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Wahoo Blue SC bike speed and cadence sensor

I last tried the Wahoo speed sensor. A brilliant design requiring no magnets and is easy to install. Sadly Garmin did not include support for speed only sensor in older devices (like my Edge 305) so I returned it.

So onto this sensor. It is the traditional speed and cadence (rate of rotation of the pedals) sensor with two magnets one on the pedal arm and one on the spokes of the wheel. The sensor itself mounts on the chain stay and has to be adjusted to be able to get at each of the magnets. There are two LEDs that light up every time it sees one of the magnets so you can see you got it adjusted right.
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And thus comes the first challenge. The magnet to mount on the pedal arm is a continuous loop elastic. The only way to get this onto the arm is to remove the arm from the bottom bracket, or remove the pedal. Either requires special tools that most people won’t have. A stupid design. The easiest way around this would be to cut the loop and cable tie it, but Wahoo did not include holes for a cable ties in the loops so all in all this is really poorly though out for all but bike mechanics.

Ok so now to put this puppy to the test to see who does (and does not) support the sensor. So I went on a 2.5 hour mountain bike ride. On a ride that is tight and twisty like this you can see the difference in distance when compared to the GPS. The sensor will always be higher as the GPS will assume a straight line between sampling points. So to test it out I used Endomondo, Wahoo Fitness app, RunGPS (all on iOS) and then I used Garmin FR70, Edge 305 and Fenix 2.

To start off with Endomondo on iOS does not support a speed and cadence sensor so the only reason for this data point is a basis for GPS only data for trying to figure out if the app/device uses the wheel sensor to figure out speed and distance.

The Garmin FR70 does not have a GPS in it, so you are guaranteed that the speed/distance data it displays is from the sensor. So using these two data points we have our comparison points.

Let’s start out comparing cadence data over this ride. ANT+ can talk to multiple devices at a time, and iOS manages multiple apps wanting access to cadence data just like it does for GPS and heart rate. So here’s the average cadence data. In order FR70, Edge 305, Fenix 2, Wahoo fitness, Run GPS are 71, 71, 47, 68, and 69 RPM. So they all agree well except for the Fenix 2, no idea what’s going on with the Fenix 2. Now looking at Max cadence the data is VERY different 145, 163, 217, 136 and 196. So to say this is inconsistent is an understatement.

So now onto the speed side of the sensor: Comparing the GPS only Endomondo with the Speed sensor only FR70 for distance over the ride we have 24.48 Vs 28.21KM, or a difference of 13%.

The Edge 305 on the same ride saw 25.57KM, so in spite of seeing the speed sensor it is not using it for distance. In the owners manual Garmin state: “The speed data is only recorded and used for disatnce calculation when the GPS signal is weak or the GPS is turned off.” So I guess they really mean it. I had seen videos with the wheel being spun and the Edge showing speed even though it wasn’t moving. Seems that is misleading. Of course this also means me returning the Wahoo speed was unnecessary. Oops.

I did a second ride because on the first I had the speed side of the sensor off on the Fenix 2. Oops. On this second ride the Fr70 saw 23.25km and the Fenix saw 22.93 or a difference of only 1% confirming that the Fenix 2 does indeed support and use the speed sensor. Yay!

Now onto Wahoo Fitness app. One would hope if anyone would get this right it would be Wahoo. Why sell a sensor and then ignore the data from it. Sadly this is exactly what they do. The distance off Wahoo fitness came in at 24.4KM spot on with the GPS data. I am very disappointed in this.

Next onto Run GPS. They have BRILLIANTLY included a setting in the app to allow you to decide whether to use the sensor or the GPS for speed and distance. Why more don’t do this is beyond me. The consumer is left guess which it’s using, or in my case running a big test.
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The data from RunGPS shows that they are perfectly using the data and it comes in at 28.4KM.

So in summary Endomondo doesn’t support the sensor, the FR70 works perfectly with it, the Edge 305 ignores (unless you turn the GPS off) and RunGPS nails it perfectly.

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September 11, 2016 - Posted by | GPS Stuff, iPhone Stuff

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