Inertial Seatbelts in an R10 - Page 2
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  1. #26
    1000+ Posts robmac's Avatar
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    Quote Originally Posted by Kim Luck View Post
    Rob! Steady on! I'm not sure who's laws of physics you are quoting or whether I missed something in class. I'm fairly sure through observation most passengers seem to be travelling at the same speed as the vehicle they're in. The passenger does not begin to slow down until after the belt has locked and begun to apply a decelerative force to the wearer, surely?
    Q:What is happening to the vehicle 13mS after impact?
    A: Decelerating at a few G

    Q:What is the vertical part of seat doing to the driver at the start of vehicle deceleration?
    A: Transferring the force to the driver's back

    The deceleration force of the vehicle is greater than and additive to the occupants initial inertia.

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    When the reel locks and the seat belt meets the occupant and stops stretching the vehicle and occupant can be considered as a single object.

    As I said earlier a study in statics and dynamics.

  2. #27
    1000+ Posts Kim Luck's Avatar
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    We can keep this going for some time much, probably, to the amusement of the gallery but you just said that "when the inertia reel locks, the passenger becomes part of the vehicle because the seat belt is now a fixed part of the vehicle and is decelerating at the same rate as the vehicle," with which I concurred, but I can't help feeling that initially the car is decelerating faster than the occupant, which is why the dashboard and is getting noticeably closer (216mm) and at the same time the seat-belt is getting tauter?
    Don't it always seem to go that you don't know what you've got 'til it's gone............

  3. #28
    1000+ Posts robmac's Avatar
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    Quote Originally Posted by Kim Luck View Post
    We can keep this going for some time much, probably, to the amusement of the gallery but you just said that "when the inertia reel locks, the passenger becomes part of the vehicle because the seat belt is now a fixed part of the vehicle and is decelerating at the same rate as the vehicle," with which I concurred, but I can't help feeling that initially the car is decelerating faster than the occupant, which is why the dashboard and is getting noticeably closer (216mm) and at the same time the seat-belt is getting tauter?
    I'm saying your calculations and premise on which they are based in post#21 are incorrect. No more no less than that.

    For the record I used Kim's timing of 12mS because I assumed he has researched the subject.

    Frankly Kim you have worn me out. So at this stage whatever you think is a fair thing is good enough for me.

  4. #29
    1000+ Posts Kim Luck's Avatar
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    Just don't forget to wear your seat-belt!
    Don't it always seem to go that you don't know what you've got 'til it's gone............

  5. #30
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    Quote Originally Posted by Kim Luck View Post
    Just don't forget to wear your seat-belt!
    I won't and I'll make it is tight.

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    Quote Originally Posted by Kim Luck View Post
    Can I point out to those who suffer from a bit of myopia that the webbing used in ALL seat belts is the same width, 50mm, has the same stretch factor and minimum breaking strain. When an inertia belt locks, which is faster than the blink of an eye, (actually 13 milliseconds,) it becomes a fixed seat belt. Please try and get your head around that fact!
    absolutely correct.
    BUT: it is fixed belt that is _longer_ than a normal 3 point harness owing to the bit wrapped round the spindle. there lies its design weakess.
    I am bewildered as to which bit of this is hard to follow. Try harder :-)

    cheers! Peter

  7. #32
    1000+ Posts Kim Luck's Avatar
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    The spindle is about 10mm diameter, maybe more, and how many turns of the belt does it have on it? Depends on how fat the occupant (or as we say down here, the victim) is. The belt may have four or five turns on the spindle when it locks. At 10mm dia, each turn is approximately 31.42mm and at four turns it will be approximately 125mm of webbing plus the webbing thickness per turn. Now the stretch factor , which is not known as we speak, may be up to 10 to 15 percent over that, which now becomes say 137.5 mm. So the end result of all this tightening up is that out of the original 305 mm that the car was going to stop in we have used 137mm in the seatbelt anchorage and we were traveling at 125mm/ms before the inertia reel locked and it's now looking like a total of 262mm out of that 305 mm available has been used up before we all stop completely!
    Don't it always seem to go that you don't know what you've got 'til it's gone............

  8. #33
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    my overall feeling is that if things like this seem simple with a definite answer, that is usually an illusion. typically, there are complexities involved which can have a significant impact on the outcome, and which are not necessary obvious to a casual reasoning process. hence the extensive use of film crash tests. so i am not adamant about anything i am saying, and only commenting to contribute to an interesting discussion, not to assert the truth of the matter.

    kim: re your calcualation. your conclusion that a body will travel 210mm in that 13ms, is based on the assumption that the 60km/h is the speed differential between said person, and the car. that, in turn, assumes that the car stops instantly at time 0, and the belt locks instantly at time 13ms. both of these are unwarranted, particularly the former assumption.

    i think rob was alluding to this already when he asked what the car was doing at time 13ms. the question is when does that 13ms start? it is obviously after the instant of first contact, but as the impact takes a measurable amount of time to occur, the start time is part way through the impact. it is quite likely that by the time the belt locks, the car is in fact still moving and the speed differential between you, and it, is still much lower than the you assume. and that the distance travelled is therefore likely less than the 216mm you noted. without running off to scour youTube, my sense is that such forward movement is not evinced in crash test video.

    re the belt stretching due to the reel: it seems intuitively beyond doubt that the belt must lengthen due to the unused part of the belt tightening on the reel. and that could be quite a bit of belt, given how far an intertia reel can extent beyond the size of an average person, to accommodate a particuarly large one. how much extra belt does that equate to? that must be a known amount. quite possibly it isnt actually very much?

    peter, are you aware of any source which quantifies that? there can surely be few things which are more studied than bodies in car collisions. there must be thousands of crash test video showing this explicitly.

    if, say, i assume there is an extra 100mm of belt let out due to tightening on the reel (which i would think an overly pessimistic assumption), and that extra 100mm is evenly distributed over the sash and the lap portion of the belt, then my rough calculation suggests the additional forward extension of the belt would be around 25mm. ie very little. given that your body is a floppy, soft and squishy thing, i reckon that 25mm is somewhat lost in the statistical noise as regards its significance for you safety. i am more than happy to explain the reasoning there, but just trying to avoid prolixity in an already long post.

    i would also think that the time taken for that stretching to occur tends to slow down the collision as if affects you, and therefore reduces the peak G applied to you by the belt. if it doesnt cause you to strike part of the car, which you would otherwise have not, that could be of benefit. without empirical evidence, i dont see that it is possible to have a firm view on the subject.

    consider this: being strapped in very firmly by, say, a 5 point harness, may seem like a great idea but it will increase the max G applied to you as you decelerate (ie due to the lack of forward movement against a lengthening belt and/or your body deforming against a 3 point harness). it also creates problems with the head flying forward and down, hence the use of those rear helmet straps used in race cars. it also keeps your body more upright, which i read causes other problems in roll overs in cars not designed with sufficient roof strength. [ie squashing your head/neck when a less restrictive belt might have allowed you to fall sideways]. so while being strapped in tighter seems intuitively safer, there are, as noted, a variety of other considerations which make it hard to say definitively which is better in a passenger car.

    from a different angle, and just fleshing out what i said earlier, my feeling is that if intertia reel belts really were fundamentally and significantly less effective than fixed belts, then it would be surprising if they had been allowed in cars for some decades. it would unquestionably be a known fact, given the amount of videod crash tests. i say that irrespective of pragmatic real world arguments about the likelihood of users not tightening fixed belts properly anyway (which i would think to be an almost universal problem.)

    personally, i would be opting for an intertial reel belt over a fixed 3 point, because they are far more convenient, will always be fitted optimally (within their own context) if the occupant is using it at all, and the nuances of fixed vs intertial reel from a safety point of view, are too complex to have firm preference.
    Last edited by alexander; 17th August 2012 at 04:42 AM.

  9. #34
    1000+ Posts jo proffi's Avatar
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    Interesting to note the fuego floor attatchment has a long metal loop that would surely deform and let more belt out in an accident.
    This is extra to the slack in the coil being taken up.
    Jo

  10. #35
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    and so too the floor attachment on the non reel end of the belts in my jeep. i was also struck that it would likely bend up in a prang.

  11. #36
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    I wonder if collar bone breakages have decreased over the years since IR seatbelts have been introduced.

    Jo

  12. #37
    1000+ Posts J-man's Avatar
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    Isn't it beneficial for the seat belts to have some give, to break the fall so to speak or reduce the impact or stress on the body as it lunges against the seat belts ? That's why seat belts often have "rip stitching" that's designed to give a few centimeters upon impact.
    Compare it with a Dodge Phoenix crashing into a modern car. The occupants of the Dodge usually receive more injury due to the impact because the car doesn't give or crumple, where as a modern car progressively crumples, slowing the vehicle down more gradually, reducing the injury to occupants. Like jumping off a chair onto solid ground compared to jumping off a chair onto a cushion.
    cheers,

    John

  13. #38
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    Quote Originally Posted by jo proffi View Post
    I wonder if collar bone breakages have decreased over the years since IR seatbelts have been introduced.

    Jo
    It's all explained here: http://hyperphysics.phy-astr.gsu.edu/hbase/seatb.html

    Make sure you read all the pages.

    The stretch in the belt limits the Gs applied to the body. As will airbags. Pretensioners help to reduce the inertia "lock up time".

    Collar bone breakages happen because of loose belts and body gaining inertia because they are not "part of the car".

    I believe the best seat belt is a, manual correctly adjusted four point harness, the type with the central round locking buckle. because:

    The belt webbing are normally wider - spreading the load over a larger area with less force/unit area
    The belt can be worn tighter - less movement of the body
    The belts spread the load to both shoulders and both hips.

  14. #39
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    that all sounds quite sensible and i think i would prefer a 4 point harness too, BUT i do note again something i read in relaiton to multi point harnesses: if they hold you more rigidly in the car, a collapsing roof then becomes a bigger threat to you. i can see this is a real danger in an old car like the one in question in this thread.

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    Quote Originally Posted by alexander View Post
    that all sounds quite sensible and i think i would prefer a 4 point harness too, BUT i do note again something i read in relaiton to multi point harnesses: if they hold you more rigidly in the car, a collapsing roof then becomes a bigger threat to you. i can see this is a real danger in an old car like the one in question in this thread.
    Ah! So you want to get on to statistics and probability?

    What is probably of an older car having a roll-over accident versus a head-on accident or a side impact accident? And what statistical chance that a serious injury will result?

    Now that would get complicated.

    Like many situations in life there is no one -solution-suits-all.

    PS- You certainly won't draw me in this discussion - because I dislike statistics as a subject- science.

  16. #41
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    Quote Originally Posted by robmac View Post
    Collar bone breakages happen because of loose belts and body gaining inertia because they are not "part of the car".
    I'm not arguing that point, but my colision with the tree was anticipated so the belt was very tight. I was just 18 years old and full of confidence and doing the rally thing.
    I was not aware of how much a head on with a tree hurts.
    The seatbelt was very firm, BUT.......it was a coupe so the seatbelt attachment point was bellow or at the shoulder, not higher than the shoulder as it is on modern cars.

    Jo

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    Quote Originally Posted by robmac View Post
    PS- You certainly won't draw me in this discussion - because I dislike statistics as a subject- science.
    I'm impressed by some of your recent attempts to not engage with some of our resident experts on everything.....

    As you say, it becomes tiring.

    Jo

  18. #43
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    Quote Originally Posted by robmac View Post
    Ah! So you want to get on to statistics and probability?

    What is probably of an older car having a roll-over accident versus a head-on accident or a side impact accident? And what statistical chance that a serious injury will result?

    Now that would get complicated.

    Like many situations in life there is no one -solution-suits-all.

    PS- You certainly won't draw me in this discussion - because I dislike statistics as a subject- science.
    gosh, cagey or what!
    i am not trying to daw you into into any discussion. i am just pointing out a risk of having a 4 or 5 point harness in the wrong car.

  19. #44
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    Quote Originally Posted by jo proffi View Post
    I'm impressed by some of your recent attempts to not engage with some of our resident experts on everything.....

    As you say, it becomes tiring.

    Jo
    I'm happy to discuss (and be educated on) the science of almost anything.

    However, the caveat is that other party/parties believe and have a basic understanding of the science.

    It becomes the "singing to pig" scenario otherwise.

  20. #45
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    Quote Originally Posted by Kim Luck View Post
    The spindle is about 10mm diameter, maybe more, and how many turns of the belt does it have on it? Depends on how fat the occupant (or as we say down here, the victim) is. The belt may have four or five turns on the spindle when it locks.
    <snip>!
    small piece of empirical evidence (both as set for average sized me):
    old fj55 landcruiser and newish forester each had about 2 feet (60cm) of unused belt on the reel

    cheers! peter

  21. #46
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    [QUOTE=alexander;1092623]
    <snip>
    without running off to scour youTube, my sense is that such forward movement is not evinced in crash test video.

    *the pics that i have seen show head well into airbag so rather a good chance of "head into hard bits if not for airbag" would be my judgement*

    re the belt stretching due to the reel: it seems intuitively beyond doubt that the belt must lengthen due to the unused part of the belt tightening on the reel. and that could be quite a bit of belt, given how far an intertia reel can extent beyond the size of an average person, to accommodate a particuarly large one. how much extra belt does that equate to? that must be a known amount. quite possibly it isnt actually very much?

    peter, are you aware of any source which quantifies that? there can surely be few things which are more studied than bodies in car collisions. there must be thousands of crash test video showing this explicitly.

    *I don't know what the stretch ratio is for various loads but, as noted elsewhere, the amount of extra belt is around 60 cm*


    <snip>

    i would also think that the time taken for that stretching to occur tends to slow down the collision as if affects you, and therefore reduces the peak G applied to you by the belt. if it doesnt cause you to strike part of the car, which you would otherwise have not, that could be of benefit. without empirical evidence, i dont see that it is possible to have a firm view on the subject.

    consider this: being strapped in very firmly by, say, a 5 point harness, may seem like a great idea but it will increase the max G applied to you as you decelerate (ie due to the lack of forward movement against a lengthening belt and/or your body deforming against a 3 point harness). it also creates problems with the head flying forward and down, hence the use of those rear helmet straps used in race cars. it also keeps your body more upright, which i read causes other problems in roll overs in cars not designed with sufficient roof strength. [ie squashing your head/neck when a less restrictive belt might have allowed you to fall sideways]. so while being strapped in tighter seems intuitively safer, there are, as noted, a variety of other considerations which make it hard to say definitively which is better in a passenger car.

    *All sensible enough but it's a trade-off. Stretch would be good if only it didn't lead to hitting hard bits with one's face. Collapsing sideways in a rollover has merit but not if it means moving laterally in a side smack. (As an aside, of my three 4-point harnesses, the Moke has a full cage, the 4CV a half cage & the Djet supposedly has rollover stiffening & gusseting - still a bit of a worry though; it also has a crap belt path over the shoulder to a lower mount.) *

    from a different angle, and just fleshing out what i said earlier, my feeling is that if intertia reel belts really were fundamentally and significantly less effective than fixed belts, then it would be surprising if they had been allowed in cars for some decades. it would unquestionably be a known fact, given the amount of videod crash tests. i say that irrespective of pragmatic real world arguments about the likelihood of users not tightening fixed belts properly anyway (which i would think to be an almost universal problem.)

    *As earlier remarked, my understanding is that legislation took usage practices into account - bad belt auto-adjusted beats good belt ill-adjusted by most.*

    personally, i would be opting for an intertial reel belt over a fixed 3 point, because they are far more convenient, will always be fitted optimally (within their own context) if the occupant is using it at all, and the nuances of fixed vs intertial reel from a safety point of view, are too complex to have firm preference.

    *odd, given that, as a thoughtful bloke, you'd adjust it properly :-)
    I think that you would enjoy a read of the "Dog & Lemon Guide" anticle; it sounds like a "webbing grabber" belt is the one for you.

    cheers! peter*
    Last edited by 4cvg; 19th August 2012 at 09:11 PM.

  22. #47
    1000+ Posts Kim Luck's Avatar
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    [QUOTE=4cvg;1093194]
    Quote Originally Posted by alexander View Post
    <snip>
    without running off to scour youTube, my sense is that such forward movement is not evinced in crash test video.

    *the pics that i have seen show head well into airbag so rather a good chance of "head into hard bits if not for airbag" would be my judgement*

    re the belt stretching due to the reel: it seems intuitively beyond doubt that the belt must lengthen due to the unused part of the belt tightening on the reel. and that could be quite a bit of belt, given how far an intertia reel can extent beyond the size of an average person, to accommodate a particuarly large one. how much extra belt does that equate to? that must be a known amount. quite possibly it isnt actually very much?

    peter, are you aware of any source which quantifies that? there can surely be few things which are more studied than bodies in car collisions. there must be thousands of crash test video showing this explicitly.

    *I don't know what the stretch ratio is for various loads but, as noted elsewhere, the amount of extra belt is around 60 cm*


    <snip>

    i would also think that the time taken for that stretching to occur tends to slow down the collision as if affects you, and therefore reduces the peak G applied to you by the belt. if it doesnt cause you to strike part of the car, which you would otherwise have not, that could be of benefit. without empirical evidence, i dont see that it is possible to have a firm view on the subject.

    consider this: being strapped in very firmly by, say, a 5 point harness, may seem like a great idea but it will increase the max G applied to you as you decelerate (ie due to the lack of forward movement against a lengthening belt and/or your body deforming against a 3 point harness). it also creates problems with the head flying forward and down, hence the use of those rear helmet straps used in race cars. it also keeps your body more upright, which i read causes other problems in roll overs in cars not designed with sufficient roof strength. [ie squashing your head/neck when a less restrictive belt might have allowed you to fall sideways]. so while being strapped in tighter seems intuitively safer, there are, as noted, a variety of other considerations which make it hard to say definitively which is better in a passenger car.

    *All sensible enough but it's a trade-off. Stretch would be good if only it didn't lead to hitting hard bits with one's face. Collapsing sideways in a rollover has merit but not if it means moving laterally in a side smack. (As an aside, of my three 4-point harnesses, the Moke by a full cage, the 4CV a half cage & the Djet supposedly has rollover stiffening & gusseting - still a bit of a worry though; it also has a crap belt path over the shoulder to a lower mount.) *

    from a different angle, and just fleshing out what i said earlier, my feeling is that if intertia reel belts really were fundamentally and significantly less effective than fixed belts, then it would be surprising if they had been allowed in cars for some decades. it would unquestionably be a known fact, given the amount of videod crash tests. i say that irrespective of pragmatic real world arguments about the likelihood of users not tightening fixed belts properly anyway (which i would think to be an almost universal problem.)

    *As earlier remarked, my understanding is that legislation took usage practices into account - bad belt auto-adjusted beats good belt ill-adjusted by most.*

    personally, i would be opting for an intertial reel belt over a fixed 3 point, because they are far more convenient, will always be fitted optimally (within their own context) if the occupant is using it at all, and the nuances of fixed vs intertial reel from a safety point of view, are too complex to have firm preference.

    *odd, given that, as a thoughtful bloke, you'd adjust it properly :-)
    I think that you would enjoy a read of the "Dog & Lemon Guide" anticle; it sounds like a "webbing grabber" belt is the one for you.

    cheers! peter*
    Peter, we all agree that seat belts give us an extra layer of protection from harm that didn't exist before they were invented. I drove cars before they were invented, like my father, mother etc. It seems the best recipe for reducing pain in a collision, (there are NO accidents) is to avoid them in the first place. All our Car Clubs would have this as their first philosophical principle and hopefully are still passing on the philosophy!
    Don't it always seem to go that you don't know what you've got 'til it's gone............

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    [QUOTE=Kim Luck;1093234]
    Quote Originally Posted by 4cvg View Post

    <snip>

    I drove cars before they were invented,

    <snip>:
    hmm! remarkable

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    Quote Originally Posted by 4cvg View Post

    hmm! remarkable
    you think that is remarkable? his parents were driving them before him!

  25. #50
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    That would be a casual slip of the pen leading to phrasing that only a dimwit would take out of context. Sorry, two dimwits!
    Don't it always seem to go that you don't know what you've got 'til it's gone............

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