Which car is safer?
Giving similar driving conditions and driving skills/habits, certain cars are much safer to ride than others. I have been working with 4 big trauma center/hospital emergency rooms in the US and am now dealing with those MVAs all the times. It is apparent to us and the EMS guys, who rescue and bring the victims to our ER, that drivers in those small mostly Japanese cars do sustain a higher rate of fatality/serious injuries than most American and Europian cars. Those ER docs drive Volvos, bimmers, SUVs, bigger American cars not because they can afford them (well, they can). Those EMS/firefighter guys like full-size SUVs and big American cars not because they are physically bigger than average guys. A very important reason is that they’ve seen enough about which car will protect the drivers poorest in an accident. So many drivers also told them that they will never touch that Corolla again!
So what makes a car safer? I am no engineer, but the following seems reasonable explanations. I welcome additions and comments.
Better design such as door-beams and heavy gauge steel bar which are more often found in Europian vehicles than Japanese vehicles. Safety features such as airbags, anti-skid, anti-rollover devices. Relative mass of the vehicles, that is heavier vehicles tend to protect their drivers better. Better handling and better reliabilities.
The Crush test score is a misleading piece of information. The reason it is being used widely is because there is no other reproducible data to use. It is just one test you can apply to any vehicles under similar conditions, in which you crash a car into a wall or a "stationary mass" at 35 or 40 mph. If there is no serious “injury” to the dummy, then the car gets a higher score. Therefore, many Japanese cars are designed to have a softer crumple zone under the lower test speed so they can get a better test score while stiffer vehicles tends to get a lower score because they claim the energies are being absorbed by the driver. As everybody can see (I hope) here, it is flawed. First of all, the crashing speed is too low. If two cars hit head-on, the effective speed is going to pass at least 60 mph. So the score obtained under the test condition which showing a good “protection” to the driver is really a bad thing because the diver is more likely to be crashed under higher speed crashes. The stiffer cars, with protective stiffer “cage” of the vehicle and pretensioner safety belt and other features, are more likely protect the drivers. Secondly, in real life crashes, you want to be in a car like the "wall" in a crash-test with larger mass, not the ping-pong car. This point I’ll elaborate more in details. So, those jap cars made of cheap plastics and thin steels will lose out big in a real life higher speed crashing into a heavier stiffer cars. But crumple zones are still a good thing and larger cars/SUVs also need them – but stiffer ones with better designs. Less stiff does not necessarily translate into better crumple zones. On a better designed vehicles, such as in Volvo’s case, crumple zones are strategically placed in front and rear portion of the frame (often appear as tiny waves), so that when enough force is applied (i.e., when vehicle is hit hard), the frame will bend at certain points, and in certain directions. In other words, the way it "crumples" is controlled, and not by relying on the random way the metal may fold. Too much or too soft a crumple zone only helps more the other vehicle who you hit. Think about that. In a head-on crash between a BOF and a unibody, the crumple zone in the unibody reduces the peak crash forces experienced by the occupants of both vehicles. This is the case because in a two-vehicle crash the vehicles exert equal but opposite forces on each other.
I like body on frame design vehicles. A BOF vehicle is more rigid than a unibody vehicle. In the parlance of the "crash research" community, a BOF vehicle is stiffer. A unibody vehicle has a deeper and more controlled crumple zone and gives more in a crash. If the intergity of the passenger compartment is not compromised, a unibody vehicle protects its occupants better in a relatively lower-speed crash than a BOF because the unibody vehicle spreads out the crash forces in time and reduces the peak crash accelerations experienced by the occupants.
Now turn back to the vehicle mass/weight issue. This is one of Newton's laws relating force and motion (you guys know better). The forces are always equal in magnitude, but if one of the vehicles is more massive than the other, the effect of the forces on the vehicles will be different. Remember that F = m * a, so a = F/m. That is, the crash deceleration of the lighter vehicle will be greater than that of the heavier vehicle because the F is the same for both vehicles, but m is different, and this is in the denominator.
The injury to the occupants of a vehicle is strongly related to the peak crash deceleration that the vehicle experiences. Consider a head-on crash between two Suburbans, identical except that one is red and one blue. Each is travelling at 30 mph relative to the ground. High and identical peak crash forces are transmitted to the occupants of the two vehicles
Now consider the same crash except that a serious but lightweight engineered crush zone 4 ft in depth has been attached to the front of the red Suburban. That is, the two Sururbans are still the same mass. The result will be lower peak crash decelerations to both vehicles. The crumple zone strapped to the red Suburban has protected equal benefit to the occupants of the blue one.
If both vehicles had had the same 4-ft crumple zone, then the peak crash decelerations would be even less because there would be a total of 8-ft of crumple zone. If each vehicle had had the same design 2-ft crumple zone it would be the same as either one having a 4-ft crumple zone.
BOF and crumple zone are not mutually exclusive. Framed vehicles can have crumple zone built in them as well. In fact, the concept of the crumple zone is often combined with providing a more rigid structure encompassing the passenger space. Mercedes has been using crumple zone technology since the late '50s.
Within the limits of what people can afford, every attempt should be made, and presumably is made by the manufacturers, to make the passenger compartment as rigid as possible. You don't want the passenger compartment to deform, nor do you want anything to "intrude" into the passenger compartment. Clearly intrusions are potentially extremely injurious. It seems to me that many Japanese vehicles using less strong gauged materials/designs could have less rigid and therefore lesser passenger compartment protection.
I personally am attracted to the idea of a body-on-frame design because I drive conservatively. I value strength and durability in a vehicle. My ideal vehicle would be a BOF "cross-over” SUV with relatively high seating position (comfortable leg angle), 3000-lb curb weight, powered by a strong diesel engine and a 6-spd manual. It would be rated to tow up to 6000 lb. It would have a variable height suspension to get best fuel economy on pavement, but be able to go on rough roads when necessary. It better has shift activated 4-wheel drive with a real mechanical rear transfer cage so I can drive nicely on snow as I have to be on-call in snow days as well. Speaking of on-call, could those old guys driving a Crown Victoria in the left lane yet still have the left blinker on all the time doing 50 mph in a 65 mph zone move to the right lane? Many people do respond to emergent situations.
