The common single-paddle headrest...
The common single-paddle headrest is being replaced by full-surround, foam-lined headrests that manage impact forces to a driver's head. In top-of-the-line models, the outer headrest support is made of carbon fiber. Note the smooth cover over the inner foam to keep the helmet from digging into the foam under load.
In the last issue, I spoke about racing seats and mounting them. The next part of the driver safety system we'll consider is the seat's headrest/head restraint.
There is a significant difference between the common seat headrest, meant only for resting your head in hard cornering, and a full headrest that provides head restraint in the event of a crash. A full and proper headrest controls the head motion in almost all angles of a crash. They are most effective in pure side and rearward impacts, but also provide guidance for the head in an angular frontal impact. Their current price ranges from about $300 and up, depending on materials.
A good rule of thumb for the operational ability of a headrest is to grab it and try to move it. If you can move the headrest around with your hand, it will not support impact loads that can easily be upward of 100 g's, or 1,000 to 1,500 pounds.
Good sources of information on the look of a properly designed headrest are the FIA, CART, or IRL rule books. Short of exploring those, I will give you a good idea of some basics.
Upgrade Your Current Seat
The average racer has a seat constructed of aluminum. The easiest way to upgrade this seat is to buy a headrest and shoulder package from a reputable seat builder. They are becoming more common, and the price is starting to drop for entry-level parts.
Aluminum outer support material...
Aluminum outer support material is an economical alternative to carbon fiber, thus making this superior headrest design available to lower racing classes. Note the aluminum structure panels on both sides that link the headrest with the shoulders. These are critical for best operation.
Before I go any further, I do need to point out one important fact: NEVER run a stiff headrest without stiff "shoulders" that can be connected to the headrest. You have to ensure that the head and shoulders move together. You do not want to run a headrest and no shoulder attachments, and you don't want to run shoulders without a headrest. Remember, this is a complete restraint system and it needs to work together.
Just like a seat, there are different methods to make a headrest work. The idea behind most headrests used in the upper racing ranks is to make the outside support structure as stiff as possible and then add load-limiting foam to the inside. These outside structures can be either aluminum or carbon fiber (expensive).
For the inside protective foam of a headrest, I recommend a minimum of 3 inches of total foam, with 2 inches of a stiff foam, such as a E-175, and a 1-inch softer inner layer of a visco-elastic foam. The visco layer does a good job with comfort and slow-speed impact absorption. The stiffer layer is for high-speed impacts.
For headrests with a stiff support structure used in the upper racing ranks, a stiffener bracket, or strut, that can attach to the rollcage can be used. You need to run at least 3 or more inches of foam inside an externally-stiffened headrest. If you don't, you will bottom the foam out in a hard impact and create a spike in the head impact load numbers. I would rather see a proper headrest structure used with no external stiffener from the 'cage.
A second type of headrest is more affordable but made out of aluminum. It is weaker than carbon fiber versions, but does seem to control head motion well in single impacts. A weaker support structure is made, and only 1 to 2 inches of foam is put into the inside of the headrest. This weaker headrest should NEVER be support-strutted to the rollcage because it does not have sufficient foam to absorb all of the impact forces. It relies on the bending of the aluminum to load-limit the occupant. This headrest MUST also be used with a good set of shoulders, and they MUST be connected to ensure they move together.