Originally Posted by
dynodonn
I have no illusions that a helmet is an absolute in saving my life when involving a motor vehicle/bicycle collision, but I personally like having some basic protection for my head since my head impacts have been without motor vehicle involvement. Again, I'm not fooling myself that a helmet is a magic bullet on saving all lives, just a couple of days ago, a local bicycle commuter died from blunt force head trauma from being hit from behind by a motor vehicle at freeway speed, and from what I deduced from the local newspapers, the bicyclist was wearing a helmet.
Dynodonn,
Thanks for the mention of the “magic bullet.” It got me to thinking about comparing the forces involved with a bullet verses the forces involved with a car in a collision with a person. We all know what a 22 caliber bullet will do to the human body. I have personally pronounced (as a deputized coroner when I was an EMT) an accidental shooting victim who was hit by a 22 caliber bullet in the chest. A 22 caliber bullet is about the lightest, and perhaps the slowest, bullet in the “normal” inventory of firearms in the USA. Here is a website that gives information about it:
http://en.wikipedia.org/wiki/.22_Long_Rifle
From this information, the foot-pounds per second can be calculated, which is the momentum of the bullet verses the car.
http://en.wikipedia.org/wiki/Momentum
We can also calculate the forces involved if we wish (I won’t do it in this post, but it is easily done), by looking at the change in velocity (acceleration) involved between a bicyclist and a car during a complete collision. This would give us a the forces involved and show that in relationship to survival of the human body, according to Newton’s Second Law:
http://en.wikipedia.org/wiki/Newton%...esultant_force
But for this post, I’ll simply compare the momentum of a car (one ton, or 2000 pounds) at 55 mph to a 22 caliber bullet:
A car traveling at 55 mph is going 80 feet per second. The auto weighs 2000 pounds, so the momentum of the car is the product of the two, 80 ft/sec x 2000 pounds = 160,000 ft-pounds/second.
A 22 caliber bullet is 30-60 grains, which is 1.9-3.9 grams, or 0.067 to 0.1375 ounces. The 22 caliber long rifle bullet travels between 350 and 1750 feet per second. 0.067 ounces / 16 ounces / pound = 0.0042 pounds. 0.1375 ounces / 16 ounces / pound = 0.0086 pounds. Multiplying the lightest weight for a 22 caliber long rifle bullet by the lowest velocity gives:
22 caliber bullet momentum (low) = 0.0042 pounds x 350 ft/sec = 1.47 foot-pounds/second
22 caliber bullet momentum (high) = 0.0086 pounds x 1750 ft/sec = 15.05 foot-pounds/second
Compare the highest values of 15.05 foot-pounds/second for a bullet with the car at 160,000 foot-pound/second at 55 mph shows the destructive value of a car in a collision with a pedestrian or bicyclist.
The car has over 10,000 times the destructive momentum of a 22 caliber long rifle bullet. Both can easily kill, but when you are saying that a bicycle helmet can help in a auto-bicyclist collision where there is a full, frontal collision is saying that you can stop a bullet with that piece of foam. So people like Closetbiker who say that a bicycle helmet in a full collision with a car will not help are correct, as far as that goes. But if there is not a full collision, just a nudge from the side which causes a fall to the ground, then the helmet can help.
John