while looking into helmets I found this guide that I thought was informative.
http://www.soloracer.com/helmettech.html
HELMET RATINGS
SA Rated Helmets:
Snell “SA” (Sports Application) rated professional helmets are designed for auto racing and provide extreme impact resistance and higher fire protection.
M Rated Helmets:
Snell “M” (Motorcycle) rated helmets are designed for motorcycling and other motorsports. They offer slightly less protection than SA rated helmets.
What are the key differences between SA and M Rated Helmets:
SA standard requires flammability test while the M standard does not.
SA standard has rollbar impact test while M standard does not.
SA standard allows narrower visual field than M standard (Some SA helmets aren’t street legal).
How do I know if my helmet meets the necessary Snell specifications?
Always check your club rulebook to determine what helmet specifications they require. SCCA Solo rules, for example, will allow you to use the current Snell specification, as well as the previous two Snell specifications. So, for the 2006 season, you could use Snell Snell M95/SA95, Snell M00/SA00, or Snell M05/SA05 helmets. Snell M90/SA90 helmets would not meet requirements after October 1, 2005.
Some other racing organizations will allow you to use the current Snell specification and only the previous Snell specification, so read your rulebook carefully.
Also, be sure that your club will allow the use of Snell “M” rated helmets. SCCA Solo does allow these helmets, although some organizations that run high speed events will require Snell “SA” rated helmets.
Where can I get more info on Snell helmet ratings?
Check out the Snell Memorial Foundation website for some very good helmet information.
BEFORE ORDERING A HELMET:
Measurement
Measuring the head is a starting point for the entire sizing procedure. Due to varying shapes, heads that are apparently the same size when measured by a tape may not necessarily fit the same size helmet.The circumference of the head should be measured at a point approximately one inch above the eyebrows in front and at a point in the back of the head that results in the largest possible measurement. Take several measurements to make sure you have the largest one. This is the size to compare on our charts.
NOTE: If you have facial hair (beard) or an unusually thick head of hair, you may want to consider ordering 1 size up from what is shown on the sizing charts…especially if you are near the top of your size range.
Helmet Size Charts
AFTER YOU RECEIVE YOUR HELMET:
Try It On
A) Grasp the helmet by the chin straps, with the front of the helmet facing you and the top of the helmet facing down.
B) Place the thumbs on the inside surface of the straps and balance the helmet with the index fingers.
C) Spread the helmet apart with the hands, and slip down over the head.
If the helmet slides down on the head with no resistance, you have your first indication that it may be too large. Obviously, if it will not slide down over the head at all it is too small. Many people unfamiliar with helmets are reluctant to pull down if they meet resistance as the helmet goes on. Only if the helmet is impossible to put on should you move up to the next size, as helmets that go on snug generally fit very well once all the way on. Remember, most people will select a helmet that is too large for them. The eyes should be approximately in the center of the eyeport with the top edge of the liner padding just above the eyebrows.
Checking Horizontal and Vertical Movement
Now that you are wearing the helmet, look carefully at the way it fits. Check to see if the cheek pads are in contact with the cheeks. Is there excess pressure on the cheeks? Look for gaps between the temples and the browpad. Check the back of the helmet where the neckroll (if the helmet has one) makes contact with the neck. Does it touch at all? Or is it pushing the helmet away at the rear causing it to roll down over the eyes in front? After you have made your visual check, grab the helmet in your hands, one on either side, and try to rotate the helmet from side to side. Note any movement of the skin while doing this, as well as the amount of resistance to movement remembering to hold your head steady.
Next, check movement up and down, again noting skin movement and resistance. If in either test there was little or no skin movement, and/or the helmet moved very easily, the helmet is too large. A properly fitted helmet will cause the skin to move as the helmet moves. And, it will feel to the wearer as if evenly distributed pressure is being continuously exerted around the head.
NOTE: Helmets are a little like shoes, in that they do break-in a little. For this reason the best attitude to have when fitting is that the helmet should be as tight as you can stand to wear it.
Retention Check
WARNING: This test may be a little uncomfortable, but it is very important! Now, fasten the chin strap, so you can check it. After the strap has been tightly fastened, while holding your head steady, reach over the top of the helmet grabbing the bottom edge with your fingers. Then, try to roll the helmet off your head. If it comes off, it is undoubtedly too large.
NOTE: Never buy a helmet that can be rolled off the head with the strap fastened.
Pressure Point Check
Finally, unfasten the chin strap and remove the helmet. Immediately after the helmet has been removed, observe coloration of the skin of the forehead and cheeks. A reddening of the skin in a small area may indicate a pressure point. Pressure points sometimes are not noticed by the wearer for several minutes, or even hours later. They sometimes cause headaches, and are at the least, uncomfortable. If you notice a pressure point, or experienced discomfort there while wearing the helmet, it’s too small. If you cannot remember, put the helmet back on for a few minutes, paying particular attention to the anticipated pressure point. If the pressure point causes discomfort either time, go to the next larger size, repeating steps four and five.