Design parameters

The test to which cycle helmets are subjected – and which they often fail – are all basically similar.  The helmet is attached to a headform (usually aluminium) which is attached to a rod with a fulcrum at one end.  The headform is dropped, rotating around the fulcrum, and at some point impacts onto either a flat surface or a profiled anvil.  The test mandates the amount of energy the helmet must absorb.  It is not supposed to break.

The amount of energy is equivalent to hitting the ground at around 12mph.

That does not mean a fall after an impact at 12mph, it means an impact with the ground at 12mph. For the average person of average height this is equivalent to a fall from a stationary or slow-moving bike – approximately walking pace at most.

Impact energy rises with the square of speed.  One helmet promoter claimed that a helmet would reduce the energy of a 30mph collision by 12mph to 18mph, but this is wrong.  It would reduce it to the equivalent of 27.5mph – 302 – 122 = 27.52.

The human skull has evolved over millions of years to resist impacts with the ground which are the equivalent of a fall while standing or walking. Very few such impacts cause serious injury, unless the subject is drunk or otherwise compromised.

This is one of the key reasons I think helmet laws fail: the test models an idealised impact which rarely if ever happens, and even if it did it would quite likely not be that serious anyway.

The wrong problem

It’s pretty obvious, then, that this is not a device designed to deal with collisions with cars, and I’m not aware of any manufacturer who sells them as such.  However, most serious and fatal injuries are a result of collisions with cars, and many of those promoting helmet use try to assert that helmets are highly effective in such cases.  This is a hard sell for me; I can believe that there may be some protective effect in some crashes, but the idea of relying on something so obviously inadequate to the task and promoting it strenuously as an effective measure, is much more problematic.

There is, to my mind, a glaring disconnect between the bogeymen of massive head trauma and high speed motor impact, and the reality of a device designed and specified for the equivalent of simply falling off. And this disconnect is further magnified when you consider that the leading cause of serious and fatal brain injury is considered to be not linear impact but diffuse axonal injury resulting from rapid twisting, such as is seen in side impacts in car v car collisions.

Broken cycle helmet
According to the Daily Mail, this helmet saved someone's life when a car ran over her head. According to me, it broke.

As an aside, polystyrene foam (which is what helmets are made of) absorbs energy fairly well in crushing, but very poorly when it fractures.  You can demonstrate this.  See how much energy it takes to crush a section of polystyrene foam packaging, then see how much energy it takes to snap it in half.  Helmets are supposed to absorb energy by crushing, but many “helmet saved my life” stories involve helmets that have failed, without extensive analysis it is not possible to say how much energy – if any at all – they absorbed.

Further reading