USE YOUR HEAD (AND PROTECT IT)
Why do we need a helmet? Well, when you have a crash, your head hits the deck, and your brain moves inside your skull, bouncing off the inner surface of the bones. If that deceleration force exceeds a certain level, the brain is damaged, and starts to swell up. Using an inch or so of stiff-yet-crushable polystyrene spreads out the deceleration over a longer time period, reducing its peak value, and (hopefully) preventing brain injury.
Another, less-obvious injury pathway is angular acceleration. That’s a posh way of saying jerking and twisting motions. If you’re sliding along the deck at 40mph, and your helmet ‘catches’ on a rough bit of road, or a drain, kerb, whatever, the helmet jerks round, and transmits that force to your head. If the force is too high, then again, your brain suffers internal damage – the tissue is ripped apart. That’s why firms like Arai are working hard on making their helmets ‘smoother and rounder’. Having fewer edges in the lidmakes these twisting jerks less likely –a smooth, round shape can slide along a rough road more easily.
Finally, there’s the penetration aspect. This is unusual in a crash – but the consequences are generally so serious that they need to be considered. Arai in particular make a big deal out of having a very strong outer shell, with all the impact absorption taking place in the liner. Some less-stiff thermoplastic shells give good impact scores, because the outer shell flexes, and absorbs some impact. But if a weak shell lets a 4in rearset footpeg stab into your skull, then it’s likely to be game over. So, penetration resistance seems like a vital part of the overall picture.
One common fallacy that’s worth scotching is about what helmets are there to protect you from. They’re not there to save you if you hit a brick wall head first at 70mph, sadly. Indeed, even hitting a wall with your head at 20mph will probably kill you: the limits on the size of a helmet mean the shock-absorbing layer can only do so much.
What helmets are mostly there for is to protect your head from the vertical drop if you fall off your bike – so a fall from around 5ft up. Generally, in a crash, that’s the impact your head suffers – the bike lowsides and you slide off, or you get flung over the highside and fall from a bit higher. Smacking your head on Tarmac or concrete, even at 5mph, without protection can easily cause serious, life-threatening injuries. And it’s this that your helmet is there to help with.