Wacky wheels

Published on July 31st, 2012
Wacky wheels

Ever noticed how the wheels on a car sometimes seem to be spinning backwards even though the car’s moving forwards? Well, it’s just an illusion, and your brain’s the one to blame…

Maybe you’ve noticed it, maybe you haven’t (but you’ll definitely notice it now that we’ve told you), but this happens in movies all the time. A car accelerates away from the cops or slows down at a traffic light and, as it changes speed, the wheels suddenly look like they’re going backwards instead of forwards! If you’re not sure what we mean, check out this video to see it in action…

What’s causes the wacky wheel change?
When you look at something, your eyes send information to your brain, and it’s your brain’s job to make sense of what you’re seeing. Usually, the information your brain receives already makes perfect sense, so there’s no problem. But sometimes, the info can be confusing and your brain has to guess what’s actually happening… and that’s what causes the wacky wheel effect.

To explain it, let’s picture a simple wagon wheel:










When the wheel turns, your brain uses the movement of the spokes to work out which way the wheel’s spinning. So, if the spoke with the star is upright at first and then it moves 20º to the right, your brain will figure out that the wheel’s spinning clockwise.

Sounds simple enough… but the whole things gets more complicated when you sit down to watch a movie. Why? Because movies are made up of still images (called frames) that refresh fast enough for your brain to see the whole scene as a moving image.

So, as we showed above, if the spoke with the red star is upright in the first frame and then it’s 20º to the right in the second frame, the brain will work out that the wheel’s spinning clockwise. But what happens if the spoke with the star moves fast enough to go almost the whole way around by the time it shows up in the second frame (let’s say it ends up 10º to the left by the time it’s in the second frame)? In this case, the brain will assume that the spoke has moved 10º backwards instead of 350º forwards. And that’s why the wheel will seem to go backwards even though the car’s still zooming forwards.

The bottom line? No matter how smart you think you are, your brain can still be fooled by a wheel. So don’t go around bragging too much, okay?