importance of the roof?

what structural support does the roof provide? I am talking about the very top part (the factory roof rack area). I know that the door pillars are important and I have a feeling that the middle of the roof provides some support too but how much?

As I understand it, the roof panel (the flat bit,) acts as a "shear panel" to help keep the upper members of the unibody located under operation. This is why I suggest to use a "halo" bar whenever anyone wants to cut out the roof, and install a cage - to keep the top ends of the pillars located.

Under normal usage, vibration and flex would act to distort the unibody if there were no provision for keeping the top ends of various structural members located - and that provision is the roof panel.

Granted - I'm an engineering student, not a fully-trained engineer, so if I'm off I'd like to be corrected. I may ask for verification - that's just the way I am!

Think of it like this - you make a square frame out of wood (call it 1x1,) and then load it in various directions. You will note that it will distort when loaded on the edges or corners - there's not much you can do about that, without adding material...

If you place a couple corner-to-corner braces, dividing the square into four triangles, you'll get much greater stiffness in edge and corner loading - but (from an environmental point,) it's still not sufficient.

Take away the triangulation braces, and add instead a thin sheet tacked down all around the frame. This is a "shear panel," and it acts to support the frame all the way around.

Granted, it will not be as strong as triangulation (what is?) but it will keep the weather out, and it will make the frame quite strong enough for normal operation - as long as you don't edge load it or place a great load upon the vertices, which would make anything fail anyhow (if stressed beyond the strength of the material used.)

I once helped my grandfather build a competition ultralight aircraft (the damn thing weighed less than I did!) with the fuselage constructed of 1/8" plywood with 1" thick styrofoam for shear panels. The plywood gave strength, the styrofoam gave support. I could walk up to the side of the thing and punch a hole in it, but it's taken several hard landings without distortion or needing repairs. The wet weight of the aircraft (minus pilot) was about 205 points - I weigh 250.

Make sense now, or have I succeeded only in confusing you? You might find it productive to search mechanical engineering boards or race chassis design fora for "shear panel," and see if they do a better job of explaining it than I just did...

5-90

thanks for the reply. Much more than I could have hoped for. One question though what is a "halo bar"? My sister recently ran mine into a sign and the sign fell on top of the car punching a hole in the top and was considering what to do to "fix" it. I didn't know how much support the roof gave. I'm thinking if I cut just the flat part out and had a "primitive" roll cage welded in. (Primitive meaning just a bar going across behind the drivers head and back seats heads, and tacked down to the floor. Also a bit of "triangulation on the bars going behind the driver/back seats) Just wondering if you guys thought that would work or not? It's be used for a bit of DDing and a bit of off roading.

XJ's are unit-body constructed vehicles. The sum of all the metal areas reinforce and share the load of the other body areas. If you are going to cut the roof off you need to replace that structure with a cage. Try this, stomp on a full can of pop or beer. Empty the contents and try it again, different results? I was at the junkyard and one of the XJ's had been cut to provide parts for a body shop. I could push on the roof and the whole body would flex about 8 inches. If you pushed and pulled it would wobble like lime Jello.

jeg242s said:

thanks for the reply. Much more than I could have hoped for. One question though what is a "halo bar"? My sister recently ran mine into a sign and the sign fell on top of the car punching a hole in the top and was considering what to do to "fix" it. I didn't know how much support the roof gave. I'm thinking if I cut just the flat part out and had a "primitive" roll cage welded in. (Primitive meaning just a bar going across behind the drivers head and back seats heads, and tacked down to the floor. Also a bit of "triangulation on the bars going behind the driver/back seats) Just wondering if you guys thought that would work or not? It's be used for a bit of DDing and a bit of off roading.

Click to expand...

Having a "tight" repair should work. The roof panel is NOT structural, but it IS reinforcement.

I'm planning on taking the sunroof out of my 88 (when I can get an OEM roof panel someday - I'm tired of it raining in my truck, but I'd like to have it look original and not hacked-up) and I'm considering getting a "no-weld" kit from Eastwood to do the job. I'll still have to "flange" the edge of the hole (so the repair panel sits flush,) but fixing the panel in place is done with a structural adhesive and rivets. Should be plenty strong for the loads on the panel - the rivets are strong in shear, and the presence of a mechanical fastener should be enough to keep the adhesive (OK, "glue") from shearing loose.

I'd expect something similiar should work for your repair - moreso, since it's smaller.

Oh - a "halo bar" is the loop at the very top of a full roll cage - forming a "halo" around your head. Thus, the moniker. If you have a full-chassis cage, you'll have a loop around the top, and that is the halo.

If you're going to use a cage for off-road without any backseat pax, I'd at least design in a "half-halo" (from the windscreen eyebrow to the C-pillar, behind the front seats) and have that triangulated in place. If you EVER think you're going to haul pax offroad, have the half-halo go to behind the rear seat (to the D-pillar.) With that triangulated, you should still be OK - but a "full" halo (from the windscreen to the liftgate) would still be best - you could support that at a minimum of six points, and triangulate everywhere.

Anything else I can help with? You might want to pick up a book on race chassis design - I don't recall the title at the moment, but Carroll Smith wrote a damn good one, and it's subtitle was "How to Build Winners That Don't Break." The man was only a lead racing engineer for 30 years or so... I have found all of his books highly useful in my researches, and I've got to get a couple to finish my set.

5-90

Im not an engineerign student , just a guy who likes to take things apart, and apart, and apart. I cut the roof off my 89 2door. I added a "b-pillar" right behind the front seats,..and added support from there to where the rear seats had been. The portion i cut from the roof was from behind he b-pillar to the gate.Including the gate. In essence i made it look like a pick up truck. I left it open and herculined the inside.So i still had the b-pillar and the over-the-front seats roof.I welded the roll-bar to the b-pillar and it also bolted to the seat-belt shoulder harnessconnection. IT seemed pretty stable to me alltho i was a bit timid about hard wheeling.Ive since sold it but i hear its still runnin about in the Outer Banks area of N.C right now.I do know that roof plays a part i nthe integrity of the XJ as a whole however one can, with the proper tools and material, recover the integrity that was lost with the removal of the roof.