Dropped Helmet, Need to Replace?

[MartianDestiny]

Helmets can be damaged even when no outward signs of damage can be seen. They are designed to compress once which increases the time of impact which decreases the impulse which is good. Once they compress they are done for and you may or may not be able to tell they've compressed.

So technically, 1 impact = new helmet. And 4ft or so to concrete does count as an impact.

All that said; I've dropped mine before too and continued to wear it. I replace my helmets every 2 years (as opposed to the 5 recommended) or at the first sign of wear to reduce the danger of significant loss of protection from all the minor knocks it gets (I commute and then haul the helmet around with me, take it on airplanes, etc).

It ends up being what you are comfortable with. If you call the helmet company though they'll tell you to replace it.