Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why the tread on tires appears to travel backward . .

  1. Dec 14, 2004 #1
    Someone asked me this and I have pondered the same question for some time. I combed the net and came up with nothing. I don't even know what the concept is called or what it would fall under. I know the "illusion" only occurs at certain speeds sort of like resonant frequencies. It kills me riding home from class everyday and watching the tread run backwards (laughing at me) and I can't explain it. Someone put me out of my misery! :cry:
  2. jcsd
  3. Dec 14, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper

    Have you noticed whether it occurs both with natural light and artificial illumnation?
  4. Dec 14, 2004 #3
    Good question. I believe I can account for it happening in both situations.
  5. Dec 14, 2004 #4
    Well, if its any help, it happens on TV if the frame rate of the cameras is slightly higher than the revolution rate of the tires, thus, every frame catches the wheel in a position slightly to the right of the last frame, and it looks like the wheels are going slowly backwards.

    I didn`t realise you could also see it in natural light though.... but maybe its a similar effect.

  6. Dec 14, 2004 #5


    User Avatar
    Gold Member

    Do you man the *tread* runs backwards, or the hubcaps? I've never noticed tread running backwards, although it's the same phenomenon.

    "I didn`t realise you could also see it in natural light though.... but maybe its a similar effect."

    Yes, but this is a relatively recent phenomenon. When the Sun was upgraded from nuclear to LED technology to conserve energy, the pulsing of the LED began to have the same effect as artificial light.

  7. Dec 14, 2004 #6
    Riiiight, so, aside from the new LED technology . . . :biggrin:

    I just spun the tire on my mountain bike about a dozen times in the kitchen and I am not getting the wanted result. I went outside and rode the bike as well, this produced nothing but skeptical neighbors. If I remember correctly it happens when it is dusk or by a street lamp, in other words when there are more shadows. Maybe intensity or angle of light has something to do with it. Anyone else know what I'm talking about?
  8. Dec 14, 2004 #7
    Hmm.. I think I remember this too. Try this:
    Repeat the kitchen/bike experiment. Get the wheels spinning as fast as you can. This is best acheived by peddaling quickly with your hand(s) (one hand is easier, since the posture for two hands pedalling can be rather awkward). Now let the wheels slow down on their own. If I recall correctly, you should start seeing the tread running backwards when the wheel slows down to some special speed.

    I think it has to do with the image processing frequency of our eyes/brain, similar to what happens with the camera.
  9. Dec 14, 2004 #8


    User Avatar
    Science Advisor

    If you see it in the light of a streetlight, then it is most likely a strobe effect of the AC used to power the light. This is similar to that mentioned by Primal Schemer in post #4.
  10. Dec 14, 2004 #9
    i've noticed this too in natural light.. wonder why its caused
  11. Dec 14, 2004 #10


    User Avatar
    Science Advisor

    I have noticed it under those orangey lights that some freeways have at night. I assumed, like krab says, that it was a strobe effect. Mercury vapor lights (if that's what they are) may flicker at 60 Hz or possibly 120 Hz--I'm not sure if they generate light on both halves of the AC cycle. I have not seen the effect under sunlight.
  12. Dec 14, 2004 #11
    I've seen it plenty of times in sunlight (hubs appear to move backawards). I'm guesing it has more to do with your eyes than AC. Besides, not all lights are AC, right? Like the LEDs on your computer, those are DC.
  13. Dec 14, 2004 #12
    It has to do with your eyes, just like TV, eyes have a focal period, so that you see x number of images per second, the images aren't done the same digital, clear 1 at a time way that images are, however, you don't see every microsecond of an objects motion, to prove this, move your hand really fast like your throwing a baseball or something, you don't see your hand move slowly, you see a blur, but there are some points where the blur is stronger than others, this isn't because your arm slowed down there, but beccause your eye got a better view of your arm at that instant.

    The same phenonmena is true not only for cars, but I've found it to be even more noticable when watching helicopters.
  14. Dec 15, 2004 #13
    Lyuokdea, your information is correct, but I don't think anyone here is doubting or denying how your eyes work. I love your example of helicopters! I see helicopters all the time here at the airport, and it's hard not to notice how it looks more or less like there's a few big black blades going very slowly. Great job coming up with a good example.
  15. Dec 15, 2004 #14
    Yes, good example. I have seen this with helicopters as well.
    So, I'm wondering if this concept of focal period is analogous to what primal schemer was saying about TV cameras. If our focal period is slightly higher than the speed of the tire then we get this backward effect. But, if the same thing is happening with blades on a helicopter then it applies to different speeds, obviously I can't make the tire on my mountain bike rotate as fast as a helicopter blade.
  16. Dec 15, 2004 #15
    Well, yes, of course it applies to different speeds. All that really matters in the scenario of a helicopter is where the blades are positioned each time your eye 'takes a picture'. Say the blades make one and 1/8th rotations in between each time your eyes 'take a picture'. To you, it will look like they only moved 1/8th (to simplify it). Even if the blades made 8000 and 1/8th rotations between each focus, it will look the same. Because from the start of one rotation to the start of the next, the position of the blades are identical.
  17. Dec 15, 2004 #16


    User Avatar

    I'm a little confused here... this sounds like an easy question from the way I interpreted it, but I guess its not judging from the trouble I'm seeing here.

    ...also, what does it being in the visible light spectrum have anything to do with it?
  18. Dec 15, 2004 #17
    I think it has to do with whether the light is flickering or not..
  19. Dec 15, 2004 #18


    User Avatar
    Gold Member

    I am fairly certain the eyes do not work this way. They do not have periods of sensitivity followed by periods of insensitivity that could be considered analagous to taking a series of pictures.

    I know that we have a retinal retention period of about 1/10th second, which is why we are able to watch TV at 30fps, I just don't think it can work as if it's a shutter.

    If we did, we would see all motion - even a fast car on a racetrack - as if by strobe: a series of stills, rather than a motion blur.

    There must be another answer.
  20. Dec 15, 2004 #19
    No, dave, you are wrong. It probably has little to do with physics, but taking psychology classes really helps on this subject. To simply put it: Our brain makes obvious corrections for us. That's why, for example, even on the old CRT monitors, we don't see a dark spot moving down the screen at all. That's why if you peer around a corner only with your non-dominant eye, you don't suddenly see the wall. Our brains will switch eyes and correct for things. Also I know someone who works in the gaming industry for a developing cmpany, and he has told me that they use the eye at a framerate of about 100fps as a rule of thumb.

    Also, as another example, if you were shown the same video once at about 300 fps, once at about 600, and another time at about 900 in a random order, chances are you probably won't be able to tell any difference between the three, even though mathematically there is a big difference.

    And no, you wouldn't see a car on a racetrack 'as if by strobe', because your eyes have a pretty high 'framerate', in fact, I guarantee it's the fastest you've ever seen!
  21. Dec 15, 2004 #20


    User Avatar
    Science Advisor

    King Nothing, the data you present only re-inforce what Dave said. Maybe you did not understand it. The eye cannot see fast intensity fluctuations because the sensors have a time constant. Not because they themselves are periodic. Otherwise, you'd get all kinds of weird effects that are not observed. For example, when my crt monitor is set to 60Hz, I can just see a flicker; when set to 70Hz, I don't. If your eyes worked at a frame rate of say 61Hz, then a 60Hz light variation would not look like a faster flicker (as it does); it would strobe at 1Hz (which it doesn't).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook