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The Physics Behind Car Crashes
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."
..that one of their findings was that cars dont explode upon slightly touching each other!
I'm reminded of an accident I almost saw several years ago. It was at an intersection where the east/west road had right of way, and the north/south road had a Stop sign. I was in a friend's shop at the time, and we heard a V8 accelerating hard, then a sudden very loud *thud*.
Running outside, we saw from the accident that a 1970s V8 Statesman with a P Plate (here, drivers get P plates to tack on their car for the first few years they're driving on their own) had obviously gone through the stop sign and hit the driver's side of a Prelude driven by an elderly driver. The young guy in the statesman was taken off to hospital, but a rescue team was needed to pull the driver out of the Prelude. Later that night the news had a piece about the accident, saying that the p-plater had caused an accident that killed the elderly guy. Even witnesses at the scene said they heard the V8 go through the intersection far too fast, and hit the prelude.
Then the next night, video shot from inside a building nearby showed the accident - the P-plater had actually stopped at a pedestrian crossing, let the people walk across, then accelerated quickly & noisily... but he was actually moving along the east/west road with right of way. It was the driver in the prelude that had gone through the stop sign at high speed, and the young fellow was just in the wrong place at the wrong time, and hit the driver's side of the prelude hard - both cars went spinning around in an impossible looking way, ending up in a position that looked for all the world like the young fellow had gone through the stop sign, even though in this case the only thing he'd done 'wrong' was make a big ol' noise in first gear. My "obvious" guess at who was in the wrong was completely off.
There's a massive amount of energy in a car collision, more than most people would expect given how much we take moving a tonne or two of steel from one place to another daily.
The newest european cars nowadays come with an extra protection agains collisions with pedestrians: The hood in front of the car is lifted a few inches after 40 or so milliseconds so the pedestrian gets lifted as well and won't get run over by the car but lands on the softer hood and might hit the car glass.
The powertrain generally takes up considerable space in the engine compartment, leaving little room between the engine and the bonnet.
To remedy this, the C6 comes with an active bonnet system as standard that automatically raises the bonnet in the event of pedestrian impact. Thanks to an impact sensor and pyrotechnic mechanism, the bonnet rises 65 mm in 40 milliseconds. A second mechanism maintains the bonnet in its raised position despite the impact and thus absorbs the deformation energy.
Example from Citroen.
Dependency hell? =>
Well, that settles it then. It's obvious we need to outlaw passenger cars. Not only will that remove millions of unsafe cars from the road, it will give a much needed economic boost to the SUV manufacturers.
Besides, if a car is unsafe around a light body truck, it must be a death trap around a standard truck or a semi-tractor trailer rig...
Any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so.
KE = (m/2)*v^2
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.
I don't know for the USA, but a chinese SUV is being imported into Europe and it received a 0 rating for safety.
So be careful! If you see one on the road, stay well clear.
I'll do it for cheesy poofs.
People are always going to consider their own selfish personal safety as a purchasing factor when choosing a road vehicle; for many people this is a very significant issue, and is played upon by advertising for autos.
We need to focus on methods of ensuring safety that don't threaten other road users: win-win situations rather than soccer moms driving trucks and declaring "I know I will win in a collision" (vaguely remembered quote from a National Geographic article on SUVs). This can only lead to a sort of arms war where we all end up driving 38 tonne truck rigs....I was particularly scared by the picture of the SUV built on what is effectively a couple of forward facing railroad rails - that's going to hurt if you're hit side on in a little compact by one of those.
The annoying thing is I am sure most of the safety issues can be solved with little cost and by improving social as well as technological aspects of road use, e.g. severely enforcing low speed limits round residential areas. As long as its cool to drive like a bastard people will.
I always like the story about how there were a lot of fatal accidents in early autos because of the bolt protruding in the middle of early steering wheels, head on crashes meant drivers suffering lots of chest damage; while this was then changed to make life safer, it's been pointed out that if there was a big spike compulsorily welded onto the middle of all steering wheels pointing at drivers then everybody would drive a whole lot more carefully and there would be a lot less accidents.....
As others have pointed out, this has nothing to do with E=mc^2.
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.
Whoever is in the car that undergoes the largest acceleration is worse off. Since mass is by definition the resistance to acceleration, the ones in the larger car win. If two equal mass cars collide they both undergo the same acceleration. Ignoring car vs SUV strength, if two cars have a head-on collision at 30mph, the people inside are just as well off (or just as injured) as if it were two SUVs.
Funny thing anout SUV's. They really are a hazard. Not only is the center of gravity higher, but they usually have misdesigned suspension (Ford Exploder, Mitsubishi Montero) that actually encourages rolling over, SUV's have higher bumpers, so cars running into them slide under, killing the occupants in the lower car. SUV's use more gas and cause more pollution. A very big problem is roof crush. SUV roofs are notorious for being frail and crushing in a roll over accident, squishing the people inside.
I for one don't like the idea of dying by having my spinal column driven into my skull.
Many of these things of course could be mitigated. There could be a standard lane change test to determine suspension quality, there could be rules on the center of gravity, there could be rules on bumper height (like on regular cars) and there could be rules on pollution, perheps making all SUV's except the kind with frugal modern common rail diesel engines (with particle filters) financially impposible to own/buy.
But this is not the case. Infact, with GM and to some extent Ford in financial trouble (and with all their profits coming from SUV's) the current US administration wil do nothing about the laws, that infact make SUV's above the law.
Right now, SUV's are excempt from current fuel efficiency laws (that are not very good in the first place, mind you), they are excempt from bumper laws (making SUV's lousy and expensive to fix in even the smallest parking lot accident) and there are no laws governing roll-overs (only tests).
With all these problems, I don't see why they even allow these on the road.
The funny thing is, I really like the idea of sitting up high in my car and being able to see further. That is probably the only reason SUV's are so popular. Looks like they could accomplish that in a macho looking vehicle without these problems.
Cross-over anyone?
Some say he is made with ascii, others that he is eyeballed daily by millions. All we know is, he is known as the Sig
Since when? Are you alluding to the misconception that a good driver can stop faster without ABS than an average driver with? With modern ABS this just isnt true anymore, even if you're a rally driver.
And ABS systems let an average driver steer their car in a heavy-braking-potential-skid situation, where the non-abs car will be skidding in an uncontrolled straight line. How many drivers are taught cadence braking these days?
The author of the study is making the mistake that safety is a boolean, and that things are either safe or unsafe.
In fact, everything is unsafe, to varying degrees. ("Life", in the words of Warren Zevon, "is gonna killa you").
The important thing, when contemplating questions of public policy, is to COMPARE one risk to the next, and make sure that we're making reasonable decisions and tradeoffs.
For example, over the last 10-15 years, a lot of states have dropped the DUI (driving under the influnce) BAC cutoff (blood alcohol content) from 0.1% to 0.08%. Lower is better, right?
Well, as it turns out, having a BAC in the 0.08 - 0.10% range has the same effect on driving ability as (a) having a cold; (b) getting a poor night's sleep; (c) being over the age of 50.
If we're going to make a 0.81% BAC illegal (and punish it with major fines), should we not also have the same punishments for driving while having the sniffles, or while being 51?
The answer is that one behavior gets a penalty because it sounds good, makes politicians look like They're Doing Something(tm) and has moralistic overtones ("get those damn drunks off the road!").
To say that "light trucks cannot safely coexist with passenger cars " is purest nonsense. We've had light trucks coexisting with passenger cars for 70 years, and the fatality rate drops every single year. Sure, if you could snap your fingers and get every pickup truck, minivan, delivery van, and SUV off the road, things would get incrementally safer for the average driver of a passenger car.
I don't know off the top of my head, but is it a level of safety comparable to every passenger car driver making sure that their tires are fully inflated before each trip? Or more, or less?
Absolutist boolean statements like "X can not safety coexist with Y" do not answer questions like this. These statements are public-policy-by-press-release and deserve to be condemned.
The SUV craze is actually the same phenomenon as the crowded theater phenomenon (where someone stands up to get a better view, so soon everyone is standing to see at all, and no one sees any better than when they were sitting). The idea is "If my vehicle is heavier than the other guy's, then in an accident I'll be in better shape than him." End result: Everyone will go out and get a bigger vehicle, because it appears that will make them better off, and as a result no one is safer.
Of course, the heavier vehicles always create a false sense of security. Trucks and large vehicles are more prone to rollovers, can't stop or swerve easily to avoid trouble, and hit stationary objects with more momentum. But like Homeland Security or MS Windows, it makes you feel safe, so people choose to go with it even if the facts are completely against them.
Really your best defense while driving is to actually use everything you learned in Driver's Ed, or if you don't remember than find books or classes on safe driving. And if there are any teenagers reading this, remember that Driver's Ed is the one class most likely to determine at some point whether you survive a situation. You know, driving at reasonable speeds (somewhere around the speed limit is usually good), slowing down before you take corners, being aware of the drivers around you, good signalling so other drivers are aware of you, etc.
I am officially gone from
And they say there is no free lunch.
Funny thing is, other then the "coolness factor," I do everything I used to do in the SUV with the van, and the van is actually better at it since it carries more. This likely includes more off road travel then most SUV's ever see.
But strangely, the bus will rapidly gain mass after an accident, from all the people who said they were on the bus and want to file an injury claim. Even stranger, this mass gain happens even when the accident is simulated.
One line blog. I hear that they're called Twitters now.
From the
National Highway Traffic Safety Administration.
"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.
Remember, You are unique...just like everyone else.
As a firefighter and first responder I can state that there's a big problem with the safety crash test ratings most consumers see. These 4 star and 5 star ratings don't tell the real story at all.
These tests emphasise not just the human safety but also the cost of repair. To some extent, the cost of human repair is the factor added to the vehicle repair to make the rating. Interesting data, but not what most of us care about, and it results in very poor decision making information.
Example: I have seen personally how effective "crumple zones" combined with airbags and safety brackets on hoods which prevent the hood from sliding directly back into the windshield in the case of a head on collision can work. I see completely destroyed cars all the time where the occupants are well protected and suffer only minor injuries. That's because much of the force of impact is used up in the act of crumpling the car. These crumple zones are amazingly effective.
The problem for insurrance companies is that crumple zones and the like TRADE vehicle damage for human damage. The low-speed destruction of bumpers, fenders, hoods, and entire engine compartments mean that these cars are a "total loss" much more frequently.
If you REALLY want to promote SAFETY -- and like me, you could care less about the damage to the vehicle if the humans are better protected -- than we need a safety rating system which ignores all things other than damage to the occupants of the vehicles.
AP
The problem with quotes on the internet, is that nobody bothers to check their veracity. -- Abraham Lincoln
Lighter cars don't grip the road? How ignorant. Explain, then, the Mazda Miata, or just about any Lotus ever built? How about the old Porsche 914 and the little MGs and Triumphs? For that matter, how about a nice little crotch rocket with really sticky rubber? How come these sub-2500 pound vehicles can exceed the lateral G loadings of say a two-ton pickup?
The suspension design, its geometry, and MOST IMPORTANT, the tires determine grip. (for normal road cars. For all-out race cars, aero is another way to generate grip.. but aero grip goes away when you slow down.)
If you put 20 dollar Wal Mart tires on your car, you deserve to die horribly in a no-grip accident.
If you treasure your life, and that of those you love, then fer $DEITY's sake, put decent rubber on your car. And that' doesn't mean "all seasons". It means summer tires for when it's not snowing, and snow tires for when it is.
Mass is the enemy. Less mass makes for better driving cars. Better grip, better control, better handling. More mass, the American Way, just ruins everything. That's just one reason why EVERYONE builds better-driving cars than America does.
Of course, if you have a small weenie, then your psyche demands you get a big honking chunk of ill-handling, ill-stopping, ill-steering Detroit Heavy Metal. To compensate, y'know.
The "Civilized World" jumped the shark ca. 1973.
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)(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 - 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.
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.
As a republican I feel it my responsibity to manufacture criminals. People need punished!