Valley Commentary : Hardheaded Adults Need Bike Helmets as Much as Their Kids Do : Children must now wear protection, but parents set a bad example by riding bareheaded. Anyone who doubts the need for it should do the oatmeal test.

Gangs? Can’t stop ‘em. Molesters? Can’t catch ‘em. Protecting our kids from the evils out there is not one of our society’s strong points.

But once in a while, we do things right--like last year when we put on the books the child helmet law for bicyclists that went into effect Jan. 1.

I pedal a couple thousand miles a year, mostly within the Valley, and I’ve seen a big increase in the number of kids wearing helmets. We can feel proud that there will be far fewer bike-riding youngsters admitted to emergency wards with head injuries.

That’s the good news.

The bad news is that increased helmet use among children contrasts vividly with the dismaying number of adult cyclists who don’t think helmets are for them.

On Valley Circle, I passed a family of three helmeted tykes followed by mommy and daddy, hair fluttering in the breeze. On Lassen Street near Winnetka Avenue, a toddler resembling a Peanuts character peered out from beneath an igloo-shaped helmet while safely belted into a kiddie seat. His bareheaded chauffeur pedaled on, oblivious to his own risk.

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Clearly, our society must try to protect our youngsters from head injuries. But what about those who should know better? Does society’s obligation extend to them?

I can already hear the wailing and gnashing of chain against cog. “Whatever happened to freedom of choice? I’m no kid. It’s my head. If I want to crack it open, that’s my business. Besides, I’m no racer--I never go over 10 miles per hour.” This last argument is similar to the one proffered by His Mayorship, the mountain biker, when called to task after being featured helmetless in a Times article last year.

No, I don’t advocate forcing adults to save their skulls if they see no need. We know you can’t legislate morality--legislating common sense is equally hopeless.

What Hizzonor and others fail to grasp is that speed is only one factor, and not the most important one, contributing to injury. The severe cranial damage, the kind that stays with you for years, comes through falling, and is independent of bike speed.

For disbelievers, let’s run a scientific experiment.

Stand beside your bike. Note that your head is about the same distance above the ground as when you ride.

Now, lock your knees and lean backward slowly. Farther . . . farther. . . . At some point gravity takes over. Your only other responsibility in the experiment is making sure the first point of contact between you and the garage floor is the back of your head.

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For a more spectacular result, try experimenting with curbs, substituting the corner for the flat garage floor. Or you might try a boulder along a mountain bike trail.

Stupid experiment, right? But if you were a properly instrumented crash dummy, you would have generated some interesting data:

* Experiment duration: about 0.5 second.

* Impact velocity: about 12 m.p.h.

* Cranium contact time: about 1/2000 of a second.

* Cranium deceleration: about 1,000 Gs.

Those cracks in your skull are just a minor irritation compared to the brain damage. Since you probably haven’t had the opportunity to sneak a peek at your brain, the next time you eat a bowl of oatmeal, poke it a little with your spoon. That blob makes a pretty good brain surrogate in size and consistency. Now pack the blob inside a skull and whack it with 1,000 Gs. The results aren’t pretty.

In the real world, 400 Gs cause brain laceration, even without skull fracture. Seven hundred Gs cause permanent and significant brain damage.

But it’ll never happen to you, right?

Twice in the last few years, I witnessed people launched over their handlebars when an object jammed their front wheel spokes. When that happens, your future hangs on the decision you made in the past, not on what you decide in that half second as you accelerate toward the inevitable 12-m.p.h. terminal velocity.

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As I said, I wouldn’t dream of forcing bike helmets on people who don’t really see the need. However, my curiosity is aroused when I see a helmetless cyclist. I’ll bet if I peeked in that person’s ear, I’d see a tiny guy in there wearing a Quaker hat.