Roll Cage for the KJ

tjkj2002 · **Posted:** Thu Jul 02, 2009 12:20 am

durangotang wrote:

Boiler wrote:

In my opinion they are dangerous. Maybe not if you wore a harness to keep you away from them.

If I were to do anything, it would be put a hoop right behind the backseat with angled tubes terminating back by the tailgate. Maybe have the hoop come forward a bit somehow.

weight problems? Aluminum all the way.

But my general view is that interior and exo cages for the KJ are more trouble than they are worth.

I agree 100%.... I see several people killed by roll cages every year at work. If you don't go ALL the way with them they are very dangerous. (all the way being PERFECT design and install, proper harness bar alignment, proper harnesses, helmets, and proper cage padding)

Yes cages can be fatal but I couldn't imagine the danger involved with using aluminum :shock:

.A cage is not about saving weight and sorry DOM will be stronger then aluminum(also cheaper).If aluminum was better it would have been used long ago and be a standard in the racing world(which it is not).

Boiler · **Posted:** Thu Jul 02, 2009 1:49 am

tjkj2002 wrote:

Boiler wrote:

In my opinion they are dangerous. Maybe not if you wore a harness to keep you away from them.

If I were to do anything, it would be put a hoop right behind the backseat with angled tubes terminating back by the tailgate. Maybe have the hoop come forward a bit somehow.

weight problems? Aluminum all the way.

But my general view is that interior and exo cages for the KJ are more trouble than they are worth.

Yes cages can be fatal but I couldn't imagine the danger involved with using aluminum :shock:

.A cage is not about saving weight and sorry DOM will be stronger then aluminum(also cheaper).If aluminum was better it would have been used long ago and be a standard in the racing world(which it is not).

The key to correctly designing with any material is to do it based on understanding of sound engineering principles, not "so 'n so uses this", that looks beefy, or other similar types of material sizing.

DOM is not a type of material. It is a process. Drawn Over Mandrel. I believe DOM tube is typically 1020 or 1026 steel. Typical yield strength of that type of material as a DOM tube is 65-70 kpsi. It can be higher if heat treated, but that's not very feasible for a cage. 6061-T6 aluminum has a yield strength of 40 kpsi. So yeah, everything equal, steel is stronger.

In comparison to 1-1/2" OD x 0.12 Wall DOM tube though, a 6061-T6 tube would be stronger if it was the 2" OD x 0.25 wall that I suggested.

1.5 x 0.12 (11 ga) has a section modulus of 0.166 in^3. Steel weight for this shape is 1.79 lbs/ft
2 x 1/4 has a section modulus of 0.5394 in^3. Aluminum weight for this shape is 1.65 lbs/ft.

yield bend load (torque) would equal yield strength * section modulus.

For the 1-1/2 x .12 steel tube: bending load to yield it = .166 in^3 * 70,000 lbs/in^2 = 11,620 in-lb

For the 2 x 1/4 aluminum tube: bending load to yield it = .5394 in^3 * 40,000 lbs/in^2 = 21,576 in-lb

So yeah, steel is stronger, by definition of strength being the stress at which a material bends or breaks. The point is when you design something properly, you size the material to reduce the stress.

My tube is about 10% lighter and can carry about twice the load. That does seem dangerous. :roll:

It is true however that aluminum is trickier to fabricate with and more expensive by the pound. Expense per strength is about equal if you size it right though. The whole reason I mentioned it is that Inc. was mentioning his weight issues.

Sport · **Posted:** Thu Jul 02, 2009 9:22 am

What Boiler said...I think :?:

Tokyojoe · **Posted:** Thu Jul 02, 2009 9:29 am

What I would like to do is make my KJ a convertible and have the roll cages like a Wrangler. It has been mentioned that the unibody would lose it's structural integrity by making it just a convertible, but would a roll cage do the trick?

I have seen an FJ and H3 turned into convertibles but they were made more for show. I contacted the company that did them (they looked professionally done and looed awesome, not redneck cheaply made), in Cali, and they replied that they won't do it. Whimps!

JJsTJ · **Posted:** Thu Jul 02, 2009 10:40 am

Boiler, please discuss bending of aluminim tubing? I don't know squat, don't claim to at all, but am curious about bending of al. tubing.

Based only on seeing it done on other unibody Jeeps, I would say that a properly designed and tied in cage could replace the roof structure strength of the KJ. I have seen XJ's w/ no roof, full cage installed and rolled badly hold up well.......bummer is that after a roll, the cage is compromised and really not good for another roll until replaced or repaired. Note that driver was properly harnessed into an aftermarket seat when roll ocurred. (iirc he had a helmet on too)

Boiler · **Posted:** Thu Jul 02, 2009 11:58 am

JJsKJ wrote:

Boiler, please discuss bending of aluminim tubing? I don't know squat, don't claim to at all, but am curious about bending of al. tubing.

At first I thought you meant how do loads bend aluminum tubes differently than steel, but after reading your question again, I think you mean how can you bend aluminum tubes during fabrication. I'll address the latter. If you were actually asking about strength in bending & deflection, I've got that answer half written up too, but its long...

Bending aluminum is much more difficult than steel. Certain alloys bend easily, while others like 6061-T6 require much more care. I won't address sheet, plate, and flats as they all have their own issues, I'll stick to tubing. The most widely available aluminum tubing is 6061-T6. It is also one of the strongest and hardest aluminum alloys. Unfortunately, it is just about the hardest aluminum to form. To bend 6061-T6 aluminum the best thing to do is exagerrate all of the same rules that you follow in steel:

1) Support the tube with a pressure plate & die that has the radius cut into it. Have the radius cut into the bending die.

2) Use a mandrel to help with wall stretching/collapse. If you support it via #1 well enough, and have a large enough bend radius, this can be neglected.

3) The easiest tube to bend is one with a small diameter and a thick wall. Basically it behaves more like a rod, which bends very easily.

4) Use a large centerline bend radius. 3x the OD of the tube is usally pretty good if the wall thickness is at least 10% of the tube diameter. Example: 1-1/2" tube with 1/8" wall: use a 4.5" CL radius or larger. If I were using a 1/16" wall thickness instead, I'd probably bump my radius up to 6 or 8 inches. This is all trial and error, and experience. I'm sure there are some tabulated data out there with better rules, but I just know what has worked for me.

Some people cheat and heat up their tubing before bending. Basically this anneals it, removing the temper (T6) which gives the material all of its strength and hardness. Annealing to bend is not necessary. What is necessary is adequate design and access to the proper bender. I've had aluminum bend at a custom muffler shop, using their compression bender. It did not break, but it necked the tube down and looked pretty raunchy. We get the 1-1/2 x 1/8, 5" CL radius tubes for my rack bent on a CNC bender like this one

.

JJsTJ · **Posted:** Thu Jul 02, 2009 2:09 pm

Boiler, That is what I was looking for but now also interested in the other info you started writing. Maybe kinda OT but this is some good tech fwiw.

Boiler · **Posted:** Thu Jul 02, 2009 11:40 pm

I'll try to remember on saturday or sunday to write one up. My parents are coming in the morning and since I'm the primary cleaner here, you can guess what a mess the house is considering I'm jacking with jeep related stuff most of my time.

Cleaning our rears off...but now I have to go work on a fab project at the shop...burnin' the midnight oil...

Roll Cage for the KJ

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