John Phoenix

I have been using a Bell speedometer. You know the kind with a sensor on the front fork and the magnet mounted to a spoke. Where on the spoke should the magnet be placed for accuracy? Also where on the frame should I put the sensor?

On the sensor, there is a little arrow close to one end - do they mean for me to center the magnet to that arrow?

Should they both be mounted in the center of the spokes/wheel? I feel where on the wheel you place these items due to its circumference will make some difference. Middle, closer to the top of the wheel or closer to the bottom?

simonaway427

The sensor measures the revolutions of the wheel - so whether you put the magnet on the top/bottom/middle of the spoke, it will still passes only once per revolution right?

There are some factors however. If you place the magnet towards the hub, it travels slower during its revlution, so there's less chance of the sensor missing it, skewing your readings. The arrow most likely indicates that you should have the magnet centred over it.

ItsJustMe

What he said; towards the hub will cause it to skip less, but that's actually usually not a problem. Apart from the sensor possibly missing a beat because the magnet is going by it too fast, it doesn't matter, the computer will measure the same no matter where the sensor is placed. Unless you have some really weird space-alien technology wheels, the center still turns the same number of times per mile as the outside.

Accuracy is dependent on how well you adjust it. You DID read the owner's manual, right? The part where it tells you to do a roll-out test to determine the number of millimeters per revolution of the wheel, and input that value into the computer in setup mode?

Giro

See Sheldon Brown's Cyclocomputer Calibration Chart.

Juha

Magnet attaches to a spoke, and sensor attaches to the fork. The position relative to hub/rim does matter in the sense that you adjust the gap between magnet and sensor that way. When placed closer to the hub, magnet will pass closer to the sensor as well. The correct distance between sensor and the passing magnet should be found in your computer's documentation. If not, a couple of mm is a good guess.

John Phoenix

I must digress fellows. Some say the distance they are placed, closer to the axle or closer to the rim does not matter. I say it must. I say this because my motor cannot do over 20 MPH. I notice top speed closer to the rim is 19 and closer to the axle is 21 mph. If the positioning along the spoke is not the cause I do not know what is. Try it for yourself. Both times the distance from sensor to magnet was measure by a nickel. A nickel is exactly the right width.

It'sJustMe, yeah, I read the book. I have been using it for months.. I just wanted to re-adjust it better than guessing.

ItsJustMe

It matters but not for the reason you think. The sensor is not detecting the speed directly, it simply counts how much time it takes between times that the magnet comes around. This does not change depending on where on the spoke you place the magnet and sensor. However, if you read our responses carefully, you'll see that one possible problem is that if you put the magnet and sensor out near the rim, the magnet is going past the sensor much faster, and the sensor may miss seeing the magnet sometimes, resulting in lower readings.

All the computer is doing is counting how many times per minute the magnet goes past. That is, and must be, the same no matter where on the spoke you place it.

If you think about it, if it DID matter, like if somehow the sensors was actually measuring how fast the magnet went past it, it would read FASTER the farther from the rim you got. And it would make a HECK of a lot more difference than a few miles per hour - on a 700c tire, you've got about 2155 millimeters around it. If you put the magnet near the axle so you had perhaps an effect 10" rim, that would be 127mm radius * pi^2 = 1252mm circumference. So if you were going 20 MPH, that's 8839mm/second so your tire would be turning 4.1 times per second, but a sensor placed near the axle would be seeing that 4.1 turns per second and thinking that meant you were travelling 1252*4.1 millimeters per second, which works out to 11.5 miles per hour.

I have tried it for myself; I've put my sensor and magnet in all kinds of different locations, and the ONLY difference that it makes is that with the magnet near the rim, at higher speeds, sometimes it misses counts and starts to read lower.

When that happens, the speed usually jumps WAY down because it thinks your tire just started turning half as fast or even less. This won't happen at low speeds, only high speeds.

There is also another problem that's less often seen where the magnet is mounted SIDEWAYS, and the sensor gets a double hit every time the magnet goes by, once for the north pole and once for the south pole of the magnet.

As far as counting on your motor to always be going exactly 20 MPH, I don't believe it. I assume the controller is set to deliver a fixed maximum amount of current to the motor. What speed that translates into is going to vary substantially given the tire pressure, wind speed and ambient air temperature (air at 32*F is about 10% denser than air at 80*F, so you'll go slower through it)

A friend of mine with a very nice ebike has a digital readout on his that is constantly showing him the number of watts being drawn by the motor, and he says that at any given speed, even a 10 PSI drop in tire pressure can result in 20% more draw on the batteries. If you have a constant current supply, that means 20% less speed instead.

Bottom line; it should make no difference, and if it does, it's not because of the sensor reading faster/slower, it's because the sensor is not picking up every rotation because the magnet moves too fast at high speeds. And you can't rely on the motor to be carrying you at exactly 20 MPH.

John Phoenix

Thanks for the detailed explanation It'sJust Me. I can understand the issue better now.

xoxoxoxoLive

I have been using one for like 6 months now, only problem I seem to have is the battery coming loose,
in the computer. the spoke you have it mounted to will not come around any faster top or bottom...
( Read the manual ) Richard ( Mine works Great )