Auto Manifesto

January 30, 2008

NHTSA Roof Crush Resistance Proposal

Published in today's Federal Register, the Supplemental Notice of Proposed Rulemaking (SNPRM) requests public comment by March 17, 2008 on a proposal to change the standard (FMVSS 216). Note: This SNPRM does not apply to convertibles.

The basic idea is a plate is used to apply a force of 2.5 times the vehicle weight to the roof over the front seat area. The roof cannot collapse more than 127 mm (5 inches), or come in contact with the head of a seated 50th percentile male dummy. The current requirement is a force of 1.5 times vehicle weight (limited to 5,000 lbs), and does not prohibit roof component intrusion.

Looks like Toyota has done their homework best on FMVSS 216. All four of their 2007 vehicles tested were ranked near the top of both tests (Scion tC, Tacoma, Camry, Yaris) with the Scion tC ranked highest on the 2-sided test. The 2006 VW Jetta had the highest overall strength-to-weight ratio (SWR) in the single-sided test.

However, it must be added that when looking at roof strength alone, the absolute highest peak strength goes to the 2006 Volvo XC90 by a long shot with a rating of 90,188 N (20,268 lbs) which is a lot more than any of the other vehicles. It just happens to weigh 2,020 kg (4,453 lbs) which cuts down its SWR a bit. There is some truth in advertising - and this roof could theoretically support an adult male elephant.

Another interesting tidbit is that, in this test, the 2006 VW Jetta roof is stronger than that of the Hummer H3, which weighs 685 kg (1,510 lbs) more. The Subaru Tribeca also did well. The lowest strength result? The 2007 Pontiac G6 at 33,393 N (7,504 lbs).

The impact of this rule (no pun intended) will probably be diminished over time, which is a good thing as that means fewer people will be killed or injured through rollovers. The reason the effect may become less pronounced, as NHTSA points out, is the mandate for electronic stability control systems (ESC) starting in September 2011. This will reduce vehicles from rolling over in the first place.

Along with many other trends, this proposal indicates that future vehicles will have to be lighter, and yet have stronger structures. However, adding strength often adds weight, and vehicles don’t need more weight above the center of gravity, such as the roof. Conclusion: Wear your seatbelt, don't rollover, and you’ll probably be fine without this standard.

(full document available here - scroll down to National Highway Traffic Safety Administration for details)

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