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post #1 of (permalink) Old 08-21-2000, 07:21 PM Thread Starter
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How to design a rollcage???

I saw cj7_304's post on the rollbar he is designing, but how can you tell if it will really work, or just look cool? I, like lots of others on this board, was scared of those pics of that rollover last week. I want to put a beefier rollcage in my Wrangler but don't know the first thing about it. I don't think I'd build it myself as I'm not that confident with a welder yet. Should I take it to a shop and have them design one or what? Thanx a bunch!! BTW, good luck cj7_304 with your design!

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post #2 of (permalink) Old 08-21-2000, 07:30 PM
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Re: How to design a rollcage???

[img]/wwwthreads_images/icons/tongue.gif[/img] Er.....Jammy; don't EVER make the mistake of thinking that just because a "shop" designs and builds something for you that it is done right. I'm sure that cj_304 has the makings of a real good system there, and once the design has been sifted through this BBS a few times, he'll have something that you CAN take to a shop and say: "Do this!" You should try to use the term "Rollover Protection System" because the whole arrangement includes much more than just the tube section.[img]/wwwthreads_images/icons/tongue.gif[/img]

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post #3 of (permalink) Old 08-21-2000, 07:31 PM
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Re: How to design a rollcage???

While we're on the subject, I have a question as well:

I'm no where near the roll bar stage of my build up but I've been running a few designs through my head. Several people have complained about the front hoop and it interfering with the legs/doors of their jeeps. I was wanting to keep the dash OUT of my jeep, so I could get a hoop installed, and have a small "S" bend so that the front hoop went through a couple holes in the dash (upper outer corners of dashboard) then proceed to the floor. I wanted to know how hard this is to have done. And as the previous post pointed out, is this a good idea or weak link......how do I know....... any imput??

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post #4 of (permalink) Old 08-21-2000, 08:23 PM
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Re: How to design a rollcage???

So far on my roll bar design I have been using all the CAD and FEA(Fancy stress solving program for us lazy engineers) tools I have learned. I started with a plain-jane front tie in kit from smittybuilt, but with 4 spreaders. I ran it through the FEA, and it could withstand 15,000 pounds with little to no effect.

I though that this might be a good number for a slow speed rollover, but then I HEARD hat the "sport bar" tie-ins from smittybuilt are marginal for anything but a side flop-over at low speed. So I now realize I need to set my strength goals higher.

Tomorrow, time permitting at work, I plan to draw up what I want to be my next design iteration and hit it with 150,000 pounds virtually. This will tell me what is fracturing under the high load. This will help in deciding what needs to be reinforced or gusseted.

It will take a few days, but I will post what I have when it gets done. I will throw all the pretty pictures I can up too. Unfortunately I only have linear FEA, which means I cannot predict what the steel ina roll cage will do once it passes it's yield stress, and starts to permanently deform. Those packages are near $100,000. Don't think the boss will sign the PO for that one, especially seeing how we do mainly automotive interior in my division.

I will keep ya'll posted. I look at everyone on here as a friend, and I would hate to lose a friend to a roll over accident trying to push the envelope.

My younger sister is a real Internet whiz-bang too, and we are currently setting up my own jeep page. This could be a nice item on there. Maybe even make itís way into the fabled Jeep CD-ROM.

Lots of people have asked what software I am using. I am using Pro-E and Pro-Mechanica. The company that makes this (I used to work for them too, so I think I owe hem a plug) software is Parametric Technology Corp. http://www.ptc.com/

Jesse

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post #5 of (permalink) Old 08-21-2000, 09:24 PM
 
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Re: How to design a rollcage???

OK, here we go...
This is going to use some engineering terms, so if anyone is offended, sorry.
I don't know how to put it any other way...

CJ Dave is very close to correct with his modified 'O' design.
CJ7-304 is pretty close, (and a great graphic) with his design.

I'm not sure the diagonal load is addressed.
The ideal design would be a 'X' design in CJ Daves 'O'.
That would take loads from both directions, and be triangulated in two of three planes.
This gives a good solid base for the rest of the cage to attach to.

CJ7, we found in sprint cars, we could make the cage just as strong by using gussets in the corners, instead of making cross bars everywhere.
You can't do that everywhere, but like around the top and in the corners on the sides.
You don't want cross bars in your escape path when the fuel cell ruptures!
Take my word for it!

Does that program you are using make flat panel gussets?

I think a front hoop is required if you are going to do anything that will send you wheels up at any time.
That will require a cross tube under the dash for support, and gussets to support the front hoop.
You can't really use any cross bracing on a hoop over the windshield, so bracing under the dash and forward tubes to triangulate the front hoop will be required.

Rear attachment to the frame/ body is sadly lacking in the stock jeep 'Roll Bar'...
It has two tubes that attach to a hoop, and go to the 'bed' of the jeep.
No cross bracing, no gussets, and several have superfluous bends that weaken the tubes even further.
Straight tubes that angle in or out would be better, so they are triangulated, but they are going to be a pain in the butt to deal with if they mount in the center of the 'bed'...

So again, I'm stumped. It's going to have to be a compromise.
What to trade off for each application is the question.

I usually like to see at least eight points in contact with the frame for heavy use cages.
And a bear minimum of 4 for light use safety hoops.
Even on single or double hoops with 4 connection points, I like to see cross bracing.
Other wise, it's going to fold up sideways.

I like to see straight legs from rear to front.
No sense in pre-bending the cage so it has a head start folding up...

Anyone else have any ideas what mandatory attributes a cage should have?




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post #6 of (permalink) Old 08-21-2000, 09:26 PM
 
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Re: How to design a rollcage???

Jesse

I would be interested in seeing what kind of numbers you come up with after you run your simulations. Material, bend radii, weld types, etc. I've been a firm believer in over-engineering ever since I did a barrel roll in an early Bronco in 72 or 73.

Up till that time I had a couple of 4x4's fall over, you know, easy like. No problem though, we just rolled them back on their wheels and motored away.

The big one was a different story. My rig went airborne at about 45 or 50 and impacted on the left front corner of the cage. The front hoop bent backwards at the dash bend and they only thing that saved me was the fact that I got it in the head.

Since then, I have religiously tried to avoid or at least minimise longitudinal bends wherever possible. I also try to make use of as many X's as I can.

A lot of people have made fun of my designs but this old 4 wheeler plans to die in bed.

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post #7 of (permalink) Old 08-21-2000, 10:24 PM
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Re: How to design a rollcage???

Hello all, I do not post often but feel compelled to give my opion on this roll bar stuff.
I will not use any numbers, big technical words, or mumbo jumbo.
The one thing I agree on, is that every one should have some form of roll over safty, wheather it be a simple single bar or an all out cage. Nascar for example has some of the most elaberate complete roll cages , and people(racers) have died. No matter how good you build it, people will push there machine harder and exceed the limitaions of there set up.
My idea of a perfect cage is this: Build or have built the best protection you can afford. Build something that YOU trust, not what somebody else says will work.
I,m not much of a writer, but I beleive there are who want protection but think that they need big bucks, big words and a degree to put something in there ride, and put it off.
For all you jeepers with jsut a regular education, a good quality cage( 2" DOM tube), tied into the frame with some spreaders should cost you around 300 to 400. So skip that winch for now and get a cage.
Just my thoughts without all the big words.]
Jimmy

post #8 of (permalink) Old 08-21-2000, 10:44 PM
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Re: How to design a rollcage???

Being an engineering student, and a Long ways from being an authority. I think that the more triangulation you can get (support in more than one direction) the better.

I also agree with the use of straight tubes as much as possible. No need to to give weakness a starting spot.

There have to be a few bends the cage however.

The Stock "roll-bar" is only 3 tubes, and has a minimum of 2 bends.

My personal idea is to do something similar to H8's front hoop. I will use the center hoop from another Jeep, put in an overhead console, outer supports (similar to windshield supports in a stock wrangler) and then run diagonal braces from the outside (where the supports meet) of the front hoop, to roughly the middle edge of the center hoop, on both sides.

I will also run bars across the bottom of the dash, along the bottom edge of the door, across the bottom of the center hoop, and (similar to CJ7_304's) from 1/2 way up the rear supports, to the middle of the center hoop.

It ties into the frame at 6 points and is well protected from just about any reasonable speed roll. (no it will not save me at 200MPH, my Jeep will not do 200 MPH)

my.02

BJ

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post #9 of (permalink) Old 08-21-2000, 11:13 PM
 
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Re: How to design a rollcage???

I agree.
Triangulation a must.
I think we will have to sacrifice some cage integrity for some visibility and some ascetics.

I'm not proposing building a tank, but if you are going to upgrade from a single, stock 'Hoop' type 'roll bar', then going to something triangulated is mandatory.

I like a six connection point double hoop much more than a single hoop with four connection points.
Also, like I and others have said, a dash bar of some kind is a must for the second hoop support.

I think we are getting somewhere...
Now, Who is going to draw this thing?

I have Auto cad and Cam Cad programs, but I don't have a clue how to use them correctly to do this drawing...

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post #10 of (permalink) Old 08-22-2000, 12:09 AM
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Re: How to design a rollcage???

TR Whats Dave's perfect 0 ? I always thought that triangles were the strongest shape. I have faint memories of building bridges out of spaghetti Noodles from advanced Physics in High school, but most of those days are a distant memory, Could ya elaborate on the that point a bit. Oh and also for us common folk, could ya explain a bit more on the gussets on the front hoop? In thinking of my future Cage, I have already planned for gussets going from the spreader bars to the hoop, actually planned on gussets on every bar that is close to a right angle. Am I close to what you are thinking of?

Also for my cage idea I am kicking around the idea of getting rid of stock seat brackets and putting to 1 & 1/2 inch bars going across the jeeps cab, that connect from on side to the other onto a set of spreaders that go from the bottom foot of the hoop to the rear hoop, then welding on new brackets and having the seats as well as the seat belts totally attached to the cage and not the body?

Any Ideas on that? anyone?

Matt



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