Roll cage questions

[Chip42]

I = pi/4*(Ro^4-Ri^4) or pi/64*(OD^4 - ID^4) (for an annulus)

reran the calcs and got different numbers, double checked and edits are correct.

thanks for catching the unit error or I would have left it wrong!

more complete list with momeny of area (I), weight per foot (assumes 0.284lb/in^3, mild steel), and stiffness:weight ratios:
__OD___wall__I (in^4)__W (lbs/ft)__I/W
1.500 - 0.120 - 0.1248 -- 1.77 ----- 0.070
1.500 - 0.095 - 0.1039 -- 1.43 ----- 0.073
1.625 - 0.120 - 0.1617 -- 1.93 ----- 0.084
1.625 - 0.080 - 0.1162 -- 1.32 ----- 0.088
1.750 - 0.095 - 0.1697 -- 1.68 ----- 0.101
1.750 - 0.083 - 0.1514 -- 1.48 ----- 0.102
2.000 - 0.080 - 0.2227 -- 1.64 ----- 0.135


sorted by stiffness:weight ratio. looks like 1.750 0.083 is the best trade-off shy of going to 2". not sure what the cost would be. Chris L, do you know?

[tderonne]

That sounds more like it.

One small thing to consider. You can't actually buy 1.75" x .080" tubing. You'll have to use 1.75" x .083". Changes the numbers slightly.

Overall, the bigger tube is a clear winner in my mind. More bending stiffness for the same weight.

[Chip42]

makes the 1.75 0.083 I=0.1514, W=1.48#, I/W=0.102

changed the above to include. thanks tim!

[TomL]

Hey, how about 3" x .020 wall? I = .2078, W = .65 lb/ft I/W = .3196

Obviously superior to that wimpy 1.75 x .095. (and 4" x .010 would be even better!)

You need to be careful about the assumptions you are making. There is a lot more to the strength of a cage than the bending strength of the tube. When you go to thinner wall tubing, buckling strength in compression can become an issue, as can the effects of localized deformation. And even in bending, with the same load, the stress on the surface tube goes up with the diameter (or maybe the square of the diameter - it's been a while since I checked). If it's a listed tube, I'm sure they're all ok, but I wouldn't try extrapolating to some thing bigger and thinner without a serious expert evaluating it.

[Chip42]

I din't make any assumptions other than a generic steel density. I answered with good, factual, and representative numbers and without getting into full engineering analysis - I added a number of listed tubes just because i thought some might find it interesting. either 1.5 095 or 1.75 080 is a fully legal tube size per SCCA 2699# and under.

Luckily the club doesn't allow crazy tubing sizes. you'll note the thinnest allowed wall is 0.080" - likely to avoid the under-thought engineering shenanigans you suggest I'm proposing.

most cage failures I've seen have been related to the weldments, and I've seen too many - so a good fabricator is the key to a safe cage, the allowed tubing sizes are ALL adequate for the task when built correctly.

[DavidM]

Taking the car to the shop this week to get the cage put in. Weighed the car tonight and it's 2480 with me in it. That's with stock driver's seat, automatic transmission, stock wheels, stock gas tank, stock exhaust, stock suspension, and lots of extra pieces on the stock motor. Have to add back in some weight for a lower front fascia & splitter, thicker sway bars, and other misc stuff. I figure subtract maybe 50 lbs from the 2480 to 2430. ITCS weight is 2630.

The cage in the current tub has 61' of tubing. Using 1.5" x .095 tubing that weighs around 87 lbs. I'm thinking it'll be another 20-30' of tubing to have all the additional stuff I want done. 90' of tubing is 129 lbs. Looks like I should have enough head room to do whatever I want to the cage.

Thanks for all the info. I'll take some pics when I get it back.

David

[erlrich]

Taking the car to the shop this week to get the cage put in. Weighed the car tonight and it's 2480 with me in it. That's with stock driver's seat, automatic transmission, stock wheels, stock gas tank, stock exhaust, stock suspension, and lots of extra pieces on the stock motor. Have to add back in some weight for a lower front fascia & splitter, thicker sway bars, and other misc stuff. I figure subtract maybe 50 lbs from the 2480 to 2430. ITCS weight is 2630.

The cage in the current tub has 61' of tubing. Using 1.5" x .095 tubing that weighs around 87 lbs. I'm thinking it'll be another 20-30' of tubing to have all the additional stuff I want done. 90' of tubing is 129 lbs. Looks like I should have enough head room to do whatever I want to the cage.

Thanks for all the info. I'll take some pics when I get it back.

David

David - just for sake of reference, what do you think you weigh?