Critique my preliminary cage ideas...

[davidt]

Do you mean how safe it is to run it, or not run it?

Not much is necessary. A B pillar hoop that is tied into the shoulder harness bolts is probably all that is necessary to keep the body from colapsing on the occupants. Everything from there is added depending on the goals.

#1 goal is always safety, #2 goal save the cab and engine, #3 goal, too look good, bling bling

 

[Roxtar]

quick question, how necessary is the bar that goes across the bottom of the windshield? I see a lot of guys that don't run them, and was wondering how safe it is.

After a lot of consulting, I chose to leave that bar off.
Since it is merely another parallel bar, without triangulation, it will do little to keep the "A" pillar hoop from collapsing into a parallelogram (It's primary goal).
This is providing that you tie the cage into the frame rails (assumed).

 

[Goatman]

I think it also mattes if the A pillar hoop is external or internal, and if an internal hoop is tied to the body. On the external it adds considerable lateral support, which is important since in virtually any rollover the rig could land directly on the A pillar hoop putting forces on the hoop both sideways and downward. With an internal cage the rig would land on the body A pillars, and the A pillar hoop would keep the top from collapsing.

BTW, in every new car the factory puts a tube across the cowl inside the dash, which adds a good deal to the safety cell concept of keeping the cabin structure intact in an accident or rollover, the older cars, like an XJ, don't have one.

 

[davidt]

I think it also mattes if the A pillar hoop is external or internal, and if an internal hoop is tied to the body. On the external it adds considerable lateral support, which is important since in virtually any rollover the rig could land directly on the A pillar hoop putting forces on the hoop both sideways and downward. With an internal cage the rig would land on the body A pillars, and the A pillar hoop would keep the top from collapsing.

BTW, in every new car the factory puts a tube across the cowl inside the dash, which adds a good deal to the safety cell concept of keeping the cabin structure intact in an accident or rollover, the older cars, like an XJ, don't have one.

my cage will be external, I will probably add it, just trying to keep it light, guess you can't have it all ...since we are on cage building 101, what are the benefits of building off of a hoop vs building off of a halo vs building it a la brettm (he used one long piece on each side, then connected them)?

 

[Ed A. Stevens]

The tubing will be 1.75" or 1.5" OD .120 wall DOM. I have a 1.5" die for my bender, but would have to beg, borrow or steal a 1.75" die if I go that route. Where the tube meets the unibody will be thick plate, with my frame rail and rocker panel tie-ins sandwiching from the underside.

Is weight going to be an issue with this design in either of my tubing sizes?

Should I bolt or weld the sandwich plates on the top and bottom of the unibody tie-ins?

So, have at it.

Here's the drawing... I did it in MSPaint, so be nice. :finger:

I like this rendering better as the initial basis for a design, because the additions made on the other drawing are not really effective to make a signigficant improvement to the design.

Use 1.75 for the B-pillar only if you plan competition (it will likely become the standard minimum for a 4000# vehicle). The 1.5 will work fine with a cage combined with the unitbody. The .120 wall can usually be replaced with thinner .095 for elements other than of the pillars and halo.

Weight should not be a problem, as long as the tubes serve a purpose to distribute loads.

The unit body does not move independent of the unitframe, so welding is better for rigidity.

Parts of the initial drawing that can be improved are the two center halo bars and the door/side bars.

Instead of the two parallel internal halo bars, place them in a modified "V" with the open end of the V at the A-pillars (this helps transmit an A-pillar shock to more cage members, and it usually keeps the bars away from the drivers head).

The 'X' in the B-pillar can be replaced with an inverted "V" to make the upper center of the B-pillar a node with the halo "V" supporting the A-pillars. This takes no more material and makes the halo and B-hoop much more rigid to protect occupants.

The two cage threads Goatman posted offer great ideas, you cannot ask for a better guide. I agree with the champions of exo-A-pillar and roof rack, and internal B & C/D pillars. The external halo/rack makes it easier to fabricate and locates where a roll-hit takes place (the A-pillar and roof) outside of the windows to minimize glass damage. If you walk junk yards you see the XJ needs external reinforcement at the A-pillar (it's the damage magnet).

The diagional door/side bars shown tie into an unreinforced node on the B-pillar (a shock could bend the B-pillar). A competition buggy cage would run from the top of the B-pillar to the bottom of the A-pillar (too much in the way for a door slammer). For the XJ these side bars do not have to be at a steep angle, they can be on the floor or along/below the door sill/rocker. Crash went a little wild with the rocker, but the concept is valid to replace a side bar.

It's not apparent in Crash's pictures, but tieing the cage into the suspension mounts adds considerable rigidity (it's there, but not detailed). A reinforcement plate on one or more side of the lower control arm mounts (from A-pillar) and reinforced leaf spring boxes (from B/C-pillar) will add more laterial rigidity to the occupant space that the side bars as drawn. The floor, frame rails, and rocker reinforcement offer considerable strength for the bottom of the cage. As long as the arm and spring hangers are part of the reinforced structure you do not "need" side bars (even if they are at non-stock long arm or linkage mount locations).

There is not much detail posted for cage tie-in's below the door sills, and it's where considerable strength can be added with the integral cage/unitbody combination. Tie-in's at the top of the unit body halo to the pillars also provides considerable improvement in combined structure rigidity. The same can be said for improving the strength of the plan view frame rectangles defined by the steering box, panhard rod frame mount and the lower control arm mounts. The stress of large tires overworks the "frame" between these mounts leading to the constant abuse of the drivers side frame. If your already bending tube and cutting tabs a little thought on reinforcement forward of the firewall in these areas will help.

Keep these cage (and other adv-mod) threads going. I have watched these build-ups for the past five years as cobwebs grow on the bender and welders (through two hobby shop moves) and can't wait to heat a little metal of my own. I plan to get my own 2-door down the hill next spring for it's much needed improvement project. In the past five years the XJ outings have all been easier JV trails or harder trails in the hills of BB or Calico, and lots of neglect. Other than a quick fix of broken shock mounts, broken steering box bolts, driveshaft scars, and crumpled rockers & corners I have only been able to take time to keep my 88 smog/street legal (not much fun). The wife's house remodel is 99% done and has sucked 99% of the XJ budget (for now), and she has called it quits for a year or two.

 

[vetteboy]

What I was asking sort of obtusely in my last post...can someone please explain the advantages/disadvantages of these two construction methods:

1) A really long bent tube that runs from the a-pillar floor, up along the roof, and down to the d-pillar floor:

2) Seperate B and C pillar hoops (this pic only has a B, but mine will have both):

I'm not interested in which one is easier per se, I'd more like to know about the structural benefit of each, or if it even really matters.

 

[Ed A. Stevens]

What I was asking sort of obtusely in my last post...can someone please explain the advantages/disadvantages of these two construction methods:

****

I'm not interested in which one is easier per se, I'd more like to know about the structural benefit of each, or if it even really matters.

With good welds and good node construction there is minimal strength difference.

Some race organization rules (SCORE & NHRA) state the one-piece A/B/C hoop method is the standard, but allow inspection approval of one piece side hoop and halo construction methods. How much hassle do you want (with vehicle demo, or inspection oversight)?

If you are not in competition (and even if you are, and have confidence in your design and fabrication skill), design and install the system with the easier overall quality fabrication using the tooling at hand. Access and sequence planning to complete quality node construction (quality welding of good nodes) will result in a better cage. Employ the design that can make you a better node fabricator with your tools.

HTH?

 

[vetteboy]

Yep, that's just what I was looking for. From an engineering standpoint I figured that as long as the nodes were solid, that it would work well either way, but I wanted to make sure before I put too much thought and effort into it.

As far as competition goes, I'm not involved with that at this point. I think I'll be putting too much TLC into this rig to beat it to death against a clock, let alone having the resources to do that all the time...but all the same, this build will be VERY thorough and well-planned. I have an XJ solely for parts that I'll be using for all the cutting and cage fitment, so I'll have a really good idea on how to put it together before I start hacking apart mine for real.

My tools would be a JD2 model 3 bender, JD2 tube notcher, 140A SnapOn MIG welder, plasma cutter, etc. I did this senior year at school:

so I've got a good amount of experience overall. Just gotta find the time and money to apply it.

If you like these cage threads, stick around...I'm sure I will be posting a ton of stuff in the near future. Just for kicks I'll probably do a whole ANSYS model of it as well because I'm a nerd like that...

 

[davidt]

excuse my ignorance, but what are nodes? I am just learning cage lingo.

 

[Jes]

I don't know about anyone else but those 420 Fab guys...
Just can't trust them I tell ya. :greensmok:

 

[Goatman]

I just hadn't gotten around to answer that particular question yet, and I agree with Ed that the various choices of which section is a continuous hoop doesn't effect the strength enough to make a difference. Design it in the way that's easiest to build, and as much as possible put the stress at weld points in compression rather than extension, and use plenty of gussets.

 

[Ed A. Stevens]

excuse my ignorance, but what are nodes? I am just learning cage lingo.

A node is an intersection of two or more tubes, a weld junction.

The structure ideal (from an analysis standpoint) is straight tubes and junction/nodes at each corner or intersection. A node of three or more tubes transfers the load on one tube to the other tubes (two or more other tubes) to distribute the stress on each member of the overall structure. More tubes in a single node results in less stress shared by each subsequent tube to resist a load.

More tubes at a node, is alot like extra helping hands to keep the structure together.

HTH?

 

[AzRick]

I don't know about anyone else but those 420 Fab guys...
Just can't trust them I tell ya. :greensmok:

That guy looks like one shady character.

Rick

 

[Roxtar]

I don't know about anyone else but those 420 Fab guys...
Just can't trust them I tell ya. :greensmok:

2X
Comon, look at this guy.
Is this the example you want to be following? :wow:

 

[davidt]

A node is an intersection of two or more tubes, a weld junction.

The structure ideal (from an analysis standpoint) is straight tubes and junction/nodes at each corner or intersection. A node of three or more tubes transfers the load on one tube to the other tubes (two or more other tubes) to distribute the stress on each member of the overall structure. More tubes in a single node results in less stress shared by each subsequent tube to resist a load.

More tubes at a node, is alot like extra helping hands to keep the structure together.

HTH?

thats what I thought it was, thanks for the help :thumbsup: