| Subject: Sigma Setup - A Little Long |
Author:
Ohio Mike
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Date Posted: Tuesday, December 04, 11:20:54pm
In reply to:
Jeff Barnhill in Northeast Louisiana
's message, "Re: Speedo Accuracy" on Tuesday, December 04, 08:43:42pm
I have often wondered just how accurately the mile markers are placed. Most of them were probably placed well before the Laser or GPS was in common use, even for highway construction crews so I would not be too surprised if they were located using the odometer on the truck that was hauling them.
Anyhow, the method I used to calibrate my Sigma involved the Voyager, a couple pieces of masking tape, and a nice, flat sidewalk. I put a piece of tape on the front tire at bottom center and a corresponding piece of tape on the sidewalk under the tape on the tire. I then put a mark on the tape and carried it down to the tape on the sidewalk (undoubtedly overkill). I then rolled the bike forward for two or three tire rotations and marked the stopping point. I then measured the total distance, divided it by the number of rotations, and ended up with a pretty darn accurate measurement of the front tire circumference. Then just plug it into the info in the Sigma manual. Much easier than trying to fish a measuring tape around the front tire and probably a good bit more accurate than the typical mile marker. Again, this is probably overkill but it seemed like the thing to do at the time.
As for locating the magnet, I mounted it on the front disc, clear of the caliper. I then mounted the pickup on the front fork, as close to the magnet as I could get it. I am thinking about upgrading to a unit with the decimal readout and may look at different mounting locations then but have had good results with the current system. The only issues I have noticed are a couple 'dead spots' where I cannot resolve to less than 2 or 3 mph. For instance, the speedo might be on 59 and there is no way it will read 60. The next reading I get would be 61. Don't know if it is a function of the pickup or the speedo computer itself.
BTW, does anyone have any first hand knowledge of how those highway mile markers are or were actually placed. I worked on a county survey team a couple summers many years ago and we used a 100 foot 'chain' (expensive tape measure) to locate cornerstones but I can't see someone walking off several hundred miles of interstate with that setup.
Mike in Celina
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