Helmets

1886

If it's a nice day I don't wear a helmet

Limey

I have a Mk5 Kevlar Army helmet, I reckon if it's good for protection against a round from an AK47 it'll be more than adequate if I fall off my bike.:D

thescouselander

There's no way, IMO, that a cycle helment can provide any significant protection. Just look at the stuff they're made from - the foam is already compressed and will not deform very much when crushed. This means there's no way they can reduce the deceleration of someones head in the event of an impact which means the wearer will still be exposed to concussive injuries.

Maybe a helmet could help with protecting the skull in the event of an impact with something that has a sharp edge but I would have though these incidents are relatively rare.

In general I'm not convinced by any of it. I've know of a few people who have come off wearing helmets and they still managed to knock themselves out. That said I am now a regular helmet wearer as it is a requirement of the cycling club I am in.

custardy

thescouselander wrote: »

There's no way, IMO, that a cycle helment can provide any significant protection. Just look at the stuff they're made from - the foam is already compressed and will not deform very much when crushed. This means there's no way they can reduce the deceleration of someones head in the event of an impact which means the wearer will still be exposed to concussive injuries.

Maybe a helmet could help with protecting the skull in the event of an impact with something that has a sharp edge but I would have though these incidents are relatively rare.

In general I'm not convinced by any of it. I've know of a few people who have come off wearing helmets and they still managed to knock themselves out. That said I am now a regular helmet wearer as it is a requirement of the cycling club I am in.

Well i had a good off in December last year.
dislocated my shoulder but the only impact I felt at the time was my head hitting the road
so at the very least the helmet saved me a bump on the head and head v tarmac cuts/grazes
in reality I would expect to have been in a sorry state, as i was easily doing 20mph when I came off and had no idea what was happening when I hit the road

thescouselander

custardy wrote: »

Well i had a good off in December last year.
dislocated my shoulder but the only impact I felt at the time was my head hitting the road
so at the very least the helmet saved me a bump on the head and head v tarmac cuts/grazes
in reality I would expect to have been in a sorry state, as i was easily doing 20mph when I came off and had no idea what was happening when I hit the road

Probably a realistic assessment of how a helmet might help. A bump on the head is not ideal but its probably not life threatening either. That's why I think it should be a matter of individual choice.

Dave_C_2

Limey wrote: »

I have a Mk5 Kevlar Army helmet, I reckon if it's good for protection against a round from an AK47 it'll be more than adequate if I fall off my bike.:D

To quote Ben Goldacre I think you'll find it's a little more complicated than that.

Long post ahead:

The Army helmet is designed primarily to stop the AK 47 bullet from penetrating the skin of the helmet rather than absorbing the impact (more of this later). The cycle helmet is designed to absorb the impact of the human head hitting the ground.

Thanks to wikipedia for the numbers
With collisions the prime concern is conservation of momentum.
where m1*v1 =m2*v2

The AK 47 bullet weighs 8 grams and the rifle has a muzzle velocity 0f 715 m/s. This gives the momentum of the round (mass x velocity) at 5.72 Kgm/s (kilogram metres/second)
The average European human is 70.8 kg which means that the bullet absorbed by the helmet will accelerate the body up to 0.08 m/s or 8 cm/s. This is reasonable as the guy firing the AK 47 is not knocked backwards from the recoil. So the helmet doesn't have to protect against sudden acceleration from an AK47 round.

Of course the bullet does not accelerate the human body as a rigid whole - I said it was complicated.

Compare this to our human falling off a bike at 17.6 mph. A little higher than the 12 mph that the helmet is designed for - more later.
The momentum of the body as a whole is 538 Kgm/s or about 100 times the momentum of the bullet. The bike helmet has to take all of the shock as the earth won't move much. Of course the body doesn't move as a whole - back to being complicated.

So the fact that the Kevlar helmet will stop an AK47 round does not necessarily mean it will be good on a bike. Weight and ventilation play a big part in helmet design.

What about energy you say, glad you asked. Your average European male at 17.6 mph has the same kinetic energy (1/2 m v*v) as the AK47 bullet - about 2045 Joules.

Hope I got my sums right

Dave

kerby_crawler

Dave_C wrote: »

To quote Ben Goldacre I think you'll find it's a little more complicated than that.

Long post ahead:

The Army helmet is designed primarily to stop the AK 47 bullet from penetrating the skin of the helmet rather than absorbing the impact (more of this later). The cycle helmet is designed to absorb the impact of the human head hitting the ground.

Thanks to wikipedia for the numbers
With collisions the prime concern is conservation of momentum.
where m1*v1 =m2*v2

The AK 47 bullet weighs 8 grams and the rifle has a muzzle velocity 0f 715 m/s. This gives the momentum of the round (mass x velocity) at 5.72 Kgm/s (kilogram metres/second)
The average European human is 70.8 kg which means that the bullet absorbed by the helmet will accelerate the body up to 0.08 m/s or 8 cm/s. This is reasonable as the guy firing the AK 47 is not knocked backwards from the recoil. So the helmet doesn't have to protect against sudden acceleration from an AK47 round.

Of course the bullet does not accelerate the human body as a rigid whole - I said it was complicated.

Compare this to our human falling off a bike at 17.6 mph. A little higher than the 12 mph that the helmet is designed for - more later.
The momentum of the body as a whole is 538 Kgm/s or about 100 times the momentum of the bullet. The bike helmet has to take all of the shock as the earth won't move much. Of course the body doesn't move as a whole - back to being complicated.

So the fact that the Kevlar helmet will stop an AK47 round does not necessarily mean it will be good on a bike. Weight and ventilation play a big part in helmet design.

What about energy you say, glad you asked. Your average European male at 17.6 mph has the same kinetic energy (1/2 m v*v) as the AK47 bullet - about 2045 Joules.

Hope I got my sums right

Dave

It's cold, it's dark, but u know what? I'm gonna go out for a bike ride!

Limey

Dave_C wrote: »

To quote Ben Goldacre I think you'll find it's a little more complicated than that.

Long post ahead:

The Army helmet is designed primarily to stop the AK 47 bullet from penetrating the skin of the helmet rather than absorbing the impact (more of this later). The cycle helmet is designed to absorb the impact of the human head hitting the ground.

Thanks to wikipedia for the numbers
With collisions the prime concern is conservation of momentum.
where m1*v1 =m2*v2

The AK 47 bullet weighs 8 grams and the rifle has a muzzle velocity 0f 715 m/s. This gives the momentum of the round (mass x velocity) at 5.72 Kgm/s (kilogram metres/second)
The average European human is 70.8 kg which means that the bullet absorbed by the helmet will accelerate the body up to 0.08 m/s or 8 cm/s. This is reasonable as the guy firing the AK 47 is not knocked backwards from the recoil. So the helmet doesn't have to protect against sudden acceleration from an AK47 round.

Of course the bullet does not accelerate the human body as a rigid whole - I said it was complicated.

Compare this to our human falling off a bike at 17.6 mph. A little higher than the 12 mph that the helmet is designed for - more later.
The momentum of the body as a whole is 538 Kgm/s or about 100 times the momentum of the bullet. The bike helmet has to take all of the shock as the earth won't move much. Of course the body doesn't move as a whole - back to being complicated.

So the fact that the Kevlar helmet will stop an AK47 round does not necessarily mean it will be good on a bike. Weight and ventilation play a big part in helmet design.

What about energy you say, glad you asked. Your average European male at 17.6 mph has the same kinetic energy (1/2 m v*v) as the AK47 bullet - about 2045 Joules.

Hope I got my sums right

Dave

Fair point but I think that if a full impact occured with the whole of the body's kinetic energy being put throught the helmet then no helmet would be adequate as you'd snap your neck. :rotfl:

I'd say a the vast majority of falls from bikes involve the head not being the first to impact, so the weight behind it would be that of the head alone.

Just my thoughts, I may be horribly wrong though.

Dave_C_2

Limey wrote: »

Fair point but I think that if a full impact occured with the whole of the body's kinetic energy being put throught the helmet then no helmet would be adequate as you'd snap your neck. :rotfl:

I'd say a the vast majority of falls from bikes involve the head not being the first to impact, so the weight behind it would be that of the head alone.

Just my thoughts, I may be horribly wrong though.

Sounds about right, with the emphasis on about
It's where the theoretical model gets too complicated and we (as a society) have to resort to experimental methods. Call in the crash test dummies.

Have bike helmets been tested with crash test dummies? A quick Google reveals this site and the rest is about the inflatable air bag helmet.

I would have expected the Road Research Laboratory to have done some tests, but then realised that cyclists don't count as road users as far as the Goverment is concerned.

Dave

kerby_crawler

Dave_C wrote: »

cyclists don't count as road users as far as the Goverment is concerned

I'm sure that can't be true - members of the government regularly appear on the news getting on or off of a bike.