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."

Comments

weight& speed are the big issue here

[jacquesm]

you don't have to be Einstein to remember that e=mv^2, and that if you want to reduce the damage to you, yours and the occupants of other vehicles you keep your speed down and your vehicle weight down.

Driving fast in an SUV loaded with kids is about as unresponsible as it gets, I see it quite often though...

Re:weight& speed are the big issue here

[Vellmont]

And you don't have to be an Einstein to know that kinetic energy isn't what kills people, it's the sudden stop that does. If I jump out of an airplane and atain terminal velocity, but deploy a parashoot I'm a lot safer than jumping out of a tree. The point is that if all that extra weight is put into engineering stronger cars that absorb the impact and slow you down more gradually then you're going to be more likely to survive a crash.

That's not to say that SUVs are safer than normal cars. The rollover risks aren't trivial. Personally I think SUVs should be banned, or at least put into some special category of business only use because of the safety risks they put on other cars. The SUV craze is literally killing more people and making driving for everyone else less safe, and this article proves it.

Re:weight& speed are the big issue here

[owen_b2]

the same is true with antilock breaks

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?

Re:weight& speed are the big issue here

[Alioth]

It is likely that this was not caused by the ABS itself, but an unintended consequence of ABS - people have a car with ABS, therefore feel safer, therefore drive faster or drive more recklessly because they think they can violate the laws of physics because they have ABS. I bet if a driver continues to drive carefully with an ABS equipped car they will be better off.

Re:weight& speed are the big issue here

[owen_b2]

Quite possibly.

But you can't ignore the fact that ABS lets you steer the vehicle in a 'panic braking' situation, and its this abilty to steer (cf skidding in a straight line) that lets people turn too hard and rollover their vehicle.

I'd have thought that the ESP systems that are becoming more commonplace would be able to reduce the problem though.

Having said all that, I still prefer my car with no driver aids.

Re:weight& speed are the big issue here

[TigerPlish]

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.

Re:Wimpifying the SUV's and Trucks is not the answ

[Vellmont]

And when the drunk moron is driving an SUV and hits you in the side, you and your family will be dead. Had he not had all the "armor" you and your family might have lived

what is the definition of "safely" ?

[tjic]

"The evidence is compelling that body-on-frame light trucks cannot safely coexist with passenger cars "

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. ...but how much safer?

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.

Re:I reckon..

[Indy Media Watch]

That only happens to American cars which will always explode in a ball of flames, but not until the hero can pull the important passengers to safety, and yell, "Watch out! She's gonna blow!"

Re:People will always buy an auto they feel safe i

[dkleinsc]

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.

Re:European car security

[smchris]

Me too (usually a pedestrian). There are significantly more pedestrian deaths in the U.S. _every_year_ than the World Trade Center deaths but I don't see people getting all weepy over it. (http://www.carwrecks.com/info8.html)

Being a long-term survivor of pedestrianism is one of the best ways to become a cynic of the human condition. It annoys me that my local media have to make a point that "alcohol and drugs were not involved" -- to which I always think, "Great, a clean kill." Running over a pedestrian is the safest way to experience the thrill of murder. Unlikely you'll even get the workhouse if you aren't too blatent about it. But be warned that if you only wing your pedestrian you could be paying off the multi-million dollar lawsuit for the rest of your life under the new bankrupcy rules. So in the end it's smarter not to run over pedestrians, ok?

[Aside from personal experience, I tend to be even more cynical because working in various places and talking about walking to work I have met _three_ secretaries who each had their father killed at a stop sign or stop light pedestrian walk.]

Re:Physics of car crashes aren't intuitive.

[the_bard17]

The biggest problem with SUVs, traditionally and still, is that they tend to roll over

Not from my point of view... their problem is this: the SUV usually wins. From my meager time in life, I've seen plenty of reckless drivers. Not all of them were teenagers... I've seen my share of reckless soccer moms, businessmen, and delivery drivers. Quite a few of these drive heavier vehicles. I suppose it makes them feel safer, being in a heavier vehicle (which they probably are).

They continue their reckless driving habits, however, now becoming that much more of a threat to the general populace driving smaller cars. They may be more likely to survive, but they're decreasing the chance of survival for everybody else.

Until everybody "wises up", goes out and buys an heavier vehicle... at which point we're right back at square one, with worse gas mileage.

Re:Physics of car crashes aren't intuitive.

[Zog The Undeniable]

The insurance companies aren't screwing you (or at least, no more than anyone else). They use actuarial tables which show the average younger driver, or any driver with no clean driving history, is several times more likely to have an accident.

Now I was never a particularly aggressive driver (and the only accident I ever caused was when I reversed into a concrete pillar) but I can safely say that in 19 years of driving I leave a lot more space in front and anticipate other drivers' bad behaviour better than I used to. Hell, when I was 25 in a little 988cc car I used to drive a section of country lane in about half the time that I would risk it now - in a car with 2.5x the power. It's all about testosterone and perceived invincibility.

Re:On SUV safety

[RosenSama]

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.

The car buying market in the US is pretty democratic. People buy what they want. We don't need more rules here to tell people what they "should" buy any more than we need more rules about televisions telling people what they "should" watch. The reason the SUV-is-a-truck-not-a-car loophole that excludes them from bumper height and fuel economy exists in the first place is because of short-sighted rules. We don't need further rules that just leave new loopholes with age. Eventually the car manufacturing regulations would be like the tax code.

Re:Physics of car crashes aren't intuitive.

[killjoe]

"Quite a few of these drive heavier vehicles. I suppose it makes them feel safer, being in a heavier vehicle (which they probably are)."

They probably are safer in their huge SUV. Too bad they are not thinking about the people they will murder though. If they were driving a lighter car which would more effectively crumple and absorb impact the people in the other could maybe survive.

I guess it re-inforces the stereotype of the SUV driver as the guy who doesn't give a shit about anybody else except themselves. Don't care about the environment, don't care about foreign oil dependency, don't care about the survival of the other guys in the crash.

Unfortuneately...

[bmajik]

the best way to be a safe driver is situational awareness.

You can choose to optimize for avoiding accidents, or you can choose to optimize for an irrational hope that the ones you do have will be survivable.

By selecting an SUV, you've done the latter.

However, by doing so, you've chosen to have more accidents, more often. SUV's, irrespective of any other factors, have less grip than lighter vehicles merely because of their excessive weight, and because the load/grip ratio on modern tires is non-linear.. there's a fall off as load increases.

Additionally, the excessive girth of most SUVs means that you're fighting significantly more inertia than other vehicles. The horrible twist and non-rigidity in the construction means that any evasive maneuvers are uselessly slow - the body of the truck twists instead of directing the tires to move.

Everything the vehicle does is slow.. numb... subdued.

The high roll center means that more weight is transferred in all accelerative movements.. when turning you move more weight to the outside edge.. when braking you unload the rear tires more.. prior to high-roll-center SUVs being commonplace, rollovers were pretty much unheard of on paved roads. (when you leave the road, that's a different story).

even if you are the most situationally aware driver, when you pilot an SUV, you are motivating a stick of butter across a pan. You're driving a numb, useless instrument instead of a precision machine with proper dynamics.

You will be in more accidents, because you're driving a vehicle more likely to roll, more likely to twist/bend under dynamic conditions, that takes longer to stop, turn, and change direction, and which has lower road-adhesion characteristics for a given tire design than a lighter car would.

Also, your increased mass will tend to cause more damage to others around you.

All of this.. because you think you are better off if someone hits _you_? You've chosen a vehicle that makes it more likely you'll get hit (because it cant evade effectively.. and it's an enormous target). Your vehicle is more likely to roll in a side impact. Your vehicle has a very weak chassis, so unless the collision hits in just the right spot, the amount of body deflection and passenger intrusion will be more severe than in a well made unibody car.

I live in the midwest, so i understand the utility and necessity of large, ladder-frame vehicles perfectly well. But i dont own one, because i am not a farmer, construction foreman, or other blue-collar contractor. Trucks and SUVs have a purpose, but passenger safety for daily driving isn't it.

Congratulations, you've been deluded.