Most people that have been riding for awhile have heard that once a helmet sustains a good fall, it should be replaced, regardless of whether you see visible damage or not.
Directly from the Samshield website:
“In case of violent impact on a helmet, the inner shell (in polystyrene) will crush and lose its shape in order to absorb the impact. From then on, the polystyrene shell will never have the required thickness to absorb another impact if you fall again. This is the reason why we have to change the helmet in case of important impact.”
So what exactly qualifies as a “violent impact”? At what point do you decide to shell out for a replacement? I tend to be on the really conservative side when it comes to this. The newest study on helmet safety seems to indicate that not all helmets are created equal to start with, much less how well they work after a fall.