This 2017 Ford Super Duty frame is typical of the current state-of-the-art. (Image courtesy of Ford.)
Americans love pickup trucks and big SUVs. The companies that make them do too, as they represent the highest margin segment of the industry and are the primary profit driver for the Detroit Three. Look at the US sales figures. The Ford F-150, Chevrolet Silverado and FCA RAM pickups took the top three places in 2019 sales, continuing a decade long trend of light truck leadership in the US market. Why?
There are multiple theories, from cheap gasoline, affluent lifestyles and even the girth of US motorists but there is an underlying technical reality that differentiates pickups from other mainstream production vehicles: the chassis. With the disappearance of GM’s B- body series and Ford’s Panther platform, body-on-frame construction has been completely replaced by unit construction in sedan and crossover production. In light vehicles, the two technologies have been competing since the mid-1930s, and in Detroit the shift away from body on frame and passenger cars started at Chrysler in 1960. Today, all automobiles and crossovers use unitized construction, with spot welded hat-section structural members forming subframes for suspension and driveline attachment and the body structure itself providing support and rigidity. Unit construction is lighter, stiffer and most importantly, lower in production cost. Yet, pickups are different market. That market is dominated by American and Asian producers who not only maintain the century-old body-on-frame system, but actively promote it as a selling proposition.
Body-on-frame does have advantages. It allows changes in cab and box structures at lower cost, requires less fixturing on the assembly line, and allows multiple wheelbase, cab and bed configurations. It also allows commonality of cab structures between Class I, II and III trucks and allows automakers to outsource frame production.
While the industry once rotated between ladder, X-section and a perimeter frame technology, in light trucks the ladder frame rules. Ladder frames are simpler, lower in cost and are easier to design for optimal longitudinal and torsional bending moments.
This 2019 RAM pickup frame uses four types of high strength steel. (Image courtesy of ArcelorMittal.)
“Simpler” however, doesn’t mean “low tech”. An example is the current generation of FCA’s RAM pickup design. Like most pickups, it uses a modified C-channel structure, stamped from advanced high-strength steel. Multiple crossmembers keep the frame rails parallel without the need for extra triangulation and allow suspension and driveline attachment points that can be easily varied to suit the multiple configurations in which pickups are produced. The RAM design uses four types of high-strength steel, representing 90% of the structure and saves 100 pounds of weight compared to the previous generation, while allowing higher payloads, greater stiffness and enough deformability for occupant protection in accidents. Crashworthiness has long been an issue with light trucks, which have traditionally been subjected to less strict standards under US law compared to passenger cars. Regardless of the regulatory environment, consumer preference and the insurance industry has demanded better occupant safety, which poses a challenge for chassis engineers. Frame type chassis are strong and stiff, which translates to high deceleration loads on passengers in both front and rear-end collisions.
The traditional engineering solution to this problem has been to design holes, stamped creases, and other features into the front and rear frame “horns” to encourage these extensions to telescope or deform progressively under impact. The RAM example shows a newer and more sophisticated approach, with the rear frame extensions formed from a specially chosen steel alloy, and the front formed from variable thickness steel.
In both unit construction and body-on-frame design, the combination of customized metal alloys, advanced simulation, and new joining technologies (including adhesive bonding and laser seam welding) makes safety comparable between the two techniques.
Some manufacturers, however, build a limited range of light trucks and do not need the wheelbase and chassis configuration flexibility of the Detroit Three. For them, unit construction is possible, and is a familiar technology for design and manufacturing engineering teams. An example is the Honda Ridgeline.
The Honda Ridgeline uses automotive type unit construction. (Image courtesy of Honda.)
The Ridgeline is designed with a unit construction chassis similar to automobile and crossover practice, with rockers, roof and A, B and C pillars that are integral to the chassis structure. This technique allows a strong yet light structure and can result in improved NVH performance compared to body-on-frame designs where cab to frame mountings and chassis flex are factors. For Honda however, this is a one chassis, one model design. A different wheelbase, cab or bed configuration requires a completely new body-in-white and an assembly line to match. For Honda, this is not a major constraint as the firm is not a major player in the light truck market, but Toyota, with a significant global presence in trucks, uses a body-on-frame strategy, as does Nissan.
Electrics on the Move
The ground rules for internal combustion engine pickup design are well-established. What about electric vehicles? Here too, different manufacturers are approaching the structural problem differently. The much-anticipated Tesla Cybertruck will be use unit construction, with some analysts predicting a monocoque, with the vehicle’s exterior stainless-steel panels stiffening the structure. A final production vehicle has not been shown, but Elon Musk’s stated intention is to avoid stamping and instead use welded, brake formed stainless sheet. This unusual method dramatically reduces tooling costs, although at a cost in higher overall unit cost if the truck is built in large volume.
The upcoming Tesla Cybertruck will use a true monocoque structure built from welded and brake formed sheet stainless steel. (Image courtesy of Tesla.)
Tesla’s major competitor in the light truck market is a Rivian, who plans to market a more conventional model in 2021. While conventional in appearance, Rivian’s pickup will use a very complex frame structure, which in renderings appears to be fully boxed and hydroformed. Although a perimeter frame, the conventional torque boxes that connect the front and rear “clip” are integral with the rails and are upswept, with compound curves. Like the Cybertruck, production ready structures have not yet appeared, and stamped, welded frame rails may still be a possibility for production. One notable advantage of Rivian’s approach is the ability to offer a full-size SUV and a pickup on same “skateboard”. Initial production from Rivian’s Normal, Illinois plant will actually be a run of step vans for Rivian investor Amazon, using a “stretch” version of the skateboard.
Rivian’s “skateboard” chassis is a chassis is a modified perimeter frame with the side rails and closing the battery pack. (Image courtesy of Rivian.)
The Dark Horse in the Race
The dark horse in the race to switch pickup designed to electric drive is Lordstown, Ohio-based Lordstown Motors. The firm operates the former GM Lordstown Assembly facility and has shown a prototype of the conventional appearing pickup with a radically different drivetrain incorporating hub motors. If successful, hub motors could revolutionize the electric vehicle industry, eliminating almost all of the conventional drivetrain. Structurally, hub motors allow a chassis whose crossmembers and triangulation do not require a compromise to accommodate motors, gearboxes, driveshafts or axles. The design of Lordstown’s frame is conventional in the extreme, lowering development of production costs for the upstart EV maker. The frame is also fully boxed and uses conventional tubular crossmembers. The current Tier One supply chain should have little difficulty in fabricating frames to this design. If Lordstown succeeds, the hub motor design should offer the ultimate in chassis flexibility, with an essentially infinite variety of wheelbases and track widths possible with very little additional costs in design or tooling.
The chassis of Lordstown Motors’ Endurance pickup is conventional and design but takes advantage of the lack of a conventional driveline. (Image courtesy of Lordstown Motors.)
So which approach will win in the end? The Japanese and the Detroit Three have developed conventional steel ladder frame design to a high art, and can now achieve rigorous safety, NVH, payload and durability goals with formed and welded steel structures. Honda has chosen to adopt passenger car unit construction technology to build a light duty pickup for a niche, non-commercial market. On the EV side, makers are gambling on new technology. Rivian’s design is advanced and will likely require hydroforming to achieve. Lordstown is using a chassis that is conventional in the extreme but is gambling on a radical driveline technology in hub motors. The big dog in the electric vehicle field, Tesla, is taking the biggest gamble of all: a brake-formed, stainless-steel welded monocoque, almost aircraft-type construction. Consumers have no idea how much engineering goes into the structure under their seats, and rarely make buying decisions based on frames. But the technology that wins may establish the standard which other automakers will feel compelled to follow.