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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
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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.
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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.
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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.