Tuesday, August 02, 2011

X Games Physics Triumph: Motorcycle Front Flip

2011 X Games competitor Jackson Strong landed the first Motocross forward flip in competition, taking home the gold for this year's best trick event.

The forward flip is one of the most difficult moves you can pull on a motocross bike. While backwards rotating flips off a ramp are almost natural, rotating forward presents some serious challenges.

For one thing, when you flip backwards, you pull back on the handlebars and push on the foot pegs as you leave the ramp. Forward flips require you to do the opposite, which is a challenge because it's not easy to pull on foot pegs.

For another, when you do a backflip, you crane your neck back and look for the ground in order to nail the landing, as you can see in this video of Travis Pastrana's double backflip.

A forward flip requires you to lean forward and hunch over the tank, which means you're essentially riding blind as you hope to time things just right.

Of course, Strong pulled it off so well that he made the front flip look easy. In addition to the initial forward rotation he gets of the ramp by pushing on the bars and kicking back and up on the pegs, he gets very low over the tank. That reduces his rotational moment of inertia, which makes him spin faster in the same way that rotating ice skaters spin faster when they pull their arms and legs inward. In fact, at the beginning he leans so far forward that his head goes down to the right of the tank. His rotation slows a bit as he lifts up early in the jump, but then he ducks down again to spin faster and nail the landing

Finally, if you look closely you can see one additional piece of the puzzle: Strong hits the brakes during the jump. That makes both the front and back wheels stop spinning. Because angular momentum is conserved, stopping the wheels from rotating forward makes Strong and his bike rotate faster. (For a backflip, you should do the opposite - goosing the engine to rotate the wheels faster forward helps the bike and rider to rotate more rapidly backward.)

It might sound simple enough, but here's how it looks when you don't get it right.

Paris Rosen had a couple problems with his attempt. He didn't get much initial forward rotation off the ramp, which got him off to a bad start. Once he was in the air, he lost contact with the bike pegs. Instead of pulling in close to reduce his rotational inertia, he kept his torso well above the tank, and halfway through the move he drifted away from the bike, which increased his rotational inertia still more and further slowed his rate of spin. On the bright side, it appears that Rosen managed to hit the brakes and stop the wheels from spinning, but there's just not enough angular momentum in the wheels to make up for the other challenges he was facing at the time.

Now that you've seen how things went wrong for Rosen, take another look at Strong's successful flip. It's simply amazing how well he put it all together.

(To be fair, Jim Dechamp pulled the front flip first, but Strong is the first person to land it in an official competition. Here's a video of DeChamp practicing front flips into a foam pit.)

No comments:

Post a Comment