Bad speed readings with Polar Power Sensor Kit

RoadBikeGuy

Hi all,

I recently installed the power kit for my Polar S710. Installation went fairly smoothly (thanks to all who posted tips in other threads!) and all the sensors are picking up acording to the blinking LEDs.

But the speed is way off. It is reading 3 or 4 times above normal. I have mounted the speed sensor on the left chain stay close to the drop out. I know that I came across something relating to this during a 'net search last week, but I can't find where it was.

I would be greatful for any help. Thanks.

jcjordan

I would suggest that it is one of two problems. you have not set the size of the wheel right or the fact that you have mounted it to the rear. Everyone i know i with one mounts the speed sensor to the front fork. I have had no problems with the speed sensor and belive it to be fairly accurate, thanks to the nice policeman with a laser speed gun. I was flying down a local hill and he pulled me over for exceeding the speed limit (is South Australia this can cost you demerit points against your drivers licence). The speed his equipment showed was 83.6km/hr and the polar said 83.3km/hr.

Tom Anhalt

The power module has a speed sensor hard wired into it that is intended to mount to the rear wheel. There's no way to mount it on the front.

You've already checked it I'm sure...but the first thought is to recheck your entry for wheel circumference.

I remember reading that you may get interference between the speed sensor and the power sensor if they are too close...but I'd expect that interference to go the other way since the speed sensor is simply a reed switch closed by the passing magnet.

tmctguer

i have had no issues with my Polar s720i & the power/speed/cadence readings. on occasion, the magnetic sensors moved a bit and stopped reading. also, i had to learn to properly mount the watch on the 2 metal sensors. but since i buttoned up those items, i've been riding for over 1 month (the time i've had it installed) without any incidents.

steve

Maybe you have the speed sensor towards the outside of the wheel and its picking up the cadance magnet?

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Steve

CyclingForums.com

Achilles180

That would be easy to see - The indicated speed while coasting would be instantly lower than while pedaling. I have not had the problem, and the magnets are 6" (15 cm) apart when mounted according to directions. I've got a second speedo mounted for backup and it always agrees with Polar's numbers.

DEM

How did you get your cadence sensor to work? Mine won't work! It's beyond irritating! bullet@lightblast.net

tmctguer

i am assuming that you are having problems with the small triangular magnet that is to installed on the crank arm.

the placement of the small magnet is the key. i mounted it on the bottom side of the crank arm (i.e., i don't see it on the downstroke, i only see it on the upstroke) slightly leaning towards the long power sensor. to hold it in the "leaning" position, i used duct tape.

until i leaned it toward the sensor, it was not reading correctly. i've read that other people have abandoned the little black triangular magnet and purchased a small magnet from radio shack. some people have placed a replacement magnet on the back of the threaded hole of the pedal (closest to the sensor). i do not have enough clearance to do this, so i had to make the little triangular magnet work.

DEM

Thx for the response.

To clarify, you think the problem may be the little magnet included w the sensor........I'm not sure. See, the sensor light "flashes" when the pedal goes by, so according to the directions the sensor is reading the magnet. Did yours "flash" but still report "00" in the cadence readout? Please advise. thanks

tmctguer

i seem to recall that when mine flashed, i got a reading. i had to keep adjusting the magnet because it would flash. later, after i had found the "sweet spot", i would inadvertently move it when clipping in. so i went to the duct tape !

tmctguer

one other thing...........make sure the "watch" is correctly mounting on the two metal tabs on the handlebar mount. this could be your problem.

i found that unless i secured the watch tightly to the handlebar mount, taking care that the two metal tabs mated with the small metal surfaces on the back of the watch, i would get no cadence or power readings.

DEM

Thanks-but I have a wireless cadence sensor, and I don't have any metal tabs on the handlebar mount.

Polar sent me an email w a service bulletin which told me to take apart the cadence sensor, adjust it, reassemble, make sure it works (give me the cadence) and then send it in to them to make sure I didn't mess up the waterproofing of the sensor.

I guess this just about cuts it w Polar. I bought the s720i because it would do everything. Honestly, if the bike store would take it back, I'd give it to them and trade out for a different brand. I have been continously disappointed by the lack of coherent documentation-basically I do everything by trial and error-that this device came with.

I'm going to give it one more shot this weekend, and then if the cadence thing doesn't work, I'm going to return that piece, and re-install my Specialized device w the cadence thing.

tmctguer

sorry, but i thought you had a power meter (which converts cadence & speed to wired), not just the HRM with speed & cadence. i will say that i had no problems prior to purchasing the power unit -- that is, when i was wireless for cadence and speed, things worked just fine.

sorry to hear of your problems.