Here is a video showing a design of self-inflating helmet.http://www.youtube.com/watch?v=Tn65Bows0Ws
The commentary uses the term " protecting the wearer", which implies that it will protect you entirely in all circumstances. However apart from being overwhelmed by high energy impacts. it also needs to deflate in order to absorb energy on impact, airbag style. This means that it is unlikely to be effective in accidents were the head is impacted more than once.
Air compresses. Tyres and air suspension don't deflate on impact. I was under the impression that vehicle air bags deflate after impact so as not to suffocate the user.
That's a good observation.
Tyres and suspension do not indeed deflate on impact, they absorb the impact and then release it back in the opposite direction. This is not desirable with a cycle helmet or an airbag as the rebound would double the total of the deceleration and acceleration forces experienced during the impact. The ideal is for the energy of the impact to be totally absorbed over the longest time period possible. The correctly performing foam lining of a cycle helmet should take about 6,000ths of a second to fully crush whilst a bare head will absorb the same energy in 1,000th of a second. If the foam lining has worked as intended then its will be totally crushed. Broken helmets or those where the lining is not crushed have in fact not performed as intended. An airbag has vent holes in it. It is the air traveling through these holes that actually absorbs the impact energy at a controlled rate.http://en.wikipedia.org/wiki/Airbag"Post-deployment"
"A chemical reaction produces a burst of nitrogen to inflate the bag. Once an airbag deploys, deflation begins immediately as the gas escapes through vent(s) in the fabric (or, as it's sometimes called, the cushion) and cools. Deployment is frequently accompanied by the release of dust-like particles, and gases in the vehicle's interior (called effluent). Most of this dust consists of cornstarch, french chalk, or talcum powder, which are used to lubricate the airbag during deployment."
9/16" Suntour BMX MP-1000 pedals / 56 tooth TA 6-bolt chainring.(I'm not bothered if the teeth are worn or damaged)