The Helmet Drop Test — Stretching the Stop
The head must stop either way — so what does the foam in a helmet actually change? A metal headform wearing a cut-away helmet hangs above a steel anvil, crown down, as in a helmet test rig; press play and it falls once and stops. Sliders set the head's mass, the impact speed and the thickness of the foam liner. The fall is shown 5 times slower and the stop 250 times slower, with the view closing in on the anvil, so the foam can be seen crushing while a force–time graph records the pulse; the timeline can be dragged to any moment.
Como usar esta simulação
- Head mass m — 3 kg to 7 kg (default 5.0 kg).
- Impact speed v — 2 m/s to 8 m/s (default 6.0 m/s); the drop height h = v²/2g follows from it.
- Foam liner thickness t — 0 mm to 40 mm (default 30 mm); 0 means a bare headform.
- Play/pause, back to the start, a timeline you can drag to any moment, and ½×, ¼×, ⅛× slow motion.
- Drag anywhere on the scene to look around.
O que observar
- Whatever the liner, the head arrives with the same momentum and ends at rest: Δp stays 30 N s at the default mass and speed.
- In slow motion the foam at the crown crushes against the anvil; 30 mm of foam ends about 9 mm thick.
- The shaded area under the force–time pulse is always Δp, but thicker foam turns a tall, narrow spike into a low, wide plateau.
- With 30 mm of foam the average force is about 3.6 kN (73 g); with no foam it is about 22.5 kN (460 g) — over six times more.
- A heavier head needs more force to stop, yet its average deceleration in g does not change.
- A faster impact raises the force with the square of the speed: 8 m/s needs 16/9 of the force at 6 m/s.
A física por trás
The headform falls from h = v²/2g and meets the anvil at speed v, so it arrives with momentum mv and ends at rest: Δp = mv whatever the liner — 30 N s for the default 5.0 kg head at 6.0 m/s (a 1.84 m drop). The foam cannot change Δp; it lengthens the stop. In the model the foam crushes by 70 % of its thickness at a nearly constant force, and the headform's skin and the padding give 4 mm more, so the head stops over d = 0.7t + 4 mm. Taking the stop as uniform, it lasts Δt = 2d/v, and the average force is F = Δp/Δt = mv²/2d. With 30 mm of foam: d = 25 mm, Δt = 8.3 ms, F ≈ 3.6 kN, an average deceleration of about 73 g. With no foam: Δt = 1.3 ms, F ≈ 22.5 kN, about 460 g. The chart shows the resultant force — the anvil also carries the head's weight — as a flat-topped pulse for foam and a narrow spike without it, and its area is always Δp. A heavier head needs a proportionally larger force, but its deceleration in g depends only on v and d. Test standards limit the peak deceleration, roughly 250–300 g.