Safe Speed Forums

Most scientists are climatologists? Methinks that you are exhibiting an ignorance of arithmetic.

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Except that, in practice, the stretch in the rope would reduce the acceleration. And where would the cinnamon come from?

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

But if the stretch in the rope reduced the acceleration, it wouldn't be 200G any more (which is part of the way that seat belts work). I assume Weepy was refering to a rope with the same degree of elasticity as a barrier - one of those magic ones made from commercially pure, hand-spun unobtainium!

You got me on the cinnamon though.

Perhaps I should have said lack of understanding or failure to comprehend rather than ignorance.

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

Oh lord!

What causes the trauma?



Did you know that if you smash into other stuff without experiencing the force of deceleration (assuming no penetration into flesh), you live and don't feel a thing?
Take the example of an oil tanker smashing into a small yacht, does the crew of the tanker suffer?

Conversely, if you suffer a large acceleration without smashing into other stuff, you still suffer and can die! You have already stated this but then dismissed it: the "organs banging together" - isn't that "trauma"?

Which leads to the obvious question: does the injury follow the smashing, or the deceleration?


Not correct. If it brakes at 200G you likely wouldn't walk away!

Toltec is exactly correct: it is the 'relative' acceleration that does the damage.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

How ridiculous! Technically, travelling along at any speed is safe so long as you don’t hit anything. Look at us all now, speeding through space at 1000mph for millions of years. Look at the safest transport of all hurtling through the air at 500+mph.

It wasn’t speed that sank the Titanic; it was the fact that it hit something. On modern ships they can go faster and safer because the technology can detect these things. Modern cars are far safer these days, as are the roads and more road furniture to protect pedestrians if they can be bothered to learn the Green Cross Code, yet the limits are constantly being lowered.

Wrestle with that notion for a moment if you will…

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

1000mph is about the surface rotation speed of the Earth Tone. It moving at over 66,000mph in the orbital track and stellar drift is much higher let alone galactic motion. All depends on your frame of reference.

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

I reckon he's barking up the wrong tree there

Seriously though, deceleration in a collision is inversely proportional to the distance in which you stop - so if you're not wearing a seatbelt so your head decelerates to zero over a couple of inches when it hits the windscreen, the g-force is probably around ten times what it would have been had you been wearing a seatbelt.

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

I'm still not buying that decelerating your head without having it hit something will cause the same level of damage as having it hit something, given the same G forces involved.

Then I'm afraid you need to improve on your understanding of applied physics, as well as what causes trauma: the "organs banging together"; this is caused by 'relative' deceleration - regardless of how it is induced.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

I think I can explain this, in brief it comes down to your experience of the world.

Being caught by a safety rope hurts a lot less than hitting the ground and since they both stop you obviously the harness which saves you must inflict less damage.
The reason it does this is because deceleration force is spread over a large area and onto points of your body better able to bear the load, it also decelerates you over a significant distance. So not only are the forces generated over a larger surface, but the energy is dissipated across a much longer time frame.

Also the idea that a moving object can stop instantly is not common sense as it is not something that happens in the real world.

Now when you think of the difference between hitting a static object and something that is touching you that then stops you automatically allow that it cannot do so. It may stop very quickly, but experience tells you there will be some give. This is why even with the premise of 'the same g force' you apply empirical reasoning to the situation and end up ignoring the premise because it cannot really happen.

Think of this another way, you are wearing a harness and are standing in front of a wall shaped to touch you at the same positions as the harness and it is two Planck lengths away. Every particle in your body and the harness is instantaneously accelerated up to 60 mph, After you have travelled one Planck length the harness stops instantly and it is so strong that no part of it or your body touches the wall which is now only one Planck length away. Will you be able to tell the difference between hitting the wall and being stopped by the harness?

ETA - had to put the c back in Planck as spell check removed it!!

ETA2 - If we assume you are on average 250mm thick and stopping instantly smeared you into a thin film then on average any part of your body would stop in 125mm, follow? So to generate an average 200G by stopping as I have described you would only need to be doing about 50mph.

In reality you would not end up as paste so the average deceleration distance would likely be what maybe half? So that means your initial speed would be just 36mph to generate 200G?

Damn forgot to convert back to mph, thought 22mph seemed a bit slow

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

And the damage occurs with the head stopping but the brain not stopping.

200g deceleration is nearly always fatal.

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The world runs on oil, period. No other substance can compete when it comes to energy density, flexibility, ease of handling, ease of transportation. If oil didn’t exist we would have to invent it.”

56 years after it was decided it was needed, the Bedford Bypass is nearing completion. The last single carriageway length of it.We have the most photogenic mayor though, always being photographed doing nothing

Why aren’t seat belts made with some ‘give’

I don’t mean elastic of course but the belt is completely the opposite. So maybe if there was enough give somewhere to allow the body to go forward, even a couple of inches, instead of nailing you to the seat it would help?

To answer my own question, I guess it gets into the realms of body weight, which would adversly effect this process, but in this day and age I would expect sensors in the seat could relay information back to, I dunno, a 'beltbracer', (© & patent pending ) which is at the end of the belt.

This mechanical/hydraulic device with servo could reduce the instantaneousness of the impact through the belt by letting it out slowly. (By which I mean fractions of a second). It would effectively do the same as one of those rides at the fair which drop you from a great height and slow you down when you’re about to crash..

Just calling Volvo; back later…

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

Seat belt webbing does indeed stretch when stressed beyond its elastic limit. It is this progressive stretch which mitigates the decelleration and protects you. Belt tensioners, which react to impact shock, actually tighten the set belt to allow it to stretch more before you hit the dashboard. Air bags are deigned to come into play as the belt stretch ends and you near the dash.

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Malcolm W.
The views expressed in this post are personal opinions and do not represent the views of Safespeed.

But the G forces involved are not the same. If your head hits something solid which only gives, say, 0.5mm, it's subject to a deceleration of twenty times that if it's decelerated over, say, 10mm. So, if the deceleration in the latter case is 200g, it's 4000g in the former case.

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

So what's the Max Planck length then? (sorry, couldn't resist)

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

1.616252e?35 meters. Or three halfpence a foot.

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Yes it would help, and yes they do! In fact, as Malc suggested, there's an insane amount of stretch in a belt, and for precisely that reason. All the time the belt is stretching, it's absorbing energy that would otherwise go into your body. Unfortunately, there's a limit to how far it can let you go before you hit the inside of the car, so they introduced "pre-tensioners". They tighten the belt at the moment of impact so that there's no slack in it - thereby maximising the amount of distance you can travel whilst having your deceleration reduced by the belt stretching. In fact, when we test the seat belt "anchorages" in a car, we generally throw the belts away and use that 5-ton ratchet strap webbing instead, (partly) because the real belts would stretch so much we'd risk running out of travel on the hydraulic rams that apply the loads to the belt anchorages! Also, to give an idea how much they stretch, I'm told that accident investigators can sometimes tell if a belt was being worn in a crash because the webbing will be narrower than the other belts in the car.

It's a tricky one, body weight. They work on a dummy of about 75kg when testing belts. Modern cars tend to see something like 20-30G deceleration in the crash test (if you pick a bit of car that isn't crumpling at the time)! That means the 75 kg occupant would see something like 20x75kg of load, for a fraction of a second - say 1.5 - 2 tonnes ( ) if they decelerated at the same rate as the passenger compartment. Clealry, the stretch in the belt webbing reduces this substantially. Of course, if you take my rather svelte (cough!) 105kg, then at the same deceleration, that's going to generate some SERIOUS forces! The belt anchorages get tested to about 3 tonnes per seating position, so I'd be in danger of ripping them out of the bodyshell! Of course there's the double whammy because not only do fat people impose much bigger loads on themselves, but they're generally, rather flabby and poorly muscled (not me, of course, you understand, other fat people, mainly)! the result is that there's little point in making the belts or their anchorages tougher, because I'd probably be dead anyway at that kind of deceleration - killed by my own flab!

Yes, there has been loads of work done on "smart" belts and airbags. Usually the other way though (e.g. reducing the airbag charge for the little old lady with brittle bones who sits very close to the steering wheel). There have been experiments with sensors on seat slides and belt retractors so that the computer can get an idea of whether it's a short, light person, a big fat person, tall, short etc and re-calculate the airbag charge accordingly. I even read of ultrasonic sensors in the seats that got an idea of the occupant's bone density! (Dusty will be along in a minute to bemoan the complexity of all this)!


Quite a few cars with airbags have them already I'm afraid! They're called "peak load limiters". They take a variety of forms, the simplest being a length of twisted or coiled / rolled metal - sometimes between the retractor and the bodyshell. As the load comes on the belt in a crash, this metail progressively uncoils / unrolls / untwists itself, absorbing energy as it does so. You're only allowd to fir them on cars with airbags because obviously, the downside is that, when combined with the natural stretch in the webbing) they let you get very close to the steering wheel / dash!

But doesn't our flab, by compressing under load, provide another buffer against those massive forces?

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Even I'm not flabby enough round my chest and collar bone to have any significant extra padding there. The extra weight (x 20 or 30) would do me much more damage than any benefit the "padding" would provide. Round my tummy, it's all fat. That thickness of muscle would be good, but fat doesn't really have any load bearing properties (as Mrs M. so often reminds me)!