I know there are a lot of purists out there who what to chastise Jeep for making all the changes that we'll most likely see on the new JL Wrangler that is expected to be released in 2017 but really, you need to understand that their hands are tied and are being forced to make these changes thanks to the government. You ever wonder why cars no longer come with pop-up head lights or why nobody ever makes T-tops anymore? Well, you can thank the NHTSA (the National Highway Traffic Safety Administration) for that and while there is a “convertible exception” in the FMVSS 216 rule that essentially allows open-top cars to "resist less force than other vehicles", this may all change and very soon. Needless to say, the possibility of the new JL Wrangler coming with some kind of fixed top may be something that Jeep will be forced to make. You can read what the experts have to say in the following AllPar article below:
2018 Jeep Wrangler in depth: Convertible/roadster or fixed roof?
by Robert W. Sheaves (edited). Part 6 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 members asked what we are likely to see as the next-generation Jeep Wrangler. This article is based on my engineering evaluation of various public sources, plus private discussions.
2018 Jeep Wrangler: “convertible” or fixed-roof?
A regulatory change in 2009 is why there are now no more T-tops.
The reason why Wrangler may not be able to remain in its current form is because NHTSA (the National Highway Traffic Safety Administration) may change its rules. Currently, there is a “convertible exception” in FMVSS 216 which allows open-top cars to resist less force than other vehicles.
Given the trend of ever-increasing mandated safety, NHTSA may require extra strength in the A-pillar during the long design life of the next Wrangler. This does not mean that the Wrangler will need to go to a fixed, steel-skinned roof like the Cherokee, but it does mean major changes will be needed — if the rules change.
The current type of body may not be practical if the exemption is overturned. Therefore, to avoid stalling the project, I expect the first direction to be an open, fixed structure with removable panels of plastic and/or cloth material, and a fixed A-pillar that no longer folds.
My opinion is that the risk of a law change is great enough that Jeep must assume the worst case — that separate standards for convertibles will be dropped. This might mean a fixed roof (not necessarily a car-style steel-skinned roof) with the higher crush loads on the A-pillar, which eliminates several load cases caused by a removable or folding top, and minimizes the load cases for a convertible should that be required later on.
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.
There might be a way to soften the blow: making a “SkySlider” (Jeep owned trademark) fully opening top panel like the later Libertys.
How engineers meet the standards
To grossly simplify: When designing a body to meet standards, one positions a special mannequin (“OSCAR”), which covers 95% of Americans in size, and follows the swept space of its joint movements in a crash. The rules state the amount of impact, in force and intrusion, that will not kill the person. 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:
•Front side glass
•Body mount and/or structure attached to a-pillar and frame (for body on frame) or subframe structure (for semi-monocoque or “unibody”)
Your limitations are:
•You cannot pass any structure through another panel (no cage tubes that go through or bolt through the dash panel/firewall)
•You can only follow the FEA (finite element analysis) load paths to minimize structure mass and complexity
•You are not allowed to split load paths and recombine into one path
•You can change thickness of materials up to 3x the thinnest section
Now, overlay the Class A “show” surfaces from Design Office, along with 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 that have failed?
You can also change the shape of the A-pillar itself, adding section height or width, but keeping the same thickness. You cannot intrude 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.
Using a fixed roof on the Wrangler allows time to be saved in the program, while not limiting options for later study.
As a side note, 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. (The 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.)
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.
Entire article with more photos and illustrations can be found on AllPar here: