Seatbelt weight limit?

[Softus]

What's your problem softus, you seem to be very insecure in that if you don't agree with something I write, you get offensive and personal.

Whether or not I am insecure is a personal matter, and not for either you or this forum, but one problem I have is the manner of some of your postings. You are the only person on any forum who has accused me of being abusive, and that's a claim that you can't make.

Do you have another forum name too, by any chance?

No. But if I did, then what incentive does your curious id imagine that I would have for telling you?

 

[AdamW]

if you have an 8 stone not too strong person things would be very uncomfortable.

Serve 'em right for being a lightweight...

At risk of sounding like a girl, "You're both right".

With a fixed seatbelt or just a plain old inertia real, softus is right as the seatbelt doesn't actually pull you into the seat.

HOWEVER, we now have seatbelt pretensioners, lovely little squib devices that pretension the seatbelt with a bang. These would have to be designed with a particular weight range in mind. If you built those to take 35 stone of girth then that would pull too tight for the 8-stone weakling. So to add some Newtonian physics, Force =mass x acceleration. With the greater mass of the fat bloke then a greater force is needed to counteract the decelerative effects of driving into a wall. So, looking at the whole seatbelt system (rather than the belt and reel subsystems), Oilman is right.

 

[Softus]

Hi AdamW

Thanks for gently coaxing us back towards the nub of the question


Newton's second law of motion (F=ma) is indeed a good starting point, as it tells us that the force applied during deceleration is directly proportional to the mass being decelerated.

A useful rule to keep on the sideplate here is Newton's third law: "Every action has an equal and opposite reaction".

So, a seatbelt rated for a particular maximum mass (aka weight, to use a term that is wrong for the purist but is colloquially accepted) will therefore cope with any mass UP TO that maximum, because the reactive force that it needs to provide (and sustain) is less for a light person than for a heavy person.

For some readers it might be interesting to consider the 'impulse', alternatively known as 'change in momentum'. Impulse is easily derived from the second law (above), and is calculated as the (force x duration of deceleration), where the force is the decelerative force being applied by the seatbelt.

Since the force reduces in proportion to the mass of the seatbelt wearer, the impulse also reduces in proportion. I acknowledge my assumption that the duration [of deceleration] is the same for any mass of seatbelt wearer, but this is not an unreasable or unrealistic assumption, IMO.

So, where does all this lead us? It means that the force felt by a big strapping bloke, applied by a given seatbelt, is greater than the force felt by a weeny little grandmother. This holds true regardless of any pre-tension in the belt, which is put there to reduce the slack in the belt and therefore start the process of deceleration more gently (rather than let the wearer 'hit' a slack belt).

If all the above is true, then it is strictly wrong to say that seatbelts are more hazardous for lighter people. There may an advantage in being overweight though, since layers of fat are capable of acting as padding and so reduce the amount of soft tissue damage, or, in extreme cases, the risk of a broken rib or breastbone.

Any comments on this? For the benefit of oilman, please resist any temptation you might feel to introduce gorillas at this juncture.

 

[AdamW]

Yes: you have trivialised Newton's Third Law Nitpicking I know, but we physicists prefer something more along the lines of "For every force, there is a force of equal magnitude and opposite direction", or simply "the net impulse of any interaction is zero". As I said, nitpicking

I wasn't too sure on how pretensioners work, so I did a quick google and came up with something interesting... apparently lots of seatbelts have a load limiter too, which limits the force that the seatbelt can exert on the passenger by letting the belt unwind a bit (happens after the pretensioner, and allows the airbag to do some work).

So, as these would appear to be relatively crude devices (it doesn't take into account the mass of the passenger in the seat), they will have one preset load limit. Now, as you have brought up impulse let's use that. J=Ft. I would agree with your assumption that for any given crash, the duration and hence decelleration would be the same, so by rearranging we come up with simply F is proportional to passenger mass.

By designing a seatbelt load limiter that is suitable for a gorilla (couldn't resist!) then we will have something that is useless for people of a less pie-hungry nature. By having something only suitable for "normal"-sized people, then we'll find very fat people wondering why they have a mouthful of windscreen.

So again, I would have to say that depending on what kind of seatbelt system you look at, either of you could be right!

 

[Softus]

Interesting stuff, AdamW. I certainly agree that, if we retrofit the load limiter to oilman's postulate, he is completely right. And he seems like a chap sorely in need of the benefit of the doubt....

...we physicists prefer something more along the lines of "For every force, there is a force of equal magnitude and opposite direction"...

Surely not for every force? What about a force that results in the acceleration of an object? Haven't you omitted from the full 3rd law definition the qualification that the system must be in equilibrium? Oh if only I were one of you physicists

Regarding the effect of the load limiter, our conjecture may be leading us into a dense forest, at night, with a fading torchlight

Notwithstanding that guileless quest for the Blair Witch, surely, if the limiter doesn't activate for reason of the passenger being 'too' light (in weight), then this would be the precise intention of the designer of the load limiter? If so, then the risk of injury to the lighter person must be considered (by the designer) to be small[er].

Surely?

 

[oilman]

I hadn't intended to imply any active devices being built into the seat belt mechanism, it's simply that the belts will always stretch when a force is applied (materials which don't deform are brittle). The belt (without any pretensioner built in) will therefore result in reduced stretch for a lighter person than a heavier one. Now you could say the force on the lighter person will be less, as the weight was less, but in stopping faster, the individual organs will have a greater deceleration applied than in the heavier person. This is ignoring the extra cushioning the heavier person's organs may also have depending on how the weight is distributed).

If you take a more obvious example, child seat belts are built of much lighter material so they give the right force to decelerate a lighter body at a safe rate. A bit odd in some respects since children can survive falls that would severely damage an adult.

I have just found this. It.s not somuch a problem of total force on a person, and a lot of the damage is prevented by air bags nowadays, but deaths caused by organ rupture would be more likely where the deceleration rate is higher.

 

[Softus]

I hadn't intended to imply any active devices being built into the seat belt mechanism, it's simply that the belts will always stretch when a force is applied (materials which don't deform are brittle).

Actually, materials that don't deform are "hard"; "brittle" materials are just not very elastic. There's a distinct difference, although some materials that are hard are also brittle.

The belt (without any pretensioner built in) will therefore result in reduced stretch for a lighter person than a heavier one.

I absolutely agree with this bit of logic, although it would be more normal to say "less" rather than "reduced".

Now you could say the force on the lighter person will be less, as the weight was less, but in stopping faster, the individual organs will have a greater deceleration applied than in the heavier person.

Nope - this is where you lose me. You say "in stopping faster", but where does that come from?

We've agreed that the stretch of the belt will be proportional (probably not exactly so, since the belt is a composite material) to the mass of the passenger. However, if I've interpreted your theory correctly, you believe (and I use the religious meaning of the word, since your postulate is not scientific, despite kindergartenphysics.com, or whatever web site you think is backing you up), that the stretch for a heaver person is so great, and the stopping distance thereby gained is so significant, that the impulse on the heaver person becomes less than that on the lighter person.

So far, have I interpreted your ground-breaking theory correctly?

If you take a more obvious example, child seat belts are built of much lighter material so they give the right force to decelerate a lighter body at a safe rate. A bit odd in some respects since children can survive falls that would severely damage an adult.

The fact that child seat belts are of a different material is not relevant to the basic point being debated, but what does "give the right force" mean? To me there's nothing "obvious" about the point you're making - "obscure" is the word that I would choose.

To re-iterate, the reactive force of the seat belt is proportional to the mass of the passenger. This means that the force of the belt on a lighter passenger is less than the force of the belt on a heavier passenger, and this holds true for ANY given belt up to the point of snapping.

I have no grasp of the intuitive reasoning that you're using to reach your conclusions. It seems to be based on oilman's first law of forums: "oilman is never wrong"; the second law, of course, would be "oilman is always right".

 

[oilman]

I could explain, and partway through your post I was prepared to do so, however since you are determined to have a dig rather than reach an understanding, or alternatively explain to me where my view is at fault in a rational way, I don't see the point.

If this was a political topic it's fair game for argument, attack and defence, but there is a lack of understanding on one side or the other of what is a matter of mechanics/physics which could be worked through for a satisfactory understanding.

You are obviously a great man..............

 

[Softus]

Nope - not great, just an ordinary bloke. But at least I'm not disingenuous, and I can take it as well as dish it out, unlike some....

 

[oilman]

Nope - not great, just an ordinary bloke. But at least I'm not disingenuous, and I can take it as well as dish it out, unlike some....

You have been "dishing it out" since YOU decided I had committed a heinous crime by using the term gorilla, you wouldn't know whether you "can take it" since I have not continually tried to ridicule every post you make.

I will make the unwarranted assumption that you are either old and grumpy, or you are comparitively young, and you haven't yet cottoned to the possibility you might do better by not going on so.

Just so you have a handle and can have another dig, please list all the posts in which I have made personal attacks on you.

 

[petewood]

Zzzzzzzzzzzzzzz.

 

[Softus]

You have been "dishing it out" since YOU decided I had committed a heinous crime by using the term gorilla, you wouldn't know whether you "can take it" since I have not continually tried to ridicule every post you make.

Ouch oilman. Beat me no longer, I cannee take it

If you haven't found any reason to criticise me, which I agree is an unlikely occurrence, then I guess you'll have to take it on trust that I can accept criticism. It seems rather unlikely that I've ridiculed all of your posts, since you've been a forum member for quite a bit longer than me. Maybe if you don't want to be ridiculed you shouldn't post ridiculous comments? Or ridicule other people?

I will make the unwarranted assumption that you are either old and grumpy, or you are comparitively young, and you haven't yet cottoned to the possibility you might do better by not going on so.

I guess that's your oblique way of asking me my age, which is no secret: 45. I'm not sure if that's old or young though. Or whether it really matters.

I'm open to all possibilities oilman, including the existence of a wild and crazy universe in which you're right and I'm wrong. It just seems so implausible though, don't you think?

Just so you have a handle and can have another dig, please list all the posts in which I have made personal attacks on you.

Thanks, but no thanks - I'm sure that I have quite enough handles (whatever they are) for my current purposes. FYI, I'm not aware of any personal attacks on me, by anyone on these forums, including you oilman. However, if you have your heart set on receiving a dig, then just write something that sounds vaguely scientific, but that is actually quite stupid, or perhaps have a go at some ethnic or social minority group, or issue a pompous put-down to a novice punter - any of those things will get a response from me. Probably.

Or, if you'd prefer to have the last word, which is something I've observed that people who are in the wrong often do, then simply append some kind of keyword to your posting, perhaps the tag "<last word>", or "<so there>", to signal that we've finished that thread, and I'll happily ignore your comments.

 

[oilman]

How wonderful and comforting it is to have such a contientious guardian of the public morals on the forum, to ensure I am kept in my place (for I'm sure there is no one else who has transgressed so).

You could become a moderator then you would be able to have even more power to force the world to conform to your vision. I suspect your time on the forum will be relatively short though as you are not as sad a case as me.

 

[Softus]

How wonderful and comforting it is to have such a contientious guardian of the public morals on the forum, to ensure I am kept in my place (for I'm sure there is no one else who has transgressed so).

Again with the sarcasm - don't you ever get tired of it? Can it really be the case that you don't know how meaningless it is?

You could become a moderator then you would be able to have even more power to force the world to conform to your vision.

No thank you. I'd like to say that it's nice of you to suggest it, but you aren't being nice.

I suspect your time on the forum will be relatively short though as you are not as sad a case as me.

So be it. It would be an exquisite jab of irony for you to be right for once.

 

[AdamW]

So be it. It would be an exquisite jab of irony for you to be right for once.

I've figured it out... Stephen Fry has joined our hallowed forum at last!