Found this which gives relivant data
According to SCCA data, the neck is the most often injured body part (31.5 percent), followed by the back (19.5 percent), and then the head (15.8 percent). Severe centrifugal forces exert tremendous shearing pressures on the brain. This causes the brain to impact on the inside of the skull, or tear at the medulla at the brain stem. Developers of the HANS used crash test dummies in their testing procedures and found the head can briefly encounter 25 'G's', amounting to about 250 pounds, in a 35 mph impact. Gravitational forces are dependent on speed, and a doubling of speed quadruples the 'G-forces'.
To determine the number of 'G-forces' in a collision, the formula is:
G's=.0333X(M.P.H. X M.P.H.) Distance. In other words, multiply the square of the vehicle's speed, in mph, times .0333 and divide it by the stopping distance in feet. This is for a direct, head on collision, and the formula is more complex in angular collisions due to the fact that the kinetic energy is expanded over a longer period of time, resulting in lower 'G-forces'.
so our case of 22.5mph 2 = 506.25 x.0333 = 16.8 stopping distance is O so the G is 16.8G
Collision, collision, collision.
There are actually three collisions occurring in a crash:
. Vehicle vs. whatever it contacts with (we can discount this)
2. body vs. whatever it contacts with
3. body tissues and organs vs. body tissues and organs
Once your vehicle strikes another object, you have suffered a collision. At that point your body is slammed into some stationary or moving object, or perhaps ejected and is thrown to the ground. At that point, your internal organs, including the brain, began a collision course of their own. Brain injury can occur without any impact to the head, whether helmeted or not. If the body comes to a sudden stop, including the head and skull, the brain continues to move and slams into the inner skull wall. Brain tissue and blood vessels can shear in this violent, twisting action. The skull, even without a helmet, can withstand hundreds of 'G's', but the brain cannot. Other internal organs, especially the heart and aorta, are subjected to these tremendous forces, and often rupture or tear. To give a graphic example, a 160 pound man will strain at his seat belt with a weight of 6,400 pounds at a 40'G' deceleration. Now you might understand why so many people die from ruptured or torn aortas in crashes. There is basically little connective tissue to anchor the heart, since it has to palpitate and move during its rhythmic beating.
'G-Force' Tolerance: Head vs. Neck
It is believed that the head can withstand 300 'G-forces', which is higher than other body parts. The deceleration of 'G-forces', movement of the head and duration of the incident all determine the amount of injury the head will sustain. It is common to have skull fracture and no brain injury, and brain injury and no skull fracture. Helmets are designed to distribute the force of the impact over a wide surface in order to reduce the amount of 'G-forces' reaching the brain. The force of inertia in a crash can cause brain injury even without an impact to the head, thus a helmet cannot protect against this event. Brain tissue and blood vessels can be torn by inertia when the head rotates, common occurrence amplified with helmet use. The weight of the head and helmet pulling at the neck can be sufficient to fracture the skull. Known as basal skull fracture (hangman's noose analogy), these injuries can often be fatal.
According to NARI, the neck is the most often injured body part in their studies. this might account for the fact that the NHTSA regional spokesman said there are more neck injuries without helmets than with, thus leading him to his erroneous conclusion that helmets might prevent neck injury. Tests using human cadavers found that the neck can tolerate about 42 foot-pounds of backward whiplash force before injuries began to occur. The muscles in the rear of the neck are stronger than those in the front, thus a forward rotating head will allow the neck to withstand about 140 foot-pounds of force. Of course, these are ideal positions, direct forward or backward movement. In a real crash, the head is bounced in all sorts of directions, and the neck is less tolerant of sideways acceleration/deceleration. In these instances, the neck can handle about 33 foot-pounds of force.
How strong is the unhelmeted head? The amount of force a head can withstand depends on several factors, including the location of the impact, the size of the object striking the head and the density of the individuals bone tissue. The frontal bone (forehead) can withstand on average, 1,000 to 1,600 pounds of force. The temporo-parietal (sides of head) bones can tolerate around 700 to 1,900 pounds of force. the back of the skull can handle around 1,440 pounds of force. The bones of the face and cheek are less tolerant, standing forces of only 280 to 520 pounds.
Remember, the brain cannot withstand the same forces the skull can, and even a helmet cannot prevent dangerous forces from reaching the brain or the brain moving within the skull cavity.
When we said that the forward rotating head can transmit energy loads to the neck, and the neck can tolerate about 140 foot-pounds of force? Well, when the engineers conducted tests on their HANS safety restraint system, they used a full human form crash test dummy. With the HANS restraint system in place, the dummy was held in position during a frontal impact collision, resulting in neck loads under 130 foot-pounds. When tested without the restraint system in place, the dummy's head rotated forward in the simulated 40 mph test collision, and the neck received loads of nearly 1,000 foot-pounds. The dummy was helmeted, and I suggest that if the spokesman for NHTSA really believes helmets can prevent neck injury, he climb onto the test sled, put on a helmet and see how his neck handles 1,000 foot-pounds of pressure.
