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Old 10-03-13, 12:01 PM   #1
ScottRae
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accurate miles? OCD?

Like many over the years I have owned different odometers. Using the chart for the tire sizes I have found that they are not always accurate. So I do a roll out measurement to get the correct number of cms. I know the chart numbers can be affected by the psi of the tire. Even after using an accurate measurement I may come up 25 yards over or under a measured mile when riding. My wife says, "Who cares?" But to me over time those extra feet add up, or take away from your true riding distance. She thinks I have OCD. Which brands on the market seem to be giving the most accurate readings?
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Old 10-03-13, 12:13 PM   #2
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Do you put your weight on the bike when doing the rollout?

Are you sure that your magnet is close enough to the computer pickup?

Who measured the mile and how?

CMs? You need to get it down to MMs!!!

I roll the tire two revolutions; measure it; then divide by 2.

Consider a GPS unit. They are often accurate to within 20 feet.
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Old 10-03-13, 01:42 PM   #3
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Listen to your wife dude! Close is good enough, just enjoy the ride and if in doubt, do a little extra.

Also, the GPS units are accurate and don't care about the wheel size etc, however if the sample size for waypoints is not short enough they can be off quite a bit on a longer ride if the ride is very curvy. Sometimes I zoom into my ride after importing it to SporTracks and can see that since it only samples like every 10 seconds or so that it will clip off a lot of the curves, the faster I go the worse it clips. It will show the curve on the map but the line that shows the route makes a straight line from one side of the curve to the other.

One big hill I ride on regularly has a real cool descent, I put the old girl in my tallest gear and hunker down and crank as fast as I can, it has a relatively tight long sweeper in the middle of it. The GPS must cut a few dozen yards off the results if it happens to sample just before and just after the turn.

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Old 10-03-13, 01:54 PM   #4
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ppl who accused me of being obsessed with bikes don't know what they're talking about. only people with a passion can understand. for me tho, I use the chart and trust it's close enough for my purposes. really what I use my computer for now is comparing the same ride with the same bike over time. so regardless of how accurate it is, it shows how/if I'm consistent and or improving. or if I sucked any particular day.
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Old 10-03-13, 01:58 PM   #5
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Where is your magnet and sensor mounted? It should be near the hub. If it's near the edge, at high speeds the magnet can move past the sensor so fast that it can miss steps.

Also if the magnet is mounted sideways, it's possible for both the north and south pole of the magnet to trigger the sensor at low speeds, resulting in doubling of readings.

Personally I don't care that much if it's a few yards off. Unless your measured mile is on a straight piece of road, the way you take the corners can make several yards difference. I do a roll-out and just go with that. Tire pressure difference can change it, but that just means you should be checking your tire pressure.
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Old 10-03-13, 03:13 PM   #6
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If you are that worried, get a GPS unit. They are more convenient anyhow and can give you altitude as well (though with less accuracy). If you alos want to get cadence measurements or use it on a trainer, Garmin GPS 510 or equivalent have the sensors for that (and heart rate).
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Old 10-03-13, 05:42 PM   #7
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I did a lot of my "serious" riding in the days before bike computers. I had one in the mid-80s, but then I took it off. Never worried too much about the accuracy. I recently got a GPS, and I like it for bike computer functions on a rare basis. It's nice to know that the distances and speeds are pretty accurate.
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Old 10-04-13, 09:03 AM   #8
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One of the routes I like to ride has a couple miles downhill, with surveyed mile markers on it. A few times a year I'll check my odometer's calibration. It changes when I change tires (new rubber), when the tire wears (smaller diameter wheel), and with temperature and tire pressure. I have a cadence computer with a rear wheel sensor to avoid front wheel swerving, and I run the check downhill so I don't measure the minor swerving from climbing; all I have to worry about is trying to keep a constant position within the lane.

After all that OCD setup, if I'm within 2%, I'm happy. That's 100 feet in a mile. GPS is usually within 1% of the odometer readings on country roads, but usually lower on rides with substantial residential mileage -- the GPS "shortcuts" turns at intersections and curves.
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