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Originally Posted by MNBoxster
In addition, an 'M' rated Helmet is tested for a single impact. They are generally less robust and easily compromised.
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Jim,
My reading of the specs years ago lead me to believe that the "M" spec helmets weren't "less robust," but suited for different types of impact. The "SA" stuff is meant to take more focused whacks, such as you mention (hitting a rollbar), versus the test protocol for "M" which I believe uses flat plate impacts, to simulate bouncing your head on the pavement. The non-SCCA-approved DOT helmet spec uses yet another impact protocol which, contrary to popular belief, is not less good than the Snell tests, but again emphasizes a different injury mechanism and makes it more suitable for road bikers.
I may be recalling this all wrong, of course, since I haven't read up on it since the early '90s, but that's my impression.
Eric