engineering" and Fred Puhn's book "How To Make Your Car Handle" so this question is just another part of my research.

[redss86]

With that many points, I think you could get away w/ 2x2 square. If you are worried about it, the biggest I would go w/ on that many points is 2x3. Most of your strength will be in the cage itself.

Again, just my .02