Vehicle-buggy accidents, Part 3: Head rests

One of the most critical parts of the system in limiting the peak accelerations experienced by the occupant's head is the head rest.

Designing the head rest is a good introduction to the differences between occupants. The head of a mature man weighs approximately 10 pounds. The head of a pre-teen weighs approximately 6.5 pounds. A system designed for a 12 G decel of an adult male head will result in an 18 G decel in the pre-teen's head...a nogo condition.

Plastic hinge theory suggests that a rectangular, ductile beam in bending will max out at a moment equal to 0.25*Width of beam*Yield stress of material*thickness^2.

The problem is that younger, shorter, lighter occupants lower the headrest and the bending moment DECREASES, thus raising the force level generated by the plastic hinge.

Having a stepped or tapered guide rod partially addresses this issue.

Position of head rest with a tall person, presumably with a heavy head.

Position of head rest with short person. The effective beam length is longer due to the increase in effective length.

It is worth noting that the upper guides don't need to be in full contact with the guide rod. That is, the guide rod does not need to be skinnied up as much as shown in the concept illustration. A groove could be ground in the backside of the guide rod and the upper, rear guide block could have a lug that only contacts the guide rod inline with the groove or not-groove. That would be similar to the groove ground in the bolt of many rifles for the ejector.

All units are inches. Bogey is 288 inch-pounds for a six pound head and 8" guide rods.

The table shown above assumes a functional bending length of 8" in the lowest position, a six pound head and 12 G target decel. The table assumes TWO rectangular, steel, strap-type guide rods for the head rest with a yield strength of 30ksi.

One might reasonably expect to get a travel of 4" from 8" long guide rods in bending. From a budgeting standpoint, we need about 30" of launch-pad to accelerate the occupant to the velocity of the bullet vehicle and a well designed head-rest gets us almost 15% of the needed distance AND gives us a way to positively discriminate between low-mass and high-mass occupants. And it is cheap and does not weigh much.

Part 4