Originally Posted by
I-Like-To-Bike
You must still be disoriented from your collision if you can't tell by eyeball and fingers if the foam has been compressed. If infinitesimally minute "invisible" compression were a credible safety factor what gives you confidence that any and every helmet that has been handled at all since leaving the assembly line (if not before) doesn't suffer from such a defect?
Please provide us with your scary credentials to state crap like that with such assurance. I don't know that there couldn't be a significant impression made evenly that was difficult to detect by touch (and neither do you), but I do know to a certitude that the helmet went through a process (crashing with a head inside it) with a relatively high likelihood of significant impact. I did not measure the thickness of the foam with a caliper before the crash, so I don't have a baseline to compare it to after the fact.
There is a small chance that the helmet I buy off the shelf is defective in a way that is hard to detect. I updated the probability of that based on the fact of it going through the crash. I determined that the risk of it no longer being properly functional outweighed the relatively light cost of replacing it (I pay about $50-60 for my helmets). Get someone to explain Bayesian probability to you.