How bike helmets are made

A bike helmet saves a head by crushing itself. Its foam liner is made to collapse in a crash.

stations: 7
things upstream: 38
raw sources: 11
steps from the ground: 8

The short version

A bike helmet is a liner of expanded polystyrene foam, often with a thin plastic shell. Polystyrene beads are expanded with steam in a mold shaped like the helmet. A shell of polycarbonate is fused on, and vents, straps, buckles and comfort pads are added. Samples are crash-tested against safety standards.

Expand the foam beads

Small polystyrene beads that contain a blowing agent are heated with steam. They puff up to many times their size and fuse into a rigid foam.

Goes in
Polystyrene
Mold the liner

Beads fill a steel mold in the shape of the helmet, with channels for ventilation. When heated, they weld themselves into a single piece of foam.
Form the outer shell

A thin sheet of polycarbonate is vacuum-formed into a shell, or placed in the mold so the foam bonds to it. The shell helps the helmet slide instead of snagging on the road.

Goes in
Polycarbonate
Cut the vents and trim

Edges are trimmed, and any extra vents are cut through the shell. More vents cool the head but remove foam.
Sew the straps

Nylon webbing is sewn into chin straps and fitted with a quick-release buckle.

Goes in
Nylon Sewing thread
Fit pads and the dial

Foam comfort pads, a retention system and the dial that adjusts the fit go in.

Goes in
Polyurethane
Crash-test samples

Sample helmets are dropped on anvils while instrumented heads measure the force. Under US and European standards, a pass depends on keeping acceleration to the head below a limit.

End of the lineBike helmet

Worth repeating

The Bell Biker, released in 1975, was the first commercially successful purpose-designed bicycle helmet, with a polystyrene liner and a hard shell.
Both the CPSC and EN 1078 standards measure only straight-line acceleration and ignore rotation.
In the US, the Snell Memorial Foundation was set up to create standards for motorcycle and auto racing helmets.