Can someone enlighten me, I never understood why dropping a helmet would make it unsafe to use? Even if it cracked a bit on the shell,isn't the important thing that the Styrofoam innards are intact, isn't that the part of the helmet that is absorbing the shock during a crash. Just curios.
Let see if this will help you understand. Most of the protection a helmet has is allowing the head to de-accelerate at a slower rate than the outer shell upon impact. This reduces the amount of "G" forces applied to the brain. Push your finger in a Styrofoam ice chest about a ½ inch and pull it out. The foam will only come part of the way back out. When you go to push it back in the same place it will free travel through the air the first part then hit the foam but that area is not as soft as it was before. So it has not absorbed that energy as well.
The outer shell prevents penetration of the helmet by sharp objects and spreads the force a bit to the outside surface of the inner styrofoam shell. An impact will cause the outer shell to flex pushing on the inner shell. Because we can not see the inner shell we do not know how much of the life the Styrofoam inner shell gave up to absorb first impact. The actual damage is hidden. There may be a void between the outer shell and the inner liner at that one point. I understand one way of checking it is using a sonic device that looks for these voids.
What is the chance the next impact will be in the same spot? There is some controversy over this in the helmet testing standards. Most think two impacts in the same place in the same accident would be unlikely"¦..in a motorcycle accident. Because the standards evolved from auto racing standards where it was more likely to have multiple impacts in the same area"¦ there is some argument on the testing standard.
It takes a pretty good hit to damage one, but it is possible. A 6 foot drop on a smooth surface should result in minimal damage, but there still could be some. Hitting smooth carpeted floor should reduce the chances.
It has been several years but I have sent in a few helmets to Shoei and Bell. Received a free replacement once, a couple back saying they were fine and one cut into two pieces. Back then I don't think they even charged for shipping. I took enough of a hit with one that I saw stars and missed 3 of 7 laps on a cross country race. When I say I missed 3 "¦. I actually ran the 3 laps, I just could not under stand why the checkered flag was there. It only had a small scuff, and it came back in two pieces. Since I quit riding/racing dirt bikes my helmets do not get as much abuse as they used to.
The inner layer can also be damaged by time, chemicals, gas fumes, ozone and other things.
For P-Willy,
All that said"¦ I'm no expert in the field so I'm not going to tell you one way or the other what YOU should do. With what you said here I myself would wear it. However it is your head, you have to decide to accept the risk yourself.