'Inner frame' makes Fit stronger, less costly

LOS ANGELES -- In creating the 2015 Honda Fit's body-in-white, Honda engineers changed the manufacturing process to make the car lighter, stronger and less expensive.

Unibody cars, or monocoques, have a traditional method of construction. The upper body, which has sheet metal skin attached to it, and the lower body, which consists of the subframes that include the engine and suspension mounting points, are built separately. The upper and lower bodies are joined on the assembly line with brackets and stiffeners at the joining points.

For the 2015 Fit, however, Honda welds the upper body -- without the sheet metal -- to the lower body as a single "inner frame." The sheet metal is then attached to the finished structure, mostly by welds but also some bolts in areas such as the doors, hood and hatchback.

The new method eliminates the need for stiffeners and brackets at the traditional upper- and lower-body joining points, said Makoto Konishi, the Fit's chief engineer.

As a result, some structural members have been eliminated, including the center roof-arch stiffener, the front roof-rail stiffener and some bolts. Honda says it saved about 9 pounds in the structural process, with the overall body-in-white weighing 44 pounds less than that of the previous model. Yet the final product is stronger overall, Konishi said.

"There is reduced weight and increased body rigidity," Konishi said through a translator. "It's also less expensive because we've eliminated all those brackets and stiffeners, and it requires less assembly manpower."

Honda body development engineer Yoshinori Nakamura noted that the Fit is not a spaceframe design. With a spaceframe, a car's body panels are merely hung on the body-in-white -- a system usually used for racing cars so the panels can be easily removed. But the Fit's outer body panels are load-bearing and provide additional strength because they are welded and bolted to the inner frame.

This new assembly technology is occurring as Honda implements its ACE -- for advanced compatibility engineering -- body structure, which uses high-grade steels and connected structural elements to better dissipate crash energy. Honda expects five-star crash-test results for the Fit.

The construction method was first used by Audi, BMW and Mercedes-Benz, and has since been adopted by General Motors and a couple of others, because it allows for better access for more and better welds, said manufacturing expert Ron Harbour, a senior partner with Oliver Wyman.

"Stiffer structures and better side-impact protection through stronger materials, more steel layers, new joining technologies and framing in stages are facilitating this change," Harbour said.

"Although the approach is not new, I would expect Honda to do it as well or better than anyone."