computer magnet position on spokes
 

The computers come with setup instructions for a value to use based on tire size. Let's pick the arbitrary 700x23. Based on where you mount the pickup sensor, the magnet could be anywhere along the spoke from outside edge to inside edge. Won't this cause some level of error? When they provide the factors, is it based on the center of the wheel spoke?

Doesn't matter.
The computer counts how many times the magnet passes the sensor for a given period of time. With this information and the circumference of the wheel it can calculate distance covered, speed and other data.

If you have the magnet REALLY close to the hub, chances of the sensor counting the magnet twice per revolution increases, especially at slow speeds, so don't put it that far down.

The magnet and pickup are measuring wheel revolutions. The computer translates revolutions to distance, based on the circumference of the tire size. The error source is that there's quite a bit of variability in circumference with tire pressure for a given tire.

Yeah, if you want full accuracy, inflate your tires to your normal riding pressure, then do a measured rollout with the bike loaded (including yourself), and enter the effective tire circumference you get. But the location of the magnet won't matter.

There is one thing to consider: distance from the passing magnet to the sensor in the fork blade. Your computer's manual should state this, it's usually a couple of mm. If you place the magnet close to the hub, you will have less distance, even to the point where magnet hits the sensor and/or fork blade. The closer you move it to the rim, the more distance you will have between the two.

As mentioned, as long as the two are correctly positioned in relation to each other, the location itself doesn't matter.

--J

Better yet borrow a Garmin bicycle computer and it will calculate the wheel size based on the wheel revolutions and the distance traveled based on the GPS data.

For one of the more "fun" discussions on BF about this, check out this thread...BF at its best (and worst)

http://www.bikeforums.net/showthread...he-front-wheel

It should be said that generally it's better to put the sensor closer to the hub. Not because it's going to measure any differently, as has been said, it's just counting how many times the wheel turns, but because if you put the sensor and magnet near the rim, the magnet is going past the sensor very fast, and at higher speeds the sensor may not pick up every revolution, leading to inaccuracy. Near the hub the magnet moves past the sensor quite a bit more slowly and you'll be less likely to have the sensor begin "skipping" counts at high speeds.