The Physics Behind Car Crashes 732
Guinnessy writes "Physics Today has an article on Vehicle Design and the Physics of Traffic Safety. The article analyzes in detail typical crashes experienced between cars, and cars with SUVs'. According to Marc Ross, Deena Patel, and Tom Wenzel, "The evidence is compelling that body-on-frame light trucks cannot safely coexist with passenger cars under existing conditions. That problem is critical because so many light trucks are used nowadays as car substitutes." They suggest some ways in which both cars and SUVs' can be redesigned to improve safety. Meanwhile Detriot News reports on a Pediatrics journal study says that claims that children are no safer in SUVs than cars because of the rollover risks."
You're both wrong. or right. (Score:5, Informative)
or the more common form: KE = 1/2mv^2
This incidentally does make it in the same class as E=m*c^2 (due to magnitude/exponentiation) but that's more or less irrelevant.
PDF warnings (Score:2, Informative)
Re:weight& speed are the big issue here (Score:5, Informative)
In addition, in a large vehicle the conservation of momentum is on your side. It's not the crash that kills you it's the sudden acceleration (your body going from 30 to 0 mph in a second). But if your vehicle doesn't change velocity much (because of its large mass, mass==resistance to acceleration) then you're fine. The other guy dies. That's why there don't have to be seat belts on buses (though it would still be a good idea). If a bus crashes into something, it's probably not going to stop very fast.
Large mass is not why TFA says SUV's are dangerous, it's because they tip over.
Re:weight& speed are the big issue here (Score:5, Informative)
bull bar safety (Score:1, Informative)
Besides the fact that SUVs can cause more harm in accidents due to their obvious bigger size and weight, some SUVs also come with bull bars (the big metalic frames in front). That stuff is a weapon. In Belgium, manufacters aren't allowed to put them on SUVs anymore. The point is that it's too dangerous and it serves no meaningfull purpose (unless you're a cowboy).
Re:weight& speed are the big issue here (Score:2, Informative)
No, I am alluding to the fact that antilock brakes keep you from skidding which increases your likelihood of flipping your car which is much more fatal than skidding into something.
Antilock brakes are best at reducing the most common and least serious of accidents -- rear end collisions. They simply do not help in dangerous high velocity situations, in fact they are more dangerous.
Re:Physics of car crashes aren't intuitive. (Score:1, Informative)
It's not simply. Nothing is ever simply because of one reason, and anyone trying to tell you there's one reason for *anything* is trying to push a political point or sell you something. Males are injured at a phenomenally high rate in cars, and in Australian statistics they are the cause of (and are involved in) around 80 to 85% of all car accidents.
Yet they only drive 54% of the miles driven. There's more to it than just more males on the road.
It's a little like the often-quoted "young people don't drive worse, it's just that there are more young drivers on the road than older drivers so we're involved in more accidents". It's an illogical excuse made up by young drivers who ignore the fact that the stats don't say "more young drivers cause road accidents" but "young drivers have a higher risk of causing road accidents"
Re:This Is Genius! (Score:3, Informative)
Here [euroncap.com] is the EURO-NCAP test-report for Citroen C6. As it happens, it's the first car to achieve the maximum score for pedestrian safety.
Very biased (Score:1, Informative)
Re:Physics of car crashes aren't intuitive. (Score:5, Informative)
There are many studies which support this (besides TFA), and I've personally seen (as an EMT) some really hideous SUV/Car collisions, and can tell you: the SUV usually wins. Sometimes there are mitigating factors, like the SUV will roll over and throw out and crush anyone that's not seat-belted, while the car will basically stay in one place, but if you have the choice between being in a Honda Civic and a GMC Suburban with similar safety equipment when they run into each other, you'd be insane not to pick the Suburban. Plus, the increased size of the car makes it likely that you'll get extricated from the vehicle a lot faster (again, assuming no rollover).
The biggest problem with SUVs, traditionally and still, is that they tend to roll over during hard panic maneuvers, or when hit from the side. Where I used to live, the biggest risk was people driving them too fast for conditions (no, your SUV does not mean you can drive on ice) and laying them over -- generally nonfatal though. But in a straightforward front-end or rear-end collision, there's something to be said for surrounding yourself with several thousand extra pounds of steel. Granted, you get that advantage at the expense of the person in the smaller car, and you have to be more careful taking it around a corner, but it's there nonetheless.
Re:Chinese SUV (Score:5, Informative)
See for yourself [paultan.org]
Re:weight& speed are the big issue here (Score:3, Informative)
While it says that the most common accidents - rear enders - were decreased, it goes on to say "Rollovers and side impacts with fixed objects - crashes that typically follow a complete loss of directional control - had the highest increases with ABS. Nonfatal crashes increased by 28 percent, and fatal crashes by 40 percent. "
Re:Two points of safety. (Score:3, Informative)
Re:Physics of car crashes aren't intuitive. (Score:5, Informative)
At the same time the Hummer is not even on the list. In fact if it was, it would have pulled a 2-2.5 star rating at best. Same as a suicidal box like Fiat Ceicento. I have seen a crashed Hummer so this unscientific opinion is based on seeing what happens when it smashes. If you are driving this POS you have a death wish.
Essentially even being in a something microscopic by American standards (like Modus http://www.euroncap.com/content/safety_ratings/de
Re:Chinese SUV (Score:3, Informative)
Re:European car security (Score:3, Informative)
This was near/on the New Mexico State University campus (University Blvd), and I was crossing the street illegally in the middle of the road. I heard the audible crosswalk signal (for the deaf) that indicated that the nearest crosswalk was allowing pedestrians to cross. This is a good cue to a jay-walker that traffic is likely not coming from that direction.
After a quick glance to ensure that no traffic was coming, I began crossing, and heard a "thump" noise. I figured someone had had a tire go flat, and looking over someone who had made a right-hand turn was pulling to the side of the road. Then I got to the center of the road, and I heard a wimpering. And I thought, "Oh shit".
Later a few other people got hit in pedestrian walk ways, and the University got on a big "jay-walking bad" rant (if you've been to College, you know how Unis can do this. They get some topic, and everyone jumps on the bandwagon, and argues for/against some stupid thing that happened months ago) And I could hardly understand how people getting hit in cross-walks related to jay-walking. I mean, if anything, it indicated that jay-walking was SAFER, because there were far fewer jay-walker strikes than cross-walk strikes.
Truck designation (Score:3, Informative)
This might have something to do with the way that the government allows trucks to be classified.
A truck is a utility/cargo vehicle, not a passenger vehicle.
Trucks are required by law to have a 5 mile/h bumper, cars 10~15mile/hour.
Fuel effiency/emmisions standards are not as stringent on cargo/utility vehicles.
Mini-vans are also classified as utility/cargo vehicles.
The only reason that there is an appearance of improved saftey is the higher kinetic energy + higher sitting position that these monsters have over a standard family-sedan.
Re:weight& speed are the big issue here (Score:5, Informative)
National Highway Traffic Safety Administration [dot.gov].
"Do cars with ABS stop more quickly than cars without?
ABS is designed to help the driver maintain control of the vehicle during emergency braking situations, not make the car stop more quickly. ABS may shorten stopping distances on wet or slippery roads and many systems will shorten stopping distances on dry roads. On very soft surfaces, such as loose gravel or unpacked snow, an ABS system may actually lengthen stopping distances. In wet or slippery conditions, you should still make sure you drive carefully, always keep a safe distance behind the vehicle in front of you, and maintain a speed consistent with the road conditions."
ABS is about control, not stopping distances. It allows you to retain control of your vehicle by preventing the wheels from locking up. As such, ABS will not reduce your velocity as much as tradtitional brakes and you will therefore impact with more energy.
However, the benefits of ABS and being able to avoid the collision outweighs the costs of reduced stopping force. So for the untrained driver who simply hits the brakes and waits for impact, ABS can result in higher impact velocities. but used correctly, ABS can help you avoid a collision altogether.
So in some cases (when you cannot maneuver) it is more dangerous, in some cases it is safer.
Re:weight& speed are the big issue here (Score:2, Informative)
So what happens when the panicced (sp?) driver sees a crash ahead, slams on the breaks and wrenchs the wheel to the left?
Without ABS he skids in a straight line towards the crash.
With ABS, he steers the car violently to the left and rolls it over
Re:Crash Ratings don't say what you think (Score:3, Informative)
The determination is risk of injury.
My favourite is the offset crash test.
http://www.hwysafety.org/ratings/default.aspx [hwysafety.org]
The standard five star rating is explained here.
http://www.nhtsa.dot.gov/cars/testing/ncap/BASC20
http://www.nhtsa.dot.gov/cars/testing/ncap/BASC20
Body-on-frame vs. Unibody construction (Score:3, Informative)
The last 25+ years, there has been a huge push for increased gas efficiency for cars, due to public consciousness, laws, etc. The easiest way to do this is to move less mass, and thus, the unibody car design is pretty much standard across the board for cars. I believe the Ford Crown Vic was probably the last car to be built using body on frame. But before this period, a vast majority of cars were built using body-on-frame, as it was easy, robust and survived crashes well (think of the boats like a Buick Estate).
Trucks are govern by a different set of rules, and technically speaking, cannot transition to the unibody design that well for practical purposes. The loads they are designed to haul and tow (whether or not they do that in real life is irrelevant) demand that a body-on-frame design be used. There are exceptions like the Honda CRV, the 99-04 Nissan Pathfinder, the new Honda Ridgeline, but in the vast majority of cases, they use body-on-frame.
So, over the last 25 years we've created 2 classes of vehicles. Unibody and Body-on-frame. The article suggests that this class difference cannot be reconciled and needs to be eliminated. But we all know that this won't happen, at least not right away. It seems like Honda is the sole pioneer of this conversion, and it was limited in part by it's lack of experience with the body-on-frame design. We need to encourage more companies to try a different route, and use more of the neat materials that science has given us in the last 25 years to bridge that gap.
Think we're still getting light? Then look at motorcycles and how they've embraced technology far better than any car out there. Ever wonder why the spark plug is so big in the car? Ever wonder why car batteries use 19th century technology and a wimpy voltage? It was because of standards created long ago, where they didn't care about size and weight and didn't have the materials andn technology that we do now. The cost and weight savings are all within grasp. Someone just needs to take the lead.
Hey I know! Let's RTFA! (Score:3, Informative)
I know, I know, I must be new here.
Sean
Re:weight& speed are the big issue here (Score:3, Informative)
I might have bought into this a decade ago, when people were still unfamiliar with how ABS worked. Early on, people would slam on the brakes, get the feedback, and pull back because it felt wrong. They were then dealing with two issues -- whatever they were trying to avoid, and an unexpected sensation from their car. This led to a number of accidents being more severe than they could have been. ABS is far more common now, and most drivers of cars with ABS know what to expect.
My first car with ABS was a 1993 Saturn SL2, and I was warned by the dealer that it was different. So I found a good-sized parking lot, would accelerate to about 20mph and then nail the brakes so I knew the feeling and wouldn't be surprised by it in an emergency. This has helped in a few occasions where I had the need to hit the brakes hard and steer, and the feedback is now in a way slightly comforting because I know that the system is working.
Re:weight& speed are the big issue here (Score:4, Informative)
That's not necessarily what's being done.
Many of the largest SUVs (Suburban for example) are on a longer redesign schedule than your standard passenger vehicle. Going 8 years between a full redesign means you're still getting crash technology that's nearly a decade old.
Making matters even better, is that the largest of SUVs, by virtue of their GVWR (a number that's made up by the manufacturer) are not required to meet several bare-minimum government safety standards.
The current Suburban, with a GVWR of 7000-8600 lbs, is exempt from government roof crush tests. (PDF) [nhtsa.com](As is any vehicle with a GVWR over 6000 lbs.) Unfortunately, passing them wouldn't mean much either - as the minimum only requires a vehicle with windshield intact to withstand 1.5 times its weight pressed down on the roof.
In a rollover, the windshield is usually gone by the end of the crash, and forces can easily exceed 1.5 times vehicle weight.
The current standard has been in place - and unchanged - since 1971, and many SUVs aren't required to pass even that lax standard. Many are also exempt from other minimum standards, like Steering Wheel Rear Displacement (only applies to cars with GVWR 4,000 lbs and less), and basic braking standards (applies to vehicles with a GVWR of 7,716 lbs and less).
Detroit, of course, insists the existing rules are adequate [detnews.com] - the rules that often don't apply to their vehicles.
Even better, it's been shown that they misrepresented data from their own tests to the NHTSA. [consumeraffairs.com]
Obviously, we all have to be more proactive in researching the safety of the vehicles we purchase - and not just go out and buy something "big" that "feels safe". Often, we may be safer in that type of vehicle, but only in multi-vehicle-non-rollover crashes - and to what expense? Raising the weight of your vehicle may reduce the risk to your family by 1/3 in some wrecks, but you've increased it threefold to the other car.
Re:Mini vans vs SUV's (Score:2, Informative)
Re:Mini vans vs SUV's (Score:3, Informative)
Of course, one can always mine the data [dot.gov] oneself. The query function is fairly easy to use.
-Adam
Re:European car security (Score:2, Informative)
I am pretty sure those are for the blind.