Originally Posted by
livedarklions
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As far as the shattering thing goes, yes, they shatter because they are dissipating the energy away from your head, just as the crumple zones in the car are dissipating the energy away from the passenger compartment. Bike helmets aren't designed to maximize their survival probabilities during a crash, they're designed to be sacrificed in order to increase the rider's chance of survival. Even if they appear intact after taking a blow, you should discard them as there may be microfractures that render them less effective.
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Sorry but that is erroneous thinking. When a helmet breaks it's because the forces on the helmet have exceeded the helmet's foam capability. A shattered or otherwise broken helmet might be offering residual protection to the head via the broken pieces but impact absorption is virtually gone.
What gets me is when a helmet shatters or breaks very early like almost as soon as the impact occurs. I had a helmet that had a large piece break off the side when the helmet fell from the handlebar of a parked bicycle. I was really surprised about that and it got me to questioning nd looking for data on just how much protection a helmet really offers. I figure it's better than none but in some helmets that protection isn't much greater.
Cheers