Placement of a Bike Computer Sensor

[zack787]

Hey. I just bought my first bike computer (or cyclocomputer?) and I have a question. It is the kind that mounts on the spoke of a wheel and the front fork. Where should I place the sensor along the spoke and fork (i.e. how far from the center of the tire)? Or does it even matter as long as the two line up? The instructions that came with it do not say and the pictures are not very good.

I ask this, because if I recall from science class, the further I place it from the center of the tire the longer the time is between each pass of the sensor, which would effect the measurements.

Thanks,
Zack

 

[alfeng]

Hey. I just bought my first bike computer (or cyclocomputer?) and I have a question. It is the kind that mounts on the spoke of a wheel and the front fork. Where should I place the sensor along the spoke and fork (i.e. how far from the center of the tire)? Or does it even matter as long as the two line up? The instructions that came with it do not say and the pictures are not very good.

I ask this, because if I recall from science class, the further I place it from the center of the tire the longer the time is between each pass of the sensor, which would effect the measurements.

I have MY wheel magnet(s) near the rim ... BUT, mostly, that is because it is easier to mount the sensor where the fork is wider.

If you take the magnet (any magnet) and wave it past the sensor, you will see that it doesn't really make any difference how quickly you do it BUT the frequency does matter ...

AND, what also matters is how close the magnet IS to the sensor to ensure a reliable reading.

 

[zack787]

I have MY wheel magnet(s) near the rim ... BUT, mostly, that is because it is easier to mount the sensor where the fork is wider.

If you take the magnet (any magnet) and wave it past the sensor, you will see that it doesn't really make any difference how quickly you do it BUT the frequency does matter ...

AND, what also matters is how close the magnet IS to the sensor to ensure a reliable reading.

So if the frequency does matter, it does matter where I put it on the spoke and fork, correct? If placement does matter, how do I get the computer to compenstate for the difference? Do I put in a different wheel size? Forgive me if I misunderstood your response.

 

[alfeng]

So if the frequency does matter, it does matter where I put it on the spoke and fork, correct? If placement does matter, how do I get the computer to compenstate for the difference? Do I put in a different wheel size? Forgive me if I misunderstood your response.

The speed/cadence/whatever IS based on the RPM, so it doesn't matter where you put the magnet in THAT regard ...

BECAUSE you have to tell your computer what your wheel circumference is, and THAT is the factor which you are probably puzzling over -- that is, the RPM & circumference result in the speed/etc.

If you are using 700x19 tires, then the value you indicate for the circumference will be less than if you are using 700x25 tires ... but, more for a 24" tire, etc.

 

[bigpedaler]

Hey. I just bought my first bike computer (or cyclocomputer?) and I have a question. It is the kind that mounts on the spoke of a wheel and the front fork. Where should I place the sensor along the spoke and fork (i.e. how far from the center of the tire)? Or does it even matter as long as the two line up? The instructions that came with it do not say and the pictures are not very good.

I ask this, because if I recall from science class, the further I place it from the center of the tire the longer the time is between each pass of the sensor, which would effect the measurements.

Thanks,
Zack

your science class recollection is wrong, sorry to be the one to tell you. one rotation of the wheel is one rotation, regardless of whether at the hub, or at the rim. the magnet & sensor of a computer work on the rpm's. the only difference would be in the amount of time the sensor had to "sense" the magnet going by, and with electronics, i don't think that's an issue.

your first supposition is correct -- as long as they line up, where on the fork matters little. they have to be close enough together, but not so much that they touch.

 

[hd reynolds]

Location of magnet does not matter coz sensor will still read it as one revolution.

Just a tip. Place it opposite where the wheel is heaviest. To do this suspend the front wheel and let it find it's way till it stops from rotating or see-sawing back and forth. Mount the magnet on the spoke on top of where the wheels rests. This way you'll more or less have a balanced wheel (or lessen the imbalance).