Bumpy ride: sporty four-wheelers are all the rage. Discover how physics know-how keeps one teen safe on the racetrack.

Veiled by a glossy helmet, 13-year-old Brittany Snider arts forward on a four-wheeled dirt racer. She's lined up next to 19 other teen riders waiting at the starting gates. The light turns green and Brittany's off: The ground trembles, and plumes of dirt spew into the air as the all-terrain vehicles, or ATVs, tear down the racetrack. Brittany is vying for a win in a National ATV Motocross competition.

Whether cruising around racetracks or motoring through the woods, Brittany has plenty of company: More than 825,000 ATVs were sold in the U.S. in 2002. But ATV-related injuries have also soared: From 1997 to 2002, the number of injured riders more than doubled to nearly 114,000.

Does that mean you have to give up your muddy rides? Not necessarily. Most injuries happen when riders don't follow safety recommendations. Brittany always sports safety gear, avoids riding on pavement, and never carries a passenger.

Brittany also relies on the laws of physics for a smooth ride. For instance, as she takes the track's first turn, she steers the bike's wheels in the direction of the bend. The wheels cause the ATV to travel in a circular motion. This centripetal force pushes the bike around the arc.

But the center-seeking force makes it tricky for Brittany to stay on the cycle. That's because inertia is keeping her body moving in the same direction it was moving before the turn--in a straight line. Immediately, she resists by leaning into the turn. Kevin Breen, an engineer who specializes in motor-vehicle safety, says that by leaning inward. Brittany aligns her center of mass--the point where her weight is focused--with the bike. That's one spill prevented. For more on ATV safety, see right.

1. LOW ON AIR?

Off-road bikes sport car-size tires. But these tires have super-low pressure--about one tenth that of a car tire. With so little air pressing out on the tire, its surface easily flexes. With each wheel rotation, the bottom of the tires flattens against the ground. This allows the vehicle's mega weight--about 225 kilograms (500 pounds)--to spread out. Result: Rather than sinking, the ATV glides over mucky ground.

The tires also have knobby protrusions to maximize friction with the terrain. "The knobs let the tires grab and pull through soft dirt," says Kevin Breen.

2. HEAD SMARTS

The most critical piece of safety gear is the helmet. If you were to plummet head first off your ATV, you would strike the ground with a certain amount of force. In line with Newton's third law of motion, the ground pushes back on your noggin with an equal and opposite force. "If you fall without a helmet, your head absorbs the energy by breaking your skull or bruising your brain," says David Thorn, a safety scientist from Collision and Injury Dynamics in California.

Helmets have a layer of foam sandwiched inside the headgear. In an accident, the foam cushions the blow to the skull.

3. CUSHION MY RIDE

Off-road trails are full of bumps and dips. What keeps Brittany from bouncing off her bike? ATVs sport fluid-filled springs.

When the cycle hits a dip, the bike's energy of motion, called kinetic energy, compresses these springs. As a result, the springs gain lots of potential energy. If all this stored energy is released at once, Brittany may pop up like a jack-in-the-box. Thankfully, as the springs get compressed, oily fluid from a separate compartment seeps into the coils. The force it takes to pump the oil into the springs uses up much of the springs' stored energy. This prevents the springs from popping back up too quickly.