Actually it doesn't matter whether the magnet is 2" from the hub or 2" from the rim.

Clearly the position of the magnet is critical for an accurate reading of speed and distance, so you would think that there is a formula somewhere, at the very minimum in the instructions.

Anyone know the answer?

Also, does anyone know if tire size affects the reading? for example, 19 vs 21?
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The position does not matter, the magnet represents one revolution of the wheel. You can place it a few inches from the hub or a few inches from the rim, either way, the end of the wheel attached to that spoke has only gone one full revolution. The distance the magnet travels is not measured, rather the number of times the magnet passes the sensor is input into a formula based on tire and wheel size. That is why the tire size reading is key.

Yes, there is a difference between 19 and 21, though small, when mutliplied over a few thousand revolutions it makes a difference. The manual that came with the bike should have the correct date to input. If you can't find the manaul I'd imagine you can find manual online.

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- Nick
Now that I know some of you guys look through the special needs bags for kicks, I'm gonna put some really weird stuff in mine. I can see it now. "What the heck was he going to do with a family pack of KFC chicken, a football helmet full of peanut butter, a 12 inch rubber dildo, and naked pictures of Bea Arthur?"

ditto on the spoke placement. Doesn't matter.

Tyre size. I think it does make a very slight difference. The only reason I know is that I bought some 23s and 20s and on the box it gave the circumference in mm, and they were slightly different.

Bobby

Put a piece of tape on the ground. Stand your bike on the tape such that the valve stem of your front wheel is at the bottom of the wheel, directly over the piece of tape. Mount bike. Roll forward the distance such that the valve stem makes two complete revolutions, and stop when the valve stem is again at the bottom of the wheel. Carefully dismount and mark the ground at the valve stem. Measure the distance between the two pieces of tape. If using imperial, divide by 2 * 39.37 (inches/meter); if using metric, divide by 2. That is the value you enter in your cyclocomputer. Repeat for each wheel you use and whenever you change tire brands/size.

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"Go yell at an M&M"

Another easy way is to use a sewing tape measure that has a metric side. Make a mark on the tire, inflate it to your operating PSI, and measure the circumference. Then enter the value, usually in millimeter, into the computer.

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I wanna go fast!

I am also under the impression that you need to subtract a small amount to account for your body weight slightly flattening the tire. the number i've heard is minute, but if you want it perfect...

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- Nick
Now that I know some of you guys look through the special needs bags for kicks, I'm gonna put some really weird stuff in mine. I can see it now. "What the heck was he going to do with a family pack of KFC chicken, a football helmet full of peanut butter, a 12 inch rubber dildo, and naked pictures of Bea Arthur?"

but then who really cares? At the end of the day, the computers aren't properly calibrated like your car anyway I would guess.

Excellent point, and I was thinking the same as I wrote my original post. And the tire may deflect differently on different surfaces, eg. new asphalt versus chip seal. Now that's gonna be tough to measure;)

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I wanna go fast!

One thing to note, because it happened to me. The closer the magnet is to the rim, the faster it travels past the pick-up sensor. I once purchased a cycle computer in Hawaii (left my at home be mistake) and I located the magnet fairly close to the rim. I found my computer giving me varying readings during my ride when I traveled over 30+km/hr. Thinking it was mal-functioning, I took it back and got a replacement. Well the same thing happened. I relocated the magnet closer to the hub and it worked perfectly. I moved it back toward the rim and it started malfunctioning. After thinking about it, I determined it was the speed at which the magnet was passing the pick-up. Too fast and the pick-up couldn't respond. Just something interesting to think about.

Actually, I would think the magnet travels faster closer to the hub. My theory is that the magnet has to be closer to hub (below the middle of the wheel) to provide a more accurate reading. I think that being closer to the rim requires some sort of adjustment on the computer for wheel size. But this is just a suspicion that I have. Haven't really thought about it and would not be able to figure out the math.

Trust us on this, so long as the computer is picking up the magnet, it doesn't matter where on the spoke it is located. The magnet is merely a representation of the outer edge of the wheel. Each time the magnet goes past the sensor, the outer edge of the wheel has gone one revolution, traveling (for a simple example) 1000mm. Now, if the magnet goes by 10 times in one second, that means you have traveled 1000mm X 10. That means you have gone 10 meters in the last second. That is then fed into the computer to figure out how fast 10 meters per second is. It does not matter that the magent has only gone (for example) 5 meters. Each time the magnet passes by the sensor, the wheel has gone one full revolution, that is all that matters.

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- Nick
Now that I know some of you guys look through the special needs bags for kicks, I'm gonna put some really weird stuff in mine. I can see it now. "What the heck was he going to do with a family pack of KFC chicken, a football helmet full of peanut butter, a 12 inch rubber dildo, and naked pictures of Bea Arthur?"

As everyone said, it doesn matter where because all its like a switch when it goes by on each revolution and doesn't measure anything other than the fact that it passed by.
Having said that, put the magnet as close to the hub as possible to get the most reliable readings as it will be traveling slower near the hub than it would at the rim. Simple physics. Thus, you will be reducing the potential for a missed reading.

Oh yea, don't use your car to calibrate either. Espescially an SUV. Car odometers are extreamly inaccurate as tire decisions made at design time don't always match those made by your dealer or tire availability at production time and almost no dealers go to the trouble to re-calibrate your odometer when you decide you want upgraded rims or tires on your car.

Find a regulation 10k running route and test calibration there. Make sure the route is certified by your local track club,

I can't believe anybody hasn't mentioned that the magnet should be placed 180 degrees opposite the valve stem. This helps to minimize the wheel imbalance and could save you 0.000000000056 seconds over a 40k time trial. Not to mention improved bike handling!

should be placed 180 degrees opposite the valve stem. This helps to minimize the wheel imbalance and could save you 0.000000000056 seconds over a 40k

Yes, but that gain is completely swallowed up by the aerodynamic losses associated with having a magnet on the front wheel, which are huge.

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"People think it must be fun to be a super genius, but they don't realize how hard it is to put up with all the idiots in the world."

Good point about being opposit valve stem.
Also, I put mine on my back wheel. (have cadence on my computer.)

Is your back wheel cadence reading based on the number of revolutions from your front or rear cassette?

I may of mislead. My computer has two magnet pickups. One on the back wheel for speed/odometer/etc. and a 2nd pickup on my left chainstay that picks up a reading on my left crankarm for cadence.

\\I can't believe anybody hasn't mentioned that the magnet should be placed 180 degrees opposite the valve stem. This helps to minimize the wheel imbalance \\

Actually, you want to place the magnet 180 degrees from the heaviest section of the wheel - this is not always at the valve stem. the valve stem is opposite the seam in the rim; depending on the rim this can still be heavier than the additional weight of the valve stem. Hang your bike up so the wheel can rotate freely and place the magnet on a spoke at the top of the wheel after it comes to rest.

additional items - the farther a mass from the center of rotation, the greater the energy needed to rotate it (1/2*I*omega^2). You save a (very) small amount of energy by having the magnet closer to the hub. This is probably also better aerodynamically since the tangential velocity of the magnet is minimized when it is closest to the center of rotation.

There are two exceptions to the rule of placing the magnet as close to the hub as possible - for cordless cyclocomputers you often get better and more reliable signal strength when the sensor is closer to the computer (i.e. handlebars), so the sensor should sometimes be positioned farther up on the fork blade. the other exception is for some fork/wheel combinations the distance between the sensor on the fork blade and the magnet on the spoke is too far when it's positioned near the hub. Moving the sensor towards the rim can move it closer to the spokes and hence enable the pickup to function. An example of a wheel I'd have to do this with is the Hed 3-spoke race wheel. here the 'spokes' are essentially flat and are too far away from the fork blade at the hub to enable a computer pickup to function properly.

Yes, I spend time thinking about proper computer placement on bicycles. ; )

J

We all need to get a life

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- Nick
Now that I know some of you guys look through the special needs bags for kicks, I'm gonna put some really weird stuff in mine. I can see it now. "What the heck was he going to do with a family pack of KFC chicken, a football helmet full of peanut butter, a 12 inch rubber dildo, and naked pictures of Bea Arthur?"