Michael Fernie profile picture Michael Fernie 6 months ago 79

Why Do Wheels Look Like They Rotate Backwards At A Certain Speed?

Sometimes, it looks as if a forward-moving wheel is stationary or even rotating backwards. So what causes this optical illusion?

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Why Do Wheels Look Like They Rotate Backwards At A Certain Speed? - Blog

Whether it’s footage online, on the television or seen in the real world with the naked eye, a wheel seemingly defying the laws of motion has probably puzzled you at some point in your life.

Watch a wheel as it accelerates and the rotation will seem normal to begin with, turning in the direction of motion. Once it reaches a certain speed however, the spokes of the wheel can sometimes be seen to be stationery or even rotating in the reverse direction to travel. So what is going on?

This illusion is known as the ‘wagon wheel effect’ and it all comes down to the function of the human eye and the way in which our brain manages to interpret and process the images it is presented with. The human eye is capable of operating at frame rates upwards of 200 frames per second (FPS) when processing light but things work differently when it comes to detecting motion. Studies have shown that the human visual system can detect changes in motion - like a wheel spinning - up to only 13 FPS.

Although your eyes can detect frame rates higher than that, the brain can generally only compute and react to 10-15 images per second, although this figure can be increased with specific brain training and depending on which part of the eye is reporting back the information.

In terms of a car wheel, it’s best to concentrate on how the spokes are moving in relation to your visual system’s processing rate. Let’s say a wheel has four spokes at 90 degrees to each other and one spoke starts vertical at the 12 o’clock position. If the wheel moves slightly forward to the two o’clock position by the time the next frame is processed by your brain - the wheel will move forward in the direction of travel as per usual.

If, however, the wheel is turning at a speed that the spoke reaches a 90 degree interval around its rotation (three, six, nine or 12 o’clock) by the time the next frame is processed, your brain will be tricked into thinking the wheel has stayed stationery as the spokes look like they are in their original positions despite their rotation.

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The reverse effect appears when the wheel reaches a speed that the 12 o’clock spoke has managed to rotate around to something like 11 o’clock. Your mind will then join the dots and come to the conclusion that the wheel has rotated backwards, which it then perceives as a smooth backward motion as it pieces the images together.

So instead of your mind interpreting the clockwise rotation to 11 o’clock, it will instead interpret an anti-clockwise rotation simply due to a lack of processing speed.

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The same effect can be seen with this helicopter rotors

This also works when filming cars driving - if a camera is filming at 50 frames per second and the wheel is spinning such that it completes a full rotation every 1/50th of a second, the wheel will seem to be stationery. A wheel rotating reverse to the direction of travel must therefore be falling just short of a full rotation within that time slot, forcing your mind into the optical illusion.

The same applies to helicopter rotors, and aeroplane propellers. Your eyes may be awesome pieces of kit, but you can’t always trust what you see!