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Offline Innerzoomer

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  • Country: England
Roadside radar speed signs and motorcycles.
« on: June 27, 2021, 09:28:12 PM »
I've recently fitted a 15T front sprocket to my GW250 and tonight I was out for a ride and saw that there were two new radar speed signs either side of a new speed limit zone, presumably to reinforce awareness of the new limit. They just tell you how fast you are going and don't take pictures. I noticed that the signs only 'see' the bike when you are really close to them; much closer than you would be in a car. I did a few runs back and forwards to see how accurate the speedometer is with the revised final drive ratio. There was another sign on my route which had showed 3mph less than my speedo, and these two showed the same, but all three only 'saw' me when very close. This will make them inaccurate, I think. The radar blips the vehicle twice within a fixed time interval and by measuring the difference in the time taken to receive the echo it calculates the distance you travel in the time and hence your speed. It doesn't much matter how far away you are from the sign when it sends the first pulse, as it is only measuring the difference between the two times. However, the radar picks up the motorcycle so late, due to the small frontal area, that the target is at a significantly different angle to the sign for the second blip. This is because the bike is being ridden past the sign, not directly at it. A car or larger vehicle is detected at a much greater distance, so there is very little change in the angle as compared with the bike. The result is that the radar speed sign will show a lower speed when a motorcycle passes it than it will when a larger vehicle passes it at the same speed. The smaller the bike, the later it will get an echo from it and sometimes the radar speed sign won't detect the bike at all, possibly because it doesn't get an echo at all, or because it only gets the first echo. I cannot rely on these speed signs to tell me how accurate my speedo now is, even though they all agree. I suspect that it is very nearly accurate, but presumably will need a GPS speedo to confirm it. Or a police speed trap!
« Last Edit: June 27, 2021, 09:31:22 PM by Innerzoomer »

Offline Robell

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Re: Roadside radar speed signs and motorcycles.
« Reply #1 on: June 28, 2021, 06:30:58 PM »
Not sure how accurate those road sign types are. Not very, I suspect. I've has some over-read my speed even compared to GPS.

You can rely on the police ones being accurate, without testing the theory  :008:
" it's better to burn out, than to fade away "


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