Polar Speed Sensor

barnesd said:

Have just bought a 725x and am going to install it later, have one quick question, you need to set the wheel size, however where do you need to install the magnets and sensor as you could put the correct wheel size in, however if the sensor was to close to the bottom of the wheel, surely this would change the increase frequency at which the magnets passed the sensor and give a false reading? A bit confused, Any help appreciated.

David

Umm... think about it. Doesn't matter where the magnet is, it still goes past the same amount of times - ie once per revolution of the wheel. The computer counts the number of times it goes past in a given timespan (the frequency), multiplies that by the number you programmed in (which equates to the diameter of the tyre, ie the distance travelled for one wheel revolution) and works out from that how fast you're going.
The only difference position makes is to the speed the magnet itself has to travel in order to make a complete revolution – closer to the hub it will be travelling slower, closer to the rim, it'll be travelling faster. But the sensor doesn't care, because it's not counting magnet speed, it's counting the frequency at which it passes.
There may be an argument to say closer to the hub is better as the magnet's moving more slowly so the sensor has more time to get a good strong count from it. But then the closer to the hub, the further the signal from the sensor has to go to the wrist unit, so you might get a problem there instead...

d'oh

Thanks

Eastway82 said:

Umm... think about it. Doesn't matter where the magnet is, it still goes past the same amount of times - ie once per revolution of the wheel. The computer counts the number of times it goes past in a given timespan (the frequency), multiplies that by the number you programmed in (which equates to the diameter of the tyre, ie the distance travelled for one wheel revolution) and works out from that how fast you're going.
The only difference position makes is to the speed the magnet itself has to travel in order to make a complete revolution – closer to the hub it will be travelling slower, closer to the rim, it'll be travelling faster. But the sensor doesn't care, because it's not counting magnet speed, it's counting the frequency at which it passes.
There may be an argument to say closer to the hub is better as the magnet's moving more slowly so the sensor has more time to get a good strong count from it. But then the closer to the hub, the further the signal from the sensor has to go to the wrist unit, so you might get a problem there instead...

barnesd said:

d'oh

Thanks

I like to install mine high up on the fork, right below the caliper, for a few reasons. 1. Superficial and subjective, but I think it looks better. 2. The speed sensor has a limit to how far it should be from the wrist unit. Too far and the unit might not pick it up. 3. the wheels I use have bladed spokes, but they are round near the nipples. The round spoke magent slides on nicely and stays put there.

PeterF said:

I like to install mine high up on the fork, right below the caliper, for a few reasons. 1. Superficial and subjective, but I think it looks better. 2. The speed sensor has a limit to how far it should be from the wrist unit. Too far and the unit might not pick it up. 3. the wheels I use have bladed spokes, but they are round near the nipples. The round spoke magent slides on nicely and stays put there.

The problem I have with putting the CS200 sensor at the top of the fork leg is that the magnet then has to be put at an extreme angle, and/or the sensor has to be turned a long way in to get it close enough to the magnet. Someone on another thread suggested getting rare earth magnets from radio shack or similar for more tolerance between sensor and magnet, so I might try that.

Eastway82 said:

The problem I have with putting the CS200 sensor at the top of the fork leg is that the magnet then has to be put at an extreme angle, and/or the sensor has to be turned a long way in to get it close enough to the magnet. Someone on another thread suggested getting rare earth magnets from radio shack or similar for more tolerance between sensor and magnet, so I might try that.

I used a Sigma Sport computer before, and actually use their wheel magnets with my Polar. They work fine. Where you place the sensor does have a lot to do with the shape of the fork as well as the alignment of the spokes.