Yes, it is a "frame"

Not to be a prick, just wanted to say yes our vehicles have frames. If they didn't, you would feel like you were driving a giant blob of jello with wheels. The frame is what holds the vehicle together. Just kinda got tired of everyone saying snide remarks about it. What else would you call those rails running under the vehicle that connect the front and rear bumpers? Unibody just keeps them from being isolated by rubber bushings, which is why Jeeps embarrass all other competition (blazer, explorer, etc) in handling and performance.

could it be considered a subframe?

no, a subframe holds something relatively small, like the engine or rear suspension on a car. This is a real, full frame with 2.5 times the torsional rigidity of common ladder frames.

Yes, it is (technically) a "frame."

However, the point that is so often made here, is that it is not a "true" or "ladder" frame from which the body can be removed. A "unibody" is also called a "uniframe" (I've heard both,) but in our case the "frame members" are made from much lighter metal, and are reinforced and stiffened by welding the body members to the frame (things to Google - <"shear panel"> and <monocoque>)

Since it's not a "frame" per se and isn't removable, it's simpler for us to say that it's not a "frame" vehicle - which everyone seems to understand. To an engineer, however, it's still a "frame" - it just can't survive as a separate unit, like a ladder frame can.

Also, the body can be considered "permanently" attached to the frame, unlike on a ladder frame where removal of some hardware will facilitate the separation of the frame and body - and that's why you can do a "body" lift on a "box and frame" vehicle far easier than you can on a "unibody" - and with less engineering.

5-90

fine explanation, just wanted to convey that there is actually a strong foundation to which everything is bolted (axles, engine, etc.), though the body is part of such foundation, rather than also bolted to it.

You, sir, are correct. I just wanted to make sure there weren't any "grey areas" leftover after your statement.

The principal difference between a ladder frame and a unibody "frame" is one of design, while on paper. A ladder frame is meant to be stiff in its own right, and mimial reinforcement or location of frame elements is required (apart from torsional reinforcement, naturally. A ladder frame is essentially two-dimensional.)

A unibody "frame" is designed along with the body, and body panels (most notably the roof, fenders, and doors) are intended to reinforce the three-dimensional "frame" that suppors everything. That's what allows the "frame" to be lighter on a unibody, and why it can be made from the same material - or only slightly heavier - than the rest of the body. However, shear panels and "filler" panels become an absolute must with a unibody - just because the "bare frame" stiffness isn't there. That's why the Bronco and Blazer (FS) could get away with a removable half hardtop, but to do so on the XJ would REQUIRE a partial cage at least - to make up for the loss of the roof (and the loss of structural integrity you'd get once the roof became removable.)

Again, I'm not arguing in the least - I just want to make sure that others don't misunderstand what you have to say. I've got a bad habit of explaining my answers - comes from school ("Why do you feel that way?" "Why?" "Defend your answer" - all very good, and very common questions on my middle school and high school tests. Pity they aren't so common anymore - when I'm teaching someone, I'll ask them to outline their thinking, right or wrong.)

5-90

interesting, sounds like you are a teacher, and a good one at that! True about the Bronco and Blazer. Out of curiosity, what do you think would happen if the top was chopped on an XJ and no reinforcements were added? Could it even drive down the road without getting twisted?

Jackhill442 said:

Out of curiosity, what do you think would happen if the top was chopped on an XJ and no reinforcements were added? Could it even drive down the road without getting twisted?

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Nope, it couldn't.

Teacher? I suppose I should be - I've probably got more hours instructing than most. Catch is, I couldn't handle teaching primary school unless I got to use the "Board of Education" every now and again - gotta keep the kids in line. I'd have to teach college - most kids are there because they WANT to be and WANT to learn - which makes a difference (but, at 33, I'm still the "old man" in most of my classes...)

As far as your question goes, it depends. There is some inherent stiffness from the "box" shape of the frame members, and that would be augmented slightly by the remaining floor panel, but you'd lose the stiffness you'd previously gained from the roof and the D pillars, plus any other losses from the "shear panels" between the C and D pillar (to include the windows. They fill those holes for a reason, you know.)

You'd get a really swimmy rear end, and I'd think this would get worse under vibration, on anything rougher than a fresh-paved road, or when turning sharply (like evasive action.) Even a "short" frame that would be tied into the C pillars would help (which would then loop around the "cutoff line" to tie into the lower D pillars as well,) but you'd be best with a semi- or full cage that would not only have posts in place of the C and D pillars, but a "cross" in the halo bar, the halo bar tied to the roof or roof rails, and at least one triangulation bar longwise down the side voids. Probably a cross in the rearward void as well, since the liftgate acts as a shear panel when closed. (Overkill, I'd admit - but I'd sooner have things not fail than things that fail.)

On terrain? I'm sure you'll bend one or both of your rear frame rails. Even a ladder frame gains some benefit from that big floorpan bolted to it - just not as much.

I've done a lot of silly things, but I'd not want to try something like that without replacing the stiffness members/shear panels you'd be cutting out. Of course, with a little design work, you could come up with an effective removable "half-roof" that would tie down to the cage. You'd be stiff with the roof on or off, and it would be easier to tie the roof to the cage than to the sheetmetal...

5-90

Jackhill442 said:

interesting, sounds like you are a teacher, and a good one at that! True about the Bronco and Blazer. Out of curiosity, what do you think would happen if the top was chopped on an XJ and no reinforcements were added? Could it even drive down the road without getting twisted?

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you must be up studying for exams, at least I am. I know the feeling of old, I am a 6th year undergrad, I already feel like a grandpa.

Nope, not even.

I got hit by a car last month (08NOV) and I'm still convalescing. I'm mostly back up to scratch, but when I've got a lot of time - or when I have some serious healing to do - I tend to revert to my more natural sleeping pattern - grave shift.

It's OK - I'll be out in an hour or two.

I'll have to get "back on days" soon - I'll be back in school after the first of the year (I wish they had grave shift classes - it would make my life easier!)

Anyhow, I've got three ribs to knit, a load of facial fractures to knit, and I'll be done. The nice thing - all the reports I've read, the word "fracture" was prefaced with "non-displaced" - so it's a lot easier than it could have been.

The XJ is fine - I wasn't in it at the time.

At least I've been getting to sleep a little earlier each night - a fortnight ago, I was up until 0800 or so, then sleeping all damn day. At least my wife understands me well enough to let me sleep when I get like that (and I do have a good reason for reverting...)

This isn't the first time (for being heavily injured, or being hit by a car in fact) - but it's the most I've been injured in a vehicle incident. Given a choice between this or being shot - I have to admit, both times I've been shot I've been up and about rather more quickly...

5-90

5-90 you need to get well soon, many of us depending on you to see what little goodie you will come up with next...speedy recovery.
As far as the frame, I know we have a frame (OEM calls it "rail"), for without this frame I would not have a place to attach my rust.....LOL
Happy Holidays...............gary

frame or no frame, it's still crap. being a unibody is the only thing i hate about my cherokee. but i wanted a jeep that my 3 dogs would fit in and would still be good offroad. so i'll have to deal with it.

Jackhill442 said:

interesting, sounds like you are a teacher, and a good one at that! True about the Bronco and Blazer. Out of curiosity, what do you think would happen if the top was chopped on an XJ and no reinforcements were added? Could it even drive down the road without getting twisted?

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This may surprise some of you. I know a guy who has a 383 V8 with about 400hp in his cherokee who chopped his top from the windshield back and DIDN'T, I repeat, DIDN'T reinforce anything. I swear!!!! He drives it like that and as far as I know he hasn't had any problems. I even asked him if he reinforced it somehow (Maybe somehow I couldn't see) he said NO. I couldn't believe it......

godplayer said:

This may surprise some of you. I know a guy who has a 383 V8 with about 400hp in his cherokee who chopped his top from the windshield back and DIDN'T, I repeat, DIDN'T reinforce anything. I swear!!!! He drives it like that and as far as I know he hasn't had any problems. I even asked him if he reinforced it somehow (Maybe somehow I couldn't see) he said NO. I couldn't believe it......

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No roll cage?

scorpio_vette said:

frame or no frame, it's still crap. being a unibody is the only thing i hate about my cherokee. but i wanted a jeep that my 3 dogs would fit in and would still be good offroad. so i'll have to deal with it.

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Out of curiosity, why do you resent the fact that XJs do not have traditional frames?

Do you actually believe a ladder frame is more rigid? Do you just like to take the body off in some instances?

I'm curious, really.

godplayer said:

This may surprise some of you. I know a guy who has a 383 V8 with about 400hp in his cherokee who chopped his top from the windshield back and DIDN'T, I repeat, DIDN'T reinforce anything. I swear!!!! He drives it like that and as far as I know he hasn't had any problems. I even asked him if he reinforced it somehow (Maybe somehow I couldn't see) he said NO. I couldn't believe it......

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There may not be any apparent tacoing (yes, a technical term) of the chopped XJ, but the fatigue induced into the remaining structure as it flexes more readily than originally designed by having removed the upper half of the vehicle and thereby greatly diminishing its torsional and bending rigidity will undoubtedly take its toll in time.

The XJ's unibody is constructed differently than most unibodies, and that's what makes it sort of a hybrid between a true body-on-frame construction and the traditional unibody.

Most unibody vehicles (typically cars) use stamped/formed/welded box sections to create the primary structure for the front end up to the firewall. At the firewall, these boxed sections end, and the load is transferred outboard to the rocker panels. (Yes, the rocker panels give the primary resistance to bending the floorpan of unibody sedans.)

The XJ's approach is more traditional. While it uses the same stamped/formed/welded box in the front end, it continues this construction for the entire length of the floorpan, all the way to the rear bumper. Consequently, on an XJ, the rocker panels serve very little to increase stiffness of the floorpan between the front and rear axles. (Ever notice how guys end up removing the rocker panels on XJ's, and yet have no complaints about body flex with the rockers removed? Thats 'cuz the rockers are NOT a primary structural element.)

Is the XJ construction bad? It depends on your measure. The XJ probably has the highest stiffness for it's weight of any vehicle in it's class, so that's a good thing. The bad part is that the DIY guys don't like the challenges that modding a unibody brings. It's easier to do aftermarket cut and weld on a body-on-frame construction.

The only thing bad about the "frame" is the thinness of the rails... I can put a dent in it if i hit it with a rubber mallet.

jackieonasses said:

The only thing bad about the "frame" is the thinness of the rails... I can put a dent in it if i hit it with a rubber mallet.

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Stop hitting it with a rubber mallet.