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.
Aluminum racing seat construction...
Aluminum racing seat construction is also evolving. Note the external support bracing welded to the outside of the main aluminum shell.
Headrest Location The headrest needs to be positioned as high as possible, but not obstruct the driver's vision out of the driver's and passenger side window. Remember, you only need to see out of the passenger window and don't need to see the bottom door bar. This positioning helps control the head motion. If the headrest is too low, the head will tend to roll over it in an impact. Newer headrests are being made to allow much better vision side-to-side also, but by wearing a head-and-neck restraint, you won't be able to turn your head as much as without one anyway.
Another important part of the headrest, recently brought to mind when I was monitoring a sled test, is a durable cover. A strong cover (vinyl) that will help the helmet slide instead of dig into the inner foam should be installed.
The choice of a long or short headrest is a personal one, but if you do run a short headrest (meaning less than 12 to 14 inches from the back foam), you should run a supplemental net restraint system. The net should be run inside the headrest's foam and be positioned to prevent the head from getting under or behind the headrest. The main idea of the nets is guidance of your head during an impact. Nets should never be used instead of a proper headrest. The net should also have a release mechanism to allow for a quick escape.
Shoulder supports need to wrap around the shoulder. They do not need to touch the occupant, but the closer you are comfortable wearing them, the better they will perform. Shoulders should be stiffer than the headrest because they control a larger body mass (torso vs. head). They need to be connected to the headrest to ensure they both move together evenly. The stiffer shoulders need to have a load-limiting foam in them also. A minimum of 1.5 inches of foam needs to be run in shoulders that can support loads of 2,000 pounds and up with less than 1-inch deflection.
Retrofit headrest kits are...
Retrofit headrest kits are available from racing seat manufacturers, so you can upgrade to a full headrest and shoulders without scrapping your current aluminum seat.
Once the full headrest and shoulders are mounted in the car, the driver needs to spend some time inside the car getting used to the new driver's compartment environment. You will need to plan out and practice escape routes to the passenger side to become familiar with them. Clearly, it's not a good idea to try to exit out the right-side window for the first time after a hard impact when the car is upside-down and on fire.
It is your safety system, be familiar and comfortable with it.