Shear Planes

[Phil]

This ties in to my "Minimum Cage" thread a little.

Instead of running tubes everywhere, how have people incorporated shear planes into their cage design? Seems like a shear plane could serve as a lightweight, compact way to replace tube in some locations.

A section of the stock roof, or the back of the cab on an MJ? Something thicker? Roof with a sunroof inside of it?

Do you consider the shear plane to be an effective structural member, or do you consider it cosmetic and run a tube over or under it?

 

[Goatman]

Phil, what's a shear plane? Any flat sheetmetal section would immediately loose most of it's structural strength if it was dented in by a rock in a roll over.

 

[vetteboy]

For one, I think the concept of a 'shear plane' is exactly what gives the unibody its rigidity in the first place. Prime example being the function of the rear tailgate in resisting the body's urge to turn into a parallelogram when flexing. Anyone who has ever tried to open/shut the tailgate while flexed up knows about this.

I think the two biggest reasons why this isn't done on a cage situation is pretty much because of the strength/weight issue, and also just for accessability reasons. Here's where mine is right now...

One example of a 'shear plane' I could include would be to attach the remainder of the roof line to the top of the c-pillar, to the b-pillar (by plug welding, I guess), and the a-pillar (also plug welding). This would take the place of a tubed diagonal 'X' along the ceiling and would be a decent way to incorporate the original unibody construction into it.

My problem with that, is anyone who has tried welding anything to the single-ply thinner sections of the unibody, can agree to what a pain in the ass it can be. Using anything short of a TIG would make it very difficult to get proper penetration on both the tube (.120 thickness) and the roof sheet (20-22 ga?). And even then, just because of the heat-affected zone on the sheet metal, you'll have a relatively thin and brittle strip right behind the welded area, which I think would probably tear under any shear loading.

So if you were to use a thicker piece of sheetmetal, say even 14-gauge (0.075"), you have a weight of just over 3 lbs per square foot, and you'd probably need at least 6-8 square feet to cover the whole roof, so say around 20-25 lbs. By contrast, you can make a tubed 'X' with 1.75" OD x .120 wall tube using around 6-7 feet (at just over 2lbs per linear foot), so you're making a much more 'structural' design and saving weight from a high area at the same time. If you crunch down on something you have a much better chance of denting the sheetmetal plane and significantly reducing its effectiveness.

The same argument could be made for closing in the c-pillar area to enclose the cab fully - however, IMO it would still be heavier, not as effective, and I really don't want to enclose it anyway.

That's my take on it, at least.

 

[Bender]

I think it could be an effective method of adding strength with minimal weight however you have to understand the trade offs. Welding sheetmetal in a window of tube is roughly equivalent to making an "X" out of flexible cable from a structrual standpoint. i.e. it only has strength in tension...unlike tube which can resist both tension and compression forces. You'd have to accept this and account for it in your design. Additionaly I don't think sheetmetal would provide as much protection from protruding objects like sharp rocks and stumps in a rollover unless it was thick in which case the tube is probably more efficient. That may not be an issue depending where it is used.

I think the vehicle manufacturers use this method because they need sheetmetal anyway to enclose the car so you might as well have it serve a couple jobs and add to the overal structure.

 

[Captain Ron]

Instead of running tubes everywhere, how have people incorporated shear planes into their cage design? Seems like a shear plane could serve as a lightweight, compact way to replace tube in some locations.

Got you a degree, and you're dragging out stuff from your last internship?

I have 3. At the rear of the cab, a bridge that also serves as a cooler deck and battery tray between the 2 main rear chassis tubes, at the rear of the chassis under the fuel cell, and on the back with tail lights in it. None of them are center line shear planes.

Current practice in areospace apps is to avoid centerline attachements. I brought mine inside of a true OD centerline by a material thickness, and the OD tangent intersect.

Longitudinal planes are going to be difficult in the rear. Lateral is what I worked on. A minimal amount of material can save you alot of tube. I punched 1.5" diameter holes on an offset grid pattern for even more.

You missed the last major build I did before Moab, but if you look at a top view of my rear layout, the outside tubes have less than 12 degrees of deflection towards the rear... not enough for any decent lateral stiffness. The solution was to add triangulation from the floor or the cab, or do a few shear planes. I saved a ton of fab and tube...

Phil, what's a shear plane? Any flat sheetmetal section would immediately loose most of it's structural strength if it was dented in by a rock in a roll over.

Then why did you put one where everyone else has? You know, that piece of prerequisite diamond plate between the A pillar and the hood bar to A pillar base...

And even then, just because of the heat-affected zone on the sheet metal, you'll have a relatively thin and brittle strip right behind the welded area, which I think would probably tear under any shear loading.

It would take a few pages to cover all the aspects of what is untrue in your post and the assumptions you're making, so I'll hit the low hanging fruit.

If you could actually weld, this wouldn't be an issue.

...
Welding sheetmetal in a window of tube is roughly equivalent to making an "X" out of flexible cable from a structrual standpoint. i.e. it only has strength in tension...
...

Wrong. This may be true if you use a sheet of paper, but in common accepted practice, shear planes incorporate at least 1 of the 6 or 7 different techniques of building compressive rigidity into the plane.

...
Additionaly I don't think sheetmetal would provide as much protection from protruding objects like sharp rocks and stumps in a rollover unless it was thick in which case the tube is probably more efficient.
...

More tube is efficient? Certainly if it's was used like so many I see here that weigh 3 tons because there's not one space left more than 6" square... Of course, use square, that'll cover some surface area.

You guys need to get your thinking out of the Mod Tech on this. Phil asked a Q, and the experts come out of the woodwork all saying "body panel". Sheesh.

--ron

 

[Bender]

I still think using tube can be more efficient than a shear plane...again like I said above it depends on the application. If you're trying to re-inforce a very large area I believe the work required to install a couple tube memebers would be far less than designing an adequate shear plan to accomplish the same task and the weight differences would be minimal.

Like I said above...using shear planes can be an effective method in re-inforcing your cage but the trade offs must be fully understood.

Ron, you obviously know a bit about the subject. What are the easiest methods for the home fabricator to stiffen a shear plan so it will handle a decent amount of compression (obviously the size of the plane would have a large effect on this so lets assume a 2'x2' area)

 

[Phil]

Got you a degree, and you're dragging out stuff from your last internship?

I guess I missed the sheetmetal design course.

I have 3. At the rear of the cab, a bridge that also serves as a cooler deck and battery tray between the 2 main rear chassis tubes, at the rear of the chassis under the fuel cell, and on the back with tail lights in it. None of them are center line shear planes.

Current practice in areospace apps is to avoid centerline attachements. I brought mine inside of a true OD centerline by a material thickness, and the OD tangent intersect.

Does the distance off of centerline matter much? Or can I be trendy just by not being on the centerline?

Longitudinal planes are going to be difficult in the rear. Lateral is what I worked on. A minimal amount of material can save you alot of tube. I punched 1.5" diameter holes on an offset grid pattern for even more.

What about larger holes in the shear plane? Say, MJ back window, or XJ sunroof? I can see the glass panel acting to stiffen up the plane like the hatch prevents the back of the XJ body from wiggling like a...plate of Jell-o, but how about when you open the window?

You missed the last major build I did before Moab, but if you look at a top view of my rear layout, the outside tubes have less than 12 degrees of deflection towards the rear... not enough for any decent lateral stiffness. The solution was to add triangulation from the floor or the cab, or do a few shear planes. I saved a ton of fab and tube...

That's what I'm looking at now, the possibility of saving time and tube by incorporating parts of the body, like the back of the cab in versions...1 and 2 (?) of your buggy. I don't recall any attachments between the tob of the B-hoop and the top of the cab. Would that just be redundant, with the sides and bottom attached? I only remember one or maybe two short attachments between the hoop and cab anyway.

Wrong. This may be true if you use a sheet of paper, but in common accepted practice, shear planes incorporate at least 1 of the 6 or 7 different techniques of building compressive rigidity into the plane.

This might be a good time suggest some of those methods. Shear planes for dummies?

--ron

 

[Captain Ron]

I guess I missed the sheetmetal design course.

...

Or can I be trendy just by not being on the centerline?

...

You did. Dumb ass. Next time, pay attention.

Trendy...

Let see. I think that's a prerequisite for newly minted ME's.

Good luck.



--ron

PS: Fawk you.

 

[Captain Ron]

Go ahead and delete this thread.

Another shining example of what you can get for doing something for someone so far beyond what any moron can do. Just go down the street and buy it.

Good luck, your welcome, go away.

--ron

 

[Matt S.]

blah blah blah I like lots of sheetmetal blah blah blah I miss phil.

Isnt that what you really want to say??


Btw, I hope he does a good job with CAD, cuz I think it will be my sorry butt putting those together.

 

[XJ_ranger]

Btw, I hope he does a good job with CAD, cuz I hope it will be my sorry butt putting those together.

 

[Matt S.]

Yeah.. good fix. I am always looking for a good reason to weld on someone elses junk.