Framing The Question

Is body-on-frame architecture on its way out, a victim of a drive for lighter weight structures? Maybe not. Smaller vehicle volumes, a desire to build more vehicles from fewer platforms, and new electronic technologies may catapult it to the forefront in a way the industry hasn't seen since Hudson introduced its 'step-down' frame in the 1950s.

Until the 1970s, when computer-aided design and engineering made the design of unitized structures both faster and easier, most vehicles –cars and trucks–were built with a separate body and frame. The construction method was simple, proven (the majority of vehicles had been built this way since the industry started), quiet (due to the isolation of frame from body), and easily supported multiple vehicles off a single base. In addition, because the frame supplied some structure, body design and development was somewhat simpler than with a fully unitized design. What’s more, the body-on-frame architecture made updates and model changes more straightforward. Despite these pluses, body-on-frame construction is now used predominantly for light trucks and SUVs, with a dwindling number of cars utilizing this structure. And, say industry executives, as buyers clamor for more “car-like” trucks and SUVs, the pressure is increasing to transfer more light-duty trucks to unitized structures, as well. Which leads to a basic question: Is the body-on-frame construction method dead?

“The answer varies based on the platform,” says Rich Marando, Advanced Structures business development manager, Dana. “When you get into the mass-produced, high-volume situation, there’s a significant challenge in making this method make financial sense for passenger cars.” For these vehicles, Marando says the trend is toward body-integrated-frame construction, where frame elements are integrated into a unitized body structure. This architecture spans the gap from unitized structures with front and rear subframes to vehicles like the Nissan Pathfinder that have frame sections welded to a unit body for increased strength. Higher crash test speeds, offset testing, and the increasing pressure on automakers to improve side-crash performance are driving the trend, helped by a consumer preference for taller vehicles. Also, front and rear subframes can be used to isolate the major mechanical components from the passenger cell. Says Marando: “It’s becoming apparent that unitized structures are coming close to their crash test limit without integrated structural support, and the move toward taller cars make it easier to package those pieces.”

The Strong, Silent Type

Two areas where body-on-frame traditionally excels is NVH isolation and heavy-duty service. “We strongly believe this construction method gives us world-class isolation,” says Mike Reed, Ford’s Panther Vehicle Engineering manager, “and is more robust in police, taxi, limousine and rental car service than a comparable unit body vehicle.” It’s also much easier to modify. Ford produces the Panther platform in two short and two long wheelbases, with the Ford Crown Victoria and Mercury Grand Marquis starting with a 114-in. wheelbase, the Lincoln Town Car with a 117-in. version. Long wheelbase variants of each model add six inches to these measurements. “We even have a program for the Town Car where we publish a set of specifications for qualified vehicle modifiers to literally saw the car in half and add as much as 10 feet to the wheelbase,” says Reed, underlining the inherent versatility of body-on-frame construction.

The fact remains, however, that as the number of cars built via this method dwindles, the cost of producing a relatively low volume line of body-on-frame passenger cars increases. Yet, to continue traveling down this road alone might endanger their financial viability. Therefore, if a majority of light trucks are built using a body-on-frame structure, is it possible to use said frame to support a passenger car program, or develop a frame structure that meets the needs of both from the start? GM is investigating those possibilities through concept and production vehicles.

Cars From Trucks

“The Bel Air concept is based on the frame that supports the Chevy TrailBlazer, Olds Bravada, GMC Envoy, and the Chevy SSR,” says Ed Welburn, GM’s executive director of Design, Body-On-Frame Architecture. Affordability was central to the Bel Air project, and Welburn’s group was charged with producing a reasonably priced convertible concept that could be produced easily and profitably using off-the-shelf components. “The original idea was not to build a car off a truck platform,” cautions Welburn, “but to build an affordable convertible that was very spirited. When we looked at the TrailBlazer architecture, we saw a range that included everything from inline sixes to V8s, two different wheelbases, and very good rigidity. So this platform rose to the top of our list very quickly.”

This wasn’t the first time Welburn’s group had traveled down this road. The hot rod Chevy SSR pickup and the Chevy Traverse concept sedan are wildly different vehicles from the same box of parts, and underlined the architecture’s ability to support a wide variety of vehicles. “The biggest challenge we faced was vehicle proportion,” says Welburn, “because everything builds up from the platform height.” Which is why perimeter frames have been the design of choice for automobiles, straight-through frames for trucks.

“Truck frames use rails that are inboard of the rockers,” says Welburn. “This is preferable for front-end crush, but it means you’re sitting above the rails, not between them.” Which meant Welburn and his crew had to get creative on how they packaged the vehicle, visually reduced its height, and dealt with the step-over height of the frame. And while these were major challenges, the use of an existing frame made it easy to develop a number of variants off the same component set. “In a way it was easy to develop the Bel Air in that they drove the chassis in, and we plopped our design on top of it. I think this project really opens the door for further exploration of this architecture.”

Dana’s Marando agrees: “You can absolutely turn a truck into a car, and this is an area where hydroforming would really be a benefit.” A single set of hydroforming dies would be used to create both car and truck frame rails, and the pieces run on the same line. “If the frame rails are hydroformed front-to-rear,” says Marando, “changing from 3-mm thick material to 1-mm thick material is as simple as changing the gauge of the tubes. The tooling doesn’t see it, and the ‘down-gauging’ lets you tune the performance away from something suitable for a run down the Rubicon trail and toward Main Street.” If weight continued to be an issue, it would be possible to substitute aluminum tube for steel at the cross members or in the main frame rails. The height of the main rails, however, remains the biggest obstacle.

Common Ground

“If you were to place vehicles like the Lincoln Navigator and Town Car on a common perimeter frame,” says Marando, “the tall SUV side rails would require an approximately 8-in. tall rocker panel on the Town Car.” This is two inches higher than the preferred six-inch rocker height, but not an insurmountable hurdle. By placing the needs of the truck and car development groups on the table at the start of the process allows them to negotiate toward a common ground. Plus, the higher sills could be an advantage in the right situation.

“We’ve looked at some convertible applications off the Panther platform,” says Ford’s Mike Reed, “but they ended up being more work than we first expected, and quickly would have become a separate platform in production.” With higher side rail sections, the frame would impart greater torsional rigidity in a convertible application, something Welburn says was central to the Bel Air project. “The frame took us a couple of giant steps in the right direction toward the structure a convertible needs,” Welburn notes, “so the car doesn’t have the cowl shake you get from so many of the unitized convertibles in production today.”

Small Volumes, Multiple Vehicles

One group, European manufacturers, are paying close attention to body-on-frame developments. “We’ve seen some OEMs in Europe interested in programs that produce 25,000 units per year, with up to five different body variations off the same platform,” says Marando. These vehicles would utilize extrusions and simple stampings rather than hydroforming. In some cases, press-brake parts would replace the stampings to keep costs down. “If you’re in the sub-25,000-unit production range,” says Marando, “automakers aren’t going to invest a lot in tooling to make the structure, unless there’s an expensive halo vehicle that can absorb the initial investment cost.” However, that may prove to be less of a problem as the volume of OEM programs continues to diminish, but the need for differentiation increases.

“What most people don’t realize,” says Marando, “is that 90% of all the perimeter frames that have ever been made are within a couple of inches in most target dimensions.” Stiffness has increased, the weight-to-performance ratio has improved, but the basic functionality and size are the same. So the potential exists for a supplier–like Dana–to provide automakers with a proprietary frame that could be adjusted in the fixtures to allow a range of variation suitable to each OEM. And the initial investment would be shared by multiple OEMs. “In some arenas,” says Ford’s Reed, “having the supplier deliver the frame as a rolling chassis makes for a decent business case, especially low volume applications.” Undoubtedly, getting the OEMs to share would be the most difficult part of the process.

In the short term, pressure to increase fuel economy through lighter weight vehicles will work against body-on-frame construction, though switching to technologies like tailored section frames–where various grades of steel are built up in either tube or sheet form to produce a frame rail with the proper gauge and grade in the right area–or moving to aluminum hold promise. “There will always be a market in the truck world for body-on-frame construction,” says Reed, “due to the service requirements of these vehicles. I’m just not 100% certain it would be a final choice for something like a new Panther platform, though it should be one of the alternatives considered given its advantages.”


Coming Attractions

One of the concepts proposed for the future of body-on-frame construction is GM’s Autonomy. That concept’s hydrogen fuel cell-powered “skate board” chassis contains the powertrain and mechanical components, while the body is a separate structure. It’s a familiar combination. “When we were developing the Bel Air,” says Ed Welburn, “the same team was also involved in developing the Autonomy concept, and we’d often comment on how the basic principles were the same in that the frame and powertrain can be rolled under a body and bolted in place.”

Drive-by-wire technology is important for the Autonomy. Electric motors, actuators, and software control the mechanical systems. There are no mechanical links between the body and chassis. This makes it easy to produce the rolling chassis in large quantities separate from the bodies, and place an almost limitless number of different body styles on top of a common structure.

BMW is working on a similar concept–minus the fuel cell powertrain–for a future edition of the 5- and 7-Series vehicles. One of the concepts currently making the rounds within BMW Technik, the company’s research and development arm, is a common frame structure that can be lengthened or widened. At a minimum, it would be mated to SUV, sedan, coupe, convertible and wagon bodies, and offer a platform for specialty vehicles, including those from BMW-owned Rolls Royce. Bodies of steel and/or aluminum would be mounted to this frame, though recent reports suggest BMW is close to producing a carbon fiber body that is extremely light weight, strong, and affordable in high-price, low-volume applications.