Bike computer setting

The odometer on my daughter's bike is a simple one. But I can't get the setting right. I looked online and got the number, but when we are biking side by side it shows her as being about twice as fast as me. (I know that her computer is the wrong one, not mine, because I know we are not traveling 20 mph on average!) I took a tape measure and marked the wheel with chalk and measured it, and got a number that was pretty close to the one I'd found online, and I entered that into the computer. Her bike still thinks we are cruising at 20 mph!

She likes it that way. I'd prefer to have an accurate mileage reading on that bike for maintenance logs. Any ideas what's wrong? It's a Sigma Sport, walmart brand bike computer. It might have cost all of $12. I've been assuming that it's me entering something wrong, but it did just occur to me that maybe the computer is actually programmed wrong, or some glitch occurred. It used to work, on her old bike.

Does it use a magnet? Any chance there are two on there?

A couple of quick things to check:
** Make sure that the computer is set for MPH and not KM.
** Sounds as though the wheel circumference number is off... magnet/sensor are reading but I truly think that the circumference is off.

Good luck!
Jackie
http://chasecyclery.blogspot.com

I agree with the wheel circumference being off. Most are automatically set for 700c wheels but I'm not sure about brands other than Cateye.

I have 700X38 tires on her bike. According to Sheldon Brown the circumference should be 2180. When I measured it, it was 2159. Those numbers are close enough. Using either number, it doesn't matter, the bike computer still thinks the bike is going nearly twice as fast as it is!

There's only one magnet on there. I knew there was only one, but I checked just now anyway just in case!

I had a computer that would give really funky readings like that if the magnet was too close or too far away from the sensor. If I tweaked the sensor position on the fork, I could get it to read normally.

I had this problem once and it turned out to be the orientation of the magnet. The sensor was detecting the magnet twice for each revolution of the wheel -- once for each pole of the magnet. I rotated the magnet 90 degrees and the issue went away. This might not be your problem but something easy to test.

Here's an article that explains this issue in more detail if anyone is interested:
Cyclometer Errors Caused by Magnet Orientation

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Originally Posted by Melalvai

The odometer on my daughter's bike is a simple one. But I can't get the setting right. I looked online and got the number, but when we are biking side by side it shows her as being about twice as fast as me. (I know that her computer is the wrong one, not mine, because I know we are not traveling 20 mph on average!) I took a tape measure and marked the wheel with chalk and measured it, and got a number that was pretty close to the one I'd found online, and I entered that into the computer. Her bike still thinks we are cruising at 20 mph!

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If you are riding 12.5mph, that would translate to ~20kph. I'd check first to be sure you have your daughter's computer set for miles (not kilometers) per hour.

I finally figured out the problem, or at least I think I did. According to Sheldon Brown, bicycle computers use one of SIX different calibrations factors! I assumed I needed to enter the circumference in mm. But this particular model wants circumference in mm divided by 1.6093! At least, if you have selected mph. Seems pretty stupid to me that the thing can't do that bit of math itself. But, I only paid $12 for it.

The puts her wheel size at 1355, and the computer seems a lot closer now. I do have the manual, not exactly the one that came with it, but I had found it online. However, her exact wheel size wasn't on their chart.

This is the sort of thing that makes it worth it to pay a little more.