Precession Torque

The spin angular momentum is along the rotation axis as shown, but the torque about the support point is in a direction perpendicular to the angular momentum. The torque produces a change in L which is perpendicular to L. Such a change causes a change in direction of L as shown but not a change in its size. This circular motion is called precession.

If the wheel is not spinning, the apparatus just rotates downward toward a vertical orientation. If you curl the fingers of your right hand in the direction of that expected rotation, then your thumb will point perpendicular to the spin axis in the direction of the torque produced by gravity. The tip of the angular momentum vector, and therefore the axle of the wheel, will precess in that direction.

Precession, Top View

The spin angular momentum is along the rotation axis as shown, but the torque about the support point is in a direction perpendicular to the angular momentum. The torque produces a change in L which is perpendicular to L. Such a change causes a change in direction of L as shown but not a change in its size. This circular motion is called precession.

Precession of Spinning Wheel

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A spinning wheel which is held up by one end of its axle will precess in the sense shown if it has that direction of spin. If the spin is reversed, it will precess in the opposite direction. The sense of precession is determined by the direction of the torque due to the weight of the spinning wheel. That torque is perpendicular to the angular momentum of the wheel.