120mph crash test

[kbui]

Visualize the problem this way. Imagine 2 exactly the same vehicle hitting perfectly head-on. The middle plane where the 2 cars meet will be the symmetry plane. Then all the mass of each car from the symmetry plane all the way to the back of each vehicle will have a velocity of 120mph at impact. The crushing of the each vehicle will start at the symmetry plane and progress toward the back of each vehicle. This happens because the mass at the front of the each car slowed down first and the back of each car keeps on coming. The progression of the crushing is slowing each car until all the mass of each vehicle is completely slowed down and rebound. Neither car's front end will pass thru this symmetry plane. Hence it's equivalent to each car is hitting a rigid/fixed plane at 120mph. All cars can only be designed for survivable accidents. Most will have their front end completely crushed at about 30-40mph into a rigid/stone wall. When you double or triple the speed, your kinetic energy is increased by a factor of 4 to 9. There is not much hope unless you have a full hi-strength steel cage or a tank. Even if you can design a structure to withstand that kind of crash, I am not so sure the occupant can survive from the head deceleration alone.

[kbui]

Originally Posted by CC Performance

I think the zones and they way they deflect the energy will definitely have an effect.. I dont think its survivable no matter what but I'd like to see them do another collision with two cars this time and lets see the results... its very interesting the way they have advanced auto safety....

We do this all the time with car crash simulation. It's also a lot cheaper with Finite Element method than actual crash tests. Auto makers worldwide have been using a product called ls-dyna (I do this for a living) to do most if not all of their crash work. Actual crash tests are only needed at the end of the design cycle to prove it to NHTSA.

[DGthe3]

Originally Posted by kbui

Visualize the problem this way. Imagine 2 exactly the same vehicle hitting perfectly head-on. The middle plane where the 2 cars meet will be the symmetry plane. Then all the mass of each car from the symmetry plane all the way to the back of each vehicle will have a velocity of 120mph at impact. The crushing of the each vehicle will start at the symmetry plane and progress toward the back of each vehicle. This happens because the mass at the front of the each car slowed down first and the back of each car keeps on coming. The progression of the crushing is slowing each car until all the mass of each vehicle is completely slowed down and rebound. Neither car's front end will pass thru this symmetry plane. Hence it's equivalent to each car is hitting a rigid/fixed plane at 120mph. All cars can only be designed for survivable accidents. Most will have their front end completely crushed at about 30-40mph into a rigid/stone wall. When you double or triple the speed, your kinetic energy is increased by a factor of 4 to 9. There is not much hope unless you have a full hi-strength steel cage or a tank. Even if you can design a structure to withstand that kind of crash, I am not so sure the occupant can survive from the head deceleration alone.

Which is why the ZL1 from the Nurburgring had a cage & race seats

[Number 3]

Kind of reminds me of the joke, "what's the last thing that goes through a bugs mind when it hits the windshield?"

This is why crash testing is done with a gazzilion ton steel wall. It is intended to simulate two cars of exactly the same mass hitting each other head on at exactly the same speed. So a 30 mph government crash test simulates two identical cars hitting each other head on at 30 mph for the unbelted test.

An unbelted 30 mph frontal crash is a pretty scary thing to watch in real time and in person, btw.

[The_cartoonist_88]

watched the clip again and at the begining he does state "2 cars hitting head on while both traveling at 120mph" so i would guess that the total force would actually be 240mph

[CC Performance]

Originally Posted by DGthe3

Note, if I've gotten any facts wrong in the above, just ignore any points I made with them

[PeeBee]

Originally Posted by hypurone

He misstated the effect being equivalent to 2 cars hitting head on at 120mph each. It should have stated as "head on at 60mph each"...

"A head on traffic accident occurs when the front of one vehicle collides with the front of another.
These types of accidents are extremely dangerous because of the cumulative speed affect of
vehicles colliding in opposite directions. Accident reconstruction experts often refer to the
combined difference in speed as closing speed or closing velocity.

For example, if two vehicles collide head on with each traveling at 30 MPH, then the closing speed
between the two vehicles would be equal to 60 MPH. If the same two cars were to collide, with one
of the one of the vehicles stopped, then the striking vehicle would need to be traveling 60 MPH to
equal the same closing speed."

Either way you cut it, it's nasty!

Erhm.... "The closing speed between two vehicles".... I think that is where you're taking the wrong decision. Indeed, if two vehicles move towars each other, their closing speed will be the sum of both speeds. Both vehicles also have crumple zones designed to absorb the forces of impact.

Take out one vehicle and replace it by a solid, non deformable object, like a concrete wall... and you're looking at a totally different perspective, since the only thing having crumple zones is the vehicle that moves, which will inevitably magnify the effect of the impact.

[DGthe3]

Originally Posted by CC Performance

I used a line very similar to that in a post few months after I joined, I've had it as a signature for about 4 years now.

Originally Posted by PeeBee

Erhm.... "The closing speed between two vehicles".... I think that is where you're taking the wrong decision. Indeed, if two vehicles move towars each other, their closing speed will be the sum of both speeds. Both vehicles also have crumple zones designed to absorb the forces of impact.

Take out one vehicle and replace it by a solid, non deformable object, like a concrete wall... and you're looking at a totally different perspective, since the only thing having crumple zones is the vehicle that moves, which will inevitably magnify the effect of the impact.

You are correct in that two cars can absorb twice the energy. But they'll also have twice the energy at impact, so it cancels out.

[GEEo]

Dammm!! that was crazy!!

[CamaroSkooter]

At those speeds, a crumple zone isn't going to help much...

[TAG UR IT]

Ouch...

[PeeBee]

Originally Posted by DGthe3

You are correct in that two cars can absorb twice the energy. But they'll also have twice the energy at impact, so it cancels out.

Keeping in mind that E = mc², I'd say the energy of two moving cars would be about 4 times the energy as one car moving and an object in rest, hence why they refer to it as the equivalent of two cars travelling at 120mph, I believe... (which brings it to, as the Mythbusters link also shows, double the force, because of the fact that the cars can absorb double the energy of one car and a non movable object, effectively dividing the energy over the mass of the two cars).

Either way, it's pretty simple.... crash into a car at 120mph (be it both moving or not) and you'll not walk away because of the immense G forces at work during the deceleration... (and that is, I believe where the "crumple zones" actually start doing their work, by dividing the energy during the transition from moving to standing object).

[CamaroSkooter]

E = mc² has nothing to do with conservation of energy. It's just a concept that the mass of an object is a measure of its energy content. The "c" in that equation is merely a constant to convert between the two.

The equation you are thinking of is for kinetic energy, which is E = 0.5mv²

In a closed system, the laws of conservation of energy dictate that the amount of energy in the system is constant. The problem with trying to apply this principle to vehicle collisions has to do with the elasticity of the collision and how the energy in the collision does not remain solely as potential or kinetic energy.

Crumple zones in a car don't really "absorb" energy. They just lengthen the time of impact. The longer the impact takes, the less G-forces encountered by the occupant.

But, like I said, at those speeds, the crumple zones won't really do much...

[DGthe3]

Originally Posted by CamaroSkooter

Crumple zones in a car don't really "absorb" energy. They just lengthen the time of impact. The longer the impact takes, the less G-forces encountered by the occupant.

But, like I said, at those speeds, the crumple zones won't really do much...

The crumple zones did something ... they crumpled. What more could you ask of them, saving the occupants lives? Oh yeah ... thats what they're there for, isn't it? Oh well. Anyhoo ...

You are right that they slow down the crash event, and that is how they save lives. But how do they slow the crash down? They slow it down by ... absorbing energy. Like a spring. But reverse the efficiency so that instead of wasting a couple percent of the energy as heat & sound while storing the vast majority as potential energy (which then gets released as kinetic energy), they convert the vast majority of the into heat and sound with a minimum being stored & released