And can you explain why less momentum is better for the passengers? The thing that injures you in an accident is acceleration (or more accurately deceleration), so you actually want more momentum.
Imagine sitting in a large truck vs a kei car and driving into a brick wall. One has the chance to deform the wall which decreases the speed comparatively slowly. The other one abruptly stops when hitting the wall.
Unless the collision target doesn't deform at all, the higher momentum is better. And if the target is completely solid, a larger typically has a larger crumple zone which is better, too.
I cant come up with any reason to prefer the smaller car/momentum, except in very constructed scenarios (driving over a cliff with a weak fence for example).
If the object is completely fixed and effectively inelastic, the main determinant in survival is the distance over which the deceleration of the occupant occurs, assuming a lack of cabin intrusion.
So the mass of the vehicle is ideally fixed in relation to its rigidity, and it’s not better to have less, or more, mass except as the ideal ratio to the rigidity of the vehicle.
What you want is a vehicle that decelerates as slowly as possible over the greatest possible time/distance.
A light vehicle that is very rigid is equally terrible as a heavy vehicle that is very rigid. What matters is how many millimeters it takes for each vehicle to come to a complete stop after colliding with the immovable object.
There are a lot of combinations of vehicle mass and rigidity that meet the ideal here, and it is not sensible to assert that a massive or a lightweight vehicle will be ideal in this case.
What you want is a vehicle that, at the speed of the collision, deforms the most completely and to the longest distance possible without incursions into the passenger compartment.
Ideally, the vehicle would have a very long and well engineered crumple zone in front of the occupants. Here, assuming similar material engineering of the impact absorbing structure, length is king. All things being equal, a car with a stubby front profile will expose its occupants to twice the acceleration as one with a 2x longer hood.
Still, the vehicle must have enough mass that the crumple zone is fully compressed at the speed of the collision. If it’s too light, it will decelerate before the impact absorbing distance if fully used.
If it’s to heavy, it will still be moving when the crumple zone is fully exhausted, and either a sudden peak of acceleration or a cabin intrusion will occur when the passenger cage becomes involved in the deformation zone.
I’m going to guess that on average, you’re still better off in a full sized SUV than In a more sensibly sized vehicle, hitting an unmovable object at speed.
You say you have a background in vehicle safety, so maybe you know something I don’t. Please explain where I’m going wrong here?
All things being equal, this is generally a little bit true, even for collisions with rigid, fixed obstacles - but vehicle design is critical here. For instance, a cab forward truck with no real crumple space does not benefit from mass, as the deformation of the pax compartment is the primary concern. A very lightweight vehicle that is very deformable and has a long crumple zone might outperform a heavier vehicle in terms of the acceleration imparted to the passengers.
The problem is that a “fixed” solid object has effectively infinite mass. This applies when the collision displacement of the obstacle can be measured in fractions of a millimeter. In these cases, the ratio of the lightweight vehicle to the effective mass of the obstacle is effectively the same as with the heavier vehicle vs the obstacle. Here, mass pays no dividends. Only the distance / time covered during the sudden deceleration, and the integrity of the passenger compartment. Of course, the efficiency and extent of restraint systems will also play a major role. But it’s not an automatic win for heavier vehicles in this case.
Notably, most collisions are not with immovable objects, but with other vehicles. In that case, mass is going to win almost every time, since the acceleration experienced by a subcompact vs a full sized SUV is about 2x, which means the forces imparted on the passengers in terms of static loads on their bodies and bones will be 4x.
For passenger survivability in most accidents, the incentives fall heavily towards using the heaviest well constructed vehicle that you can afford.
