Just a thought: I tend to wear a helmet on the road but not on the mountain. Why?

Most of the time, an accident on the road will tend to be with a) the road surface which is usually from 0 to 5 degrees of incline relative to the force of gravity b) another object, probably stationary and relatively immovable.

A crash on the mountain usually consists of loss of control of the bike and turning into a short range projectile, but now the surface is commonly 10 to 20 deg of incline relative to the force of gravity. The dynamics are different, such that I see a much larger chance of head injury crashing on the road than on the mountain. I've taken many falls from my mountain bike and _none_ have resulted in faceplants, concussions, or any other direct head/ground impact. There's either too much or not enough rotation before intersection with the ground.

That said, I still wear a helmet in _really_ technical rides because the chances of falling backwards onto large rocks are increased when the climbing angle gets severe.

Really, I get more nervous on steep road descents, especially past 28-30mph, than on hairy singletrack. Sure, without a helmet the 7mph fall onto rocks will suck, and might concuss you, but the 30+mph impact/slide across asphalt is just gonna be a BAD day.

My 2 cents.