Help me understand my cyclecomputer
 

So, I got a bike (road bike, 700cc wheels), and the bike is on an indoor trainer. Also, I got a cyclecomputer. A Cateye Mighty 8, to be specific. I mounted it to the chainstay, as I obviously wouldn't get a reading off of the front wheel while stationary. Now then, my first question is, why does it seem to not matter where on the wheel you install the sensor? I would think the calculations would differ greatly depending on whether the sensor was closer to the hub or closer to the rim; but the installation instructions didn't specify at all where on the wheel to put the sensor. My second question, given that the instructions said to install the computer on the front wheel, will the readings I am getting from it being on the back wheel be different?

Can ayone help me sort these questions out?

Thanks!

it does not matter what position the sensor is because the computer calculates speed based on how fast the wheel makes 1 complete turn...when you were setting up the computer, it must have asked you for a number, that # is either the diameter or radius of your wheel + tire

Calculation wise, it doesn't matter where the sensor is located. That wheel magnet will take exactly the same amount of time to make 1 revolution whether its mounted near the rim or the hub.

Place the sensor as far from the hub as possible. And by as far from the hub as possible, I mean where the distance between the magnet and the sensor is the largest possible where the sensor still registers the magnet. This is pretty easy to do - takes no more than a couple of minutes to adjust to perfection.

The point is to make the magnet pass by the sensor as fast as possible, to minimise the time the sensor is "tripped". It could possibly affect the accuracy of the measurement.

It is, of course, true that each revolution takes place simultaneously at any distance from the hub, along the same radial, but the rotational speed is much different, while the magnet's "sphere of influence" has the same size regardless.

Readings will be the same front and rear, assuming same tire size. They probably tell you to mopunt it on the front because the wire isn't long enough to reach the rear.

Uh wow.

So completely opposite of what you said basically, and it makes sense actually.

+1 to what operator / Sheldon Brown says. Closer to the hub is typically better. On my 3x Open Pro wheels I place the magnet right above the second spoke cross, maybe about 1/4 along its length.

Thanks guys.

So, after I posted I was riding the bike, thinking about it and figured it out. Sorry it was kind of a bonehead question, but I guess I had to ask it in order to really wrap my head around it.

You're never going to leave the room while it's on a trainer no matter what the Cyclocomputer displays!:lol:

Not really completely opposite, but a bit more cautious.

I still argue that my approach is better. But try for yourself. I get erratic - or rather, strange - readings when the magnet is too close to the hub...

[QUOTE=samspratlin]My second question, given that the instructions said to install the computer on the front wheel, will the readings I am getting from it being on the back wheel be different?

QUOTE]

Rear wheel/front wheel function will be the same, assuming that the wire is long enough.
But in terms of accuracy the circumference calibration number should be slightly smaller for the rear wheel because the rear wheel carries more weight, the tire is squashed more, and the effective circumference is slightly smaller.

Al