Garmin cadence sensor

Bar Shaker

Please show post your fit files then, I am happy to post loads of mine.

Mine, above, is the raw data file from my 800. Every raw Garmin data file will contain the same, unless using power and HRM sensors too. These will then also be recorded. No speed is ever recorded. Why would it be? Location is recorded to 6 decimal places and speed is a simple function of the distance between any two points, over the time. I don't have speed recorded in any of mine and you won't have it in yours. Fact.

A poor reception will not lead to a slow speed display if there isn't a speed sensor input, it will lead to an erroneous speed display. I had a max of 85mph on a cycling app, in a known poor reception area near me. That's not slow!

I am always happy to be proved wrong with fact. We can see plenty of assumptions but no facts.

drlodge

I find my Garmin 800 tends to show a slow speed display when it loses some satellites e.g. under tree cover, something around 0.5 to 0.8 of the speed I'm actually going. However, this doesn't seem to show up on Strava when uploaded (and I'm not using a GSC10).

apreading

<Trackpoint>
<Time>2014-07-26T07:16:51.000Z</Time>
<Position>
<LatitudeDegrees>xx.44063436426222</LatitudeDegrees>
<LongitudeDegrees>-yy.030544936656952</LongitudeDegrees>
</Position>
<AltitudeMeters>62.400001525878906</AltitudeMeters>
<DistanceMeters>13.979999542236328</DistanceMeters>
<Extensions>
<TPX xmlns="http://www.garmin.com/xmlschemas/ActivityExtension/v2"&gt;
<Speed>3.558000087738037</Speed>
</TPX>
</Extensions>
</Trackpoint>

apreading

I think what you have done is export a GPX from Garmin connect - this doesnt include the speed. If you export a TCX or look at the original FIT file (not so straightforward) the speed is in there:

<Trackpoint>
<Time>2013-08-25T06:46:12.000Z</Time>
<Position>
<LatitudeDegrees>51.38284902088344</LatitudeDegrees>
<LongitudeDegrees>-2.359195277094841</LongitudeDegrees>
</Position>
<AltitudeMeters>14.199999809265137</AltitudeMeters>
<DistanceMeters>56.63999938964844</DistanceMeters>
<Cadence>65</Cadence>
<Extensions>
<TPX xmlns="http://www.garmin.com/xmlschemas/ActivityExtension/v2"&gt;
<Speed>5.181000232696533</Speed>
</TPX>
</Extensions>
</Trackpoint>

chris_bass

Agree to disagree?

Dippydog2

I do not care about speed, so do not even have if displayed on my 800.

What I care about is effort, and I get that from heart rate and perceived exertion.

Speed is just a by product of these things with extra factors outside of your control like tempersture, wind etc.

Just live with the after the fact GPS calculations, they are more than enough. If you are watching a speed display change as you ride under trees you are missing the point.

Or if you are really serious get a power meter.

Bar Shaker

apreading wrote:

I think what you have done is export a GPX from Garmin connect - this doesnt include the speed. If you export a TCX or look at the original FIT file (not so straightforward) the speed is in there:

<Trackpoint>
<Time>2013-08-25T06:46:12.000Z</Time>
<Position>
<LatitudeDegrees>51.38284902088344</LatitudeDegrees>
<LongitudeDegrees>-2.359195277094841</LongitudeDegrees>
</Position>
<AltitudeMeters>14.199999809265137</AltitudeMeters>
<DistanceMeters>56.63999938964844</DistanceMeters>
<Cadence>65</Cadence>
<Extensions>
<TPX xmlns="http://www.garmin.com/xmlschemas/ActivityExtension/v2"&gt;
<Speed>5.181000232696533</Speed>
</TPX>
</Extensions>
</Trackpoint>

Yes I have only looked at the .gpx files, so I take your point that it is recorded in the .fit file.

GPS time stamps are 'only' to 8 decimal places (100,000,000th of a second) and I am sure you understand the significance of determining bike speed to an accuracy 10,000,000 times greater than that... with a magnet attached to a spoke.

apreading

The key to using wheel revolutions to measure speed/distance is that this removes the inaccuracy of GPS readings and can take better account of distance travelled due to changes in altitude and not moving in a straight line between two GPS points. It also knows when the bike really isnt moving, whereas GPS always seems to detect a little drift and has to guess whether this is movement or spurious readings.

Bar Shaker

But if the log data of is derived from wheel speed, it would imply the GSC10 is accurately measuring the trigger point to within one thousand million, millionth of a millimeter... or very roughly the diameter of a hydrogen atom?

GPS location will give spurious readings (mostly due to differing gravitational fields/atmospheric densities near or on the path that the satellite signal travels through - the speed of light is slower through higher gravitational fields) but these can be considered a reasonably constant error so location values taken 1 second apart can be treated as being very accurate. I suspect that the speed data is only taken as a mathematical product of the distance moved in that time and GSC10 speed data is not recorded at all.

By the way, I mentioned that my experience was repeatable. If anyone lives in an area that does have reduced satellite coverage (high DOP), it would be interesting if they could repeat my set up and see if they get the same results. To do so, you would need to remove your spoke magnet, slide your GCC10 along the chainstay (mine sits underneath it to miss my Di2 battery so the arm is on the path of my crank magnet) and move your speed sensor arm out so it was approx 5mm-10mm away from the path of your crank magnet. Ride around at 80rpm and go through some areas of poor reception.

It would be interesting to see if you get the same results.

kingstonian

The cadence sensor I really like is the new Wahoo RPM, you can clip it to your pedal rather than have to attach it to the crank etc which makes it super-easy to change between bikes.

Bar Shaker

The new Garmin one is similar in that it goes around the crank with a rubber band and doesn't need a magnet. The speed sensor goes on the hub in a similar way.

Simple to swap between bikes.