Framing the question

Automotive Design & Production, Feb, 2003 by Christopher A. Sawyer

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 basi c 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.