Ok... I think you did the right thing getting your son to wear a helmet - but the general point you are trying to make is complete nonsense - because you know nothing about what is actually a pretty complicated subject.
This is my 10year old sons Bell helmet after a major-league stack at Afan on Saturday. Somersaulted, and the first thing that hit the deck was the lower right back of his head. The helmet took it all, and failed as it should do.
How do you know the helmet took all the energy? Do you even know much energy the helmet is specced to take? It's quite a bit LESS than the amount likely to lead to a concussion, which is what usually leads to people being "kept in hospital over the weekend."
He was battered, bruised and scraped, but able to get back on his bike, and ride 8 miles back to the car.
I hate to think what would have been the outcome if he hadn't been wearing it, but he'd have spent the weekend in hospital as a minimum I reckon.
This is what the guy who runs the main helmet test for Europe and who is probably the most respect forensic witness says:
It's not surprising that people who've been through a crash on their bike and escaped serious consequences but found helmet damage often believe strongly that the helmet has “saved their life”. However, the number of helmet users with this experience seems very much greater than the number of bare-headed cyclists who ever suffer a head injury. This suggests that the reality might not be so straightforward.
How cycle helmets work
The principal protection mechanism in a cycle helmet is the polystyrene foam, or styrofoam, that covers the head. When this receives a direct impact force, the styrofoam is intended to compress and in this way spread and reduce the force that is passed onto the skull, thus reducing linear accleration of the brain.
However, it is common for helmets to break without the polystyrene foam compressing at all. A major helmet manufacturer collected damaged childrens' helmets for investigation over several months. According to their senior engineer, in that time they did not see any helmet showing signs of crushing on the inside (Sundahl, 1998). Helmet foam does not 'rebound' after compression to any significant extent. If the styrofoam does not compress, it cannot reduce linear acceleration of the brain. The most protection that it can give to the wearer is to prevent focal damage of the skull and prevent minor wounds to the scalp. It is not likely to prevent serious brain injury.
[picture]This helmet has split along the ventilation slots, which is common. However, the thickness of the styrofoam has not been compressed.
It most likely gave no more than superficial protection.
Some dissipation of impact force might occur from the action of a helmet breaking, but in most cases this is likely to be small. Helmet standards require the foam to start to compress at a level of force less than that which might be expected to lead to brain injury. While it is known that many helmets do not actually meet the standards to which they are supposed to be accredited (BHRF, 1081), it follows that if the styrofoam does not compress at all, the direct linear force on the helmet was minimal and it's quite possible that the cyclist would not have received any injury if the helmet had not been worn.
So... It's VERY possible that your son's helmet had no effect. THEY USUALLY DO NOT. They always look trashed because they get that way really easily; you cannot use this to say that the helmet worked.
1. You should still wear a helmet for serious offroading. Because you are more likely than usual to come off the bike, and scalp lacerations are no fun. It's unfortunate most helmets don't work in practice - that's due to the poor test standard - but there you go.
2. That goes double for children - not for the reason someone else said, but because their skull bones aren't yet knitted together, so a bang on the skull can be much more serious.
3. The impact energy limit for a cycling helmet is so low that it is virtually impossible to for it save an adult from serious injury - oh, not a downhill helmet, but the usual 300g foam hat. To quote that Helmet Guy again:
In cases of high impact, such as most crashes that involve a motor vehicle, the initial forces absorbed by a cycle helmet before breaking are only a small part of the total force and the protection provided by a helmet is likely to be minimal in this context. In cases where serious injury is likely, the impact energy potentials are commonly of a level that would overwhelm even Grand Prix motor racing helmets. Cycle helmets provide best protection in situations involving simple, low-speed falls with no other party involved. They are unlikely to offer adequate protection in life-threatening situations.
How much energy will a helmet absorb? If you standing up and trip over without managing to use your arms, you are pretty much on the limit - assuming you land so that the helmet takes the impact of your head only, and does so against a flat surface. It's important to understand this, just like it is the limits on any piece of safety equipment. Oh - and I'd suggest buying a Snell 95 cert helmet, because the European standard is so low.