greenhouse

Weight ratios between the cars will typically be dominant factor in head-ons with modern cars. Then probably matching bumpers. Both favor SUVs. A ford Excursion and 911 hit at 50 each, I'd much rather be in the Ford. It looks like the passenger side took it a little worse here. Was the BMW a 3 or a 7-series?

I buy the drunk and loose thing; seen drunk guys get clocked and roll with it.

Your example, assuming equal weight is like 70 to 0, not 140 to 0, i.e. if both are going 70 it's like hitting a stone wall at 70.

If you're hitting a wall/tree etc. it's all about how much time you have to decelerate- it's an impulse problem, it's a big part of what a helmet is designed for since your skull doesn't give much time to decelerate. The design of a car's crumple matters, but so does the length of the hood- more distance means more time to decelerate.

A well-strapped body can withstand 20+ g's, but 3pt seatbelts aren't NASCAR approved.

So yeah a 911 may be well-designed, but it's light, has a low bumper, and a not-long nose. In a 70 mph pure head-on, no deflection, you're dead. 55 mph vs a 7-series, the same. In car-to car collisions, I thought getting T-boned was the worst because there is no crumple and the force is in a direction your body doesn't cope with as well.

texas911

And I'm sure you guys were the model of maturity at age 18.

OCBen

Good point. And worthy of discussion.

Two cars equal weight (ie equal mass) traveling down the freeway in the same direction in the same lane. The car in front is traveling at 65 mph while the approaching car behind him is traveling at 70 mph and rear-ends him. The speed of collision is 5 mph, and the damage done to both vehicles is from a 5 mph collision.

Same two cars. But this time traffic up ahead has slowed to a stop. The guy doing 70 is texting his girlfriend and is not paying any attention to traffic, and plows right into the stopped car in front at 70 mph. The speed of collision is 70 mph and the damage done is from a 70 mph collision.

Same two cars again. But this time the young driver is driving drunk and gets on the freeway at an exit ramp and stays in the "right-most" lane doing 70 and plows head-on to the other car traveling 70 mph in the carpool lane. The speed of collision is 140 mph and the damage done is from a 140 mph collision.

Now let's compare it to a car doing 70 and plowing into a 10 ft deep solid stone wall. Let's also assume that the collision is purely inelastic (ie a plastic collision with no elastic "bounce") and that the car just crumples into the wall and stops dead at the wall without bouncing off it, and that the wall is unfazed by the collision. Now let's remove the wall and draw an imaginary dotted line where the wall was, and let's have the two cars of identical mass and 70mph speed of opposite direction meet and collide head-on at this imaginary line with the same plastic collision assumption - i.e. no bounce. Both cars would come to a complete stop at this imaginary line, just as the one car did in colliding into the wall. Now while the speed of collision was 140 mph in the head-on collision and 70 mph in the wall collision the two collisions are indeed identical in terms of conservation of energy. The kinetic energy in each case (½*m*v^2) was brought to a complete stop and was absorbed internally by the car resulting in the crumpled up mass at that imaginary line of collision and at the stone wall. So, yes, colliding into a solid stone wall is equivalent to colliding head-on with another car of the same mass and same speed.

Here's what's interesting. In the first example of the car in front doing 65 and the car behind doing 70, and this time assuming a perfectly elastic collision, the car being rear-ended will get a bump in speed and the car behind a drop in speed. If the masses are equal, the car in front will be sped up to 70 mph and the car behind slowed to 65 mph. This follows from the law of conservation of momentum, which of course requires ideal conditions.

In the second example, assuming ideal conditions and elastic collisions, the car at rest will end up going 70 mph while the car plowing into it will come to a complete stop. It's difficult for most to visualize the ideal conditions but this popular toy, Newton's Cradle, demonstrates the principle quite simply. But in actuality the car in front will end up several yards forward of the point of collision and the car doing the rear-ending will end up several yards forward of the point of collision.

So the bottom line, I suppose, is that it is better to run off the road to avoid colliding head-on with a massive Mack truck and take your chances with whatever stationary objects are in your path. In the head-on collision with the truck your car will end up being sent backwards from the point of collision and the energy absorption will be greater and thus deadlier. Trains of course are the deadliest objects to be involved in a collision with, they being the most massive moving objects a car can encounter.

greenhouse

Yeah, that's pretty much it, Ben. I like to normalize to the stone wall thing because then things like mass ratios don't matter; decleration from speed v to 0 instantly at the front of the car.
Newton's laws explain it all as you say.
Perversely, this argues that once a critical mass of SUVs are on the road, you want a SUV to protect your kiddies too, and you get some sort of safety feedback loop because of that m1/m2 ratio.

OCBen

Yup, and you can imagine how I feel driving my tiny Mini Cooper down the freeway next to these big SUVs and trucks. I'm just one notch up better than a motorcycle. And not by much.

But thankfully, the equalizer seems to be the price of gas, as more and more people are shying away from big and buying small because of fuel economy. Of course, that won't have much impact on the presence of 18 wheelers on the road. You'll always want to leave enough safe distance between yourself and these big rigs, as they sometimes lose control. And when they do, you don't want to be in front of them or between them and whatever it is they'll be sliding into.

OCBen

I've never looked at it that way before. Interesting perspective.

But you know, I've been thinking about this comparison and have come up with something else that's also interesting and worthy of further discussion. If instead of a stone wall imagine there to be a solid steel wall just as huge in its place. Now imagine the face of it to be polished smooth like a mirror in reflection quality. And now imagine a test vehicle - of any type and speed - heading squarely for it. The approaching speed of the car with respect to its reflection that is approaching the plane of the "mirror" from the opposite direction at the same exact speed will always by double the actual land speed of the car. Here then is the proof that colliding head-on with another car of equivalent mass and equivalent speed doubles the speed of collision, and is equivalent to colliding squarely head-on with a solid immovable object, both of which are likely to result in death of the occupants.

So of what practical use is this information? Mainly in that if you drive a car that is lower in weight than the average vehicle on the road (this includes the 997, the Mini, etc.) you will likely fair worse in a head-on collision with the average vehicle than if you were to plow straight into a solid concrete wall. And that you should do all you can if possible to avoid such a collision, including running off the road if need be.

Granted, not all head-on collisions are square impacts. The one in the OP hit the 997 just to the right of center of mass, which would have caused it to spin around in the yaw direction after impact, and likely explains its resting orientation with respect to the road as seen in the photos. This fact, that it was not a square hit, may have also contributed to the survival of the young lad.

Be careful out there on the roads, folks. Stay alert. Be mindful of road and driving conditions. Drive defensively. Expect the unexpected. This has been a public service announcement.