Computer sensor placement....
 

My sensor for the computer is about two thirds down the fork (most look like the are about in the middle).
Does it matter? I was going to hold on to someones car to test for accuracy. Do I need to?

Um. What computer, specifically? Holding on to a car? :eek:

Just a basic cateye computer. Don't know if the accuracy is dependent on the placement on the fork.

nope, as long as the correct tire size is entered in the computer, it doesn't matter...

As long as the computer is picking up the signal from the sensor you're fine. The sensor just ticks off every time the magnet goes by, then multiplies by the circumference of the wheel to calculate speed/distance. 100 rpm at the hub is the same as 100rpm at the rim.

If you do that holding on to a car thing, could you video tape that for us?

I like mine high on the fork, putting the magnet as far from the hub as possible. But I guess the rotational weight guys would say put it as close to hub as possible. I don't think it really matter. No testing required, just measure your wheel/tire combination.

thanks..........ok.

I don't know. Ask this guy:
http://www.cyclingnews.com/photos/20...ghai066/17.jpg

Ouch! Got a feeling that if he was a lefty before, he's a righty now. :eek: <hurts Vega to look>

I like my sensor high on the fork if it is wireless so the transmitter and the reciever are as close as possible.

I like to place mine as close to the hub as possible. It reduces the imbalance effect that the magnet has on the rotating wheel, and (more importantly) allows me to get the sensor-magnet gap close without having to push the sensor into the area between the fork and the wheel.

BTW-mount the sensor in such a way that if it gets pushed into the wheels by accident the next spoke to hit it would push it back out of the way again.

If you feel compelled to test clibration against a car, have the carmeasure exactly a mile, and then see ifthe bike computeris highor low: don't try to match speed by holding on or otherwise.

But realize car speedometer calibrationis not that accurate. One, tire size (which might notbe OEM, or may be an OEM option not calibrated for) affects it, and Two, manufacturers often purposely make speedomters read a tad high.

ok, I guess I don't have to know EXACTLY how fast I'm going.................ouch!

Why not just ride pace with a group and all compare moment in time speeds and then see if your mileage is close to theirs at ride's end. Works for me if I have doubts.

mmkay. We've now established that you're a very smart roadie. :)

LOL. wow this made my day.

True, but the magnet is moving faster relative to the sensor at the rim than the hub. I try to get as close to the hub as possible.

Oh my god, did he make it out alright? It looks like the rear tire is about to run over his shoulder or possibly his head...

^+1 would like to hear more on the matter?

All that the computer sensor does is to count the revolutions that the wheel makes. The hub makes exactly the same number of revolutions as the rim so it doesn't matter how high on the fork you mount it.

What does matter is how close the sensor is to the wheel magnet. Mount the sensor wherever it's necessary to get it to within a couple of millimeters of the wheel magnet.

One other thing. Mount your sensor on the front of the fork. That way if it ever gets bumped into the magnet the rotation of the wheel will push it out of the way rather than into the spokes.

The sensor doesn't "care" how fast the magnet is going - just if it's going past.... I guess it could mis-read if it was faster....

I set it out quite far near the rim - as close to opposite to where the valve stem is to balance the wheel :) is it only cyclists that worry about the most pointless details??:)

Interesting. I've found that most of the time the valve stem is the lightest part of the wheel. I assume that's mostly due to the steel pins that are used to pin the rim joint together opposite the valve stem. When I put a bike with a nicely adjusted hub on the workstand, the front wheel usually rotates until the valve stem is near the top.

Apparently the guy DID end up (relatively) OK. Here's the link to the whole series of photos...
http://www.cyclingnews.com/photos/20.../qinghai066/14

Does it not matter where you put the sensor when calculating the speed and total distance???

By setting the radius of the wheel, you know the circumference of the wheel. The magnet/sensor combination works by counting the number of revolutions that the wheel makes. From this it computes a rotational velocity with the help of a clock (i.e. revolutions per second). Using the radius of the wheel it can find the tangential velocity...

For the distance it just counts the number of revolutions. Each revolution means that the bike had gone a distance because that is how far any point on the wheel has moved in a circle, and since the wheels don't slip, the bike has moved that far...

In both cases, the positioning of the magnet/sensor was not needed, as the total number and frequency of the revolutions are all that is needed given the radius of the wheel...

I put my wheel magnet at the location needed for the sensor to sit on the chain stay where it can pick up my cadence magnet on the pedal axle....