swansonj
Guru
The human race
The CycleChat Helmet Debate Thread
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swansonj
Guru
Adrian me old mate, I regard you as a friend and I study all your posts in the knowledge I will almost invariably learn from them .... but I'm going to guess that physics may not be your strongest subject?
What if the branch hits the side of the helmet?
mjr
Comfy armchair to one person & a plank to the next
- Location
- mostly Norfolk, sometimes Somerset
So why are you concentrating on trying to mitigate the effects if you have the same accident yet again, rather than preventative measures?
mjr
Comfy armchair to one person & a plank to the next
- Location
- mostly Norfolk, sometimes Somerset
What if the branch gets stuck in the vent?
Not even if it gets snagged in a side vent, my speeds off road is quicker than some manage on the road. Normally about 14mph but often up to 25
swansonj
Guru
My quibble, which may not have come over quite as light-heartedly as I intended
was not about what happens but how we describe it.The same person with the same speed has the same momentum.
They then make contact with a surface. If they hit head-on a nearly vertical surface - a wall or the side of a car - such that the impact force is aligned with the neck, it is possible for there to be no rotational component. But if we compare coming down onto a horizontal road with hitting a branch with the top of the head, there is inevitably a rotational force applied to the head.
At the first instant of impact, the forces are pretty similar. The head doesn't know whether the thing it hit is a road or a branch, it just knows there's a force being applied. And the dimensions of the head are the same, so a given initial force produces the same initial rotational moment.
What happens next differs depending on the specifics of the situation. The coefficient of friction could be different. The likelihood of snagging could be different. The branch is likely to deflect more than the road does. And what happens to the rest of your body is likely also to be different.
I think the commonest scenario is probably that the branch deflects or the neck bends sufficiently for the head to pass underneath or a combination of both so that limited damage is done. But that's not really a function of the momentum of the body and the same thing is also true of impacts with roads - the commonest scenario is that the body-and-head combo just slides and limited damage is done.
So I wonder if the main difference between branches and roads is simply that branches tend to be more flexible? Consider the thought experiment of hitting, not a branch of a tree, but the underside of a concrete bridge. It seems to me all the usual arguments about helmets and roads would be equally applicable. The helmet, if it crushes rather than cracks, will absorb some energy which cannot be a bad thing - but the amount of energy, if you are travelling at more than a walking pace, will be trivial. But also, by increasing the diameter of the head, the helmet makes an impact more likely in the first place, and increases the radius at which the impact force acts and therefore the angular acceleration of the head. Your head may just slide with minimal rotation - but that's a consequence of the detail disposition of forces, directions and coefficients of friction, not determined primarily by your momentum - and applies to coming off onto a road as well.
The main similarity between coming off onto a road and hitting a branch seems to me the rather important one that in the vast majority of cases neither results in anything worse than possibly a headache or some superficial grazing.
swansonj
Guru
A follow on, because I was re-reading your posts and realised that one of the differences you were noting was that on the road the sliding of the helmet along the ground can be prolonged whereas with the branch any contact is over quickly - then I got summoned to go and eat my pain au chocolat and drink my cappuccino before I could incorporate that into my reply.My quibble, which may not have come over quite as light-heartedly as I intended
The same person with the same speed has the same momentum.
They then make contact with a surface. If they hit head-on a nearly vertical surface - a wall or the side of a car - such that the impact force is aligned with the neck, it is possible for there to be no rotational component. But if we compare coming down onto a horizontal road with hitting a branch with the top of the head, there is inevitably a rotational force applied to the head.
At the first instant of impact, the forces are pretty similar. The head doesn't know whether the thing it hit is a road or a branch, it just knows there's a force being applied. And the dimensions of the head are the same, so a given initial force produces the same initial rotational moment.
What happens next differs depending on the specifics of the situation. The coefficient of friction could be different. The likelihood of snagging could be different. The branch is likely to deflect more than the road does. And what happens to the rest of your body is likely also to be different.
I think the commonest scenario is probably that the branch deflects or the neck bends sufficiently for the head to pass underneath or a combination of both so that limited damage is done. But that's not really a function of the momentum of the body and the same thing is also true of impacts with roads - the commonest scenario is that the body-and-head combo just slides and limited damage is done.
So I wonder if the main difference between branches and roads is simply that branches tend to be more flexible? Consider the thought experiment of hitting, not a branch of a tree, but the underside of a concrete bridge. It seems to me all the usual arguments about helmets and roads would be equally applicable. The helmet, if it crushes rather than cracks, will absorb some energy which cannot be a bad thing - but the amount of energy, if you are travelling at more than a walking pace, will be trivial. But also, by increasing the diameter of the head, the helmet makes an impact more likely in the first place, and increases the radius at which the impact force acts and therefore the angular acceleration of the head. Your head may just slide with minimal rotation - but that's a consequence of the detail disposition of forces, directions and coefficients of friction, not determined primarily by your momentum - and applies to coming off onto a road as well.
The main similarity between coming off onto a road and hitting a branch seems to me the rather important one that in the vast majority of cases neither results in anything worse than possibly a headache or some superficial grazing.
Firstly, I really doubt there is often prolonged sliding at significant contact force between head and road. We frequently point out that even the initial impact is nearly always with the body and rarely with the head - how much more true of subsequent sliding? And second, except possibly in some remote scenario where the head gets tucked under the body and effectively carries the whole weight of the body, once the initial contact is over and we're into the sliding phase, the head is probably contacting the ground with only the force of its own weight, and therefore generating frictional forces of a similar magnitude, forces the neck is well adapted to cope with. It's the initial impact where I suspect the damage is (or more usually is not) done.
There is a campaign in the US - "Smoother, rounder,safer"
Modern helmets have sharp edged vents and edges where" aerodynamics" have led over safety, a smoother, rounder helmet is safer
These can catch and prevent a helmet sliding, in some cases causing the helmet to eject from the head as well as causing abrupt deceleration and rotational moments
Of course the answer in testing has been to address this by "Gaffer taping" the helmet in place, but unless you wish to tape your helmet in place, t is not a real - life solution to the issue
Modern helmets have sharp edged vents and edges where" aerodynamics" have led over safety, a smoother, rounder helmet is safer
These can catch and prevent a helmet sliding, in some cases causing the helmet to eject from the head as well as causing abrupt deceleration and rotational moments
Of course the answer in testing has been to address this by "Gaffer taping" the helmet in place, but unless you wish to tape your helmet in place, t is not a real - life solution to the issue
I am quite prepared to accept your argument and that there is a benefit, if you are wearing a helmet
Now all we need is an explanation why a pedestrian hitting their skull on the tarmac does not have he same benefit, and therefore the same justification for wearing one
Profpointy
Legendary Member
i'm afraid you've rather missed a key point. I used to share your view - the "it's obvious innit?" argument if I may be slightly flippant. The trouble is, if you look at the numbers post-cumpulsion in Australia, there does not seem to any net benefit - even though numbers wearing helmets went up from something low like 10 or 20% up to 90% so something odd is going on.
We can speculate a number of possible reasons - obviously your head is a lot bigger - nearly twice as big in terms if cross section, so a lot of near misses become hits. Also there's more leverage which could well cause more neck or other rotational injuries. And maybe cyclists and motorists take more risks. Whatever the true reason - they don't seem to work on average across the population. Of course they may well help in a given accident, but seemingly that's balanced by another accident where they've made things worse.
keithmac
Guru
All I can say is my injuries were to the soft part that surrounds the skull, and the helmet helped the second time as it did contact the road.
Accidents are wide and varied, it is possible the helmet could be detrimental in some (if it gets ripped off the head etc).
I personally have fallen over when walking very few times, but if I did and split my head open on the floor then a helmet would have been benificial at that time.
It's for everyone to make their own choice at the end of the day, I happilly chat to helmet wearers and none wearers at the traffic lights. Main conversation recently had been about the Gtech, never a mention about head gear..
Accidents are wide and varied, it is possible the helmet could be detrimental in some (if it gets ripped off the head etc).
I personally have fallen over when walking very few times, but if I did and split my head open on the floor then a helmet would have been benificial at that time.
It's for everyone to make their own choice at the end of the day, I happilly chat to helmet wearers and none wearers at the traffic lights. Main conversation recently had been about the Gtech, never a mention about head gear..
Profpointy
Legendary Member
helmet might well have helped in your particular instance, but conversely I can relate an incident where my head missed the floor by something like the width of a helmet However in my view it's wiser to base decisions on the wider picture - which I suggest is represented by the Australian statistics. It convinced me to stop wearing one, and I speak as an early adopter of cycle helmets
Brandane
Legendary Member
- Location
- Costa Clyde
That's good. Unfortunately, I have read more than one post on this forum along the lines of "I will acknowledge all other cyclists, apart from those who are not wearing a helmet". More recently, non helmet wearers have been referred to as "twats" by a newbie poster.
These are the types who really get my back up. If they care to actually research the subject, they will find that helmets are not the magical life savers that some presume them to be. Yes, they may well help to reduce injury given a very limited set of circumstances; but the chances of such circumstances coming together are no more than they would be walking along the street. So why pick on cycling as some high risk activity, when it is not?
I am not anti helmet, other than the fact that wearing one might give the wrong impression with regards to the risk factor. What I am VERY anti is the preachy nobbers who come on here (usually newbies) and wade in with tales of how a piece of plastic with a thin foam lining which conforms to a very basic safety standard (google BS EN 1078 if you're interested) saved their lives. These are the idiots who would have the wearing of said helmets made mandatory, and probably drive cycling numbers into a nosedive.
If people want to wear a helmet for whatever reason, whether it be simply because it makes them feel better, or because they want to look like a TDF rider, then fine. Just don't preach to me that I should wear one too, or look down your snotty nose at me just because I have chosen not to. I would wager a bet that I do a LOT more miles per annum on a bike than your average helmet evangelist.
