I'll be Earnhart's belts would have worked fine at 20G, but failed at a higher level. If the SkullSaver Supreme slips out of the harness at a high G level, isn't that a major non-linearity which a low-G crash wouldn't demonstrate?
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It could be, but that probably depends on the seat. The SSS slipped out of the harness at Delphi at 68Gs using a bench seat. But that was a 30 degree offset test, meaning the lateral component was 34Gs (sin30 = 0.5). If you can get as high as 20Gs (just guessing) you may see the same thing.

Seat selection will be critical, and I doubt anyone here wants to use a bench seat.