Some Roll Cage Math
In another post we have been discussing the pros and cons of tube vs. pipe. Here is some simple math, and you can decide for yourself. I am comparing 1-1/2" sch 40 ASTM A500 ERW tube to AISI 1020 or 1026 1.75 OD x .120 wall. The section modulus (a propertied determined by the cross-section of the tube) of the pipe size ERW is .326 in^3, the corresponding figure for the tube is .235 in^3. Lets say we take a piece of a cage 42" long, possible a spreader bar, and apply a load of approximately 1500 lbs to the center of the span. We will assume that the ends are free to rotate (this is not the case, but this is only a simplified model). With this load the stress due to bending in the pipe will be 46000psi, the stress in the tube will be 67350psi (values are rounded for simplicity). Now we have stressed the pipe to the point of yield, (i.e. it is bending). Let's look at the properties of the tube (I am using values from my Ryerson catalog which meet or exceed published standards) 1026 hot rolled seamless has a yield of 47000 psi, it is bending. 1026 cold drawn DOM has a yield of 72000psi, it is not bending. 1020 welded DOM has a yield of 60000, it is bending. 1020 ERW has a yeild of 38000, it is definitely bending.
Now consider the weight savings, the tube is approx. .7 pounds lighter per foot.
Now consider the cost, the tube is hugely more expensive than the A500 pipe, and it requires an expensive bender to form it.
My contention is: pipe is adequate for a recreational rollcage when care is taken in material selection and fabrication. In many ways the tube is better, and you have to decide for yourself whether or not the pro's outwiegh the cons.
This was a very simplified model, but it illustrates the point.
Cage Up, Wheels Down
All my Jeeps are in pieces! Except for one!!