It seems to me that I've written this article a couple of times in my life. It's time for another iteration on selecting helmets for Autocross, Driving Schools, and Racing. This one has more detail on the actual testing procedures than years past; it's all the boring stuff right at the start.
There are four different tests for helmets in use in the US, and since 1985, there has been a distinction between Motorcycle and Racing (SA) helmets. We're going to start by actually outlining the tests that are used, since this seems to be an ongoing topic of very confused discussion all over the Internet. After that we'll look at other issues affecting helmets, including how to buy one in a proper and sensible manner.
The US Government is responsible for one of the oldest of the Standards, DOT FMVSS Number 218. This is a motorcycle helmet standard. It is not of any direct interest for any modern motorsports activity, but establishes a number of definitions and procedures that are used in other more directly relevant standards. Among its specifications are two types of steel test anvils (one hemispherical and one flat) and the general style of testing. DOT FMVSS 218 requires two impacts of a helmet on a flat anvil when dropped from 183cm and two impacts of a helmet on a hempsherical anvil when dropped from 138cm; for the helmet to pass, the headform inside the helmet must not be subjected to more than 400G. In addition, a 3kg steel striker is to be dropped on the helmet from 300cm, and must not contact the headform which is "wearing" the helmet at the time of the test. Finally, there is a chin strap retention test which preloads the chin strap with 23kg, and then adds a test load of 136kg. The chin strap must not extend more than 25.4mm (one inch).
A more directly useful standard is ANSI Z90.1-1992; this is a update of the original ANSI Z90.1 standard of 1966. ANSI Z90.1-1992 is accepted by many organizations as an Autocross helmet, and is appropriate for SCCA Solo II cometition. ANSI Z90.1 depends on many of the definitions of DOT FMVSS 218; it specifies pre conditioning of the helmets with low (-10 degrees C) and high temperatures (50 degrees C) before testing as well as testing at ambient temperature. A water immersion pre conditioning is also specified (between 4 and 24 hours in water at between 18 degrees and 27 degrees C before testing.) The standard specifies the velocity of the impact rather than the height from which the helmets are to be dropped, but the flat and hemipsherical anvils from DOT FMVSS 218 are to be used, and the headform accleration standard is tighter, with the limit at 300Gs. The ANSI Z90.1 standard also adds an "edge" anvil to the hemispherical and flat anvils. The chin strap retention test has the same 23kg preload, with a dynamic test load consisting of a 38kg mass dropped 120mm to provide an abrupt load on the strap. The Z90.1 standard does not appear to include the dropping of a striker on the test helmet. It does, however, include a solvent test which involves wiping the helmet with a 50/50 mixture of Toluene and Benzene. The actual purpose of the solvent test is unclear to me.
The Snell standards are the ones most widely accepted for Racing and Driving School applications, and recent Snell standards are acceptable for SCCA Solo II competition as well. The Snell Foundation updates their standards for Motorcycle and Special Application (Road Racing) usage every 5 years; for example, 1995 Snell standard conforming helmets started to appear in early 1996. Before 1985, the Snell Foundation only had one standard for vehicular helmets, but in 1995, the Motorcycle and SA categories were separated from each other. A helmet's conformance with Snell standards is indicated by the presence of a Snell sticker inside the helmet liner (some helmets have a cloth Snell "sticker" sewn to the chin strap, but this is rare.) The Snell sticker is brightly colored and the year and level of protection is in white agasint the colored backdrop. Currently, SCCA Solo II permits any Snell 85 or newer helmet (M or SA), but SCCA Road Racing now requires Snell SA90 or SA95. Other organizations will vary, but many take their cue from the SCCA.
The Snell Foundation requirements are similar to the ANSI Z.90, but there are some additional requirements; for example, the ANSI standard doesn't cover painting helmets, but the Snell Foundation requires that they be paintable without compromising the protection. Furthermore, the Snell Foundation requires SA helmets to have Nomex liners instead of the more common and cheaper Nylon (if you didn't know, Nylon does really, really, really bad things to skin in a fire.) The Snell Foundation has the 3kg steel striker test, and a chin bar test for helmets with chin bars, consisting of dropping a 5kg weight onto a helmet from a height of 0.62 meters.
Many claim that there are differences between Motorcycle and SA helmets based on the difference between a "hit and slide" motorcycle accident and a "bounce around inside the vehicle" accident. There is nothing that I can see in the standards themselves to support this claim, although a manufacturer certainly might implement motorcycle helmets differently from SA helmets. It is almost certain that helmets from the top manufacturers would pass both standards except for small details like the Nomex liner requirement.
SFI has been in the automotive & racing standards business for a long time; their standards for suits, window nets, "ballistic blankets" for flywheels in drag cars, etc., have been the accepted norm in racing for some time now. They are a new player in the helmet standard game, and at the present time, are not a guiding force. They may be at some time in the future, and are certainly no slouches at safety standards.
I described the standards above. Now, let's talk about how to select a helmet. First of all, if you're going road racing, get the latest Snell SA model; this is a no brainer. If you're doing Marque club schools, you might as well get the latest Snell SA model, as you may decide you want to go road racing and if that happens, you're going to need one anyway.
For autocrossing, it's less obvious. If autocrossing is all you're ever going to do, then you are unlikely to need an SA helmet, as serious autocross fires are virtually non-existant. But if you think you might go road racing some day, then seriously consider SA helmets. The additional cost to get from Snell M to Snell SA is typically about $30.
The SCCA has generally tended to want to make older helmets go away, especially in Road Racing (this year they tightened up the Solo II standards for the first time in a while.) Since there hasn't been a lot of change in the Snell standards since 1985, one might wonder why. The reason is that helmets do age, particularly the foam liners. I have seen the liners in old Snell 75 helmets coming out in pieces. The liner (which is the primarly protective piece) is worthless long before it actually starts breaking up. Helmets should be retired after they've served for 5 or so years of serious usage, because you simply don't know how well they're going to work after they're that old.
Helmets need to be reasonably tight, or they won't do their job properly. When you put on a helmet, you need to wiggle it about to see if the skin on your forehead moves a little; if it doesn't, the helmet isn't tight enough. But don't get a helmet that's too tight; it'll give you a headache and ruin your concentration. If you are going to wear a balaclava with the helmet (the SCCA requires them for road racing drivers with facial hair) then use one when trying on helmets. Sizes vary between manufacturers; Bells run smaller than Simpsons, in my experience. Fit is an important enough issue that you probably want to avoid mail order until you've found out what your size is for a particular manufacturer. You may find that helmets from one manufactuer fit you better than another, and good fit is the primary concern, so don't buy a poor fitting helmet just because you like one name brand better than another.
This depends on a lot of things. Drivers in open cars lean towards Full face helmets, although the SCCA still permits goggles with Open Face helmets. Drivers in closed cars are starting to lean towards full face helmets, as large objects have been known to penetrate windshields. I personally wouldn't use anything other than a full face helmet, and i drive a Sedan in ITB.
If you do buy a Full Face helmet and you wear glasses, then make sure that the eyeport is large enough. It can be very difficult to put on glasses with some of the expensive Full Face helmets.
Full Face helmets tend to have fog problems. The remedy for fogging that has worked for me has been to have an Extremely Clean visor with Fog City anti fogging agent liberally applied to the inside. That, and some Rain X on the outside to help with rain in an open car can make visibility a lot nicer; if you try and drive an openwheel car in the rain with your visor cracked open, you'll get a face full of water for your trouble the first time your turn a corner.
First time you bounce your helmet off the pavement when you're mad about your session, you've just written it off. Throw your driving gloves; they're harder to break. A good thing to keep in mind is that helmets, like seat belts, are use once devices. Once they're overstressed, it's time to retire them.
The Kevlar ones are more expensive, and lighter. It's a matter of taste and wallet heft.
A helmet bag is a good idea, especially a padded one. Helmets are expensive; don't abuse them unnecessarily.
That's a dump of the stuff that I know about helmets. Keep in mind that any of the major helmet companies makes a respectable, safe helmet that meets the Standards. Pick a good one that fits properly, and take good care of it.