Question of vehicular safety 5

[Lion06]

I've been having a discussion with my wife regarding the safety of automobiles. My wife hit a deer today with her car and later made the comment that it's a good thing the car didn't crumple like an accordian. I tried explaining that the crumpling is what makes the car safer to the passenger because it helps to dissipate the energy of the impact as well as decelerates the car at a slower rate (which, in turn, imparts less acceleration and force to the passengers). She is convinced that a car that could make it through a head-on collision with no damage is the safest car to be in (while I told her that is actually the most un-safe car to be in) Let's for argument's sake say a 1970 chevy nova is in a collision with a 2010 toyota camry - she believes the passengers in the nova will make out better. She suggested I come here to pose this question. I already know what the answer is, but I would appreciate if some of you could weigh in.

 

[MintJulep]

Controlled, designed crumpling, combined with a rigid cage around the passengers is what makes a safe car in a collision.

The goal is - as you correctly noted - to dissipate the energy of the collision in a controlled way. The reduces the deceleration experienced by the occupants.

The super stiff car subjects the occupants to much higher decelerations and corresponding injury.

The 1970 Nova might crumple in an uncontrolled way, including deformations that reduce the survivable space in the passenger cabin. Uncontrolled crumpling is bad.

Numerous sophomore or junior engineering design competitions involving eggs demonstrate this.

 

[msquared48]

In any collision, there is a total amount of energy to dissipate before equilibrium state of no motion is acchieved. There are many ways to do that...

Perhaps she should ride in a car with no shock absorbers or springs to experience the instantaneous accelerations to her body, then ride in one with them to see which ride she prefers.

No different in an accident. Any energy that anything in the accident absorbs rather than transfers, is less energy your body has to deal with.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jmw]

Of course, if she was driving a main battle tank and hit a passenger car, she'd be OK.
She probably wouldn't even notice the impact.

Or if she were driving a big rig.... big rig drivers also have been known to wipe out a passenger car and not notice.





JMW

http://www.ViscoAnalyser.com

 

[ivymike]

as noted above, she should try dropping eggs until she finds a way to keep them from breaking.

might show her this one (59 bel air vs 09 malibu)

http://www.youtube.com/watch?v=joMK1WZjP7g

 

[BrianPetersen]

You can find crash test videos on youtube that will illustrate the difference between a modern car (very stiff "cage" around the occupants with designed-in crush zones outside that) and an old one (in which the passenger compartment deforms).

Here is one you don't want to be in ...

http://www.youtube.com/watch?v=8kuSghb7P7U

Here's what happens when a new car hits an old one ...

http://www.youtube.com/watch?v=fPF4fBGNK0U&feature=related

Take particular note of the interior views. The interior of the new car is hardly deformed. The old car is quite another matter.

Here is another good one comparing a modern vehicle with an older one.

http://www.youtube.com/watch?v=uvoumToSP5c

 

[gruntguru]

One reason your wife may be having trouble, is she is relating to her experience of hitting a deer. There is a big difference between hitting a deer and colliding with something having similar mass to your own vehicle. Colliding with a deer would be no problem in any vehicle with: 1. sufficient mass and 2. sufficient strength in the passenger cell. Not much need for crumple zones.

The most valid collision scenario is one between two identical vehicles. Two identical, rigid vehicles produce a less survivable collision regardless of mass. Mass becomes irrelevant - it is the deceleration distance (crumple dimension) that determines the magnitude of the deceleration the occupants are subjected to.

 

[tygerdawg]

Besides the crumpling issue (which is correct), a few are dancing around the physics involved with a collision. If you'll recall from that beastly Dynamics class:

Kinetic Energy = (1/2)mV^^2

So for unfortunate Bambi crossing your path, the KE is almost zero. Versus your poor car buzzing along at 45mph...no contest. Masses AND velocities come into play. Some of the modern weaponry these days rely heavily on the V^^2 component to deliver enough KE to damage a target sufficiently. Think of the NATO 5.56mm round.

Then also, remembering "Conservation of Energy"...all that KE(car1) and KE(car2) must go somewhere. Hopefully car1 and/or car2 can take all that KE and turn it into mechanical deformation energy and heat. But usually there are some other components of re-directing one KE vector into another KE vector, which produces those abrupt changes in momentum ("accelerations") that snap necks and such. Think of Dale Earnhardt's accident when he hit that wall at almost 200mph. Now most all them boys wear those collars.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[TheTick]

Mortals are silly!

 

[rmw]

StructuralEIT, you are making 2 mistakes. (I am making the assumption that the EIT in your handle indicates your age to be young to some degree.) I hope what I say here will help you.

First, you are arguing with a non technical person. I make this assumption based on her position in this case. A technical person would know better right off.

Second, you are arguing with a woman. Sorry, but to most of them that I have ever met, logic and facts mean nothing, emotions mean everything.

Lots of good and valid information has been presented here by experienced professionals, but I don't think it will matter a twit to further your case.

The operative words are "Yes Dear, you were right, I am sorry." Then go to work and impress your technical buddies with your technical gleanings from this thread.

And hope she never gets in a head on collision.

rmw

 

[SomptingGuy]

... before trying to explain impact mechanics, try warming up with something simpler, like heat transfer as it relates to home heating/cooling.

- Steve

 

[Lion06]

I tried some simple examples - here is one: I asked her if someone were going to hit her in the stomach with a bat, would she rather have a book tight to her stomach or a pillow. Of course she goes and says she'd rather a book thinking that the person would hurt their hands when they hit it!!!

The second was I asked her to put a book tight to her nose. I asked if she would rather I punched the book or rather I replace it with a pillow.

Neither of these examples got my point across.

 

[GregLocock]

One of the odder results we got from a deep dive into NHTSA crash data was that not only are heavier vehicles safer than light vehicles in collisions, as you'd expect, but also heavier variants of a given model were (slightly) safer than their lighter brethren. This is despite the fact that often these heavier variants were convertibles.

Newton's laws really work.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[zekeman]

Maybe one vote for your wife.

A stiff car collides with a soft car. The forces of impact are equal so deceleration is inversely as the masses and thus I will choose the larger stiffer car as safer against a smaller softer car, if that is the test.If the cars are of equal mass, then the decelerations are equal and there is no benefit of the softer car, but only for this test.


So your wife is not exactly wrong for this .

However if you choose to hit a tree, the reverse is true.

In summary,I'll take a tank any day for this test.

So, go buy your wife a Hummer.

 

[aafuni]

If you have a heavy stiff car and everyone else has a crumpley small car, your car will benefit from the crumpley cars design (although not as much as if you both had crumpley cars). As long as your wife is selective in what she hits (only small crumpley cars and soft squishy animals) the stiff car will be a cheaper choice as it will suffer less damage.
-AAFuni

 

[IRstuff]

write this 100 times in neat handwriting:

I'm right [≠] she's happy

TTFN

FAQ731-376

 

[jmw]

And that is why engineers get divorced, they spend too much time focussing on the problem and not the client (in StructuralEIT's case, his wife).
A mistake I also made in this thread....a well earned star for RMW.

This is one time the client is always right.....
This is one time when you could be IK Brunel and a world expert and win (?) the argument but lose the war....

This is also an illustration that we should be wary of being focussed on the wrong problem by the client.
In our case the client is StructuralEIT who presented an engineering problem which we all tried to solve rather than a marriage counselling problem which it really is and RMW spotted.

Listen to the rest of us and pretty soon Structural would be sitting his wife down and lecturing her with Power Point presentations and You Tube videos helpfully provided by the Eng-Tips members.... only for StructuralEIT to end up as road kill.

You are obviously in real trouble.
Offering to put a book in front of the wife's nose and punch it is not recommended. It does suggest that this has escalated already into a fairly intense argument.
The more you argue, the more stubborn she gets.
She will never ever accept the real answer (unless you can get it printed in the Sunday Supplement horoscopes).

No, a sudden reversal on your part will only make things worse.
If you've been saying "its only simple mechanics that a three year old could understand, so why doesn't she?" then you are way past "Hey, guess what, I was wrong and you were right honey."
Because if she was baring her teeth before, she might skip straight past going home to mum and on to her own experiment with you as the soft squishy animal.

Your only hope now of saving your skin and maybe even your marriage is to generate a plausible explanation that shows your wife to be right.

So, everyone, heads together and fake up a way to show, scientifically, that the wife is right, complete with PowerPoints, graphs, a hockey stick chart and You Tube videos and all planted on the internet somewhere (OK, you can create a fake web page and run it in IE6).

It can't be all that hard, I mean, look what they've been able to do with a few tree rings and a some ice cores.


JMW

http://www.ViscoAnalyser.com

 

[btrueblood]

Phew! Jmw, star for the excellent analysis of the problem, and outline for solution. Good luck to Structural...

 

[aafuni]

Well in the collision with a deer she is right, she would be better off with a rigid car. Colliding with something with a mass much less than the car will not kill you from rapid deceleration. The danger in an accident with a dear comes mainly from the possibility of the deer coming through the window at you. For this reason the ridged car is the better option as it will sustain less damage and be a cheaper repair, although it will make mush out of the deer.

For the purpose of this conversation neglect the car vs. car situation and stick to the deer situation, in which your wife is sort of right, it is best that the car didn't really get that damaged.

 

[drawoh]

StructuralEIT,

Part of your problem is that your wife is looking at a specific accident, and you are looking at wide range of potential accidents. She actually is right, that a sturdy car sustains less damage when she hits a deer. This also is true for supermarket fender benders. If she hits a concrete abutment, or another car head-on, this is not true.

Think about all the people who bought SUVs because they were bigger, therefore, safer in a crash. They focused on the head-on collisions, and they ignored roll-overs, which is what makes a lot of SUVs more dangerous than cars.

Quite a few years ago, I have driving on a limited access highway at 100km/hr when a deer ran across in front of me. I was in a 1988 Honda Civic. If I had hit the deer, it probably would have gone through the windscreen, and it would have been very dangerous.

JHG