Speedometer
 

Got to make sure i ask enough annoying newbie questions as possible :p ok i put on a bike computer and my speed seems like its right most of the time... but then sometimes it'll jump way off.. like it said i was doing 18 then jumped up to 40mph and then back to 18.. so i'm assuming its on there wrong. The instructions didn't really say where to put it on the spokes... does it matter as long as the magnet lines up with the sensor or what? any other ideas why the readings might be wrong, or maybe just sometimes they mess up? man i hate being new and ignorant, thanks for any help.

Where have you put the magnet ? The best place is as close to the hub as possible, both to reduce the effect of the weight off balancing the wheel and because it brings it closer to the sensor on the fork.

Check out this thread as well:

Setting up a bike computer

its pretty close to the hub... i'm not sure if its as near to the hub as possible... i'll look into that. The magnet gets close to the sensor though, for a little while it was actually rubbing against it so i adjusted it a little. Thanks for the site, I'll check it out.

Any problems I have had with wide variations in my speed have resulted from magnet placement. Correcting magnet placement has always corrected this problem. Good luck.

John

What brand and model cyclocomputer did you get? That information will help diagnose your problem.

I have a Cateye Astrale on my commuter and a Vetta R77 on my road bike. No problems with either. :)

Sounds like magnet problems.

i'm not exactly sure, its a Bell... i guess i'll get back to you on that one when i'm at home and can look at it

Is this it?
http://www.nashbar.com/profile_morei...gory=&sku=6334

Have you cleaned the contact area on the handlebar mount and the computer unit? Remove any tarnish on the contacts. An eraser works great.

its all new, so i assume its all clean, it looks clean anyways

This may not be of any help, but it has happened to me so I will tell my story. I have noticed my speedo will drop to zero kph when I am riding on certain streets, then go back to the normal reading. I assume it is magnetic interference from either radio waves, hospitals or mobile phone towers. I have actually had my speedo reading 56kph/35mph while standing inside the house with the speedo in my hand, this is definitely interference of some kind.:confused:

This could be a factor in your erroneous readings, maybe.

CHEERS.

Mark

Mark, is yours a wireless computer? I have seen this happen to wireless computers near sub-stations (power stations?).

I just realised that if your speedo is not aligned properly or there was a battery problem, the speedo would read a slower speed not a higher one. ie: if there was a sensor problem the speedo would only intercept every 2nd or 3rd rotation so you would get a much lower speed reading. So my interference theory may not be as wacky as it sounds.:)

CHEERS.

Mark

Yes it is.

Hmm! I never thought about the battery, how long should one last? 2yrs, 5 yrs? 10,000 miles? :)

I havent replaced my battery for the life of my computer, now about 5yrs old with a little over 20,000 miles. Just this week i have noticed it turning on and off durring the ride, however it has always showed the correct speed when working.

A sign that the battery is going is when it fails to read on cold mornings. Last year, when riding to work in winter my speedo would take 30mins before it would give any speed readings, although the elapsed time and clock were still working, once the weather warmed up the speedo would start to work. I eventually bought a volt meter for $5 from a car shop, this way I get the most out of those expensive batteries.

The volt meter also makes it easy to work out which battery needs changing, the top or the bottom one. With three bikes and three speedo's it gets used quite a bit.

CHEERS

Mark