Our local police force, here in grayest Michigan, has one of those mobile radar units (I assume it's radar) that officers can drop off at a location and zap everyone that passes by. The don't use it for enforcement, it's simply got a big speed limit sign on it and a large display that indicates your speed. If you're going over the posted limit, the display flashes your speed in big red numbers. I guess the idea is to build awareness of both the speed limit and the fact that big brother is keeping an eye on you. I suppose if Michigan required a front license plate, this type of unit could be used as photo radar; but to my knowledge, it's only informational (and a bit of a scare for detector users, I'm sure (though I don't use one...). <p>Anyway, I've passed this device six or seven times at various locations in my neighborhood this week, both in our '98.5 A4 1.8tmq and in our '95 Integra GS-R. When I'm in the Acura, as soon as I see the device's display -- which I assume is the moment the device sees me as well -- it reads within a mph or two of my speedo. When I approach the device in the A4 however, the display reads either a speed well below my actual speed according to my speedo (9 mph when I'm showing 45, for example), or doesn't display a reading at all until I'm literally a few yards away. <p>This occurs in both daytime and night, in rain and in dry, whether accelerating towards the device or braking...uh, with the radio on or off...groceries in the trunk or not.... I can't figure it out. I'm wondering if there's some combination of the angle of the front of the car and the height of the device, or the size and position of the radiator, or, I don't know, the pearlized paint (?) on the car that's causing the radar to scatter. Has anyone else noticed this? Any ideas?<p>Mind you, I'm not complaining. If I could prove that I really did have a stealth car, I could save a lot of time on my commute. : )<p>Thanks,<p>Peter

What do you mean grayest Michigan?<br>The sun is shining today, here in southeast MI. We've seen the sun more this past year than our normal, though.<br>Where are you seeing this radar speed board? I see one sometimes, on Hickory Grove in Bloomfield Hills.<br>Unfortunately, it picks my speed up pretty much dead on. Maybe it's my red A4....they always 'look' like they're going faster. At least to the cops!<br>-C.

The device I pass is complements of the Meridian Twp. police department (near Lansing). <p>Hmmmm...maybe it is the paint (pearl). Maybe that's the *real* reason Audi doesn't paint cars that color any more. : )<p>Remembering once hearing that Lansing has fewer sunnier days than Seattle, <br>Peter<p><br>

A radar detector measures the rate your distance changes in relation to the detector. You have the calculate the angle that you are to the detector. For example, if your heading directly towards the detector then the speed shown should match your spedo. However, if your car happens to be exactly perpendicular to the detector when the gun is fired, no matter how fast your going the reading will be zero because the "rate of change" between you and the detector is zero.<p>Corey

...they position it just like they would position their patrol cars in a speed trap. <p>The mystery continues... <p>Thanks,<p>Peter<br>

nt<br>

nt<br>

Its the angle. If I could draw it for you it would be alot easier to understand. Draw it yourself. Take a piece of paper and draw a straight line for the road. Next put a dot right next to the road to simulate the radar gun. Now put several dots along your road (line) to represent your car at diffrent positions. Finnally get a ruler and connect the dots. You want to connect the several car positions to the one radar position. See how the line changes angle? There you go! <p>Corey<br>

Despite the location of the radar -- regardless of what street or corner they place it -- I get the same result. AND, more importantly, I get two totally different readings driving by the same radar position following the same path, but in two different cars. <p>I suspect it's some characteristic of the car and not of the radar position, because the results don't vary by position. Only by car. See what I mean? <p>Sorry if I'm not being clear... : )<p>Thanks again,<p>Peter

I share the confusion over the explanation so far. What is unique about the A4 that would cause the different observed result?<br>

The Beatles sing "hey you get off my cloud"<p>A Scot says "hey McCloud get off my eue"

Yes, its happened to me several times. I even did it in reverse (well, as I was departing the unit's sweep area) and I was indicated doing 8 mph in a 20 (on Camp Bullis, a US Army post outside San Antonio) <p>Head on I register 42 in a 55. Wow! If its proportional, this means I can do 90 in a 70 and not get caught? I don't think the radar is all too accurate.<p>BTW, the calibration check involves a tuning fork. This is struck to produce vibrations and held in front of the gun. The gun averages the velocity of the moving fork tines. Could the A4 have a resonant frequency which alters the reflected signal? Hello sparkies! Some input as this mechanical engineer only knows vibes, not too much e-mag.<p>krj

The angle issue is sometimes simply referred to as "cosine error". What it means is that the highest speed a radar gun can detect (the car's true speed) is when the car is moving at an angle of 0 or 180 degrees relative to the radar. For example, a car moving at 40 MPH straight toward the radar should result in a readout of 40 MPH on the radar readout. A car moving at any other angle will have an indicated speed on the radar that is less than the car's actual speed, because the radar can only detect the component of the car's velocity that is in the 0/180 degree (straight-on) direction. If a motorcycle cop is sitting at the side of the road, and he waits until the car is right next to him before pulling the trigger on the radar (or laser) gun, the car's speed will be zero, because the radar signal will be traveling perpendicular to the direction of the car's travels, and thus there's no component of the car's velocity in the direction directly toward the radar. Hence, indicated speed equals zero. That's why cops are taught to clock people head-on as much as possible, because it indicates the highest possible speed.<p>But that's a different issue.<p>It sounds like the real issue being discussed in the previous posts has to deal with the radar cross-section of the A4 vs. the Integra. Unless a radar (or laser) can get a good, strong reflected signal off a car (or fighter plane or ship or whatever), the radar can't measure anything. Shooting a radar signal at the front of an 18-wheeler with a big, boxy, square metal front end, the police can get a strong enough signal reflected back even when the truck is far, far away. Shooting the radar at a motorcycle at the same distance would be pointless, because the reflected signal would be too weak for the receiver in the radar gun to detect. A passenger car is somewhere in between those two extremes, and all cars have different radar reflectivity (also sometimes called radar cross-section). Many of the aspects of vehicle shape that make one car more aerodynamic than another also make a car harder to pick up on radar (pointy nose, short overall height, certain materials in the nose, angled front license plates, etc.). So it sounds like the A4 and Integra have some real variation in radar cross-section, and until the A4 is close enough that the radar simply can't miss, the radar won't know what speed to indicate. It might guess based on weak signals, but most cops can easily tell when the radar doesn't really have a good lock yet, because the value wavers.<p>One more thing: The strength of that reflected signal varies inversely with the square of the distance between the car and the radar. Loosely, this means you've gotta be pretty close to the radar before they can get a strong enough signal to clock you accurately. That's why early warning from a radar detector is so helpful. A single, slow "beep" on my Passport means that there's a very weak signal coming towards my car, but that same signal is much, much weaker by the time it gets reflected back to the radar gun.<p><br>Me: BA in Physics<br>Brother: Police officer<br>Sister's brother-in-law: State police district patrol commander

Rolling Stones sang "Hey..Hey..You..You...Get off of my cloud"!

...And the Audi design crew.

nt<br>

It's beginning to sound like an issue of radar cross-section -- which is surprising to me because if I had to guess which of the two cars -- the Acura Integra or the Audi -- had the lower radar cross section I'd probably guess the Acura. <p>Maybe this is a bit like the mid-80s Corvette -- a car that urban legend suggested (or maybe it was demonstrably true) was very difficult to pick up on radar in part because of its fiberglass construction and angled radiator. <p>Anyone have a radar gun I can use to test this theory? :)<p>Thanks for all the input so far!<p>Peter<br>