ETA:  Never mind, I figured it out.  


Ok, I bought this thing tonight.

It's a little bike speedometer / odometer.

I installed it on my bike, and the instructions did not specify any particular distance to place the magnet and sensor from the center of the wheel.

Just put the sensor on your fork, put the magnet on a spoke, and go.

Now, I am familiar with Hall Effect sensors and the basic theory of operation; the damn thing counts how many times it sees the magnet and derives a rate of speed from that count.

While the cyclocomputer has a setting for wheel size, it can't possibly know how far from the center of the wheel I am installing it, so how in the hell does it know my speed?

click on the live chat help

Quoted: click on the live chat help

I'm gonna take a shot in the dark and guess that the phone salesperson isn't going to be able to explain the math behind how this thing works without a consistent hub to magnet distance on various installations.

The tiny elves inside are very sensitive to motion.  Don't shake it though, or you might kill them.

RPM is RPM... if you know the circumference of the wheel and how many times it turns in a given time period - voila.

The spoke near the hub and the spoke near the tire pass the same plane at the same time. It's not measuring how fast it goes by the sensor only that it goes by it and how many times a second it goes by that sensor, I would guess its for a 21" tire (is that the common bike size).

-JTP

DOH!


Never mind.

It just occured to me that while the speed of travel of the magnet will vary with the distance from the hub, the number of revolutions it makes is the same no matter where on the wheel it's located.

Damn, I'm full of dumbass tonight.

Quoted: RPM is RPM... if you know the circumference of the wheel and how many times it turns in a given time period - voila.

Quoted: The spoke near the hub and the spoke near the tire pass the same plane at the same time. It's not measuring how fast it goes by the sensor only that it goes by and how many times a second it goes by, I would guess it for a 21" tire (is that the common bike size) -JTP

Damn, you guys beat me to it.  

Quoted: While the cyclocomputer has a setting for wheel size, it can't possibly know how far from the center of the wheel I am installing it, so how in the hell does it know my speed?

Because a rotation of your wheel is a rotation of your wheel..... If it knows the size, it does not matter where the magnet is. All that it is measuring is how fast the tire goes around or how many revolutions there are on the tire.

Or am I thinking of it wrong?

Yo. Chill out. You're thinking too much. It doesn't matter how far the magnet is. The Cateye is simply counting revs and mutliplying them by the circumference of your tire. 216 is typical for a roadbike.  Cool little units.

Stay safe

They will pass each other at the same RPM, no matter where it's spaced on the wheel.  

height=8

Quoted: Damn, you guys beat me to it.  I will confess that I asked this same question of an applied math phd several weeks ago and also felt like a dumbass :)