Helmet Cams and the integrity of cycle helmets

This sub-forum all discussions about this "lively" subject. All topics that are substantially about helmets will be moved here, if not placed here correctly in the first place.
nez
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Re: Helmet Cams and the integrity of cycle helmets

Postby nez » 15 Oct 2015, 5:42pm

Ah yes. We have established by experiment on the rugby pitch, the Welsh are harder nuts than the English :shock:

TonyR
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Re: Helmet Cams and the integrity of cycle helmets

Postby TonyR » 15 Oct 2015, 6:14pm

nez dans le guidon wrote:
TonyR wrote:
Elizabethsdad wrote:I am pretty sure this arrangement would have meant the camera breaking away in the event of an impact.


I doubt it given the mechanics. The camera will be punched into the helmet in five thousandths of a second with a force equivalent to being hit with a one tonne weight.

70kg x 20kph x under surface of camera = being hit on the head with a hammer.


Unfortunately not. Try to work out the forces to bring 70kg from 20kph to zero in 2cm (the thickness of the helmet). My numbers were for 4.5kg from 12mph in 2cm. Yours will be a lot more.

nez
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Re: Helmet Cams and the integrity of cycle helmets

Postby nez » 15 Oct 2015, 6:17pm

TonyR wrote:
nez dans le guidon wrote:
TonyR wrote:I doubt it given the mechanics. The camera will be punched into the helmet in five thousandths of a second with a force equivalent to being hit with a one tonne weight.

70kg x 20kph x under surface of camera = being hit on the head with a hammer.


Unfortunately not. Try to work out the forces to bring 70kg from 20kph to zero in 2cm (the thickness of the helmet). My numbers were for 4.5kg from 12mph in 2cm. Yours will be a lot more.

I think we're agreeing Tony. But I'm afraid 45 years since the last physics class has put the sum beyond me, if I ever could have done it.
Last edited by Vorpal on 15 Oct 2015, 9:35pm, edited 1 time in total.
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[XAP]Bob
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Re: Helmet Cams and the integrity of cycle helmets

Postby [XAP]Bob » 17 Oct 2015, 7:58am

20kph is 20,000m in 3600seconds or ~6m/s

70kg 6m/s is 1/2.70.6^2 = 1250 joules

Force * distance is energy, so 1250J/0.02m = 62.5kN or about the weight of 6ton

Fortunately the body isn't rigid, and this is not actually the force involve - but it illustrates why helmets have such a limited working range.
At 10kph the force is divided by 4, at 30 it is 3 times greater than above.
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nez
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Re: Helmet Cams and the integrity of cycle helmets

Postby nez » 17 Oct 2015, 12:19pm

Thanks for the demonstration Bob. My top speed on my regular run is 57kph! Like many of us I try to average 20kph minimum suggesting I spend a lot of time at or above 30kph

nez
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Helmet Cams and the integrity of cycle helmets

Postby nez » 17 Oct 2015, 1:16pm

1/2.70.6^2 .
what's this bit? I was just trying to apply it to a 130kilo rugby player doing 10ms...

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[XAP]Bob
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Re: Helmet Cams and the integrity of cycle helmets

Postby [XAP]Bob » 17 Oct 2015, 1:25pm

Kinetic energy(J) = 1/2 * mass(kg) * speed(m/s)^2

When doing algebra I use . instead of x for multiplication, to avoid confusion in written work. Also * is three keystrokes on the iPad, . is just one ;)
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.

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[XAP]Bob
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Re: Helmet Cams and the integrity of cycle helmets

Postby [XAP]Bob » 17 Oct 2015, 1:31pm

nez dans le guidon wrote:Thanks for the demonstration Bob. My top speed on my regular run is 57kph! Like many of us I try to average 20kph minimum suggesting I spend a lot of time at or above 30kph


And the difference between 10 and 30 is a factor of 12 - go to 60 and it goes up by another factor of 4 from the higher calculation of 30 earlier - I.e 4 times 3 times the 6ton or 72ton (but the human body isn't rigid)
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.

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Re: Helmet Cams and the integrity of cycle helmets

Postby TonyR » 17 Oct 2015, 1:53pm

[XAP]Bob wrote:
nez dans le guidon wrote:Thanks for the demonstration Bob. My top speed on my regular run is 57kph! Like many of us I try to average 20kph minimum suggesting I spend a lot of time at or above 30kph


And the difference between 10 and 30 is a factor of 12 - go to 60 and it goes up by another factor of 4 from the higher calculation of 30 earlier - I.e 4 times 3 times the 6ton or 72ton (but the human body isn't rigid)


3/10 for maths young Bob. The difference between 10 and 30kph is a factor 3 squared or 9

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[XAP]Bob
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Re: Helmet Cams and the integrity of cycle helmets

Postby [XAP]Bob » 17 Oct 2015, 2:05pm

My mental gymnastics as I pressed submit are clearly faulty, and then carried through... The 20 was calculated, the 10 is a trivial factor, but you are right going from 20-30 should be (9/4), not whatever I said above.

Of course 20-60 is a direct factor of 9 -> 54 ton
Apologies...
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
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pjclinch
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Re: Helmet Cams and the integrity of cycle helmets

Postby pjclinch » 17 Oct 2015, 3:33pm

Though unless you hit a horizontal constraint (like a wall) you won't actually scrub off all of that speed in the thickness of a helmet. It's not how much KE you have when you hit, it's the rate at which you lose it.
Come off directly square on to a wall at 57 kph and that will do far, far more damage than hitting a flattish road with space to slide at the same speed, because in the latter you'll scrub off speed more slowly. You'll scrub off more skin in the process, but that's not as bad as skull to wall well above what it's evolved to take.
And a helmet might reduce the peak deceleration, but quite probably not enough: they're not designed for that sort of hit.

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irc
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Re: Helmet Cams and the integrity of cycle helmets

Postby irc » 17 Oct 2015, 4:51pm

pjclinch wrote:Though unless you hit a horizontal constraint (like a wall) you won't actually scrub off all of that speed in the thickness of a helmet. It's not how much KE you have when you hit, it's the rate at which you lose it.



Very true. Which is why track racing mitorcyclists survive high speed crashes when they slide after leaving the bike. While on the road it's a lottery. The slide often being interrupted by walls or lampposts with fatal results.

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Mick F
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Re: Helmet Cams and the integrity of cycle helmets

Postby Mick F » 17 Oct 2015, 4:57pm

pjclinch wrote:............ they're not designed for that sort of hit.
Absolutely.
We all know this. (or we should know this)

However, is there any statement of what they are designed to "hit"?
Is there a mission statement?
Kite Mark?
E Mark?
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pjclinch
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Re: Helmet Cams and the integrity of cycle helmets

Postby pjclinch » 17 Oct 2015, 7:44pm

Mick F wrote:
pjclinch wrote:............ they're not designed for that sort of hit.
Absolutely.
We all know this. (or we should know this)

However, is there any statement of what they are designed to "hit"?
Is there a mission statement?
Kite Mark?
E Mark?


It's in the spec for the helmet (usually EN1078, sometimes the better Snell). With this being the design requirement, this is the very specific thing they are built to achieve. More expensive helmets typically achieve the same standard but with better ventilation (i.e., less stuff, more holes).

Can't remember exact numbers and fine print, but the basic idea is a fall to the ground from stationary with impact square to the front of the helmet, securely fastened to a headform.

In that sort of accident you can reasonably expect a helmet to make you considerably less uncomfortable than without it. However, you could also expect an unhelmeted but otherwise identical strike to leave you with nothing worse than a marked bump and a bad headache. You're also somewhat in the dark about aspects not tested, and nothing about how much the extra size might lever your neck in a disadvantageous manner. Also the case that not all over-the-bars will have the impact taken on the lid: last time I did it (over 10 years back) the bit of my head that took it was my chin... thanks to the extra weight of the lid it would have been a bit harder a whack than without it.

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Tonyf33
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Re: Helmet Cams and the integrity of cycle helmets

Postby Tonyf33 » 17 Oct 2015, 8:10pm

I only just read 'cycle' last night, I thought it was very telling what camp the author was in, immediately it was blindingly obvious with the title...helmet cams, why not just video cameras for cycling. That the go pro and the Virb at least were (originally) designed to fit directly onto bars and also that the pros have them fitted there too yet there was no mention at all of handlebar fitment. :roll:

There was one minor mention of fitment to the bike, the reviewer couldn't be bothered to mention the very real safety issue, ultimately in a worse case scenario it negates massively any potential safety aspect a helmet has by mounting one. That's pretty bad for a test in the CTCs magazine!
Overall the review in itself was poor, so much missing in useful terms and lack of information, also none of the low to mid range cameras tested either which can and do do an effective job for a whole chunk of money less.