what is the difference of DOT and SNELL?

DOT is the basic motorcycle helmet standard on the market. The Dept. of Transportation has minimum requirements for helmets that is called DOT. Helmet manufacturers are not required to submit their helmets to the govt. or an independent testing facility for testing, they are allowed to just put the DOT sticker on them if they believe they meet the standards. In some mandatory states, DOT sticker is required on the back of helmets; while some do not require wearing helmets.

Here in California you can buy little 'beanie' helmets that are not DOT approved (and they say so right on the box), but there is a DOT sticker in the box in case you want to put it on yourself.

On the other hand, the Snell standard comes from a race driver who was killed by a head injury in the 1950s. He had a DOT helmet but it didn't protect him. So racing drivers and helmet manufacturers formed an association to set standards for helmets that were stricter than the DOT standard, and to have mandatory testing for those manufacturers who wanted to join the association and test their helmets. The standards are more strict than the DOT standard, but not measured by the government. Therefore, if you import helmets from overseas, DOT is a must for custom clearance, but SNELL is not.

Now, Snell helmets are designed for racing, not for regular riding. If you are a casual rider, an DOT approved helmet is enough and much cheaper too. The foam in a helmet compresses when your head hits something. Snell helmets have firmer foam to protect against a harder impact. But in normal motorcycle riding, the impact is usually about the same, falling from about 5 feet. If you fall off the bike, your head falls about five feet to the pavement.

You might be going 70 mph, and you might slide a long ways, but the actual force of impact is the same as if you were stopped at a light and fell over. DOT helmets are designed for this force. Snell helmets are designed for more shock, so the foam is firmer, so they don't actually do as well at this amount of shock.

OTOH Snell helmets are made more carefully because the manufacturers know they will be tested. They don't submit samples, the samples are actually taken from store shelves!

SNELL helmets are usually going over US$250 for many famous brands. SNELL is more superior, but not necessary for normal riders.