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by Robert W. Sheaves (edited). Part 3of 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 approach I would take to design a unibody Jeep for 2017 model year — and what Chrysler is likely to do.
This article is based on my engineering evaluation of various public sources, plus private discussions.
Think of the body of a car as a tube with the ends closed off. The body of the vehicle is required for structural rigidity, to resist load inputs in torsion, bending, and linear crush (I apologise for oversimplifying this), among other forces. A round cylinder provides the stiffest shape, with the minimum material used to resist these forces. Cut a hole in this tube and you weaken it to where it will collapse (“collapse” means “deform along force vectors.” I am going to use "collapse" as a generic term for this result of applied force).
To demonstrate this, take a round paper towel roll and tape off the ends with masking tape. Hold the roll either horizontally or vertically (your choice) grasping each end with both hands and twist in opposite directions with each hand. The paper tube will resist collapsing until you exceed the stiffness of the tube, then it will crush in a spiral pattern. This is called torsional deformation and represents the twisting of the body of the car, or unibody ( semi monocoque technically).
Now, as you cut more openings (doors, hood, tailgate, and the removable roof), you will have a piece of Swiss cheese that can barely support itself at rest, let alone under any form of induced loading. You have two options:
1. Add a load bearing member to the structure to transfer this loads elsewhere in the body.
2. Add local reinforcement to the opening. The classic case is a Mustang Fox-body convertible; the bodies were built without the roof panels, sail panels, interior structure, and roof reinforcements for the coupe or hatchback, or back seat and C-pillar to floor reinforcements, then shipped to McLaren to be converted into a roadsters. The issue of stiffness is clear in these cars; the weakest point was the B pillar, because of the missing structure above the beltline (rear quarter glass, aperture reinforcement, and roof header reinforcement), so that closing the door flexed the remainder of the B pillar (supporting the striker assembly) to flex inwards almost an inch, every time the door was closed. Now think about that...an inch of movement, each time the door was closed.
How many times do you open and close your door every day?
Opening reinforcements come in a couple of design variations; one approach is using the “sport bar” to reinforce the open section of roof. A common problem of aftermarket “roll bar kits” is that you can easily overdo the structure and cause body sheet metal fatigue failures, such as body cracking and separation at the attachments.
Like McLaren did with our example, you must use a finite element program to replace the structure needed only and spread those loadsout on the body shell to prevent this condition.
Just remember, regardless of how bleak the outlook, there is always a way to improve and advance. It may not be in a direction that people approve, but that does not mean you have failed.
The XJ Cherokee was a unibody; it had subframes welded to the underbody structure, making the assembly a true semi-monocoque design, just like Chrysler's Unibody with its bolted-on front subframe.
The short answer: Body on frame, using a high strength hot-rolled steel frame.
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.
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).
3. Full monocoque. A single structure relies completely on stressed body panels with no subframes at all.
4. Roll bar. 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.
5. Spaceframe. A series of extrusions that form a full load-carrying stressed structure. Usually a basic box shape with flanges added.
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 with cost variance as the determining value (not factor, but value).
The best answer I could give is the way 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 space frame.
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 RU-based-crossovers will give you the volume for a full monocoque over the BOF 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.
The Jeep JJ is an example of exactly where the JL is now. Chrysler lost roughly $1 billion when JJ was cancelled in favor of Grand Cherokee ZJ. 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. (Rick Anderson: “Walk before you run, you can do aluminum bodies, you can move on to aluminum space frames?”) Exactly.
On the issue of the J8, with it's unique hotchkiss rear suspension and semi floating Dana 60, I would expect to see the axle changed to a light duty 60, but the hubs to change back to the older full floating style and a higher rated GVWR link coil package to meet NATO needs. I do NOT see a version for the US military from FCA.
On a side note regarding J8, I could see the pickup in 2 versions....a standard pickup with light duty set of floating axle hubs as today and a "heavy duty" OPTION (meaning NOT a seperate model) using the full float setup from J8, taking advantage of the IS to only require a wheel end swap on the line in the front.
You would not have any four door pickup, strictly a 2 door version with some extra space behind the front seats, like a Dakota or MJ Comanche, and not a full “Club Cab,” either. You would probably have about 6-7 inches to allow for a rear crash clearance (for your head not to impact the bed access glass).
For the expected two door pickups, I expect that to also be about a production of about 40,000, though split differently (7,000 CKDs and 33,000 complete vehicles). The CKD also figures into the African sales, through AAV in Egypt (discussed here). The limits on the size due to line spacing will keep the pickup’s sales limited (and incidentally prevent it from competing with Ram 1500).
Due to the spacing of carriers remaining the same distance apart, I do not expect a length change to allow a four door pickup. This is one of the compromises customers and consumers will have to live with; without an increase in the spacing there is no room to build a longer vehicle.
frame width adaptation for a chassis cab version. In order to mount a different bed, of whatever type, you need to have the rear frame rails spaced at 34" across the top flange. This will require a pair of adapter rails that bolt to the top of the existing frame rails similar to the older Dodge D and W c&c trucks. It is more involved than just pulling off the box or just not installing a box in the rear, because of using the full body's frame assembly under all of them (this also means a change to the J8 export military truck to this new design from the existing design).
In other words, your pickup will be about the size of a Ford Ranger in length, but a little wider — the passenger room of today's two-door Wrangler, with narrower mirrors and fender flares, and built with a seperate bed. The bed would probably not be unitized to the main body for structural reasons.
Just like the first and second generation Dakota, you would have around a 1,000 lb payload and up to a 6,000 lb tow rating with a Pentastar V6 (this doesn’t mean a diesel would be precluded).
The short answer: A convertible is not likely, but possible. A hardtop is the initial direction, but a soft top is not precluded at this time.
A regulatory change in 2009 is why there are now no more T-tops.
NHTSA has two different standards for roof crush and A-pillar deformation on rollovers: one for a convertible, and one for a fixed roof car. A fixed roof car has to resist a greater amount of force than a convertible.
Every open-top car takes advantage of this exception, not just Jeep, but the question “How long will this exception last?” must be answered. Jeep has been able to use this for every redesign since 1968, the first year of the FMVSS (Federal vehicle safety regulations) in the US. Given the current trend for ever-increasing mandated safety, NHTSA may well require extra strength in the A-pillar.
Right now I would bet on the law not changing as hard tooling is under way. That does not mean that Chrysler is sitting still, doing nothing. You can also bet the lobbyists are working on this also.
My opinion is that the risk of a law change is too great, so Jeep must assume the worst case — a fixed roof with the higher crush loads on the A-pillar. This allows elimination of several load cases caused by the folding top and minimizes the load cases for a convertible should that be required later on.
The removable doors cannot be crushed by the platen used for testing crush. The doors, if installed, must remain operable, so they cannot provide crush support in the load path.
Realize that roof crush protection is not a bunch of round tubing, but would be mill shaped extrusions. A roll cage adds more weight without an equivalent stiffness increase.
The Jeep JJ is an example of what I expect to be the configuration for JL. This means a body on frame, with the structure of the aluminum body designed to support the crush test standards for the conventional car, while still allowing an open top feel.
In this manner we can follow all the NHTSA rules by certifying the car to 216 without any exemptions.
Customers have no say in this. A fixed roof that is removable does not meet the expected regulatory change. A soft top does not meet the regulatory change.
There might be a loophole: making a “SkySlider” (Jeep owned trademark) fully opening top panel like the later Libertys.
To grossly simplify: When designing a body to meet standards, one positions a special mannequin (“OSCAR”) and follows the joint movements defined by the particular OSCAR model used (which covers a size-range of 95% of people), then defines the swept space of OSCAR movements in a crash. The rules state the amount of impact, in force and intrusion, that will not kill the person (represented by OSCAR). The designer must create a structure that will not deform so much as to kill off the occupants.
In addition, FMVSS 103, 104, and 111 all and the relevant SAE standards all show what and how you need to see out to safely operate your car.
In rough numbers (I am going to illustrate the problems, not solve for real numbers) you need to be able to support 25% of the gross vehicle weight, placed at a compound 45° angle, in all planes to the body, to pass the deformation test in compression. To do this, you can play with the:
Your limitations are:
Now, one must overlay the Class A “show” surfaces from Design Office, along with what is called the master sections, which show you the desired construction at key locations around the body.
Once these are in place, you get to start sweeping the sections into each other to determine the actual metal shape you will cut stamping dies from.
As you are building all these surfaces, you are also analysing the structure for adherence to the FEA load paths.....so you don’t end up with an oops and create a buckle condition under load, or a formability issue for fabrication.
Where you get a buckle, twist, or crack, you now have to decide on how to repair the issue. For example, can you stiffen the part by adding thickness to the base material — do you need a double panel to “glove” (this means be slightly smaller in size to completely fit inside your main panel), or can you add a separate piece to transfer the load elsewhere in the structure to another part to act as a bridge between the two parts you have failed?
You can also change the shape of the A-pillar itself, adding section height or width but keeping the same thickness. What you have to watch out for is intruding on any of the prohibited zones — commonly called DNE for "Do Not Exceed" zones.
You also avoid funky load paths like the split A-pillar used on the Fiat 500L, and extremely fragile glued quarter glass required to bridge between the dual A-pillars on each side, with all that attendant structure. In other words, the fixed roof allows time to be saved in the program while not limiting options for later study.
The short answer: No. Narrower by sanctioning and better aerodynamics by closing gaps and pull in track into wheel houses. Requires IFS minimum due to costs.
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.
The brake booster would have to change to a Delphi hydroboost with relocated pressure ports. (FV 45 degree down and top bleed screws). 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.
[Note: Sergio Marchionne talked about using a smaller engine, but we are assuming he means the 3.2 liter V6, or a future Wrangler, ten years away, which may be even lighter and able to perform well enough using a four cylinder. An in-line engine would help with packaging.]
The short answer: Around 300 pounds at curb weight over current, and around 3-4 mpg better on the highway with the current V6 (which might not be used). Due to the expected use, 3 mpg is more likely, as 4 mpg would require a full aero kid as used on Dart (this might be part of a less capable model).
Why not just use a VM V6 diesel with an eight speed? The engine does not fit. That transmission does not fit. There is no transfer case to mate to the ZF transmission. The frame is not long enough to have any rear driveshaft at all.
Why not use a hybrid: Because of technical risk. Look at stop-start systems, for example; you start with a bolt-on system, such as a Bosch package on a mule, to assess the risks, then develop changes to make the idea work in one specific vehicle. Then you start to simplify the system to eliminate costs and adapt the components to the next vehicle.
During this time, you are also deciding where this product is best applied (to maximize the impact on the customer, while minimizing the impact on production, parts sequencing, and retraining the line techs that have to put this “thing” together, not to mention the impact on dealers that have to repair and troubleshoot in the field).
You must minimize the technical risks associated with every part and subsystem change to keep from going bankrupt, should your engineers fail to meet the targets.
Do you really want to further compromise Wrangler by adding yet another major technical change to the mix, by adding a hybrid into aluminum, downsizing the outside, and losing capability?
Sometimes you have to shoot the engineers and begin production. This doesn’t mean a hybrid won't happen....just not at the start.
Lets take performance requirements first. Just what does Wrangler HAVE to do?
To answer this you need to refer back to an SAE spec called J688 (which I mentioned earlier in this series of articles).
(Note to those that follow closely....yes this spec is obsolete and has been superseded by J2188. The 688 spec is easier to understand by novices and I will continue to use J688 for this reason).
J688 is a specification used to predict vehicle performance in the context of highway speeds, fuel consumption, drag, and other variables related to the vehicle performance envelope.
GM Truck and Bus division used to have a publicly available guide that showed how to spec out a vehicle from minimum requirements to perform to the "customer's" needs (there is that word again....I said it would show up again...hehehe).
Unfortunately I have lost the link to it or it was removed when 688 was obsoleted. This is why I included the SAE link to their supporting documentation....so the student can use it to follow along.
Related Jeep Wrangler pages
Inside the Wrangler
Variants and related...
The 2018 Jeep Wrangler JL: suspension • aluminum vs steel • open or fixed roof • pickup
body engineering • weight, strength, and safety • transmissions • engines
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