Video: 50mph BMW 535i vs. Volvo 960

[DueyT]

I think you mean 120mph. Even worse.

No, 60 mph relative to the inertial frame of reference within wich the vehicles are being considered. While it is true that their closing speed relative to each other is 120 mph, this is not at all equivalent to one vehicle standing still, and the other hitting it at 120 mph.

Conservation of energy proves this. Let's say v (velocity) = 60 mph, and both cars equally weigh mass, m. Total kinetic energy just before collision is:

Car 1: K.E. = 1/2 m * v^2
Car 2: K.E. = 1/2 m * v^2
Total: K.E. = m*v^2

If someone said that's like one car hitting a stationary car at 120mph (2 x 60mph or 2v) the energies would be:

Car 1: K.E. = 1/2 m * (0)^2 = 0
Car 2: K.E. = 1/2 m * (2v)^2 = 1/2 m * 4v^2 = 2m*v^2
Total: K.E. = 2m*v^2

m*v^2 does not equal 2m*v^2, so the 120mph effective impact (Energy wise) is not a valid construct.

To make the conversation more intersting, when do you steer for a wall and when do you still hit the oncoming car?

Conservation of momentum (product of mass times velocity, mv...not kinetic energy, 1/2mv2) makes us consider the head on vice wall crash carefully. If you have greater momentum than the other car, then you are better off hitting it, than a concrete wall. Your greater momentum means you will slow down less than the other vehicle...which would actually reverse direction of travel. The difference may be slight, for example, say your car weighs 3500lbs and the oncoming vehicle weighs 2500 lbs. Calculating conservation of momentem, you would slow from 60 to 10mph, a -50 mph change, while the other guy would slow from 60 to 0 then be pushed backwards by you at 10mph opposite to his original travel...overall change of 70mph vs. your 50mph change...ouch!

Now, if the vehicles have the same mass... then both come to 0 mph...exactly as if you had hit a wall (interesting!)

OK, what if you have less mass (against say, a Brinks armoured truck full of coin!)...now you're heading backwards and you would have been literally better off hitting a brick wall.

Interestingly enough, a wall (or other unyielding structure) is actually a fully variable force structure...if strong enough (so it's structuraly integrity is not compromised) it will "push back" against the car with "just enough" force to counter the car's energy...perfectly! No less (or it would yield, crumble, chip, etc... and the car would continue forward) and no more (otherwise the wall would actually "push" the car backwards after the collision.)

Food for thought...


Cheers,
Duey

[Craig]

Duey-

Great proof! Never thought to actually work it out, I always just ASSumed. Neat, thanks!

BS Physics, University of Washington 1998

[henryw525i]

I reckon the rear seat passengers in the BMW would have been "alive" It was an M20 525iA with newer version gauge cluster...and cruise control.

Daym I wish I lived in the UK and could buy one for a grand!!!

[DueyT]

The frontal quartering (a.k.a. offset frontal) impact is one of the worst out there...well, aside from being T-boned.

Any greater overlap between the cars and both engines would have submarined below the body and both frame rails of each car would have more evenly absorbed/disappated the energy.

Not that either car was in good shape, but I think the BMW still did slightly better... Now if they did a head-on collision test with a BMW (or Volvo) with, say a 2004 Chevrolet Cavalier...the meat-wagon would definitely be pulling up to the Chev.

Cheers,
Duey

[Brian C.]

Luckily I just take photos at the crashes I go to. The Traffic Investigation Guys are the ones that do all sorts of skid and vehicle weight calculations. There's actually one old guy who's been doing it for 30 years who still uses a slide rule! Hey.....if it works....

Brian C.

[DueyT]

Brian, do your TCI guys have TotalStation? That's a real sweet setup...it backs up the slide rule really well. My buddy loves it because he can survey a scene literally in a few minutes...that sure beats hours with a 100 yd measuring tape!

p.s. Do the guys think that the Cavalier/"Death-fire" (a.k.a. Sunfire) are vehicles you might want to stay away from?

Cheers,
Duey

[Dan in NZ]

I haven't actually watched the video, screw downloading 50mb... But if the BMW was in "better shape" visually, would that mean it's structure has deformed less, increasing the rate of deceleration, meaning greater injuries to the occupants? Assuming equal vehicle masses etc etc etc.

[rickm]

They didn't seem to focus too much on the rear seating, but if you were sitting up front in either car you'd now resemble chopped clams.

Dan: the Volvo ended up on it's roof, the BMW rolled once. The top of the BMW looked ok (anyone want a sunroof?), the Volvo you couldn't see. I wish the Volvo was a lighter color so you could see more details...that burgundy blended all too well with the undercarriage (though i'm sure the video compression didn't help). Either way that was a nasty wreck. I want to see a 7 series against a Ford Festiva.

An hour of these videos would make a great PPV. Or some good material for the driver's ed classes.

[Brian C.]

...The Traffic guys use it a lot and seem to think it's just peachy, but the Crime Scene Guys in my office like to refer to it as Total Waste of Time. I think it's better suited to large scenes like roadways and intersections as opposed to inside a house or building. It's one of those toys that the CSIB people all roll their eyes about when somone asks if they've "total-stationed" the scene yet. I've never personally used it. In an interior scene they have laser-tape measures that will give them interior-to-interior wall dimensions, and then they do a rough sketch. Then they will do a more thourough scale drawing back in the office. 75% of these people have been doing this same job for 10-20+ years so they are not too conducive to new things. I know that the TS data can be downloaded to a CAD plotter and do a drawing that way but they don't do that here yet. And if you get your scene done too fast, the overtime is'nt adding up as quickly!

Brian C.

[Hector]

I remember COE and COM being shoved down my throat cuz I think the prof had a relative who analyzed distance traveled of totaled cars in traffic accidents and such. So we ate this stuff 3 times a week for a month!! I can still taste it :o)

No, 60 mph relative to the inertial frame of reference within wich the vehicles are being considered. While it is true that their closing speed relative to each other is 120 mph, this is not at all equivalent to one vehicle standing still, and the other hitting it at 120 mph.

Conservation of energy proves this. Let's say v (velocity) = 60 mph, and both cars equally weigh mass, m. Total kinetic energy just before collision is:

Car 1: K.E. = 1/2 m * v^2
Car 2: K.E. = 1/2 m * v^2
Total: K.E. = m*v^2

If someone said that's like one car hitting a stationary car at 120mph (2 x 60mph or 2v) the energies would be:

Car 1: K.E. = 1/2 m * (0)^2 = 0
Car 2: K.E. = 1/2 m * (2v)^2 = 1/2 m * 4v^2 = 2m*v^2
Total: K.E. = 2m*v^2

m*v^2 does not equal 2m*v^2, so the 120mph effective impact (Energy wise) is not a valid construct.

To make the conversation more intersting, when do you steer for a wall and when do you still hit the oncoming car?

Conservation of momentum (product of mass times velocity, mv...not kinetic energy, 1/2mv2) makes us consider the head on vice wall crash carefully. If you have greater momentum than the other car, then you are better off hitting it, than a concrete wall. Your greater momentum means you will slow down less than the other vehicle...which would actually reverse direction of travel. The difference may be slight, for example, say your car weighs 3500lbs and the oncoming vehicle weighs 2500 lbs. Calculating conservation of momentem, you would slow from 60 to 10mph, a -50 mph change, while the other guy would slow from 60 to 0 then be pushed backwards by you at 10mph opposite to his original travel...overall change of 70mph vs. your 50mph change...ouch!

Now, if the vehicles have the same mass... then both come to 0 mph...exactly as if you had hit a wall (interesting!)

OK, what if you have less mass (against say, a Brinks armoured truck full of coin!)...now you're heading backwards and you would have been literally better off hitting a brick wall.

Interestingly enough, a wall (or other unyielding structure) is actually a fully variable force structure...if strong enough (so it's structuraly integrity is not compromised) it will "push back" against the car with "just enough" force to counter the car's energy...perfectly! No less (or it would yield, crumble, chip, etc... and the car would continue forward) and no more (otherwise the wall would actually "push" the car backwards after the collision.)

Food for thought...


Cheers,
Duey