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#11
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"Mark Leuck" wrote
I use White-out on the concrete garage floor Yes. The weight of the rider on the bike changes the wheel circumference by a small amount. Some people resort to putting a spot of paint or other material on the ground, riding through it, and then measuring the distance between the paint imprints as the wheel rotates. Where a surveyed accurate known distance course is available, such as perhaps a running track or a highway speedometer test zone, you can simply ride the course, note the mileage, and apply a correction to the cyclometer setting in ratio to the actual vs. reported distance. Repeat as desired. I generally don't bother with more than a simple single rotation measurement when I get a new cyclometer or a new tire. I recently rode through a five-mile marked speedometer test zone and found even my relatively casual setting was within 2%. I haven't bothered to adjust the setting... When this subject has come up before, some people mention use of GPS distance measurement or automobile odometer readings for comparison. Both are subject to errors, of course. GPS constructs a route by "connecting the dots" of its sampling period and doesn't account for the curves it cuts out. Some are better than others. Automobile odometers are often off by several percent and are subject to tire size changes and inflation differences. For most of us, close is good enough. Even a single wheel rotation measurement is more accurate than using the wheel/tire size tables that come with most cyclometers. Jon Meinecke |
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#12
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I've used the measured course method as told by Sheldon Brown
http://www.sheldonbrown.com/cyclecom...l#measuredmile. I use the same course, a twisty section of road about 4 miles long I regularly ride. I've plugged the formula into the excel spreadsheet and calibrate my bikes. I use the mile markers on the side of the road, and during testing, I keep the speed down so I can stay about the same distance from the edge of the road, no cutting corners. Very curious thing, the small wheels provide a very repeatable result. I run the route and collect the data, enter it into the spreadsheet and adjust the computer and retest to find more accuracy. 16 and 20" wheels work very well. 26 and 700 wheels get a bigger error than started with. I've no idea why. My GPS, a Garmin Vista mounted on the handle bars, has almost no error, .01over the 4 miles, so long as we don't drop satellite contact. |
#13
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Jon Meinecke wrote:
When this subject has come up before, some people mention use of GPS distance measurement or automobile odometer readings for comparison. Both are subject to errors, of course. GPS constructs a route by "connecting the dots" of its sampling period and doesn't account for the curves it cuts out. Some are better than others. True, I've seen as much as 10% error between two GPS receivers measuring the same twisty route (when subject to variable tree cover etc.). However, they can be used to get a very much more accurate measurement if you can use them to create mile markers over a straight line. Choose a time when there are many satellites visible in a location without buildings or tree cover. Halt at one end of the route, allow some time to average the position and waypoint that location. Note the display of estimated position error (this isn't totally reliable but you can at least try to ensure it's only a few feet). Then move to the other end of the straight stretch, again stand still to get the best out of the position averaging and use the function which measures distance to the first waypoint. This value should be accurate to within 2x the claimed accuracy of the receiver, which is far better than the odometer function. You might want to repeat, tweaking your location for an exact mile etc. It's also possible to use projected waypoints to get an exact mile but the principle is similar - you're using the comparison of two averaged locations rather than the sum of a sequence of estimated movements. It's always best to use a straight line - mile markers on a twisty route are not necessarily reliable unless you follow the same line with the bike as the original marker took. -adrian |
#14
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wrote
Very curious thing, the small wheels provide a very repeatable result. [...] 26 and 700 wheels get a bigger error than started with. I've no idea why. We can speculate... Cyclometers work by counting pulses or switch closures from the wheel sensor. It is possible that they may miss a pulse or see one pulse as two. With larger wheels, the resulting error in computed distance is larger than with smaller wheels. However, smaller wheels would result in a higher pulse rate and perhaps more opportunity to skip or miscount! %^P Hmmm.... One could design a cyclometer (and perhaps they do) that used frequency sampling and averaging over a time increment to reduce the effect of skipped/miscounted pulses.... In any case, it seems unlikely that such miscounts could account for a significant error, though. What size is the deviation/variation you see over what distance? In my five-mile surveyed markers route, I saw the same 2% error for each of the miles. This was with a 650c wheel mounted sensor. I also have bikes where the sensor is on 16" and 20" wheels and haven't noticed significantly more or less accurate distance readings for the route I commonly ride. Jon Meinecke |
#15
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On Thu, 28 Jul 2005 12:02:24 -0500, "Jon Meinecke"
wrote: In any case, it seems unlikely that such miscounts could account for a significant error, though. What size is the deviation/variation you see over what distance? Working from memory here because my notes are about 100 miles away.. When set to the printed suggestions tat came with the computer I was about half mile off over the measure course. After running the numbers through the spreadsheet, I was over a mile off. Suggested number was something like 82.2 inches and corrected number was 100. I guessed at a new number, based on the fact the 100 seemed to go the wrong direction with the error, and set the new number at 75 and I've not had a chance to recheck, since this current calibration is a work in progress, not to mention, I changed from a large tire on the wheel to a narrow tire yesterday while doing some maintenance, so I will have to start all over again. Mike |
#16
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wrote
On Thu, 28 Jul 2005 12:02:24 -0500, "Jon Meinecke" wrote: What size is the deviation/variation you see over what distance? I may have misinterpreted your comments on differences in repeatability differences between cyclometer sensors on small wheels and big wheels. I thought you were saying you saw more variance in the cyclometer measured distance over the same course when riding with larger diameter wheels. When set to the printed suggestions tat came with the computer I was about half mile off over the measure course. The correct setting is equal to the old setting times the actual distance divided by the distance as reported with the old setting. Since it is a simple ratio, it doesn't matter what the setting number unit is (circumference in cm, mm, in, diameter in whatever...) A = actual distance in miles D = distance as reported with S setting S =setting (that generated D) C = correct setting A / D = C / S C = S * A / D For instance, if the actual distance is 10 miles, the reported distance is 10.5 and the setting was 82.2 then the correct setting is 78.3. In my case, my cyclometer shows I've ridden 1.02 miles for an actual 1 mile course. I don't recall the exact number, but say my current setting is 1995, the corrected setting would be 1956... I changed from a large tire on the wheel to a narrow tire yesterday while doing some maintenance, so I will have to start all over again. The known measured course need only be long enough to show the error within the precision of the cyclometer. Shorter may be better, as then there may be less variance, wandering on the course. Ride it once and apply the formula. I've never found the suggested setting numbers in cyclometer instructions to be very accurate. In fact, I've always found them to be too large for a given wheel/tire size. That makes sense, if they're going to err, err on the side that makes people think they're riding faster and further! %^) I've also noticed the price scanning errors are most often in favor of the store.... %^P Jon Meinecke |
#17
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On Wed, 27 Jul 2005 15:33:38 -0500, "bent_pilot"
wrote: "Mike" wrote in message hlink.net... Dex wrote: Accurate calibration of cycle computer I measure the distance convered by several revolutions of the wheel that I am calibrating. Make a scratch mark on the surface with the valve stem at the bottom of the wheel. Make several revolutions of the wheel and make another scratch mark with the stem at the bottom again. I usually use 10 wheel revolutions. Measure this distance and record it. If the distance was measured in feet, you usually have to convert it to centimeters to use it in setting your wheel circumferance. First, find your wheel circumferance by dividing the distance by the number of wheel revolutions. Then multiply this answer 30.48 -- this will be the number you use to set the wheel Where does this number come from? The conversion I use is: millimeters = Inches * 2.54 This is the number (30.48) that I think I have been using to convert feet to cm. -Dex |
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