do you believe your instruments?
Discussion
What speed are you doing?
well,
Your speedo tells you one thing (apparently they over-read, by anything up to 10%) but how do you know that?, plus they have a latency. they tell you the speed that you were doing about 1 second ago. Add into that the effect tyre size, wear, and inflation has on road speed.
A Lastec 2020 will tell you another, but the again there's cosine error (you can't be perfectly perpendicular to the ballatino in the number plate), and they are only accurate to about 20cm or so, which over 0.3 of a second is 'more or less' +/- 1MPH.
A GPS receiver. Calculates your position to (classified) accuracy, and by seeing how this position changes with time calculates speed. The clock in the GPS is always locked to the satelite, which is atomic (pretty good) and even taking into account the diminished selective availability, the variations still aren't quick enough to detract from the location acuracy and therefore a speed reading.
latency is about 2 seconds.
Then theres a measured mile. The speed is an average anyway, and unless you've got the kit your down to your own eyesight and pressing the button on a stopwatch. not very accurate.
So chaps, how do you know your speed? (apart from the NIP telling you)
well,
Your speedo tells you one thing (apparently they over-read, by anything up to 10%) but how do you know that?, plus they have a latency. they tell you the speed that you were doing about 1 second ago. Add into that the effect tyre size, wear, and inflation has on road speed.
A Lastec 2020 will tell you another, but the again there's cosine error (you can't be perfectly perpendicular to the ballatino in the number plate), and they are only accurate to about 20cm or so, which over 0.3 of a second is 'more or less' +/- 1MPH.
A GPS receiver. Calculates your position to (classified) accuracy, and by seeing how this position changes with time calculates speed. The clock in the GPS is always locked to the satelite, which is atomic (pretty good) and even taking into account the diminished selective availability, the variations still aren't quick enough to detract from the location acuracy and therefore a speed reading.
latency is about 2 seconds.
Then theres a measured mile. The speed is an average anyway, and unless you've got the kit your down to your own eyesight and pressing the button on a stopwatch. not very accurate.
So chaps, how do you know your speed? (apart from the NIP telling you)
rsvmilly said:
I've checked my car an bike by GPS;
Clio Sport 99 at indicated 100mph.
Aprilia 98 at indicated 100 (although I've shortened the gearing. They have been tested as 100% accurate at 167mph)
Kawasaki ZX6R 93 at indicated 100mph - which is pretty crappy.
...all done on a race track of course...
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