View Full Version : Got Clocked Going 18 Today

08-23-2008, 09:31 PM
Anyway, a bunch of us were going up a hill today and there were two cops with a radar gun at the top of the hill. I asked them how fast we were going. He pointed his gun at me and said...18. My computer was showing 14. I asked the guy next to me and he said that his computer was also showing 14. Aside from the fact that the cop may have just been trying to make us feel like we were flying up the hill, i was wondering how accurate radar guns are and whether it matters what angle they are clocking or how close the vehicle is. Does anyone know.

08-23-2008, 10:31 PM
Radar guns don't read over. If anything, they read less than actual speed. The equation is 'radar speed' = 'actual speed' * cos (angle). Where angle is the angle between your direction of travel and the direction of the radar waves. If you are headed directly at the radar gun, the angle is 0 degrees and cos (0) = 1 so the radar speed equals your actual speed. At any other angle, the cos (angle) will be less than 1 and the measured speed will be less than your actual speed. At 90 degrees (radar waves perpendicular to your direction of travel) the gun will read 0 mph.

So...angle does matter.

Distance to the gun should not matter.

In other news, I rode past a 'Your Speed' radar road sign today on a 25 mph side street and it indicated 18 mph which was identical to my polar computer.

Peter P.
08-24-2008, 05:38 AM
Fierte Poser has it right. "Cosine Angle Error" can reduce the actual speed reading. Officers are trained in this, as I was as a police officer in the military.

Also, part of my daytime job is to recertify radar units.

First, I measure the radar's frequency with a microwave counter, and it must be within spec based upon the radar band.

Then, I measure the tuning forks' frequencies, to verify they correspond to the speed stamped on them.

Lastly, I generate precise tones using a function generator with the antenna stuck in an anechoic chamber, to confirm the tones I generate produce the correct reading on the radar display.

They are accurate to +/- 1mph. Very, very rarely are they out of spec. They usually work or don't work.

08-24-2008, 07:14 AM
Ask a question on the Serotta Forum, and bingo, someone will give an expert answer. Very impressive. When I climb a hill, police never point their radar gun at me...too busy betting whether I will get to the top. :rolleyes:

King of the Mountains,


08-24-2008, 07:34 AM

POTD, Sandy.

Of course, it's still early...

dvs cycles
08-24-2008, 08:52 AM
Could a bunch of bikes with spokes spinning distort anything?

Ken Robb
08-24-2008, 09:04 AM
since we all know that many bike tires measure smaller than the advertised size it should be no surprise that our computers read high if we use the advertised size to set the distance factor when we set up our computers.

Even if we do a "roll-out" measurement of our wheels on an unladen bike the number measured would still lead to optimistic speed and distance readings because the tire would compress a bit under rider weight when ridden.

08-24-2008, 10:53 AM
my sunday ride the last couple of weeks has taken me past a portable speed display trailer used by the sheriffs dept. each week have gone thru it at 21-22 mph. good to know as i don't use a computer. at least i am consistant!

08-24-2008, 12:08 PM
I can't believe you can go 15 mph, let alone 18 mph, up a hill! Way to go! Yesterday the fastest I got up a hill was 7.4 mph and I felt i was really pushing it. At the Finger Lakes I only went 5-6 mph up hills :(

08-24-2008, 01:51 PM
glad to use a garmin now. no more fussing with tire size for accurate speed readings--accurate enough for me. MOF today I threw it in my jersey and let it collect data out of sight as I was on my third road bike and don't have that many extra mounts for the unit-yet.

as for the moving parts question above--the radar unit is working of an "echo", there will be lots of them, but it's going to register the strongest one and that's going to be from the mass of the rider. Hypothetical: If you're a really small rider and there's a really big rider riding a different speed in your immediate vicinity--the radar is likely to register his/her speed and not yours atmo-depending on lots of things. Not with laser, but with the old standards.

08-24-2008, 02:15 PM
I can't believe you can go 15 mph, let alone 18 mph, up a hill! Way to go! Yesterday the fastest I got up a hill was 7.4 mph and I felt i was really pushing it. At the Finger Lakes I only went 5-6 mph up hills :(

Not all "hills" are created equal. To some people, an Interstate overpass is a "killer hill". To others, Rose Hill and Bumpy Oak don't signify as "hills". There are plenty who believe the Indian Head 100 is a "flat century".

08-24-2008, 04:23 PM
That was good. Thanks everybody!!