True motion
True rotation: 300 RPM
Strobed view
Apparent rotation: +13 RPM
Adjusting this resumes the strobe if it was off
Why spinning wheels appear to freeze, slow down, or reverse
A wheel with N identical spokes repeats its own appearance N times per revolution, so what a strobe light — or a video camera's frame rate — actually samples is that repeating pattern, not the wheel's raw spin. Change the spoke count, rotation speed, or strobe rate below and watch the wheel on the right appear to freeze, creep forward, or spin backward, even though the real wheel on the left never does.
The left wheel always spins at its real, continuous speed, no matter what the strobe is doing. The right wheel only updates at each strobe flash (the instants where the wheel is actually seen by the eye or a camera sensor under a flickering light.)
Because the spokes are identical, the wheel looks exactly the same every 1⁄N of a turn, so what matters isn't the strobe frequency versus the wheel's rotation speed, it's the strobe frequency versus that faster repeating pattern. Match them closely and the wheel appears to freeze; drift the strobe rate up or down by a little and it creeps forward or slides into reverse.