Vehicle mass: What effet does it have in an accident?

[geckoGT]

Originally Posted by Brazen

Instead of ANCAP smashing cars to an imoveable wall- which is the equivalent of a Fiesta hitting an oncoming Fiesta and a Patrol hitting an oncoming Patrol.

They should smash the car into a moveable object which simulates the average car, ie the Fiesta is now hitting an object the weight of a Commodore and the Patrol is now hitting an object the weight of a Commodore.

I think it would be scary to see the difference in ratings...There is probably a bit of social pressure to not do this, as it would just encourage people to buy heavier and heavier cars.

You may have a very good point there as it would change things to a degree, the problem is as your mass and speed increses, your chance of survival decreases.

By that I mean all cars are built to withstand a certain force of impact and maintain good survivability. So a 5 star car maintains good survivability at a equivalent force of impact at 64 km/h. Once you go over that speed or increase the force from that level, chances of survival rapidly deteriorate as the structural integrity and crash protection of the vehicle is exceeded. Similarly if you were to keep the speed the same but increase the mass, perhaps 200 kg of cargo, your chance of survival decreases.

In this situation the vehicles own mass works against it in terms of vehicle safety. Think of the passenger compartment as the critical zone, the more mass in front and the less behind the critical zone, the less force that will push through the critical zone. The more mass behind the critical zone, the more force that will push through it once vehicle structure integrity has failed. Think of it this way, large rear wheel drive vehicles have a greater percentage of mass behind the passenger cell, small FWD have a higher percentage of mass in front of the passenger cell.

In a stark and over exagerated demonstration of this to illustrate a point, review these two clips.

The first is a smart car into a solid concrete barrier at high speed of 70 mph (112 km/h).
Smart Car Crash

Notice how on impact the car changes direction and actually bounces back off the wall without the back of the car pushing through the passenger cell, even to the point that the doors can still be opened. I would not want to be the occupant in this vehicle but considering the maintained integrity of the passenger cell, survival is possible.

In this next video it is a heavily loaded dual cab truck That was actually used as a wall test for future crash testing. In this video the test was apparently conducted at 65 mph (104 km/h).
Truck Crash

In this crash you will note how the cab completely fails, the fatality is almost certain as the heavily loaded back continues to move forward. With such momentum that it plants the truck against the wall and keeps pushing until there has been enough crumple zone to slow it, which occurred way past the passenger cell.

Now I will concede that the are many inequalities in the examples I have given, most notably it is a 1980's vehicle with 1 star rating if you are generous against a 2000's vehicle with a 5 star rating. Also the wall is of different construction but the smart car wall did maintain enough integrity to bounce the car.

Having stated those inequalities, I still think these videos do demonstrate the concept that mass behind the passenger cell can work against the aim of crash survival when the speed is enough to overcome the designed safety features of that vehicle, when crashed into a solid immovable object. Albeit to a spectacular level and not a level commonly seen on the street.

[geckoGT]

Originally Posted by cheap

Thanks for the ANCAP link, here are some interesting FAQ's

Question: If a large, heavy sedan and a small, light sedan both receive five stars and the same ANCAP scores, is the large sedan safer for the occupants than the small sedan?

Answer: It is not appropriate to compare ANCAP ratings across vehicle categories, particularly if there is a large weight difference. The reason is that in car-to-car crashes, the heavier vehicle has a theoretical advantage (due to the physics of the crash). Similarly, a higher ride height might be an advantage in a car-to-car crash. However in single vehicle crashes, such as with solid fixed objects, the weight might no longer be an advantage. So it depends on the type of crash. Also some small cars do remarkably well in crashes with larger vehicles as they have very strong passenger compartments and advanced occupant restraint systems and these features make up for the mass disadvantage.

Question: Is it better to have a small car with a 5 star rating or a medium car with a 4 star rating?

Answer: It is not appropriate to compare ANCAP ratings across vehicle categories, particularly if there is a large weight difference. The reason is that in car-to-car crashes the heavier vehicle has a theoretical advantage (due to the physics of the crash).. Similarly, a higher ride height might be an advantage in a car-to-car crash. However in single vehicle crashes, such as with solid fixed objects, the weight might no longer be an advantage. So it depends on the type of crash. Also some small cars do remarkably well in crashes with larger vehicles as they have very strong passenger compartments and advanced occupant restraint systems and these features make up for the mass disadvantage.

Question: Can ANCAP results be used to compare the relative safety of the vehicles tested?

Answer: ANCAP results can be used to compare the protection offered to occupants in the event of a severe frontal and side crashes for vehicles of similar size and weight. [B]Care must be taken when comparing results for different vehicles as only those vehicles of similar mass can be correctly compared. As a heavier vehicle will generally provide better protection in a collision with a smaller and lighter car, any result comparison should be restricted to cars of a similar class. To assist with the comparison, ANCAP publishes the kerb weight of the cars tested.

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So if you're about to have a collision. Vehicle A is a 5 Star ANCAP rated 4WD/2600Kg and vehicle B is a 5 Star ANCAP rated Focus/1300Kg, both traveling at 50kph and they're about to have an unavoidable head-on crash with each other.

Which car would rather be in, the 4WD or the Focus?

Newton's 2nd law (momentum) is such a wonderful thing.

I think those points are excellent ones and I do not see how they could be countered in that situation, except on a few counts.

One is many have posed the question that due to lesser braking, slower acceleration and decreased handling, are you more likely to be in a crash in the first place in the 4WD?

Also most automotive buyer are constrained by budget and it is more difficult to buy a large 4WD with a 5 star rating, a lot easier to buy a hatch with 5 stars. For example you have $30k, you could buy a 5 star Subaru Imprezza or a 4 star Great Wall X240, cut the budget down to $20k and you can still have a Fiesta but you would have to go for an old 3 star 4WD.

I guess my point is it is just not that simple that vehicle safety comes down to purely mass and size.

[sudszy]

Originally Posted by geckoGT

Similarly if you were to keep the speed the same but increase the mass, perhaps 200 kg of cargo, your chance of survival decreases.

In this situation the vehicles own mass works against it in terms of vehicle safety........

In some cases yes, but if you are a very safe driver then you most likely collision scenario is not going to be your fault and will most likely happen when you are rear ended by some numbnut tailgating, then it will work for you.

[cheap]

Originally Posted by geckoGT

I think those points are excellent ones and I do not see how they could be countered in that situation, except on a few counts.

One is many have posed the question that due to lesser braking, slower acceleration and decreased handling, are you more likely to be in a crash in the first place in the 4WD?

Also most automotive buyer are constrained by budget and it is more difficult to buy a large 4WD with a 5 star rating, a lot easier to buy a hatch with 5 stars. For example you have $30k, you could buy a 5 star Subaru Imprezza or a 4 star Great Wall X240, cut the budget down to $20k and you can still have a Fiesta but you would have to go for an old 3 star 4WD.

I guess my point is it is just not that simple that vehicle safety comes down to purely mass and size.

The laws of physics are not good enough, ANCAP's own explanation isn't sufficient and the numerous responses from the informed members are they an inconvenience?

Now you want to divert to vehicle dynamics and financial constraints.

You don't want to concede do you?

[sudszy]

Originally Posted by cheap

The laws of physics are not good enough, ANCAP's own explanation isn't sufficient and the numerous responses from the informed members are they an inconvenience?
?

Im unable to follow what the above is about(even after reading your previous posts), what case are you arguing/ disputing?

[geckoGT]

Originally Posted by cheap

The laws of physics are not good enough, ANCAP's own explanation isn't sufficient and the numerous responses from the informed members are they an inconvenience?

Now you want to divert to vehicle dynamics and financial constraints.

You don't want to concede do you?

I guess that is what happens when you have such a broad question.

[AussieAV]

Originally Posted by sudszy

Im unable to follow what the above is about(even after reading your previous posts), what case are you arguing/ disputing?

Fairly obvious isn't it? Cheap is saying that since gecko did'nt win the discussion based on physics, ANCAP, or member comments (at least in Cheap's view), Gecko is now resorting to other topics to try and make his point, i.e vehicle dynamics and relative cost of small versus large cars. (again, in Cheap's view)
He finally suggests that Gecko is unwilling to concede that Cheap is right (again, in Cheap's view).

Now I'm not saying I agree or disagree with anyone's posts, just deciphering the strange language Cheap was using for you sudzy - I believe its called English. (To further translate, I have used sarcasm here to have a not too subtle dig and your comprehension skills, since you seem to be implying that Cheap is not making his point clearly, while I can't see how anyone could have diffuculty in understanding it).

Hope I have been able to help in your understanding of the post you were having difficulty with.

[cheap]

Originally Posted by AussieAV

Fairly obvious isn't it? Cheap is saying that since gecko did'nt win the discussion based on physics, ANCAP, or member comments (at least in Cheap's view), Gecko is now resorting to other topics to try and make his point, i.e vehicle dynamics and relative cost of small versus large cars. (again, in Cheap's view)
He finally suggests that Gecko is unwilling to concede that Cheap is right (again, in Cheap's view).

Now I'm not saying I agree or disagree with anyone's posts, just deciphering the strange language Cheap was using for you sudzy - I believe its called English. (To further translate, I have used sarcasm here to have a not too subtle dig and your comprehension skills, since you seem to be implying that Cheap is not making his point clearly, while I can't see how anyone could have diffuculty in understanding it).

Hope I have been able to help in your understanding of the post you were having difficulty with.

If we weren't on the opposite side of OZ - I'd buy you a beer. Well said!

[chevypower]

I would say that it is more inertia more than momentum. Momentum suggests that the higher the velocity, the less mass plays a part. Eg a Ford Fiesta hits a Hummer H1 in the side at 60 km/h. There was no momentum on the part of the Hummer. The Fiesta is still really going to come off much worse than the Hummer. So it's neither force (m x a) or momentum (m x v). It is inertia, which is totally independent from velocity.

[geckoGT]

If you read back through my posts I have actually agreed with cheap in a number of areas and told him that he is right , mass is a strong element.

However, his so called clear win from ANCAP is not as good as he thinks, let me change his highlighting a bit to demonstrate.

If a large, heavy sedan and a small, light sedan both receive five stars and the same ANCAP scores, is the large sedan safer for the occupants than the small sedan?
It is not appropriate to compare ANCAP ratings across vehicle categories, particularly if there is a large weight difference. The reason is that in car-to-car crashes, the heavier vehicle has a theoretical advantage (due to the physics of the crash). Similarly, a higher ride height might be an advantage in a car-to-car crash. However in single vehicle crashes, such as with solid fixed objects, the weight might no longer be an advantage. So it depends on the type of crash. Also some small cars do remarkably well in crashes with larger vehicles as they have very strong passenger compartments and advanced occupant restraint systems and these features make up for the mass disadvantage.

Is it better to have a small car with a 5 star rating or a medium car with a 4 star rating?
It is not appropriate to compare ANCAP ratings across vehicle categories, particularly if there is a large weight difference. The reason is that in car-to-car crashes the heavier vehicle has a theoretical advantage (due to the physics of the crash). Similarly, a higher ride height might be an advantage in a car-to-car crash. However in single vehicle crashes, such as with solid fixed objects, the weight might no longer be an advantage. So it depends on the type of crash. Also some small cars do remarkably well in crashes with larger vehicles as they have very strong passenger compartments and advanced occupant restraint systems and these features make up for the mass disadvantage.

ANCAP insists on a good result in its side pole test for a 5 star occupant protection rating. This requires head-protecting side airbags or curtains. A 4 star medium car might not have this life-saving technology. ANCAP recommends that people choose a 5 star vehicle.

So although weight will give an advantage when the is large weight difference, there are still factors that can negate that advantage and even times that weight and the way it is distributed throughout the car is a disadvantage.

So, to make myself clear as it seems I was not before, yes a heavier car does have an advantage in crash survival if it has similar structural features as the smaller car, in many crash scenarios. A heavy car does not however have a clear win over the small car due to a number of reasons, this is backed up by ANCAP. I am sure they know what they are on about, they do this stuff for a living.

I am sorry, but I think based on my experience, research and the evidence I could never say that a large car has a clear advantage over a small car, it is just not 100% true and it would give people a false sense of security. I would hate to have to admit my mistake as I pull their severely wounded body out of a wreck.

So Cheap, you have your win, but a conditional one.

[sudszy]

Originally Posted by geckoGT

So although weight will give an advantage when the is large weight difference, there are still factors that can negate that advantage and even times that weight and the way it is distributed throughout the car is a disadvantage.

. A heavy car does not however have a clear win over the small car due to a number of reasons, this is backed up by ANCAP. I am sure they know what they are on about, they do this stuff for a living.

I am sorry, but I think based on my experience, research and the evidence I could never say that a large car has a clear advantage over a small car, it is just not 100% true and it would give people a false sense of security. .

Okay, I get what each is saying now. Gecko, did you take in the numbers that I provided for you in this post:http://www.fordforums.com.au/showpos...7&postcount=87

Its slam dunk case for the larger vehicle? given that cheap was seeking comparison with 5star ancap ratings for both vehicles?

You have waxed lyrical about how the smaller car might be more manoeuvrable and make it less likely to be involved in a collision, however, that was never the answer to the question being asked “which was what effect does vehicle mass have in an accident”, not what effect mass or lack has in helping avoid an accident.
How much lower ancap rating could the 4wd go and still be more survivable than the 5star small car?

Its impossible to work out from the ancap ratings, they have not backed up what your claim, would need to look specifically at how it behaves in a front on collision.

Im not sure of how the regs are now, but in terms of australian standards for frontal impacts 4wds in this country were considered as work/commercial vehicles(both tax and safety wise)(Jim Goose raised earlier) and didnt need to meet the same front on as regs as passenger vehicles, so one has to perhaps look at how each individual vehicle presents in terms of the front on testing rather than just the overall ancap rating.

I feel ancap has been deliberately vague in not spelling out the basics numbers and consequences when larger vehicle meets smaller vehicle,.

It is really stuck between a rock and a hard place due which I did allude to in another thread http://www.fordforums.com.au/showpos...1&postcount=68.

[sudszy]

Originally Posted by chevypower

I would say that it is more inertia more than momentum. Momentum suggests that the higher the velocity, the less mass plays a part. Eg a Ford Fiesta hits a Hummer H1 in the side at 60 km/h. There was no momentum on the part of the Hummer. The Fiesta is still really going to come off much worse than the Hummer. So it's neither force (m x a) or momentum (m x v). It is inertia, which is totally independent from velocity.

Yes, I agree, though your point has been confirmed by myself and others a number of times throughout the thread: "there is no substitute for mass in a collision" (let's assume the only gyroscopic devices we have in vehicles are the wheels and the engine)

I do feel however, though you have used the term inertia correctly(a very rare thing in this world), (though inertial mass would be even better) it will confuse the average person as most throw around the terms of inertia and momentum not realising there is a difference, just using "mass" and leaving the term inertia completely out of the discussion is much simpler for the "masses"

[colinl]

Originally Posted by sudszy

Okay, I get what each is saying now. Gecko, did you take in the numbers that I provided for you in this post:http://www.fordforums.com.au/showpos...7&postcount=87however, that was never the answer to the question being asked “which was what effect does vehicle mass have in an accident”, not what effect mass or lack has in helping avoid an accident.
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I think the simplest answer to that is, the more mass involved the greater the amount of energy released. Energy can't just be removed, it has to transfer. In the case of a car crash it can transfer through the car or the people in the car. It does this by converting the kinetic energy of the car into thermal and acoustical energy. What makes the difference from a safety aspect is how easily the car manages this process. The more kinetic energy the car uses to create heat and noise, the less energy transference occurs through you.

So in a single vehicle accident, and forgetting about dynamic safety strategies, the heavier car will be less safe than a small car with the same level of energy dispersion.

[OLDFORDNUT]

Originally Posted by bigsta

Its simple physics really.

First when hitting a immovable object in a bigger car there is more crumple zones and more metal to adsorb the impact as well as room for engine bits and steering racks to go before the force and said steering racks make their way to you.

When a 3t car hits a 1 ton car this is where the physics are really on your side in the big car as your momentum and force is repeatedly adsorbed by the smaller mass vehicle so in a sense you are pushing into them and using their crumple zones where as the small vehicle is experiencing more of a impact with a concrete wall as they are not only adsorbing the impact from their kinetic energy but the big cars as well. This is why its quite common to see in a impact between a 4WD or a truck and a smaller car the 4wd has almost no damage but people are being cut out of the excel.

There is a few variables that can change things such as speed angle of impact etc the worse possible accidents are those that stop you dead on such as a head on or a t bone.

Thank you, i was about to write the same thing.

[GasoLane]

Lets face it. If it's your turn to die, nothing you're driving will stop it.

[302 XC]

^^^^^^
Absolutely bang on

[colinl]

Originally Posted by OLDFORDNUT

Thank you, i was about to write the same thing.

Presuming the larger vehicle does have crumple zones.

[colinl]

Originally Posted by GasOLane

Lets face it. If it's your turn to die, nothing you're driving will stop it.

In the words of Tony Abbott, "**** happens". But often it is a result of a number of factors leading up to it. I tend to enjoy riding motorcycles in a spirited manner, and have come off a few times doing this. I have no doubt that in these cases I would have been very badly injured or killed if I hadn't made the choice to wear full leathers, body armour, decent gloves and boots. It is certainly true that despite everything, you may die; but why increase the risk through making poor choices.

[sudszy]

Originally Posted by colinl

I think the simplest answer to that is, the more mass involved the greater the amount of energy released. Energy can't just be removed, it has to transfer. In the case of a car crash it can transfer through the car or the people in the car. It does this by converting the kinetic energy of the car into thermal and acoustical energy. What makes the difference from a safety aspect is how easily the car manages this process. The more kinetic energy the car uses to create heat and noise, the less energy transference occurs through you.

So in a single vehicle accident, and forgetting about dynamic safety strategies, the heavier car will be less safe than a small car with the same level of energy dispersion.

Colin, I dont think I could have made the case any simpler:
http://www.fordforums.com.au/showpos...7&postcount=87

Colin, in most collisions a lot of the vehicles KE goes into rearranging the metal work. This doesnt guarantee the safety of the occupants, its how effective that process is in reducing the acceleration of the occupant, having them come to a gradual stop, rather than a quicker one by hitting something hard inside the car, like the windscreen.

Ive written about the "energy" thing in another thread:http://www.fordforums.com.au/showpos...0&postcount=54

[colinl]

Originally Posted by sudszy

Colin, I dont think I could have made the case any simpler:
http://www.fordforums.com.au/showpos...7&postcount=87

Colin, in most collisions a lot of the vehicles KE goes into rearranging the metal work. This doesnt guarantee the safety of the occupants, its how effective that process is in reducing the acceleration of the occupant, having them come to a gradual stop, rather than a quicker one by hitting something hard inside the car, like the windscreen.

Ive written about the "energy" thing in another thread:http://www.fordforums.com.au/showpos...0&postcount=54

I tend to agree with you. The better the structure of the car is at dispersing the KE of vehicle over a greater time frame, the less energy is transferred to the occupants. I also agree that a vehicle with more mass when colliding with a vehicle of lesser mass will suffer less deceleration than the smaller car. In the case of a large ridged vehicle colliding with a solid object, then the KE energy will more directly and suddenly be transferred to the occupant. I take your point about the body movement and the force at which it moves and the value of airbags at decreasing the bodies deceleration; but I would tend to think that the better the structure of the car is at dispersing the KE, the less work the airbag needs to do.

[sudszy]

Originally Posted by colinl

I tend to agree with you. The better the structure of the car is at dispersing the KE of vehicle over a greater time frame, the less energy is transferred to the occupants. .

Colin, its apparent from this that you didnt take read the link about the transferring energy to the occupants myth?

Its all about how we quickly we remove the motion or the relatively small amount of kinetic energy from the occupant, we either have the car extend its collision time by crumpling or an big cushion for the occupant.

You still have the idea that if the kinetic energy of a badly designe car is not "absorbed" well by the crumple zone then it will be transferred into the occupant, no.

Perhaps think of what would happen if the same car was crashed with no occupants, using your logic, the kinetic energy instead of being transferred into the occupant would be transferred where?

[geckoGT]

Hey all,

Been seeing all these articles on how a lot of the new small safe cars are better to be in a crash than in say a 4wd and I have question...

so we say for example that a there is a collision between a small say 1Ton car and a 3Ton 4wd at say 100kmph does it mean that the force encountered is the same for both vehicles?

From what I can gather, wouldn't there be say 3 times more force applied to the car than the 4wd? I would expect the 3Ton 4wd to slow down to a stop in this collision and the car to end up going backwords..

Does this sound right/can someone clarify?

Cheer

Lets see if we can get this back on track a bit and then I will try and explain my position a bit as some do not seem to understand (which is ok, perhaps I have not said it very well)

In your original post all your points are very close to the mark if you simplify vehicle safety down to vehicle mass and crash energies involved, you are on the right track. The only thing that I will add is although it is not a case of forces being equal for both cars, it is how they manage that force that matters.

Now the problem with this is overall vehicle safety is no where near as simple as a question of mass and energy, there are many other factors including active crash avoidance systems (ABS, TC, DSC, EBD etc), grip levels (tyre size and performance), weight distribution on both a longitudinal and vertical plane, crumple zones, air bags, seat belts, pre tensioners, glass standards, glass positioning, interior surfaces etc, the list goes on. Now yes the ANCAP rating is a good guide but lets not forget it only tests four different scenarios, drivers side frontal, side, pole and pedestrian. Just because a car performs well at the frontal crash test and scores 5 stars, that does not mean it will perform the same as another 5 star car if the crash speed is 20 km/h more.

The Ancap test also does not test crash avoidance systems, sure you get points for having them but the function of them are not tested so this is of little value.

My primary point here is just looking at the star rating and mass of the vehicle with the associated physics calculations gives an incorrect impression of safety because it is grossly oversimplifying a very complex issue.

Now some of you are thinking what the hell does this have to do with the question? well, a lot actually.

Yes sometimes the big heavy car will be the hands down leader in crash safety, but another 5 star car of similar weight , due to other elements of car safety may perform poorly compared to a car half its weight once forces are outside the manufacturers design. I hope that makes sense.

Suggesting a large car is always safer (even when comparing only 5 star rated cars) gives a false sense of security which I am sorry I can not support. Not all 5 star cars are created equal and mass is not the only factor. The problem we have here is that all the factors are too complex to really discuss here and most of the engineering and physics is way beyond what the lay person will understand (me included).

What I can say with some authority is what I have seen in my profession and the multitude of crashes that I have attended. This is a good time to define what I consider a serious crash. Personally I maintain a occupant based focus and consider a serious crash one that involves a serious injury or fatality. A fatality is obvious but a serious injury is not. Someone that gets taken to hospital with injuries that only require minor assessment and folowup with their GP is not serious injury. Injuries that involve internal structures of the head, thorax, abdomen or involve fractures of 2 or more major bones (eg femur, pelvis, multiple ribs etc) are serious injuries. My observation is that of all the serious crashes that I have attended, only a handful involve a small car (focus size or smaller), the rest were large cars (camry size or larger) or 4WD/SUV. One thing I will concede is the vast majority of serious crashes I have attended involved forces far beyond standard testing. So why is that? I am not sure, perhaps it is something to do with vehicle dynamics, mass and forces beyond the capabilities of vehicle design, driver attitude or act of god, I do not know for sure. I just know the results I have seen and I can say that the two most amazing lucky escapes I have seen involved small cars and big trucks, talk about mass difference.

So I apologise if it annoys people that I refuse to simply answer the posed question without challenging the thoughts of every one here with other elements of vehicle safety. I see defining it down to purely mass and size is over simplifying the issue and the result is misinformation and a poor perception of vehicle safety

So my final answer is yes differences in mass can and often do change the dynamics of a crash. Yes sometimes the larger car does have the advantage but inversely sometimes a small car has the advantage in a crash. Does size and mass equal a safer car? Not with any reliable certainty that can be broadly applied.

Originally Posted by colinl

I tend to agree with you. The better the structure of the car is at dispersing the KE of vehicle over a greater time frame, the less energy is transferred to the occupants. I also agree that a vehicle with more mass when colliding with a vehicle of lesser mass will suffer less deceleration than the smaller car. In the case of a large ridged vehicle colliding with a solid object, then the KE energy will more directly and suddenly be transferred to the occupant. I take your point about the body movement and the force at which it moves and the value of airbags at decreasing the bodies deceleration; but I would tend to think that the better the structure of the car is at dispersing the KE, the less work the airbag needs to do.

Very well said.

One thing that many do not mention is seat belt pretensioners, this little devices combined with good anchor points and a belt with adequate stretch will prevent the occupant from touching the air bag in most crashes at urban speeds. It is these devices that made the biggest difference in the reduction of some of the previous life threatening seat belt injuries.

[colinl]

Sudzy,

I read your two posts, are they the links you referred to?

Physics isn't a strong point with me, but I can't help thinking about an exercise I did years ago in a TQM class. We were in teams and had to design a capsule to house an egg made out of drinking straws, then drop them on the ground from height. The object was to see who's design didn't break the egg. In this case the designs were very similar in weight, but individual effectiveness varied greatly. The most effective designs in this circumstance were the ones were the straws were arranged to collapse around the exterior of the egg compartment.

With regard to the question of an empty vehicle. Wouldn't the total KE still be absorbed? The addition of people just add to the mass and KE. In this case the concept maybe similar to a chain breaking. When a chain breaks it is the weakest link that fails. In the crash the deformation starts in the weakest structure, then progressively deforms stronger structures. At some point in that process the occupant is next in that chain of progressive destruction and starts to suffer damage. If there is no occupant, than the damage just transfers to the next level of structural strength and the damage occurs over a greater area of the structure.

[geckoGT]

I can see what you are trying to say but a few points.

Its all about how we quickly we remove the motion or the relatively small amount of kinetic energy from the occupant, we either have the car extend its collision time by crumpling or an big cushion for the occupant.

The amount of kinetic energy involved in the average 80 kg adult traveling at 60 km/h is not by any means small, it is way beyond what any body structure can withstand if bought to sudden stop against a surface without first being slowed down.

You still have the idea that if the kinetic energy of a badly designe car is not "absorbed" well by the crumple zone then it will be transferred into the occupant, no.

Well yes it will as while the force is badly controlled and the force shortens the passenger compartment, the occupant becomes a part of the crumple zone. Whilst that passenger cell crumples and involves the passenger, the force that is breaking bones and bursting organs is absorbing energy through the production of sound and heat. Think of your body as an airbag for the back seat. That is why a modern car has crumple zones built into all parts of the car except the passenger cell, this cell is designed to not crumple or shorten enough to involve the occupants. For an example of this think of a formula one car, the drivers cell remains intact whilst the rest of the car disintegrates.

Perhaps think of what would happen if the same car was crashed with no occupants, using your logic, the kinetic energy instead of being transferred into the occupant would be transferred where?

Further rearward to the back seat etc.

[sudszy]

Originally Posted by geckoGT

The amount of kinetic energy involved in the average 80 kg adult traveling at 60 km/h is not by any means small, .

Perhaps have a think about the word relative in this context

Originally Posted by sudszy

Its all about how we quickly we remove the motion or the relatively small amount of kinetic energy from the occupant,

Originally Posted by geckoGT

Well yes it will as while the force is badly controlled and the force shortens the passenger compartment, the occupant becomes a part of the crumple zone. Whilst that passenger cell crumples and involves the passenger, the force that is breaking bones and bursting organs is absorbing energy through the production of sound and heat .

Okay, this should be easy. Perhaps think about what would happen if a 80kg person were in the vehicle without a seatbelt, and the vehicle collides with a brick wall at 50km/h. The person has about 8kJ of kinetic energy(about the same as the calorific value of a can of diet cola), this kinetic energy is removed very quickly when they hit the wall by a large force over a short distance, the amount of energy removed by the force is 8kJ.

Now if we have the same case where the person is arrested by their seatbelt then the amount of energy removed is the same, though they wont be beat up because the force was smaller but operated over a larger time and yes the vehicle has to crumple to allow the seatbelt to do this. If the vehicle doesnt crumple the force from the seatbelt gets larger and or the moving person hits part of the vehicle that has now become stationary, the person doesnt get extra energy transferred to them from the stationary bits of the vehicle.

Notice how in both cases that it doesnt really matter if we were in a 5 tonne truck or a 500kg toy car, its only the speed that the occupant actually hits something that causes the damage/injuries

[geckoGT]

Originally Posted by sudszy

Perhaps have a think about the word relative in this context





Okay, this should be easy. Perhaps think about what would happen if a 80kg person were in the vehicle without a seatbelt, and the vehicle collides with a brick wall at 50km/h. The person has about 8kJ of kinetic energy(about the same as the calorific value of a can of diet cola), this kinetic energy is removed very quickly when they hit the wall by a large force over a short distance, the amount of energy removed by the force is 8kJ.

Now if we have the same case where the person is arrested by their seatbelt then the amount of energy removed is the same, though they wont be beat up because the force was smaller but operated over a larger time and yes the vehicle has to crumple to allow the seatbelt to do this. If the vehicle doesnt crumple the force from the seatbelt gets larger and or the moving person hits part of the vehicle that has now become stationary, the person doesnt get extra energy transferred to them from the stationary bits of the vehicle.

Notice how in both cases that it doesnt really matter if we were in a 5 tonne truck or a 500kg toy car, its only the speed that the occupant actually hits something that causes the damage/injuries

It is not relatively small when you are the relatively dead occupant, I have seen a few and they did not seem to like it.

As for the other thing, thanks for the physics lesson but I did say I saw you point. Now you need to read mine.

As the structure collapses , it is not just the energy stored in the occupant that does damage to him, it is also the energy stored in the seat back, back seat, parcel shelf and loose items in the car etc. In large scale crashes it is not just the occupant moving and hitting surfaces, surfaces also move and hit the occupant. At Least that is what happens at the crashes I have been to, nearly every fatality was a result of the loss of structure to the passenger compartment and the passenger now occupying a much smaller space than before, energy was transferred from the car to the occupant.

I am sorry, but that is not pepsi can physics calculations, that is real life crash experience.

As for this discussion, I believe the question has been answered and this is now OT. Perhaps it is time to move on.

[sudszy]

Originally Posted by geckoGT

It is not relatively small when you are the relatively dead occupant, I have seen a few and they did not seem to like it..

Very emotive, but the context was that you and others claim that the energy of the vehicles themselves is converted to that of the occupants in a collision. I pointed out the folly of that, its only how quickly the kinetic energy of the person is removed that is important, a RELATIVELY small amount compared to what is in the whole collision......perhaps you can give another alternative to what I meant by that,other than misquoting.

Originally Posted by geckoGT

As the structure collapses , it is not just the energy stored in the occupant that does damage to him, it is also the energy stored in the seat back, back seat, parcel shelf and loose items in the car etc. In large scale crashes it is not just the occupant moving and hitting surfaces, surfaces also move and hit the occupant. .

Sure parts of the car, loosely stored luggage can be killers in any mass of vehicle, but that is not what you or others were claiming nor what I was refuting.

Originally Posted by geckoGT

At Least that is what happens at the crashes I have been to, nearly every fatality was a result of the loss of structure to the passenger compartment and the passenger now occupying a much smaller space than before, energy was transferred from the car to the occupant.
.

This is the bit giving you most trouble.
You believe that if the passenger cell remains okay then the passenger survives? A cell can be designed to survive a 100km/h head on, but the passengers wont.

When the cell does crumple, there are parts of the car that are now in the passenger compartment that werent there before, but they are not moving(front end components etc), they have hit the wall and become stationary, its the person still moving towards them at speed that hits them which incurs the damage due to their own moving energy.

Stationary objects have no kinetic energy to give away...

Ive deliberately left this as the car(any vehicle) into the wall example, but the same concept applies occur whether its car on car or truck on car.

Originally Posted by geckoGT

As for this discussion, I believe the question has been answered and this is now OT. Perhaps it is time to move on.
.

So true, and you are an ambo? keep up the good work.

[chevypower]

Originally Posted by sudszy

I do feel however, though you have used the term inertia correctly(a very rare thing in this world), (though inertial mass would be even better) it will confuse the average person as most throw around the terms of inertia and momentum not realising there is a difference, just using "mass" and leaving the term inertia completely out of the discussion is much simpler for the "masses"

Yes I should have said Inertial Mass. It seems like people are still getting it wrong. Even adding emotion in to the debate, like "how would you feel killing some poor person?"
It is inertial mass! That's the cause! Mass on it's own doesn't do it. But mass affects inertial mass, which affects momentum. But momentum, velocity, acceleration or force are NOT needed here. If you drive a Kia in to a stationary bus, the bus will be ok, the Kia will become a pancake.

Inertial Mass is the difference.
Velocity: The bus has zero
It is not Momentum (because you need velocity to have momentum).
It is not force, because that only affects change in velocity / acceleration of the vehicle.

The personal injuries are as a result of the sudden change in velocity the person is experiencing (or g-force). The person is traveling at (say 60km/h) and suddenly the car is doing 0. The person keeps traveling that speed until they hit the dashboard or seatbelt. The vehicle with the larger inertial mass is going to be less resistant to a change in velocity than the smaller car (that's a scientific definition of Inertial Mass), so the occupants are experiencing less change in velocity than those that are in the smaller car. This is why the level of personal injury differs.

[prydey]

Originally Posted by Brazen

Instead of ANCAP smashing cars to an imoveable wall- which is the equivalent of a Fiesta hitting an oncoming Fiesta and a Patrol hitting an oncoming Patrol.

They should smash the car into a oncoming moving object which simulates the average car, ie the Fiesta is now hitting an oncoming object the weight of a Commodore and the Patrol is now hitting an oncoming object the weight of a Commodore.

I think it would be scary to see the difference in ratings...There is probably a bit of social pressure to not do this, as it would just encourage people to buy heavier and heavier cars.

well what do you know, you have finally said something i agree with.

although i don't see an issue with your last sentence. if it highlights that the small 5 star cars aren't necessarily the safest option, then those buying them for that reason wouldn't be misinformed. as highlighted by the ancap website itself, even they claim that you can't compare cars across different categories but i believe its the manufacturers claims that confuse people more, by really pushing the 5 star thing.

[Olbucko]

Originally Posted by Brazen

Instead of ANCAP smashing cars to an imoveable wall- which is the equivalent of a Fiesta hitting an oncoming Fiesta and a Patrol hitting an oncoming Patrol.

They should smash the car into a oncoming moving object which simulates the average car, ie the Fiesta is now hitting an oncoming object the weight of a Commodore and the Patrol is now hitting an oncoming object the weight of a Commodore.

I think it would be scary to see the difference in ratings...There is probably a bit of social pressure to not do this, as it would just encourage people to buy heavier and heavier cars.

ANCAP do not smash cars into a large immoveable wall, the car hits a large crushable metal block which is designed to collapse in the impact.
see clip, www.youtube.com/watch?v=cfhkrSYh9pc