Why Do Car Tires Seem to Rotate Backwards While Driving?

Understanding Optical Illusions in Movie and Video

Did you ever notice your car tires seeming to rotate backwards while driving at high speeds on the interstate? This phenomenon, known as a rotational illusion, is an optical illusion that can be quite fascinating. In movies and TV, this effect is common as a result of the frame rate of the camera used to capture the video. Movies and TV are created from a sequence of still frames that are played back in rapid succession. This sequence can sometimes lead to illusions, such as the wheels of a moving car appearing to spin in the opposite direction.

The Role of Frame Rate in Video Capture

The frame rate of a video camera is the number of frames per second (fps) that the camera captures. For instance, a typical camera might capture 24 frames per second, while higher-end models might reach up to 60 fps or even 120 or 240 fps. If the camera’s frame rate is too slow, it can introduce an optical illusion where the wheels appear to move backward. Conversely, if the frame rate matches the wheel’s rotation rate, the wheels will appear to be motionless, creating a stroboscopic effect.

Stroboscopic Effects and Optical Illusions

Imagine a scenario where the wheel moves exactly one revolution for each frame captured by the camera. In this case, the wheel will appear to be motionless, like a strobe effect. If the wheel moves slightly slower than one revolution per frame, it will seem to move backward. This happens because your brain interprets multiple frames of the wheel in a certain way, leading to the illusion of reverse rotation.

Proportional Movement and the Beat Phase Effect

The beat phase effect refers to the timing between the wheel’s rotation and the frame rate of the camera. This effect is particularly noticeable when a vehicle starts moving from a stationary position and accelerates. You might observe the following sequence: the wheels initially rotate correctly, then blur, followed by the wheels appearing to move backward, then slow down while still moving backward, and finally, the wheels stop and reverse direction. All these phenomena occur due to the wheels’ rotation rate being out of sync with the camera’s frame rate.

Practical Examples: Film vs. Live Video

To better understand this phenomenon, let’s use film as an example. A film camera typically captures 24 frames per second. If a clockwise turning wheel makes only 7/8 of a full rotation between frames, it will appear to move backwards counterclockwise by 1/8 of its circumference. This small deviation occurs repeatedly over time, leading to the perception of the wheel moving in reverse.

Conclusion

Understanding the relationship between the wheel’s rotation and the camera’s frame rate helps explain why car tires can seem to rotate backwards when viewed through a video camera. Whether on a vehicle, a stagecoach, or a video, the cause is the same: the frame rate of the camera versus the actual rotation rate of the wheels. This optical illusion is a fascinating phenomenon that can be observed in both real life and digital media.