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2019 Jeep Wrangler in depth, Part 5: Body design (frame, unibody, etc)

by Robert W. Sheaves (edited). Part 5 of a series. Written in mid-October 2014.

Bob Sheaves was responsible for 4x4 suspension design at the Jeep/Truck Engineering's PreProgram Engineering Department from the AMC days until 1993.

Allpar's owner, David Zatz, asked what Chrysler is likely to present as their next-generation Jeep Wrangler, given their public statements and my own experience. This article is based on my engineering evaluation of various public sources, plus private discussions.

The short answer: Wrangler is likely to remain body-on-frame; they will probably use a high strength hot-rolled steel frame (the image is from the 2013 Ram but shows a sample frame).

Playset Toy

The body design choices are:

  1. Body on frame. A separate body structure is built on a full-length frame; the visible body panels are "unstressed," not needed for rigidity. This is how current Wranglers and full-size Ram pickups are made (a Ram frame, with some additions such as brakes, is pictured above).
  2. Semi monocoque. A single structure has stressed body panels and a bolt-on or welded front subframe (e.g. any Chrysler car from the mid-1960s onwards, or the Jeep Cherokee XJ four-door wagon-style SUV).
  3. Full monocoque. A single structure relies completely on stressed body panels with no subframes at all.
  4. Roll cage. A separate structure provides a stress path through round tubes passing through (and independent of) the body structure, and tied to the main frame-either separate or subframes. One example is Norm Layton's custom-made "Jeep" used for off-road endurance racing, pictured below.
  5. Spaceframe. A series of extrusions (metal formed by forcing it through a die) that form a full load-carrying stressed structure; it's usually a basic box shape, with flanges (ridges to add strength or hold different things together) added.

Vehicle Machine tool Chassis Crane

Joel W. Jackson wrote in his book Fast Days that he originally wanted the Dodge Viper to be based on a spaceframe, as a real racing car put into production. This was not done, possibly to ease production or control costs. At the time, Chrysler had very little free cash, and many pundits were predicting the company's bankruptcy - just before its resurgence and record profits.

We know what force we need to resist in the body to keep it from becoming a pretzel factory. We have an idea what our worst case load path is through the body and it is now time to make a decision on the best technical solution; cost variance is the determining value (not factor, but value).

Vehicle Wheel Transport Tire Automotive tire

The best answer way I could think of to approach this issue, which is pure speculation, would be to rethink the entire aluminum body design and go to a true spaceframe design of aluminum. Not a sport bar, not a roll cage, but a spaceframe. The aluminum space frame is more advanced than a monocoque or unit body, as it uses less material for greater strength in torsion and stiffness. Efficiency beats brute force. The large crossovers will give you the volume for a full monocoque over the body-on-frame to make it practical.

The problem is that, outside of Audi, the expertise does not exist anywhere to accomplish this. Not Chrysler, not Ford, not GM, and not Fiat. The point I am making is that just because something is possible, that does not mean you can do it in the time before introduction, and definitely not before a decade has passed. The complications go far deeper than just the skin. Even with the HMMWV body (which we designed at AMG and was on a steel frame), it took over a year just to perfect the toggle lock method of attachment of multiple sheets of aluminum together without welding, riveting, or adhesives.

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The Jeep JJ is an example of where the JL is now. Chrysler lost roughly $1 billion when JJ was cancelled in favor of Grand Cherokee ZJ.

The next Jeep Wrangler, "JL," has 20 other programs dependent on it completing successfully - that is, relying on technical expertise being developed in JL. If JL fails, they all fail.

One has to do aluminum bodies before one can move on to aluminum space frames, which is likely the optimimum future for the Jeep Wrangler.

As for the body appearance, it will likely be made narrower to present a smaller "face" to the wind, and be more aerodynamic. They can do this by closing gaps and shortening the track to pull the wheels back into the wheelhouses; it would actually be cheaper to switch to an indendent front suspension to do this, than to narrow the axles.

There is a limit to how much the body can be narrowed, both in terms of interior space and the ability to fit V-type engines, now that the Jeep I-6 is long gone. Fitting in these engines is why the tub would stay the same width and the fender extensions and mirrors would take all the sectioning, allowing the track change to narrow the overall width.

Sectioning to make the body narrower, for the layman: Take the CAD (computer drawn) model of the fender, hold the fender mounting surface in space where it bolts to the sheetmetal, then link all the 3D styling surfaces together (to hold tangent and contiguousness), and finally translate the outer flanges with the side marker lamps and make the outer flange contour boundaries narrower, to reduce the overall width. You literally take out a section of the middle of the fender, while keeping the styling the same. This is actually a much more complicated process than it seems - it would take around a month to modify all four fenders and the mirrors.

The brake booster would have to change to a Delphi hydroboost with relocated pressure ports. The re-porting would narrow up the booster to allow the accumulator to go up top. The only problem would be getting Delphi to agree without charging at least an arm for it.

How to build it?
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Steel or aluminum?
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New suspension
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Open or fixed top?
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Weight and strength
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True Jeep
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Original Jeep
More to come?


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