We have seen this before and are not strangers to the phenomenon: the wheels of a fast moving vehicle appear to move slowly and sometimes will even appear to spin backward. This is evident when an accelerating wheel will seem to rotate forward at first, but as the car gains more speed, the wheel will appear to slow down. As the vehicle moves faster, the wheel will then appear to spin backward. This goes against our logic because the wheels seem to be spinning backward when we know for a fact they are rotating forwards.

Wagon-Wheel Effect
What we are experiencing is the Wagon-Wheel Effect. It is an optical illusion in which a wheel appears to rotate differently from its correct position. This entails appearing to rotate slower, appearing stationary, or appearing to rotate in the opposite direction. The name comes from people first seeing this effect on wagon wheels in movies and on TV.

It is relatively easy to explain and straightforward to understand. Video cameras record footage by taking a series of pictures in quick succession. To those techy people out there, this is known as the “frame rate.” Most movie cameras use a frame rate of 24 frames per second and thus take 24 pictures per second. This frame rate is highly significant in understanding why a wheel appears to spin backward at high speeds. Suppose a 24 spoke wheel is being recording by a camera with a frame rate of 24 per second. When the rotation of the wheel’s spin matches the frame rate of the camera, the wheel will rotate quickly and will appear to be in the same position every time the camera captures a frame. In the end, the wheel will seem to be motionless.

If the wheel rotates faster than the frame rate, then the wheel will appear to spin backward instead of forwards. In each frame, the spokes have rotated and seem to be a few degrees behind the position it was at when the camera last imaged it. This is commonly known as the Reverse-Motion effect. If the wheel rotates even faster, the spoke will appear to be a few degrees ahead of the position it was at when last imaged by the camera. This will make the wheel seem to rotate forward rather slowly.

This was a highly simplified explanation of the Wagon-Wheel Effect and I hope you understand other factors must also be taken into consideration. The camera’s exposure time, the frame rate setting (some cameras have features to increase or decrease the frame rate), and location with regards to the wheel will affect the appearance and strength of the effect. For example, a wheel with 24 spokes rotating at 24 revolutions per second will appear still if shot with a camera shooting at 24 frames per second. Similarly, if the wheel were rotated at 48 revolutions per second, the wheel would also seem to be motionless.


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