Fabian Cancellara sparks helmet debate on Twitter, says all cyclists should wear one

HarryCallahan wrote:

nuclear coffee wrote:

HarryCallahan wrote:

nuclear coffee wrote:

HarryCallahan wrote:

nuclear coffee wrote:

HarryCallahan wrote:

If the helmet isn't there where does the energy that cracks the helmet go?

Tell me, how much energy, in J or kCal if you like, is required to create a few square inches of crack in expanded polycarbonate?

Too much stupid to bother replying to everything.

Look up "impact test" about anywhere really. Energy is absolutely relevant, as is stress. And no, you absolutely cannot drive a steamroller over a helmet and have it survive, idiot.

Just quickly.

What is the answer to your question?

Normally, I'd be charging for that information, since it took labour to acquire.

Pro bono, to crack a helmet in two you'd need to create at least 0.01 +/-0.005 square metres of crack (on mine anyway), you'd need about 200 +/- 100J. Really of course you're not going to create the one crack, and I haven't taken into account the covering, so 200J is highly unlikely to cause a major failure. Since unlike you I'm not actually trying to prove anything about helmets themselves, for any more I will have to charge. Given that unlike you I'm not trying to prove anything about helmets themselves, it's pretty shameful that I've done the work and you haven't.

nuclear coffee wrote:

Normally, I'd be charging for that information, since it took labour to acquire.

Pro bono, to crack a helmet in two you'd need to create at least 0.01 +/-0.005 square metres of crack (on mine anyway), you'd need about 200 +/- 100J. Really of course you're not going to create the one crack, and I haven't taken into account the covering, so 200J is highly unlikely to cause a major failure. Since unlike you I'm not actually trying to prove anything about helmets themselves, for any more I will have to charge. Given that unlike you I'm not trying to prove anything about helmets themselves, it's pretty shameful that I've done the work and you haven't.

You could have shown some evidence you did some work and didn't pluck numbers out of thin air.

What physical formulas are you using here?
What application of force (to the helmet) is performing 200J of work?

For a materials scientist you seem to have grossly simplified the problem.

For instance, a surgeons scalpel applied to the foam would require far much less force than a hammer to break it. (less work, joules).

A different experiment might try to laterally shear it, that has its own unique set of parameters.

You seem to have your materials science mixed up with collision physics. A full helmet doesn't lend itself to materials analysis i.e. they wouldn't work out the tensile strength of "expanded polycarbonate" (your original question) by stress testing a helmet.

You've provided unexplained answers to a problem which hasn't even been defined.

And it's still irrelevant because the foam is there to cushion the impact - it's supposed to give! (crack/compress)

HarryCallahan wrote:

nuclear coffee wrote:

Normally, I'd be charging for that information, since it took labour to acquire.

Pro bono, to crack a helmet in two you'd need to create at least 0.01 +/-0.005 square metres of crack (on mine anyway), you'd need about 200 +/- 100J. Really of course you're not going to create the one crack, and I haven't taken into account the covering, so 200J is highly unlikely to cause a major failure. Since unlike you I'm not actually trying to prove anything about helmets themselves, for any more I will have to charge. Given that unlike you I'm not trying to prove anything about helmets themselves, it's pretty shameful that I've done the work and you haven't.

You could have shown some evidence you did some work and didn't pluck numbers out of thin air.

What physical formulas are you using here?
What application of force (to the helmet) is performing 200J of work?

For a materials scientist you seem to have grossly simplified the problem.

For instance, a surgeons scalpel applied to the foam would require far much less force than a hammer to break it. (less work, joules).

A different experiment might try to laterally shear it, that has its own unique set of parameters.

You seem to have your materials science mixed up with collision physics. A full helmet doesn't lend itself to materials analysis i.e. they wouldn't work out the tensile strength of "expanded polycarbonate" (your original question) by stress testing a helmet.

You've provided unexplained answers to a problem which hasn't even been defined.

And it's still irrelevant because the foam is there to cushion the impact - it's supposed to give! (crack/compress)

As previously stated, I do not spend my own time explaining formula, finding data and doing calculations for lazy and ungrateful people for free. Nothing I used was propietary, and my question was entirely well-defined, thank you very much, so if you can't answer it that is entirely down to the work I have done and you have not.

I suppose what remains is for me to call you a passive aggressive douche for asking a question with a clear implication ("where does the energy go?") without providing clearly relevant information (such as the amount of energy we might be talking about), and an ungrateful douche for your behaviour towards someone who has provided you with at least some of the relevant information.

HarryCallahan wrote:

Go do a simple experiment using a hammer, a helmet and your head. Which hurts more, with or without helmet?

No. Since you're the one with such faith in the powers of this supposed safety intervention in bike accidents, you go and do it, then get back to us with the results.

Not that it will mean anything if you do, of course, as it's got nothing to do with cycling.

congokid wrote:

HarryCallahan wrote:

Go do a simple experiment using a hammer, a helmet and your head. Which hurts more, with or without helmet?

No. Since you're the one with such faith in the powers of this supposed safety intervention in bike accidents, you go and do it, then get back to us with the results.

Not that it will mean anything if you do, of course, as it's got nothing to do with cycling.

Well, quite. The actual analogy here is that my head will be much better off not being hit by the hammer at all, never mind whether I'm wearing a helmet.

Hence spending my energy campaigning for better cycle infrastructure, rather than the marginal intervention of encouraging helmet use.

nuclear coffee wrote:

HarryCallahan wrote:

If the helmet isn't there where does the energy that cracks the helmet go?

It gets focussed into a raw concentrated form of idiocy that's then handed out to people like you.

Tell me, how much energy, in J or kCal if you like, is required to create a few square inches of crack in expanded polycarbonate?

It's not the energy in the crack creation/propagation itself which is the main factor (although I guess some enterprising engineer might tell us as the Youngs modulus, fracture toughness, surface energy etc are probably easily available).

If the crack occurs but the foam underneath is not compressed then it's likely that the fracture itself provided little protection (it may have slowed things a little or hardly at all).

If the foam is compressed then the helmet will likely have slowed the impact - although that may not be sufficient to prevent injury (or maybe irrelevant to the outcome).

HarryCallahan wrote:

"velocity of any impact would in all likelihood be well above the maximum stipulated under EU standards"

Doesn't mean the helmet isn't effective. Better to have concussion than mashed brains.

Except the impact mitigation for cycle helmet material is not to lessen the impact, but in fact to take it all unto itself

This results in the rule that ANY impact involving your helmet has almost certainly resulted in it splitting

In a real crash, impact directly to the helmet will result in it splitting like a watermelon hit by a hammer - providing NO protection to the rider

HarryCallahan wrote:

"velocity of any impact would in all likelihood be well above the maximum stipulated under EU standards"

Doesn't mean the helmet isn't effective.

Indeed, there often appears to be an issue with understanding how helmets work to try and mitigate physical damage in these debates - and equating bike velocity with velocity of the head in accidents, impact mass with total body mass, efficacy with KSI tables and so on and so on.

There certainly are issues with helmets and helmet use, physically and psychologically, but IMO they aren't always sensibly dealt with by some of the more simplistic arguments that tend to appear.

I think one issue which most or many of us can agree on is that blanket compulsion is not the answer.

Das wrote:

Well all I know is that when I was hit and knocked off by a motorist I was glad I had my helmet on. The damage to the helmet was minimal, but without it my head would have connected with the tarmac.

Or maybe your neck would have been able to support a lighter helmet-less head and avoided any impact with the tarmac at all? Would you like to repeat the experiment multiple times with and without helmet to verify the anecdote?

It's sort of outside the real debate though, because not even helmet makers claim they offer any protection in collisions or falls onto spiky things at the minute. They're nearly all designed for protection in standing falls onto flat surfaces and personally, I've not done that since I learned to ride a bike.

a.jumper wrote:

Das wrote:

Well all I know is that when I was hit and knocked off by a motorist I was glad I had my helmet on. The damage to the helmet was minimal, but without it my head would have connected with the tarmac.

Or maybe your neck would have been able to support a lighter helmet-less head and avoided any impact with the tarmac at all? Would you like to repeat the experiment multiple times with and without helmet to verify the anecdote?

It's sort of outside the real debate though, because not even helmet makers claim they offer any protection in collisions or falls onto spiky things at the minute. They're nearly all designed for protection in standing falls onto flat surfaces and personally, I've not done that since I learned to ride a bike.

Maybe it would have, maybe it wouldnt have. I have no idea. All I remember is saying to myself "the car has stopped, hes seen me.........WTF why am I lying in the middle of the road!!!!" I never fell on a spikey thing, I fell on to a perfectly flat road surface. It was a sideways motion, like a scraping motion, so I doubt the instant hit allowed my body to tense up, so yes im still happy I had a helmet on. I pretty sure the outcome was much better than a scalping and "Gravel rash" through my hair.

Das wrote:

I never fell on a spikey thing, I fell on to a perfectly flat road surface. It was a sideways motion, like a scraping motion, so I doubt the instant hit allowed my body to tense up, so yes im still happy I had a helmet on. I pretty sure the outcome was much better than a scalping and "Gravel rash" through my hair.

A baseball cap or even a trendy retro cycling cap would probably offer enough sacrificial protection to avoid gravel rash on the head.

And yes, you fell onto a flat road surface, but in a manner outside the design parameters of current casual cycling helmets. It's just chance that it seems to have helped.

Are you sure?