Roll cage questions

[DavidM]

This is a tech question along with a rules & regs question, but I didn't feel like making two different threads. I'll be having the roll cage built for the new tub in a few weeks and had a couple questions.

- The cage in the current tub does not have a dash crossbar. There is a stock "dashbar", but nothing welded to the cage. Is this something folks would recommend for the new cage? I'm concerned about it changing the handling of the front end.

- The ITCS weight for the car is 2630. This is what should be used to determine cage tubing right?

- The chart in the GCR says cage tubing for 1701-2699 lbs is 1.5 X .095 or 1.625 x .080. The sentence before the chart says "The following table shows the minimum allowed tubing outer diameter and wall thickness by vehicle weight". Does that mean 1.5 X .120 tubing could be used? Along the same lines, is the 1.75 X .095 or 1.625 X .120 from the 2700 lbs and up legal for lighter cars?

Thanks.

David

[Chip42]

yes - your cage is 1.5"/0.095 wall or the listed alternatives, minimum.

you can always build the cage with larger tubes than required. thus the word minimum. the older cage rules (1994-2007, see GCR appendix I, if you are interested) required 1.5" / 0.120 wall from 2500#s so there are a lot of "heavy" cages out there. just for reference, the weight per foot of common DOM sizes (assuming0.284#/cubic in):
1.50/0.95, 1.43#/ft
1.50/0.120, 1.77#/ft
1.625/0.080, 1.32#/ft
1.625/0.120, 1.93#/ft
1.75/0.080, 1.43#/ft
1.75/0.095, 1.68#/ft


as for the front crossbar, I like adding safety, particularly if you already have to add weight. it might change the handling, but probably not by much. consider bracing the rear better, too. use heavier tubes lower if you are only increasing for weight. no sense placing weight by the ceiling.

[Matt93SE]

My train of thought is that you can never have too strong of a cage, or too much chassis reinforcement.

i.e. use the bigger tubing, add the dash bar.

[joeg]

In IT you do not need the dash bar. A good thing to have as it is somethiong to whichj you can mount a center net.

I really like 1 5/8 tubing and as you can see, it gets the best weight break at.080 wall!!!

[Lael Cleland]

I have built a bunch of cages out of .120&.095 1.5in DOM, the last Lemons car I did was 1.75x.095.....I like it more than 1.5....I cant explain why, maybe different radius, thinner and welds well @ lower temp, OH OH More Manly, Thats it!!!!!

[betamotorsports]

A dash or knee bar is a very good safety addition to any roll cage. It helps distribute side impact loads and provides additional support for the front of the door bars.

[erlrich]

David - IMO if you can afford the weight definitely add the dash bar, as well as the two braces to the firewall. Is this another 240SX? If so, I would also suggest running the rear braces to the shock towers & then running a cross brace between the towers, as well as a diagonal or X-brace to stiffen that whole structure.

I'm not sure why you think the extra bars might affect the handling though - is it the extra weight on the front end you're worried about?

[DavidM]

Yes. It's a new 240sx tub. I wanted to do a bunch of stuff and decided it was easier just to start from scratch. The cage in the current car has the rearward tubes on the shock towers with a cross bar as well. No X brace, though. No dash bar either, but it does have legs to the firewall. Bob built the car when the weight was 2530 (-180 for cage weight) and I think he built a minimal cage to save weight.

I need to put the car on scales to see what is the current weight and know how much weight I've got to play with. I've got it mostly stripped, but still have all the emissions stuff on it and it's an automatic at the moment. I'll have to subtract some off for that stuff. My thought was to build a robust cage with the extra weight.

My thought on the handling is that the dash bar will stiffen the front more than on the current car. Essentially linking the two sides more than is done now. The front pushes in low speed stuff as is and I was thinking a stiffer front would increase that. It may be minimal and not noticeable, though.

David

[Tristan Smith]

David, I would use the 1.75/.080 tubing. Yes I would put a dash bar in.

I would strip the car, weight it and see where you are for weight. Build the cage to the weight you want, but remember that things like cool suits are a great to have and allow you to put some weight where you want it.

The good news is you will not have to worry about slapping barbell weights in the passenger floor well.

[betamotorsports]

IMHO... if you need to add weight, add the weight with safety in mind - additional cage bracing, doors bars, etc. Yes, its a little higher up in the chassis then some barbell weights bolted to the floor, but barbell weights won't save your ass when things go bad.

[DavidM]

For those that haven't seen my car, there are two 45 lb barbell weights bolted to the passenger floor. That's how the car went from 2530 to 2630. The idea with the new tub is to use the extra weight for a beefier cage along with a cool suit and other odds and ends. I don't want to make it too heavy though.

Thanks for the info.

David

[spawpoet]

For those that haven't seen my car, there are two 45 lb barbell weights bolted to the passenger floor. That's how the car went from 2530 to 2630. The idea with the new tub is to use the extra weight for a beefier cage along with a cool suit and other odds and ends. I don't want to make it too heavy though.

Thanks for the info.

David

To give you a bit of reference on the total weight difference for cages done with different tubing, we dropped around 40lbs (+ or - 5lbs) in going from a cage with 1.75"x.120" tubing to a cage with 1.5"x.095" tubing. The cages were a bit different in design, but in terms of total length of tubing used I think there was only around a 5% difference.

[DoubleXL240Z]

Something to consider as well, is the cost of material/ to weight. The cost of material to me varies greatly!! I used some 1.75x063 on a cage (non necessary tubes) it was $2.75 per foot different. 1.62x.080 is almost $5 per foot more than "standard" 1.5x.095.
Figure using roughly 80 feet of tubing for a cage @ 1.50x.095 vs a 1.62x.080 would save approximately 7.5 lbs and cost approximately $320 more!! Almost $10 per foot vs a bit over $5 is a lot of money!! Close to $800 for material alone!!

[DavidM]

Interesting. the 1.5 X .095 and 1.75 X .080 tubing are shown as having the same weight per ft. Are these equivalent as far as "strength" for building a cage? I'm inclined to go with the smaller tubing provided they're equivalent.

David

[Chip42]

in terms of bending they are very simillar:
generally you look for L/EI, a relationship of the material, length, and cross section. deflection can be calculated from this value and the loading of the system.

sparing you the garbldy gook and assuming everythign else is equal - loading, length, material (fair assumptions in this case)
then the number of interest is "I" the first moment of area.

I values:

1.5" 0.095w = 0.1039 in^4
1.75" 0.080w = 0.1467 in^4

for a counter point:
1.625" 0.080w = 0.1162 in^4

higher is better.

[tderonne]

Hmmm formula I have for bending has a 4th power in in. Larger tubing wins every time. I gotta dig it up again...

[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.