Here is some information from the Snell foundation web page on the differences in helmet ratings. They have a load of information on helmet testing on their pages. Home
What are the differences between the SA, M and K standards?
The SA standard was designed for competitive auto racing while M standard was for motorcycling and other motorsports. The K standard was released to accommodate helmets used in karting. There are three major differences between them:
The SA standard requires flammability test while the M and K standards do not.
The SA and K standards allow for a narrower visual field than M standard (Some SA and K certified helmets may not be street legal).
The SA and K standards include a rollbar multi impact test while M standard does not.
Why does Snell make my racing association upgrade to the newest Snell Standards?
In short, we don't. Snell always makes itself available to explain our standards and programs. We may even offer recommendations on some issues regarding head protection. Each association and/or track has the responsibility for the safety of its members or participants, which generally creates a unique set of issues that must be dealt with, and rules to be set accordingly.
How do helmets work?
Helmets are normally comprised of four elements; a rigid outer shell, a crushable liner, chin straps or a retaining system and fit or comfort padding. The rigid outer shell when present adds a load-spreading capability, and prevents objects from penetrating the helmet. It's kind of like an additional skull. The liner, usually made of EPS (expanded polystyrene) or similar types of materials absorbs the energy of an impact by crushing. The chin strap when properly buckled and adjusted along with the fit padding helps the helmet remain in position during a crash.
Helmets work kind of like a brake or shock absorber. During a fall or crash a head is traveling at a certain speed. Since the head has weight, and is moving there is a certain amount of energy associated with the moving head. When the helmet along with the accompanying head impact an unyielding object; a rock, a wall, a curb or the ground the hard shell starts by taking the energy generated by the falling helmet (head) and spreads it over a larger portion of the helmet, specifically the internal foam liner. The foam liner then starts to crush and break which uses up a lot of the energy, keeping it from reaching the head inside. Depending on how fast the head is traveling, and how big, heavy and immovable the object is the faster the head slows down, and the more energy is present. In short everything slows down really quickly. A helmet will effectively reduce the speed of the head by breaking and crushing which reduces the amount of energy transferred to the brain. The whole process take only milliseconds to turn a potentially lethal blow into a survivable one.
Why should you replace your helmet every five years?
The five year replacement recommendation is based on a consensus by both the helmet manufacturers and the Snell Foundation. Glues, resins and other materials used in helmet production over can affect liner materials. Hair oils, body fluids and cosmetics, as well as normal "wear and tear" all contribute to helmet degradation. Petroleum based products present in cleaners, paints, fuels and other commonly encountered materials may also degrade materials used in many helmets possibly degrading performance. Additionally, experience indicates there will be a noticeable improvement in the protective characteristic of helmets over a five year period due to advances in materials, designs, production methods and the standards. Thus, the recommendation for five year helmet replacement is a judgment call stemming from a prudent safety philosophy.