Do I need to replace my bike helmet after a crash?

Yes, if it took an impact. The expanded polystyrene liner protects by crushing permanently, so the protection in the struck area is already spent. The polycarbonate shell frequently hides this completely — a helmet can look undamaged with fully crushed foam underneath.

Why it happens

EPS works by collapsing at a near-constant force across its thickness, converting impact energy into permanent deformation instead of passing it to your skull. That mechanism is single-use by design. Unlike foam in a seat, it does not spring back, and crushed cells provide no further protection.

The in-moulded polycarbonate shell is bonded to the foam during expansion and is there to spread point loads and let the helmet slide rather than catch. It is thin and flexible enough to look intact over foam that has fully compressed, which is why visual inspection is not a reliable test.

The other thing a crash can damage invisibly is the retention system. Webbing that has been shock-loaded and a ratchet cradle that has been impacted may not hold position in a second crash.

What to do about it

1. 1Replace after any impact to the helmet, whether or not you can see damage.
2. 2If you are unsure whether an impact occurred, press firmly around the shell and feel for soft or dented areas beneath — but treat a negative result as inconclusive, not as clearance.
3. 3Some manufacturers run crash-replacement schemes offering a discounted unit. Worth checking before buying at full price.