Slamming the rear brake in mid-air will make the front of
the bike rotate down (or counterclockwise).
Because the mass of the motorcycle is much greater
than the rotating mass of the rear-wheel components, the
frame’s acceleration is slight, but it causes enough rotation
to move the front of the bike down. Remember: you are
basically weightless; it only takes a little force to move the
bike when both wheels aren’t pinned to the ground.
WHAT HAPPENS IF THE REAR
WHEEL SPEEDS UP?
The same rule of physics (for every action there is an
equal and opposite reaction) causes the front end to rotate
upwards when you twist the throttle in the air. When you
spin the rear wheel faster (action), the reaction to that
acceleration causes the rest of your motorcycle to rotate
in the opposite direction (as the wheel accelerates counterclockwise, the frame will spin clockwise). The more abrupt
the acceleration, the greater the force of the reaction—and
the faster your front end will come up.
Most motocrossers call this “panic rev”! You are midway
over a big double when you realize that you are going to
land front wheel first—and it’s going to hurt. You panic rev
the engine. That’s smart! Revving the engine spins the rear
wheel, which exerts an equal and opposite action on the
frame, raising the front wheel. If you panic rev the engine
soon enough and hard enough, the front of the bike will
rise (and you will live to make some other mistake later
in the day).
WHAT’S THE KEY INGREDIENT
TO A BIKE’S ROTATION?
Acceleration. It isn’t the spinning per se, but the rapid
acceleration or deceleration that brings about the reaction.
It is all related to that Newtonian thing about “an equal
and opposite reaction.” Equal means that the harder you
THE SCIENCE OF THE