Originally Posted by
Big_e
With all due respect, are you sure about that? I would think that different placements could give false readings on the speed. I placed mine right above the spot where the spokes cross as they near the axle. That's kinda how the small illustration has it placed.
Ernest
Absolutely certain, yeah
It's a question I have to answer a lot (LBS mechanic since 1989).
If the computer were trying to detect the
linear velocity at which the magnet is travelling, then it would matter, because the further out the magnet is located, the faster its linear velocity. But that's not how the computer computes speed. It looks at the
frequency of sensor impulses, which is independent of the magnet's location on the spoke. For a given ground speed, any point on the spoke will pass by the sensor at the same frequency.
And that's a good thing, or else it would be very difficult to set up bike computers accurately!
On a separate note, I like to put the magnet at the crossing point of the spokes too, for wired computers (for wireless, I put the sensor as close to the bars as possible). Many spoke magnets can engage two spokes at the crossing point, keeping the magnet from getting bumped out of position as time goes by.