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Old 07-30-09, 03:39 PM   #1
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My favorite "cheap" computer: Bell F12

I just got a wired Bell F12, $11 at the local Target store. Have had it on my Trek 7700 for a week now, and it has been perfect (well, a week isn't enough time to determine reliability, but so far so good).

There is an error in the manual that came with it. It says that, to reset the trip odometer (not the main odometer), press both sides of the button for three seconds. This is wrong - that resets the ENTIRE computer, blowing away your trip odometer, main odometer, the calibration number that sets your wheel size, the time of day etc. Then you get to re-enter all that stuff, and your main odometer starts from zero as well as your trip odometer.

The correct way to reset just the trip odometer, is to hold down ONLY the left side of the button for three seconds. This resets the trip odometer, the elapsed-time indicator, and the max-speed only. The main odometer, wheel-size factor, time of day etc. are unaffected. This isn't a bug in the computer, it's a bug in the documentation. I've seen complaints about this going back for a year or more - apparently Bell isn't interested in revising the manual. Dommage.

Coolest thing about this computer, is that the odometer and trip odometer display distances with THREE decimal-point digit accuracy! Where the Cateyes I've seen (great computers) display their odometers with only a "tenths" digit, and their trip odometers with "tenths" and "hundredths" digits and no more... the Bell F12 has "tenths", "hundredths", and "thousandths" of a mile in both their main odometer and trip odometer! So the distance (in miles) rolls along as:


At normal bicycling speed (for me anyway, 10-15 mph) that last digit changes several times each second.

Makes me wonder how the computer decides when to change that "thousandths" digit. The way the computer knows how fast you're going, of course, is a sensor on the fork that "feels" a magnet on the wheel pass by, once every revolution. A 700c or 27" wheel moves about seven feet every revolution. But the "thousandths" digit has to change about every five feet.

If the computer is going to change its "thousandths" digit every five feet, but the only info it is getting is "Whoops, the wheel just moved another seven feet! Whoops, there went another seven! Whoops, another!..." etc.... how does it know exactly when to change that "thousandths" digit?

My guess is, it measures the time between magnet pulses, figures an approximate speed, and waits for the time that should have taken the bike to move five feet at that speed, and trips the "thousandths" digit then, sort of reading between the lines.

And looking closely as the display as I rode (no, I didn't run off the path from not looking where I was going), I noticed that the "thousandths" digit didn't always change smoothly. Sometimes it would jump from 7 to 9, etc., though I wasn't changing speeds appreciably. So I'd guess that, in addition to calculating when the "thousandths" digit should change, the computer kept track of REAL distance by simply counting the actual magnet pulses, and corrected the "thousandths" digit when necessary.

It's still very cool to have that "thousandths" digit along with the hundredths and tenths. It is a pretty good indicator of when the "hundredths" digit is about to change, and makes calibrating the speedometer for exact wheel size, much easier.

Plus, it's just plain fun to see the distance clicking rapidly away.....
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Old 08-07-09, 07:35 PM   #2
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Heh, nice long post. No one gave a shat though. I have the same bike computer as you except in wireless. Got it for 20 bones at walmart. Put 1000 miles on it in the last 1.5 months. No problems yet.
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Old 08-08-09, 02:25 AM   #3
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sounds nifty, so hows it look when its on the bike? i would like something like this so that i can keep track of my miles throughout the day, instead of approximating when i get home. i was also thinking of just making one myself since i dont really need a lcd screen, i just need total miles traveled.
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