Wheel Spin

[Phoenix]

Hello everyone and good day. I was watching some videos of super cars accelerating when I noticed their wheels and how the spin of the wheels seemed to be going backwards, and then forwards.

I am sure everyone has seen this phenomenon before, either on the highway or whatnot. Anywho, I was wondering, what exactly causes this visual deception to happen?

[joseff]

it depends on the spoke pattern and display framerate. A PAL TV has 30 frames/sec.

So imagine a 6-way symmetrical wheel like the OZ F1. That means, each 1/6 revolution of the wheel, the pattern is identical. If (wheel rpm * 6) is a multiple of 1800 (frames/min TV framerate), the wheel appears to be stationary. At (n + 0.5) * 300rpm, the wheel appears as a 'blur'. Anything between these 2 values, the wheel appears to be spinning forward or backward.

Given the F1 tyre diameter as 660mm, the circumference would be 2073mm and hence we can calculate the speeds at which the wheels appear to be stationary, spinning forward, backward, or blurry.

so for example:

Code: Select all

rpm     kph     appearance
600   74.63     stationary
                spinning forward
750   93.28     blurry
                spinning backward
900   111.94    stationary
                spinning forward
1050  130.6     blurry
                spinning backward
1200  149.3     stationary 

as you can imagine an F1 car goes through these speeds in a flash, that's why the spinning backward/forward thing is only visible at higher speeds,or in slow-motion replays (effectively modifying the framerate). If you have wheels with a different level of symmetry, modify the calculations accordingly.

[Phoenix]

Wow. That's some pretty comprehensive information joseff!! Thank you very much, I've always wondered about that. But riddle me this, what explains viewing a wheel spinning backwards (while the car is traveling forward) with your own eyes? Is there a limit to how much our eyes can process also?

[joseff]

Hmm... how about this. A repeating pattern moving to the right:

Code: Select all

time pos
0  x---x---x---x--- *
1  -x---x---x---x--
2  --x---x---x---x-
3  ---x---x---x---x *
4  x---x---x---x---
5  -x---x---x---x--
6  --x---x---x---x- *
7  ---x---x---x---x
8  x---x---x---x---
9  -x---x---x---x-- *
10 --x---x---x---x-
11 ---x---x---x---x
12 x---x---x---x--- *
13 -x---x---x---x--

If you take a snapshot every 3 time units (marked *), it'll appear as if it's moving to the left like so:

Code: Select all

time pos
0  x---x---x---x---
3  ---x---x---x---x
6  --x---x---x---x-
9  -x---x---x---x--
12 x---x---x---x---

You can experiment with different speeds (like a speeding wheel) and still take a snapshot every 3 units (as a camera's framerate is constant) and you'll end up with patterns (ie. spokes) apparently moving left or right

[Steven]

Any ideas on how fast the human eye can see that?
I mean, the same rotation change can be detected with the naked eye too.

[Gecko]

Any ideas on how fast the human eye can see that?
I mean, the same rotation change can be detected with the naked eye too.

If you can see that, then it is very likely that you are watching a night race where it is the frequency of light flickering that causes the strobe effect.

[vyselegend]

Sorry if I disgress a little with the topic but I've been wondering about this subject recently.

I was told in school that our brain is analysing 24 img/sec. Which is supposed to explain why we eventually see backward move with a forward moving wheel, as explained by joseph.

However, I think the eye can percieve much more images by seconds. Look at video games, I remember for instance the first Tekken (not arcade version) on PS1 in PAL. It was horrible, the framerate was only 24 fps and it made your eye feel very incomfortable and even provoked headaches. If you're looking at a game or film even with 100Hz TVs, you can still percieve that the image isn't static and is at least "vibrating".

So IMO our brain is analysing only 25 fps, but the fact it isn't synchronised with a tv makes it percieve images changes even with framerate x4.

[Sawtooth-spike]

Film and US TV is all all done a 32 FPS (last time i check) UK tv is done at 25 FPS. I might be 1 or 2 out, it has been 4 years since i sat through a Video Lecture.

You have to Remember your eye makes most things up. which is why you sometime double take when you look at people becasue you brain sees something that looks kinda like something else and put that something else there. when you look back your eye has a proper look at relises the differance. Very very odd things the human eyes and brain.

[joseff]

Yes, you are correct and I was wrong: PAL is 25fps, NTSC is 30fps.

More digression:
In the balance, PAL ends up with a larger resolution than NTSC:
PAL 352 x 288 vs NTSC 352 x 240
Hence, PAL is also known as "Pay for Additional Lines" with reduced framerate.
NTSC = Never The Same Colour, alluding to its problem with blue/breen tint.

Cinematic NTSC is almost 24fps not 32.

[Sawtooth-spike]

I thought i was close, Thanks for that.

Good old tint problem. got to love it.

You have lots of fun playing with Things that mess up your eyes. Try putting a Fan in front of a CRT Screen, looks really odd.

If you notice they do the wheel thing alot on top gear, they just have it to the Wheel is moving slowly back wards. Looks neat.

[Saribro]

I was told in school that our brain is analysing 24 img/sec.

Find a new school, seriously.
The eye perceives 24fps and faster as fluent motion due to persistence of signals in the rods and cones in the eye, but it's capable of detecting change at a higher frequency (somewhere in the 60-80Hz range at least, just watch a CRT at 60Hz and feel the headache coming). As most TVs are still CRT, you want a high refresh-rate to avoid screen flicker, regardless of the 25fps or 30fps from the videosignal. LCD screens can run at 60Hz because of pixel persistance. I won't comment on plasma displays because I'm not familiar enough with it.

[joseff]

actually 12-15 fps is more than enough to perceive continuous motion. For example many anime DVDs are actually 15fps.

Despite the old-tech, I prefer my plasma tv to LCD, thanks.