In a 100km/h collision, seatbelts...

[Pezzoni]

Should not come close to fracturing bone?

This being the result I've got from a calculation for some physics coursework. I find this kind of hard to believe. The average pressure on the body I calculated to be 0.18MPa, with the breaking stess of bone being around 130MPa.
I'm sure I've gone wrong somwhere. It'll be a bit tricky to post my calculations directly into here (Word Formula editor), so I've uploaded them to here:
http://www.rollingchat.com/help.doc

Thanks for any ideas... I'm sure I've gone majorly wrong somwhere!

[Lord Zentei]

I can't see anything wrong with your calculations perse. The possibility exists that you have made some bad assumptions, particularly with regards to the stopping distance. Consider that the force is not a constant over this time period. Consider what happens if another passenger is present and is not wearing a seat belt: he will be hurled forward into the windscreen and be decellerated much faster, hence subjected to greater force.

[Ariphaos]

Should not come close to fracturing bone?

Older seatbelts often had those little sewn-together folds for exaclty the reason Zentei said - the vast majority of the stoppage occurs at the last moment of the collision, so the seatbelt is designed to have a bit more give to it in the case of a serious emergency, and spread the force of impact over a longer course of time.

Also, the seatbelt is not covering ~800 square centimeters of bone. It's covering a far more complex system of such and in the case of impact nearly all of the pressure on the upper body is going to be on the shoulder and upper ribs.

Are there many accidents where pelvic damage occurs in comparison to rib, neck, back, or shoulder damage?

[Broomstick]

I believe the most common seat belt injury in car collisions is injury to the abdominal organs - spleen, intestinces, kidneys, etc. - from malpositioning of the belt.

Collar bones and ribs are likely fracture points as those are not particularly massive structures. On the other hand, a properly designed belt will spread the load over these structures, and ribs in particular are designed to flex much more than many other body structures, and can shift much more before actual breakage.

It's pretty hard to break a pelvis - it's one of the larger and more massive boney structures in the body, after all. You certainly can break one, people do, but it has to be a very violent impact.

As already mentioned, seat belts are designed to stop you more slowly than, say, impact with a dashboard would. They don't stop you a LOT more slowly, but little things count in a collision.

You also have to consider bone density. It is significantly harder to break the bones of a young, male athlete compared to an 80 year old with advance osteoporosis. In fact, the little old lady may break a rib by coughing, much less a high-speed impact.

[Pezzoni]

Great, thanks for all the ideas: I can mention them as sources of innacuracy.
One thing I thought of in addition was leverage? Does the fact that the seatbelt is positioned at the outside of ribs leave them more susceptable to breakage due to the fact that leverage will be gained, allowing them to break around the sternum?

Thanks again.

[Broomstick]

The point where your ribs join your sternum is actually a joint and thus deisgned to flex. Ribs will break along their length, but if they separate from the sternum it's actually a dislocation of a joint. Which doesn't mean you won't wind up with broken bits, just that it's a slightly different problem getting everything fixed up.

Yes, leverage can also be factor - which is why most new vehicles have sliders to move the upper point of the belts so you can adjust the angle at which the shoulder strap comes over the upper torso. When properly adjusted this reduces the risk of collar bone breaks and neck injuries