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Old 05-21-2001, 10:40 AM   #31
luvmyvfr
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Default Re: Police Question their Own Radar Gun

You must be one of those guys with square disc brakes that don't spin, 'cause that could reflect a radar signal back to the gun, or one of those bikes with square wheels that just skids on the ground and makes a nice flat reflective surface. Or maybe you're one of those guys who doesn't know what he's talking about, got flamed, and felt he needed to justify his stupidity.



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Old 05-22-2001, 06:13 AM   #32
banda
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Default Re: Police Question their Own Radar Gun

Hey physics-boy,



A flat surface presents a poor radiation return, because it reflects all the energy in one direction. If you're not prefectly perpendicular to the flat surface, all the return energy goes somewhere else: no reading.



However, the interior of flat surfaces at right angles to one another makes an excellent reflector. Some examples of that on a motorcycle wheel would be:

[*]the place where the spoke of the wheel meets the hub[*]the place on the wheel where the spoke meets the rim[*]the place where the rotor spokes meet the rotor[*]the holes in the rotors themselves



All of these structures are relatively flat metal surfaces joined to other surfaces with an internal angle of 90 degrees. All of these structrues are alternating their speeds between 0 mph at the bottom of their travel and twice the speed of the vehicle at the top of their travel.


If you ever get the chance to take a close look at an actual motorcycle, you can quickly confirm what I'm telling you. Perhaps a trip to a motorcycle dealership, or even a friend or acquaintance who has a motorcycle could show you.


See, I'm big enough to forgive you for your ignorance and help you understand what the adults are talking about. That's a life lesson you can use throughout your childhood.
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Old 05-22-2001, 01:20 PM   #33
luvmyvfr
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Default Re: Police Question their Own Radar Gun

Since you're such a rocket scientist, and those areas mentioned are such great reflective surfaces, I suppose that is where the reading comes from on a radar gun, right? The SPOKES? For crying out loud, read the earlier post, the readings come from the light reflectors on your bike, not the wheels. The surfaces you mention, if they could get a reading off them at all (maybe low speeds) would be so INSIGNIFICANT compared to the reflection off the parabolic (more or less) reflectors in most headlights, that they would be overwhelmed. More than likely, they show up as static, and oh-so faint static at that. I could let you look at my motorcycle, if you need to see what the whole bike looks like, and not just the picture of a tire you've been using for reference. Look at the big picture before you get your feelings hurt and need a banda-aid.
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Old 05-23-2001, 05:15 AM   #34
banda
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Default Re: Police Question their Own Radar Gun

The parabolic reflector of a headlight would, by design, reflect all of the energy at a single point in space, namely, the headlight bulb. They're made that way for a reason.


Have you ever seen a police officer calibrate a radar gun? They use a tiny little tuning fork that would fit in a shirt pocket. Only the very tips of the tuning fork vibrate at 60mph, yet the miniscule movement of the tiny prongs is more than enough to give them a strong reading.


Have you ever seen "corner reflectors" used on an experimental aircraft? They are itty bitty metal devices attached to an aircraft to amplify its radar signature. They are far smaller than the aircraft itslef, but return thousands of times more radar energy.


So, you see, unless a radar gun is very accurately aimed, it could be measuring returns from parts of the vehicle that are moving at up to twice the linear speed of the vehicle itself. The size of those parts is far less important than the materials and angles that characterize the parts. (As can be shown with the tuning fork and corner reflector examples)


To understand why this could happen, you have to remember that the radar gun isn't measuring light from streetlights reflecting off the bike, and it isn't measuring the light being emitted from the bike's headlights or tail lights. It is beaming a coded signal at the bike and measuring the change in the amount of time it takes for that same coded signal to be returned. If any part of the wheel above the axle returns that signal, the reading will be higher than the actual speed of the bike. Of course, any other good radar reflectors on the bike will reflect the radar energy as well, but, as is shown with the tuning fork, the radar gun reports the fastest signal it receives. (Hint: the bottom of the tuning for isn't moving at all, the very tip is changing directions back and forth with a maximum velocity of 60mph, and the radar gun returns a reading of 60mph)


Dude, it's Ok to learn something.
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Old 05-23-2001, 02:11 PM   #35
luvmyvfr
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Default Re: Police Question their Own Radar Gun

O.K. Sport, I didn't really have the time or inclination to have to explain all of this to you. I guess a little knowledge *is* a dangerous thing. First of all, your only half right on most of your arguments, and the majority of the rest, you leave out vital information.



The whole 90 degree arguement you use, is almost right, but wrong. It does relate, however to your aircraft argument. You see, the reflectors on the aircraft you mention have 90 degree angles in them, but they are configured like the cut-off corner of a box, or a hollowed out pyramyd, 3 90 degree sides coming together into a point will reflect 100% of the energy directly back to the source it comes from. I know all about the aircraft reflectors, you see? You won't find this kind of reflective surface on a motorcycle.



If you *did* have one of these reflectors however, and you mounted it somewhere with about 2-3 other pyramids on a wheel (not the motorcycle wheel, but something free spinning), you would make the perfect radar jammer, and it would be 100% legal in any state, because it would create a doppler shift much slower than your bike travels, and it wouldn't be emitting any energy itself, but the bike would look a little goofy (it may even work on laser if it is shiny enough). The air would "blow" the wheel backwards, and since it would have such a strong signature (as you correctly mention in your aircraft argument) it would completely overpower the signature of the rest of the motorcycle.



Regarding the parabolic (note I wrote *more or less*) reflectors, I know how they work, it's pretty basic geometry, but, you'll note that your headlight does not shine in one very well defined circle on the ground ahead of you. This is due to several things. On modern motorcycles, like mine, there is a computer designed reflective surface on the inside of the headlight that scatters some of the beam in all directions, and the light coming straight out is refracted by the lens, which is designed to have variable thickness and angles cut/molded into it. That's how the light works from the inside out. From the outside in, it is not the exact same, and to assume that it does, is incorrect. I won't go into it, but I'm sure an obviously educated individual like banda can figure out that an extremely reflective surface (reflective to much more than just visible light) such as is found on the more or less parabolic dish of a headlight would receive a scattered signal, scatter it more (as it is not entering from straight in, but at an angle, thereby distorting the parabolic effects of the lens) and shoot more of it back, than that which the dull (or even shiny) paint on the front or side of the motorcycle, and, yes, even that tiny signature of the wheel.



As for the tuning fork, I don't need my intelligence insulted any more than it has been. A cop holding a tuning fork directly in front of his radar gun would reflect a signature not unlike that of a huge warehouse travelling at 60 mph straight at the gun. I'm saying that same tuning fork 200+ feet out (or even 50 feet, I would guess) would barely show up, even if there were no other objects in the line of the radar gun, AND the fork would have to be exactly perpendicular to the gun (which at that range would be a challenge). You see what I mean?



I even thought about doing the calculations of how much of a second would it take a wheel to rotate, that was travelling at 60 mph, figuring out how long a spoke would be in the line of sight of the gun during that fraction of a second (because it would only be subject to the radar for 1/2 the rotation of the wheel, and the disc brakes, front suspension, and parts of the fender would get in the way of the radar during much of the spokes acceleration) and seeing if the gun could read a doppler shift from 0-120 mph (using your 2X speed of bike at top of rotation) in that amount of time, but I thought it would be over-kill. Especially when you take into account that there are 2-3 other spokes reflecting the a totally different doppler shift either before or after the subject spoke. I think it much more likely that the RIM of the motorcycle (which would be travelling at 60mph) would give a much cleaner reflection of the radar energy emitted by the gun, than a spoke on the wheel as described above.



I think it may be an issue if your motorcycle only has one spoke with a radar reflector on it, or a disc brake with a single large hole that somehow could reflect back to the gun the signature code it emits. I don't really think this is possible, and THAT is why I think your whole argument is silly. Intellectually, I agree that there is some accuracy in what you say, and much of your theory is sound, but it can't be brought to the real world and expected to change a radar reading that much.



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Old 05-24-2001, 03:43 AM   #36
banda
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Default Re: Police Question their Own Radar Gun

Good points.


So let's say you and I figure out how to attach corner reflectors to our spokes and we'll call it a day.

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Old 09-13-2001, 08:22 AM   #37
jackbird
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Default Re: Police Question their Own Radar Gun

I loved to ski on LSD, also seeing the Grateful Dead on LSD, and I'll admit hang gliding on it was lots of fun, but riding a motorcycle? On a high speed run? Cool! NOBRKES, you're a braver man than I!

Fastest for me was 165+ for 7 miles on my Super Blackbird (had a lot left but was getting airborne too much to go faster) for which I saved $2 by getting to the toll bridge in time, while motorcycles cross free. I'd love to open this motorcycle up some day. Haven't tried it on LSD yet though.
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