IIHS says small cars are more dangerous

[Neil]

Physics.

Chemistry.

Chemistry is nothing more than applied physics anyways.

[Iormlund]

As I said, the big shock to me is how dangerously misleading the official safety ratings are.  Yes, they do make a disclaimer that frontal impact results can be compared only against like cars.  However, would you really know from that disclaimer that a car with perfect safety ratings can still be up to three times as likely to kill you compared to a heavier car?

Crashes often involve things like trees, trucks or pillars which are not going to give way, no matter how big your car is. In that sense the standard test is much more accurate.
Mind you, the article is unclear on a very significant point, I suspect these car to car tests were carried out at a combined velocity of 80 mph. That is a lot, and it is no wonder lighter cars suffer heavily, since they experience much more decel than in standard tests against a barrier, thus the comments from the manufacturers.

[KRonn]

As I said, the big shock to me is how dangerously misleading the official safety ratings are.  Yes, they do make a disclaimer that frontal impact results can be compared only against like cars.  However, would you really know from that disclaimer that a car with perfect safety ratings can still be up to three times as likely to kill you compared to a heavier car?

That is scary and the comparisons here were just between the very small cars and mid size cars, not vs large cars or large SUVs. Plus the single car accidents hitting trees, poles or what ever which can be bad; I kind of think those kinds of accidents aren't counted in the safety stats? Not sure though. Cars are a lot safer though over all.

[DGuller]

Yes?

So why is the result only valid against a car of similar size?

The physics of a car hitting an immovable object at 40 mph is equivalent to the physics of the car hitting another car of the same size head-on when both are going 40 mph in opposite directions.

[DGuller]

Mind you, the article is unclear on a very significant point, I suspect these car to car tests were carried out at a combined velocity of 80 mph. That is a lot, and it is no wonder lighter cars suffer heavily, since they experience much more decel than in standard tests against a barrier, thus the comments from the manufacturers.

If two cars of the same weight crash at 80 mph combined velocity, then the result is the same as crashing one car at 40 mph into immovable barrier.  They shouldn't experience more deceleration in one test compared to another. 

The small cars will suffer more damage in car-to-car tests if crashed against bigger cars, but that was the whole point.  In the real world, if you're in a small car, you're likely to crash into a bigger car when you have a head-on, not into a car of the same size.

[Iormlund]

Mind you, the article is unclear on a very significant point, I suspect these car to car tests were carried out at a combined velocity of 80 mph. That is a lot, and it is no wonder lighter cars suffer heavily, since they experience much more decel than in standard tests against a barrier, thus the comments from the manufacturers.

If two cars of the same weight crash at 80 mph combined velocity, then the result is the same as crashing one car at 40 mph into immovable barrier.  They shouldn't experience more deceleration in one test compared to another. 

That's impossible. You can't bypass the laws of physics.

The small cars will suffer more damage in car-to-car tests if crashed against bigger cars, but that was the whole point.  In the real world, if you're in a small car, you're likely to crash into a bigger car when you have a head-on, not into a car of the same size.

I guess that depends largely on your city. On mine it is rare to drive on a fast, two-way street that has no physical separation in the middle. Crashing onto a tree or a lamppost is much more likely than into oncoming traffic.

[The Brain]

Mind you, the article is unclear on a very significant point, I suspect these car to car tests were carried out at a combined velocity of 80 mph. That is a lot, and it is no wonder lighter cars suffer heavily, since they experience much more decel than in standard tests against a barrier, thus the comments from the manufacturers.

If two cars of the same weight crash at 80 mph combined velocity, then the result is the same as crashing one car at 40 mph into immovable barrier.  They shouldn't experience more deceleration in one test compared to another. 

That's impossible. You can't bypass the laws of physics.

Elaborate.

[DGuller]

That's impossible. You can't bypass the laws of physics.

I'm not trying to.  In what way do you think I'm bypassing the laws of physics?

[derspiess]

A friend of my wife's bought a smart car.  Definitely one of the most inappropriately named vehicles ever.

[ulmont]

That's impossible. You can't bypass the laws of physics.

Force - Colliding With a Wall
Consider case A, in which car A collides with static unbreakable wall. The situation begins with car A traveling at a velocity v and it ends with a velocity of 0. The force of this situation is defined by Newton's second law of motion. Force equals mass times acceleration. In this case, the acceleration is (v - 0)/t, where t is whatever time it takes car A to come to a stop.
The car exerts this force in the direction of the wall, but the wall (which is static and unbreakable) exerts an equal force back on the car, per Newton's third law of motion. It is this equal force which causes cars to accordion up during collisions.

It is important to note that this is an idealized model. In case A, the car slams into the wall and comes to an immediate stop, which is a perfectly inelastic collision. Since the wall doesn't break or move at all, the full force of the car into the wall has to go somewhere. Either the wall is so massive that it accelerates/moves an imperceptible amount or it doesn't move at all, in which case the force of the collision actually acts on the entire planet - which is, obviously, so massive that the effects are negligible.

Force - Colliding With a Car
In case B, where car A collides with car B, we have some different force considerations. Assuming that car A and car B are complete mirrors of each other (again, this is a highly idealized situation), they would collide with each other going at precisely the same speed (but opposite directions). From conservation of momentum, we know that they must both come to rest. The mass is the same. Therefore, the force experienced by car A and car B are identical and are identical to that acting on the car in case A.

http://physics.about.com/od/energyworkpower/f/energyforcediff.htm

[Iormlund]

The cars are not mirrors of each other. So if one weights much less than the other conservation of momentum dictates it'll experience much more deceleration during a crash. There's no way around it.

[Iormlund]

Also, there is certain drawback from massive cars: the amount of energy they need to absorb in a crash against something like a tree or a bus is accordingly greater.

[Neil]

Also, there is certain drawback from massive cars: the amount of energy they need to absorb in a crash against something like a tree or a bus is accordingly greater.

True, but they are generally much better built than smaller cars.  Steel has a much better strength/weight ratio than unibody plastic.

[Razgovory]

The cars are not mirrors of each other. So if one weights much less than the other conservation of momentum dictates it'll experience much more deceleration during a crash. There's no way around it.

What if one weighs more but the other has more mass?

[DGuller]

The cars are not mirrors of each other. So if one weights much less than the other conservation of momentum dictates it'll experience much more deceleration during a crash. There's no way around it.

I was talking about cases where they were mirrors of each other.  Re-read the post you quoted when you said it was impossible.