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2019 Jeep Wrangler in depth, Part 1: Aluminum and Steel

by Robert W. Sheaves (edited). Part 1 of a series. Written in mid-October 2014; updated September 2016

Bob Sheaves was responsible for all the 4x4 suspension design done at Chrysler's Jeep/Truck Engineering's PreProgram Engineering Department from the AMC days until 1993. I have a wide and deep knowledge of vehicle design overall, with an emphasis on suspension and driveline for 4x4s.

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.

Both the production process and plant are probably finalized by now. Hard tooling creation for the first production mules requires around 75 weeks (before volume production, or "BVP"), which is around a year and a half, and the first Wranglers are due in calendar-year 2016. The more complex the system, the less you can change. It gets more complex as we go along. What is designed in is what you are stuck with.

Jeep's actual choice is at the end of this story..

Body material: plastic, aluminum, steel, fiber?

The short answer: aluminum alloys, to reduce weight without excessive cost, and to build corporate expertise in working with aluminum (much of which has been lost since Daimler took over in 1998).

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Why not plastic? The Saturn and Fiero both had a semi-monocoque body from steel with non-stressed skin. From a production standpoint the major problems were:

  • Alignment at assembly
  • Thermal expansion, causing distortion of body panels (the panels expanded and contracted more than the underlying structure; this was partially corrected with a changed resin and manufacturing process at Saturn)
  • Adhesives failure under extreme heat and cold (corrected by the end of Fiero production).

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By the time these issues had been worked out, Saturn had become a victim of the shot-lived GM "brand management cabal" and was no longer allowed to go its own way. Instead, it sold revised versions of Opel/Vauxhall cars.

Consider this first use of aluminum not only by itself, but in terms of knowledge transfer to other programs. Without these changes, the rest of the corporation suffers. These things are the cost of doing business. Stand still and you get run over. Managing change can only hold you in place - innovation keeps you in the game.

They have the product that needs the change, the timing is right for such a massive change, and FCA must develop a massive amount of lost knowledge from Daimler's extinction of Prowler and the loss of most of its development team.

That said, the weight savings will be reduced somewhat by the need for changes to accommodate the aluminum. As one example, the F-150 body-in-white is up to 700 lb lighter with aluminum than steel; but issues like noise and vibration require thicker glass, thicker sound deadening, structural damping, and other materials, so Ford is unlikely to deliver pickups anywhere near 700 lb lighter than current models.

Looking at the 2002 Audi S8 (all aluminum) and the 2005 Chrysler 300C AWD (all steel), the Chrysler is actually 46 pounds lighter.

Why is the Wrangler the prime candidate for aluminum?

The main reason why the Wrangler is being considered for aluminum first, and why Ford is using it mainly for its trucks, is because both are body-on-frame, which adds a lot of weight to a vehicle. The body on a body-on-frame car or truck needs to be lighter in order to reduce the overall weight. This is why aluminum frames have been used in Class 8 trucks (Peterbilts, Macks, and such) since the 1950s.

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An aluminum body on frame structure that meets FMVSS216 would eliminate problems caused by the possible elimination of the "convertible exemption" for rollover crush strength.

There is an unlikely combination which must be eliminated: a plastic frame.

In 1979, GM shocked the heavy truck industry with a specially built GMC General OTR truck. Boasting an aluminum cab and a plastic frame, it was intended for Class 8 loads of 80,000 pounds. The result was a truck frame that weighed just under half of the then-current lightweight material, aluminum, and less than one third that of a comparably stiff (there is that word again) steel frame. This worked......for a while.

You may now be asking yourself, "Just what does a decades old GM truck have to do with a new Wrangler?"

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The Alfa 4C carbon-reinforced plastic technology is so far from production capable at the numbers required for Wrangler, it was never seriously considered for use. The technology is not scalable at this time, the environment is not suitable for plastics use, and most importantly, the technical risk is off the scale.

Future research is warranted, but based on my own experience and the state of the art in aircraft (arguably far more advanced than automotive), you would be hard pressed before the next full revision of Wrangler to even consider carbon-reinforced plastic for a production design.

That leaves many materials that can still be used, but within the limits of:

"Technical risk" has a specific meaning in engineering. The simplest form is a spreadsheet with all the pros and cons of a decision, each having a numerical value. These values are added to score the probable risk.

  • Cost
  • Technical risk
  • Production viability
  • Plant viability
  • Environment ("where does this thing have to live and work?")

Exotics like boron steel, titanium, ceramics, and ferroceramics fail on expense. This leaves you with two basic options for the underbody: some common alloy of steel or some common alloy of aluminum (we will ignore specialized alloys of aluminum and steel, due to cost).

One could minimize galvanic corrosion between the steel frame and aluminum body the way Rover did it 60 years ago-through the design of the body mounting isolators.

"SouthPawXJ" wrote, "You could design it with an aluminum frame, but the concern would be stiffness: one third that of steel. This would produce a larger frame, not a problem unless the frame is too large for what you are doing), cost, and assembly."

An aluminum body on frame is a prelude for the use of a semi-monocoque shell for Durango, Wagoneer, minivans, etc., in turn as programs reach the stage where body design is to be frozen. It all relates to not having the expertise to do the job all at once.

All these changes have to be "learned" and Wrangler is just the first to try some of the needs by a staff that does not yet know how do do it.

Aluminum vs steel production

Aluminum and steel differ in stamping (steel suffers from "springback," or resistance to retaining the die shape), corner formability (steel has the ability to form a short radius corner for hemming without cracking), and ability to hold compound curves (aluminum, when heated, retains a shape that curves in more than one plane without overspending), and other properties.

In addition, while aluminum weighs one third of steel, but it is also only a third as stiff as steel (these are rule-of-thumb numbers that are roughly valid for any alloy in the automotive industry).

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This means that for given stress, if a steel part is only stamped from a thickness that is too thin to manufacture accurately, aluminum will need to be three times as thick to withstand the same load.

"Strength" refers to tension, "stiffness" refers to compression.

As another example, compare compacted graphite iron (CGI) and aluminum with gray iron in an engine block. CGI and aluminum can reduce weight because you are not able to manufacture the grey iron thinner to reduce weight. In aluminum you can go to a thicker wall for the stress, and with the CGI you have the pressure of the press to make the wall thinner than a conventional sand casting.

To show a practical application now, let's look at an A-pillar, which has both Class A surfaces (which are visible to the customer) and Class B (hidden) surfaces. Anywhere you have a corner (such as where the A-pillar turns a corner to form your door seal surface), there will be a radiused surface that must match the door skin's edge, where the inner and outer door panels are bent over each other. You have 3 corners to be formed and the radius is different, depending on your choice of material. Which material will form each corner easiest?

What Jeep chose

According to aluminum producer Alcoa, the 2019 Wrangler will be the first vehicle to use the new C6A1 high form alloy; Alcoa 6022 and A951 alloys will be used for the door inners (front and rear) and for the hood (outer and inner). Aluminum hoods are nothing new; nor are aluminum door inners. FCA has already announced that the Wrangler will have a steel body and frame.

Additions from readers

cd36 wrote:

The modulus of elasticity for aluminum is 10,000,000 psi, and steel it is 30,000,000 psi.

When calculating deflection of a profile, the only material property to be taken into account is the modulus of elasticity, so you can't just say "use a different steel/aluminum alloy" to reduce deflection, because it does not change with common alloys. So if you have a profile that is deflecting by half an inch in aluminum, you can't pick a stronger aluminum alloy and reduce the deflection. You have one of three choices:

1) Change the profile to move more material further away from the neutral axis (e.g. I-beams)
2) Add more material to the existing profile
3) Change to steel

You need to make sure your material is strong enough so it will not break, but also you need to make sure to not have so much deflection that it may cause issues or be seen as quality issue by a customer. Fixing one doesn't mean that you have fixed the other.
Rick Anderson added:

Steel in the complex tight radiuses, aluminum for the outer radius and surfaces. Where the A pillar has to join with the hem of the door, aluminum might leave an unsightly gap, but you could add a plastic trim piece to the large radius to sharpen the radius, or design the door to overlap the large radius and close the gap. For production, joining multiple materials can be difficult, despite advances in adhesives, and coatings to protect against corrosion (which two different metals joined and adding the right third corrosive/galvanic component can make corrosion run wild). I didn't think it would viable for mass production in the numbers for a Wrangler.
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xjgary wrote:

Looking at two airplanes, many of the brackets, landing gear, and some tubing are steel (4130), but there is no sheet steel except for the firewall. Since aluminum of the thickness needed for a Jeep body easily cracks when bent tight and does not like compound curves, steel would be easier for this, and much cheaper, but heavier. In an airplane, we would use a piece of aluminum angle for the bend and rivet (or glue) the skin to the angle. 6061-T6 aluminum, if it was thick enough, could be pieced together and welded. The much stronger 2024T-4 and 2024 T-6 Aluminum can't be welded but it is about 2/3 the tensile strength of 4130 steel, so the strength to weight ratio is great. Aluminum can stretch easily, however, so in a press, with some heat, you might be able to get compound curves and maybe even tight bends on material that thick. But I would be afraid of cracking over the long term unless that part of the structure is supported with a doubler or other framework.


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