Car going forward but wheels going backwards!!!

robbie2003ie

Hi,

Always wondered why on some cars when its driving forward, that when you look at the wheels they sometime look as though they are going backwards!!

Anyone have a explanation for this?

I think its something to do with , for every wheel evolution going forward, the next evolution doesn't go all the way around so thats why looks as though its going backwards!! ( I don't really know what i'm talking about do i?!!!!)

Thanks

Rob

Seth Brundle

from http://en.allexperts.com/q/Physics-1358/Wheels-spinning-backwards.htm

This effect is called "aliasing." Aliasing means that a phenomenon characterized by a given frequency is measured (or appears to be) of a different frequency.

Measurement devices (including your eye/brain vision system) can be characterized as having a measurement rate. For your visual system, this measuring rate is about 15 Hz (samples per second).

If you are observing a rotating wheel that exactly matches 15 Hz, you will not see motion. What you will see is a uniform, gray wheel. Your visual system is averaging the signal over an entire cycle of the wheel motion, thus returning a completely uniform signal. This is aliasing: the 15 Hz signal appears to be 0 hz (uniform).

Now is a good time to bring up "difference frequency." In the above case, your visual system sees the difference in frequency between the source and the measurements. Since they are both 15 Hz, you see 0 Hz.

If you slow the wheel down a little bit, say to 14 Hz, the difference frequency is 14-15 = -1 Hz. Because the difference frequency is negative, it will appear that the wheel is turning very slowly in the backwards direction. In real terms, this is because the wheel turns almost, but not quite, a complete revolution in each measurement time. It looks like the wheel has just moved a tiny amount backwards. As you slow the wheel down, it will appear to increase in speed, but in the backwards direction.

When you reach 7.5 hz, things change. That frequency is called the "Nyquist frequency." It is the maximum frequency that can be resolved by a measurement device, and is equal to one-half of the measurement frequency. At the nyquist frequency, every-other measurement is identical. As a result, the apparent direction of motion is ambiguous. If the frequency is held constant, you would be able to interpret the motion of the wheel as being at the proper frequency in either direction.

At frequencies below the Nyquist, you preferentially see the source frequency rather than the difference frequency (your visual system prefers to lock on to the lower of the two frequencies). Therefore, you see a forward moving wheel at the actual source frequency. You may briefly perceive both the forward motion and the reverse difference frequency at the same time.

Now, assembled in reverse, you get a feel for what happens as the vehicle accelerates from a stop:
1. Accelerating, forward-moving wheel.
2. Fast-moving wheel, ambiguous direction.
3. Decelerating, backward-moving wheel.
4. Motionless wheel.
5. Accelerating, forward-moving wheel (at the difference frequency).
6. Blur.

Bet you wish you didn't ask!

Seth Brundle

This makes it a bit clearer:

Many people notice that the spinning rims of a car shown in a commercial or film appear to spin backwards even though the car is clearly moving forwards. It is especially noticeable when the car is just beginning to speed up or slow down. During an extended shot of the car driving down an open highway, its spinning rims may even appear to stand still and then rotate backwards. There are actually several different optical illusions at work which cause spinning rims to appear to spin in reverse.

There is a fundamental difference between real motion and the sort of animation used to create the illusion of motion on film or video. When a real car's wheels start to move forward, our eyes can usually follow the motion of individual spokes until the car reaches a certain speed and they become blurred. After that, our eyes can only catch glimpses of certain spokes if they are illuminated by an outside light source. This provides our brain with enough information to recognize the forward motion of real spinning rims.

Animation, however, works on a different principle. The actual car's motion is only represented by a series of still images moving at 24 or possibly 30 frames a second. Our eyes see the same spinning rims, but now the information is entering the brain at a different rate. If an individual spoke in a series of animated images appears in a different position, the brain interprets it as moving in a certain direction. If a spoke appears lower in subsequent frames, it will appear to move backwards in our mind's eye. Even if the actual wheel is clearly moving forward, the individual spokes of the spinning rim may appear to move in reverse.

Besides the brain's interpretation of the moving spokes, our eyes can be fooled by an optical illusion called the stroboscopic effect. A stroboscope is a device which can be calibrated to send out pulses of light at a designated rate per second. If a stroboscope's light is focused on a fast moving object, such as a car's spinning rim, individual elements of that object could appear to stand still, move forward or move backwards. In a car commercial, an outside light source such as the sun or movie lamps could create a stroboscopic effect as individual spokes are illuminated. Even though the actual tires are clearly moving forward, the stroboscopic effect can create an illusion of backward motion or even a complete standstill, much like dancers can appear motionless when illuminated by a strobe light.

When the information is limited, the brain often makes assumptions based on what it knows. In the case of spinning rims, the relative position of the individual spokes and the light source which makes them visible can combine to create an optical illusion of backwards motion.

http://www.wisegeek.com/why-do-spinning-rims-appear-to-be-rotating-backward.htm

robbie2003ie

HA HA


Jesus lads, i think i'm gonna have to wait until the morning to read that!!! Its too late in the night for my Brain to take in all that.

Thanks Lads

2 Espressi

Sodium monochromatic streetlights also have a stroboscopic effect, so you noticed the effect more in urban areas.