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Computer Magnet Location (?)(9 posts)

Computer Magnet Location (?)DWridesGT
Feb 28, 2002 1:07 AM
I have the Topeak Panoram bicycle computer. The pick-up magnet is to be attached to a single front spoke, but the directions mention nothing on exact location. My high school math class tells me placing the magnet down near the hub will give me a different reading verse placing it up near the rim. I know I'm a product of "public education" but where exactly does the d@mn thing go? Any thoughts....

I bet Woof the Dogg knows

re: Computer Magnet Location (?)Jack Daniels
Feb 28, 2002 4:20 AM
It doesnt matter where you place it on the spokes. The mag will still turn around the hub the same amount of times if it is near the hub or the rim.
It does not matterpmf1
Feb 28, 2002 4:37 AM
Think about it. your spoke passes the same place anywhere along its length while the wheel is spinning. Closer to the hub it travels more slowely, but has less far to go than closer to the rim. The opposite is true close to the rim.

So just figure out where on your fork you want the sensor and put the magnet in the corresponding place on your wheel.
It does not matterweiwentg
Feb 28, 2002 5:30 AM
indeed; one revolution is one revolution. the sensor magnet counts revolutions, not tangential velocity.
normally I mount my magnet near the hub to decrease rotating weight. is this being picky? yes. does it cost anything? probably not.
re: Computer Magnet Location (?)JimP
Feb 28, 2002 7:18 AM
Yes, the above replies are correct about revolutions. The magnet causes the sensor to close as it passes by. If the magnet is placed closer to the rim, it will be passing the sensor faster and the sensor may not pick up the click. The theory of placing it close to the hub works for both the speed angle as well as rotational weight. Also, the sensor mounting band that goes around the fork may not be long enough to work if it is not close to the hub.
re: Computer Magnet LocationChen2
Feb 28, 2002 7:23 AM
It's going to give you the same speed no matter where you place it on the wheel, BUT, you can use the magnet to help balance the wheel. With the bike off the ground, spin the wheel to see which side stops at the bottom, it may be where the valve is or it may be the opposite side where the rim joint is. Put your magnet on the light side.
re: Computer Magnet Location (?)DY
Feb 28, 2002 9:12 AM
There was discussion about this a while back. The topic was distance from the hub as it relates to the speed that the magnet passes the sensor. Some mentioned that there were known cases that if the magnet was placed too close to the rim, it would be traveling too fast at times and the sensor wouldn't be able to read it. On my computer, the book says to place it no more than 10 cm from the hub. It is hard sometimes to get these sensors to fit, as the shapes of carbon forks are different and the clearances get pretty tight sometimes. BTW...all the sensor is doing is counting the number of times the magnet goes around. Your instincts about feeling the magnet would be going faster if placed farther away were right, but it would not do anything to alter the nuber of times the magnet passed the sensor.
same number of revolutions everywheresalmonwheel
Feb 28, 2002 9:25 AM
The computer counts how many times the magnet passes the snsor per minute (rpms) not how fast it's going. So where you place it shouldn't effect the readings. The speed of the magnet will be greater the further you are from the hub, and I've read (somewjere) that placing it closer to the hub is better becuse the magnet is moving slower and it reduces the risk of missing the read. But on ne computer I got they recommended putting it as far away from the hub as possible, which I assumed was to reduce therisk of picking up false reads.

In my experience it realy doesn't matter.
re: Computer Magnet Location (?)mhinman
Feb 28, 2002 10:25 AM
Here is another reason to place it as close to the hub as possible, I am a big rider, and when I am pulling hard on the bars, I distort the front wheel significately, that can cause the magnet to hit the pickup or move too far away to register accurately. The closer you get to the hub, the smaller the distortion.