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Why do the wheels on a car appear to rotate in the opposite direction to the tyres?

Why do the wheels/alloys on a car appear to rotate in the opposite direction to the tyres?
Michelle Bray from Bedfordshire (Age 45-54)

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3 Responses

  1. Michelle, I need more information to answer this. When have you observed the effect – at night under street lighting for example?

  2. Hi Dr. Naish

    I have seen this effect when cars are driving along and if you study the wheels (alloys) they appear to spin in the opposite direction to the wheel – it is almost as if the eye cannot take 2 movements in at once and separates them

  3. Hi Again Michelle,

    The big question here is what sort of conditions bring about the effect. You still haven’t told me exactly when you see it. A plausible guess would be under street lighting, which is not constant but flickers (because mains electricity alternates 50 times per second). Although the flicker is too fast for our eyes to detect it, in fact we keep getting a series of glimpses of objects, as the light comes and goes.

    If you see a series of still pictures, each slightly moved from the previous position, they run together and look like something moving – that’s how the film works at the cinema. The brain detects what it recognises as the same object in a slightly different position and works out that it is indeed the same thing that is moving. This can produce ambiguities for a moving object that has a repeating pattern – for example a revolving wheel with identical spokes. If you see a spoke position in one flash of street light it will have moved on by the next flash. At some speed each spoke will have moved on so far that it is almost up to the position that the adjacent spoke occupied in the last flash. Now the brain has a problem! The spokes all look the same, so it can’t tell which part of the image has moved to where – should it assume that the matching spoke is the one that has moved on quite a long way, or is it the one that is just next door, but slightly behind? It goes for the nearest and the wheel seems to move backwards.

    Depending on the speed of a wheel, number of spokes and the flash rate of the light a wheel might seem to stop, go slowly forward or slowly backwards. The patterning on a tyre is quite different from the spokes, so if you got a similar effect at all it would certainly be at a different speed. Thus, the two appear to move differently.

    Hope that helps. If you get the effect in daylight we will think again.

    Best wishes, Peter

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