Critique my preliminary cage ideas...

[NotMatt]

I'm tired of wheeling with nothing but a tin roof over my head, so I'm building a cage in my new project rig. The rig is a '91 2-door Jeep Cherokee Sport. This will be the first cage I've built, and I would like some advice from the experts on my design and methods.

The tubing will be either 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.

I want the following things from the cage: First and foremost, safety. I want to be able to take this rig wherever my ego and my right foot desire, and feel safe doing it. I also want extra unibody stiffness out of this. Also, in the future it would be nice to add some seat mounts into the equation should I upgrade from stock XJ seats, and possibly harness mounts down the road.

The things I'm NOT so concerned about: the back seat. I haven't used the back seat in ages in my 4 door, and plan on doing away completely with it in the 2 door. Don't have kids, don't like taking more than 1 person wheeling with me, so more room for cargo and my spare, no back seat. I'm not concerned about the body in the event of a roll (at this point). In the future I may add-on to this to include some kind of external protection, but I want to take care of the safety aspect first.

Ease of entry/exit and weight lie somewhere between the 1st two lists... it's not a DD, will be a dedicated trail rig, but I will drive it to and from the trails and around town. I think what I've drawn should allow for easy enough entry/exit. The weight issue I'm not sure about... I don't want a top-heavy rig, it's not lifted a lot for the tires I'm running, and I'm keeping the spare down low in the back. I may move the tank up, haven't decided yet. Is weight going to be an issue with this design in either of my tubing sizes?

Stuff I'll be doing at the same time include boxed unibody frame rails, and boxed or boat-sided rocker panels... both will be tied into the cage, and I have a good idea how I will do that in my head already. Should I bolt or weld the sandwich plates on the top and bottom of the unibody tie-ins?

So, have at it. Give me your best "it'll fold like a taco" lines... but give me some ideas where I can improve, or stuff I don't need... and sorry for such a long-winded post.

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

 

[5-90]

I don't see a LOT of trouble offhand - you've got triangulation in between the B-pillars, and triangulation between the rear and the pillars.

You might want to watch for the forward slope bars - they might get in your way getting in and out.

A couple things I'd add, given a "clean sheet of paper,"
A full "halo" bar - tie it into the pillars and the roof rails.
A crossbar for tying front-seat harnesses into the cage.

Here's an oddball idea - if you're bending and welding it yourself, why not punch the pillar bars through the floor and into longwise bottom bars? Make the cage a full structure, and you're really going to be in good shape. Tie in crossplates or crossbars to mount the seats to, and even if the body falls apart around you, you'll be connected to something else (I'm not saying you'll survive a fall-apart, since neither you or I are engineers - but I'm working on it...)

5-90

 

[NotMatt]

5-90: Good points.

The door bars are something I don't have set in stone (well none of it is bent up yet so I guess nothing is set in stone). I have a friend with a cage in his rig, and I really hate getting in and out of the front seats because the A-pillars come straight down in front of the dash, and the door bars are kind of are curved to match the seat/hip area. The straight A-pillars are the worst part though. I'm not convinced this design will be better, but just looking at it seems it should be. I guess I'll get to that point and tack some stuff up and see how getting in and out is.

A full halo tied into single A/B pillars is something I thought about as well. Is it well-known that halo-style cages are "stronger" than this type, with a B-pillar hoop and seperate bars for each A-pillar? I figured this might be a little simpler to build overall, but the halo design intrigues me and if I get some good reasons I might revise and go that route (keeping most of the rest of the design).

Seat mounts may come later, I'm still using factory seats and I don't want them to mount any higher than they already are. I'm not an extremely tall guy, but I feel like they're already high enough. Maybe a couple cross bars for seat mounts and some custom-built seat mounts for the stock XJ seats would work.

Your idea on the frame/rocker tie-ins seems to be right in line with what I had planned, but I may just skip the sandwich plates and go straight through like you suggested. I like the sandwich plate idea better though, so as not to have holes in the floor that will be open to water and such, and I think the more places the cage is tied into the unibody, the better (more stiffness added)

Also, here's a pic of something someone from another site suggested to tie the rear bars in a bit better. I plan on another bar between the two rear green bars, but haven't decided where because my spare will be directly behind the front seats.

 

[5-90]

Cut, weld, and seal properly, and you should end up with "punches" that don't leak.

Also, you could go with seat bars under the floor, and it might be a good idea to put in a driveshaft loop or mount points for a skid plate while you're about it.

The extra bars under the door bars aren't really needed - you'd be a lot better off doing a three-pillars mounting setup, with a halo bar, and then triangulate where needed (for instance, between the last two pillars.) It would also be good to tie the triangulation bars into the corners directly, instead of a short distance away. That divides the section into two triangles, rather than a triangle and a quadrilateral (triangles are great structural shapes - quadrilaterals are not.)

I'd set up the rearmost crossbar to be readily removable (for access to cargo and such,) or move it to the bottom. It might not also be a bad idea to add equipment or spares mounts to the cage, but that's not something I can design (I haven't seen your typical loadout...)

5-90

 

[NotMatt]

It would also be good to tie the triangulation bars into the corners directly, instead of a short distance away. That divides the section into two triangles, rather than a triangle and a quadrilateral (triangles are great structural shapes - quadrilaterals are not.)

Can you clarify what you mean here? I've read it over a couple times but I can't quite understand what part you're talkin' about.

Anyway, after plating the frame rails, I will also be welding in a clayton crossmember and re-mounting my DPG skid, since I already have it and it has worked great for me. Might work a driveshaft loop in there somehow if it works out.

 

[5-90]

The idea behind a "triangulation" bar is to make triangles - because they are strcturally more sound than anything else.

Look at your rear triangulation bars (from the rearmost pillar to the next one forward,) and see what's going on. You're going right to the corner on the lower end (which is workable,) but the forward end of the bar will want to be much closer to the joint between the vertical and the horizontal - which will effectively divide the area into triangles - making it much more rigid in a "fore-aft" direction.

The crossbars behind the seats are all right - you're cutting the rectangle into four triangles, and giving side-to-side stability to the cage.

I'd want to tie the cage not only in the floor (and the underbody,) but into the cieling/roof rails (at least the sides.)

While you're about it, you might want to delete the small (6" actual?) bars in the lower rear corners - they aren't going to do anything for you, and they're extra fabrication.

I might want to consider replacing the longwise rails in the overhead for traingle braces - you can maybe add (some load-bearing, rather than structural) members. That's another argument in favour of a full halo bar - just in case...

5-90

 

[NotMatt]

Alright, gotcha. Makes sense to me now.

I'm going to let this sit here and see what others have to say... I posted it on PBB and a local forum as well, so I will update with the other suggestions I get as well.

 

[NotMatt]

Ok... here's what I've got now. Others have said the same about the "roof", so I changed that, and removed the black bars in the back... the rear of the cage may end up coming back first and sloping down instead of going straight back to the floor from the B-pillar hoop... I haven't decided, but either way I think this will work. This area is not of critical importance to me because I will be carrying no rear passengers, so I removed the black bars, I don't feel they are necessary.

Tie-ins with the roof structure and such may come as I work on it, but I'm not too worried about it yet... that's cosmetic IMO to keep the roof in place, but I don't know if at this point I care much... I just want to walk away if I roll it. I'll deal with cutting the top off if I have to.

Here is my thought about the windshield/dash area... yay or nay? I haven't looked yet at the rig to see how much room is available on the floor in this area, I don't want it to interfere with the throttle or anything else in there, so I may ditch the dashbar to floor/trans tunnel bars and make the windshield V come to one point on the dash bar which would make it stronger. Thoughts?


ALSO, has anyone ever ripped apart the dash and run the A-pillars straight down through the glovebox and dash on the driver's side? I'm considering this, as well as some engine-bay tie-ins, which would stiffen the front end up some... just a thought, not sure what's in there that I would have to move or deal with.

 

[vwxyz_xj]

also have a 2 door and had this idea. I plane to half cab mine and it will have a solid roll bar in the bed. Did this real quick on paint program. Only thing I didnt do was on the lower cross brase in the front it will have another piece bent to go around the tranny tunnel.

http://www.imagestation.com/album/pictures.html?id=2115981598&idx=1

 

[Goatman]

A few thoughts.

While the X on top between the A and B hoops will make it stronger, it also makes it much more likely that you could hit your head on it. When safety is the main concern, not hitting your head on the cage is as important as the structure holding up.

On the safety issue, it doesn't take much cage to make it damn safe. The stock structure holds up very well. So, comments that would be true for a stand alone cage (like on a buggy) won't necessarily apply to an internaly built cage in an XJ. In other words, it doesn't have to be all that strong to be safe. More tubing can actually make it less safe, though more rigid, due to my above comment.

This is my opinion, so take it for what it's worth. Since chassis stiffness is also a main goal, if you design the cage for maximum chassis rigidity, it will easily end up being safe enough.

Considering chassis stiffness, the D pillar and the A pillar become important, and what can be done to go in front of the A pillar. It's been shown over and over again on our XJ's that stiffness needs to be added as equally as possible along the entire length of the frame. Otherwise, stiffening one section will cause increased flexing of another section leading eventually to metal fatigue and cracking.

I don't think the single down bars from the C pillar to the rear floor (D pillar) will add the stiffness you're looking for. A single bar will flex, so it needs to be triangulated.....possibly a bar running for and aft at the floor between the C and D pillars.

The A pillar is always a problem due to access and to stiffening. Cracks can form where the front frame sections meet the firewall, and cracks can form where the front cross member connects to the two frame rails. So, if stiffening is a goal, something needs to run forward. You can pull the dash down and run the cage to the firewall. The very first cage I ever saw/heard off in an XJ did this. It helps both access and rigidity, moving the base of the A pillar hoop forward, and would allow for tie ins to the front.

Personally, I like exterior roof hoops. Especially on a mostly trail rig, why go through the trouble of building a rig, and building a cage, and then trash the roof, windshield, and more in a rollover? It only takes a little more (punching the upper cage through the roof) to gain quite a bit of benefit.

After looking at many cages, my perfect cage recommendation would be a combination of what CRASH and GaryE did on the B, C, and D pillars, and what I did on the exterior A pillar and forward to the front bumper.

 

[Goatman]

CRASH's cage thread and pics

Gary E's cage thread and pics

CRASH

Gary E

Goatman's

 

[NotMatt]

Goatman, thanks for your input as well. I think since I'm doing this, I'm going to do it right, and run the A-pillars down through the dash. Not only because it will make it easier to get in and out, but because the dash in my project rig needs to be taken apart anyway and dried out... due to a leaky windshield.

At the same time, I'm going to do a 90 degree right off the A-pillar sort of like you did, and run it forward either inside the engine bay or underneath the fender skin and tie it in somewhere in the front around the front bumper mounts or directly to the bumper. Not sure yet... trying to keep this thing as uncluttered in the engine bay as possible for easy repairs, but I want the extra rigidity in the chassis.

I also like your ideas about external roof protection in the event of a rollover, and I'm still undecided what I want to do here. I am thinking safety first, and I want to keep the amount of tube down to keep weight down, but for my own personal reasons I want the inner cage to be built as stout as I can without being a head-wound risk (it will be heavily padded in any case). On top of that, I still have not decided whether I will cut the back half of the roof off on this like I've seen others do with 2-doors... and in that case I don't want to waste tube on the back roof.

I guess my goal is, like I said, to get the safety aspect there first. I can always go back and add onto the cage to include other niceties like that... this rig will have no headliner, no interior other than seats and dash, so it's not a big deal to add/remove later.

Again thanks everyone for the input. Watch for some build-up pics starting after the holidays probably.

 

[Captain Ron]

AAAAAAAHHHHHH Crap, another cage thread.

Or,

Are you in my Fugu??!??!

Or,

My dad insists on three wraps on head sail winches.

Or,

Red, right, returning, except in MDR, when you have your fenders down.

Seriously, after pondering 14+ "E" pages of Shuttle crew containment drawings, and bidding on the "upgrades",

I have an all new appreciation for "sheetmetal".









Even if it is "CF".

--ron

 

[Captain Ron]

At some point, I'll get around to posting something completely out of the realm of the "homebuilder", and totaly discredited by the "experts" on this board, regarding sheetmetal, and the efforts involved by all here at "improving" on it, adding to it, and "sheetmetal cages."

I know you all can't wait.

Metal Plastiques

From the local HR supplier, I'm sure. Fawk, why bother, BillaVista I'm sure has covered it in absolutley no detail.

--ron

 

[vetteboy]

I'm in the design stage of my project XJ as well, and I've started doing some rough cage layouts. A question I have is about the orientation of the hoops: is it better to do a individual hoops at the B pillar and C pillar, tied together with lateral members that intersect at the hoops, or to do a single piece of bent tube that runs the entire length of the vehicle, with vertical and transverse members to support it?

(if that makes no sense, I can MS paint it)

Also, after experiencing my first XJ rollover this weekend:

I've got some mixed reviews on the unibody. Granted things are a bit different in a traffic roll at 30 mph than a slow off-camber roll, but here's some things I noticed:

Obviously I'm fine. I got no more than a small cut on my knuckle from climbing out through the windshield, which I kicked out from the passenger seat (I wasn't driving). The passenger side a-pillar took the brunt of the impact, which blew out the passenger front window and crumpled the door in half as well, but the seatbelt worked well and I was restrained close enough in the seat that I was protected from the roof and sides.

Given something like a 4 or 5 point harness, with a cage with adequate a-pillar and roof support, this particular roll would have been fine. The problem with the wrecked XJ, after putting it back on its feet, is that pretty much everything on it is tweaked now. There's not a single panel or door that lines up, although the only busted glass was the windshield and passenger window. Would a properly designed cage (for both stiffness and safety) help prevent this as well, or would it have caused damage more isolated (i.e., cage mounting points) but more severe?

Just some random thoughts. Still kinda shook up from the whole thing.

 

[Goatman]

I like cage threads, even though we've had a bunch of them. There are many aspects to consider, and there is a lot that can be learned. Definitely cages need to be well thought out, and the goals clearly defined. I'm a firm believer that if you're going to go through the trouble of building a cage, you might as well protect the body and not just the passengers. To protect the passengers, nothing more than the simplest of cages is necessary.

Here's my roll experience, just for more info. Very minimal damage on a decent roll off of a ledge, then had to roll it the rest of the way to get it back on it's wheels, so ended up going over 360*.

A pillar cross bar fixed by pulling it out and capping it with a half round section of the next size bigger tubing.

 

[Captain Ron]

...Definitely cages need to be well thought out, and the goals clearly defined.

...If you're going to go through the trouble of building a cage, you might as well protect the body and not just the passengers. To protect the passengers, nothing more than the simplest of cages is necessary.
....

A pillar cross bar fixed by pulling it out and capping it with a half round section of the next size bigger tubing.

Thread killer. MS paint is the bomb.

Just for that, it's gonna be a few weeks of 8.8 threads before I tell you how NASA solves its crew survival "Problems".

Long live 1.75 X .120 HREW. Home builders.

--ron

 

[davidt]

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.

 

[NotMatt]

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.

I don't know, but when I build mine, this is one piece that will be easily removeable... we had a hell of a time replacing the cd player in my friend's rig because his dash bar crosses right in front of the removeable dash bezel portion around the gauges and heater controls and cd player and such, making it non-removeable.

 

[Goatman]

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.

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.