What is the visual mechanism that makes wheels on a car appear to be turning backward while the car is moving forward?
I notice this alot in commercials and have always wondered.
Mismatch between wheel rpm and camera frame rate.
The spokes are just short of being in the same position per frame.
Thank you. You have vastly improved my Super Bowl commercial viewing experience.
To expand a bit on runner pat’s answer, video recording works by taking pictures at a certain rate (say, 50 pictures/second - it depends on the medium). The wheel on the car also rotate at a certain rate depending on the speed (say, 24 rotations/second).
In this scenario, every time a picture is taken, the wheel has done slightly less than 1/2 of a full turn. If the wheel has 4 spokes, you can’t actually see the difference between slightly less than 1/2 turn or a slight rotation backwards. But your vision interprets it as a backwards rotation because the forward (correct) interpretation is much faster than what our brain is set up to easily deal with.
Example with 2 spokes:
picture number:
1 2 3 4 5 6
wheel "spokes":
- / | \ - /
The above is what you’d see if the camera rate matches the rotation rate and the spoke was turning anti-clockwise, but it also matches what you’d see if the camera rate was 0.75 times the rotation of the spoke and it was rotation clockwise. This works for many close ratios and also depends on the symmetry of the “wheel” (basically, the number of “spokes”).
I meant to say here: if the camera rate was 4 times the rotation rate.
Thanks for the answers. I couldn’t decide if it was a film/video artifact or if they do this IRL.
I never thought to look for it when driving. You know, the whole “look where you’re going” thing interferes with me looking at other’s wheels while on the road. 
You won’t see it in real life, just on film/video.
You can sometimes see it in real life, due to the microsaccades your eye performs while viewing a scene.
Nevertheless, checking this while driving is contraindicated.
Thanks for asking, loshan, I’ve always wondered about this – but never wondered hard enough to ask.
nm
Just a nitpick…you will see it in real life if the wheel is illuminated by a strobe light, or a florescent fixture, which acts like a strobe light. And the strobe light acts like a motion picture camera shutter to produce the same effect. Admittedly, this isn’t going to happen outdoors on a sunny day.
One can see the “wagon wheel effect” at night under certain streetlights. The lights look like they’re steady, but they’re really going off and on at 60 cycles per second due to the alternating current frequency.
edit- Too slow!
So I guess that makes me a wanker? Cool. I always thought having girly bits excluded me from wanker territory.
And, no problem twickster, I’m glad to be of some use around here 
This is an instance of a more general phenomenon known as aliasing.
I have seen this a few times rolling down the highway, its one of those things you don’t want to quit watching as you just know its a once in a lifetime thing. I always thought it was a steady path and over a certain speed thing.
Nitpick:
don’t you get a 120-Hz flicker, because the light illuminates when the current is moving in either direction?
I first noticed this as a kid when I saw Chitty Chitty Bang Bang. Very disorienting.
Always fun: