Quote:
Originally Posted by Ed in Maine
I don't believe it is possible to set up the speedometer to be accurate over the full speed range. When a spring is used to establish a desired output value on any device, you have what is referred to as "proportional band offset." In other words, you can make the speedometer repeatable and accurate for a given speed but at other speeds the indicated value will be slightly higher or lower than the calibrated setting. This is because the spring isn't perfect, its spring rate (force vs. displacement) isn't linear. I would set up the speedometer to read 40 mph at 40 mph and just know that your indicated speed will be higher at lower speeds and lower at higher speeds. I hope I didn't get too windy on this post!! Ed
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Are you speaking from actual hands on experience from calibrating speedometers, or theory.
Yes springs are not linear, however the effect of the magnetic field from the rotating magnet on the cup also is not linear, with practice you can learn to balance out these forces and make it linear. when I calibrate speedometers I adjust the spring and the magnet and can usually get a linear reading from 20 to 60 MPH plus or minus 1 MPH. It is not east to do and takes some time but it works most of the time.