Car wheels

QUESTION:

Why do the wheels on car commercials on television appear to go backwards? Ed G

ANSWER:

            It is called the wagon-wheel effect or stagecoach-wheel effect. The old oater westerns where the stage pulls into town, and the spoked wheels are turning in the wrong direction. When the stagecoach slows down a bit, the wheels are seen turning in the correct direction. The same phenomena  is observed in airplane propellers and helicopter rotors.

            Many modern cars have wire wheels that mimic stagecoach wheels. It is a stroboscopic effect. Movie cameras take 24 individual pictures every second, while television cameras take 30 pictures every second.

            Consider the following. A wheel  has only one spoke and it is turning clockwise. A movie camera is filming at the normal rate of 24 frames per second. The wheel is turning at a rate of 24 times every second and the film camera shutter opens each time the spoke is at the 12 o’clock position. When viewing the film, the wheel would appear to be stationary. They spoke would only be seen at the 12 o’clock position.

            If the rotating wheel slowed slightly, so that the spoke made it only to the 11 o’clock position, it would appear as if the wheel rotated backward.  The wheel would be behind the position it was in when the previous frame was taken. It would appear that the wheel turned slightly counterclockwise, even though it was turning clockwise.

            Let’s say the wheel rotates slightly faster and would make more than one rotation before the camera shutter opened, and the spoke would appear at the one o’clock position. The wheel would seem to be moving clockwise, in the “correct” direction, but much slower than its actual rotation rate. 

            Wheels have many spokes. How we view the rotation of the wire wheel on a car or stagecoach wheel depends on the position of those spokes and when the camera shutter takes a picture.

            Researchers are of varying opinions on what is going on in our perception and our brain. The wagon wheel phenomena can be complex and is not thoroughly understood. They use such terms as beta movement, Schouten’s theory, and temporal aliasing theory.

            This strobe effect is used in industry. The ignition timing on some cars uses a strobe. This is especially true on older models. The printing industry uses a strobe to adjust the rollers as newsprint comes off the press machine. The stroboscope flash rate is adjusted to the rate at which the newsprint comes off the rollers. Tweaking can be done to make sure the colors are lined up properly. Strobes are used to measure the rotation rate of motors, shafts, and rollers. They usually read out in revolutions per minute (RPM).

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