I’ve been ignoring this problem for a while, hoping that it would be fixed in a firmware update, but it persists, and I finally got aggravated enough with it to write this post (and to engage Garmin support). The problem is simple: my Garmin Fenix 3 triathlon watch will not reliably record data from the Stages power meter I have on my bike.
A quick digression: there are two major standards for wireless exercise sensor connectivity, Bluetooth Low Energy (aka BLE and Bluetooth 4.0) and ANT+. Some devices support one or the other, and some devices support both. For example, my heart rate monitor (the excellent Scosche Rhythm+) simultaneously transmits both ANT+ and BLE signals, but my Wahoo speed/cadence sensor is ANT-only. When I ride, I usually use two devices: my old iPhone 4 on the handlebars, in a Wahoo case that has a built-in ANT+ adapter, plus my Fenix 3. The iPhone is too old to use BLE, and turning on BLE on the Fenix 3 dramatically drops its battery life, so I’m using ANT for all the sensor data. Having two devices means that sometimes I forget to start or stop one device or the other at various points, so I often have mismatched data between the two.
A picture will illustrate the problem most clearly. When I use the Fenix 3, I end up with ride data that looks like this:
As you can see, the power graph has a few spikes with lots of flats– and an average power of only 23W. (I’ll get to why the average is important in a minute). By contrast, here’s what the ride looked like when captured with the Strava app on my iPhone 4. Note that the power data much more closely tracks the speed, cadence, and HR data.
So why is this important? First of all, as a techie, it annoys me when two things that are supposed to work together won’t. More importantly, I actually use the power data from these rides in two ways. While I’m on the bike, I use it to gauge and adjust my level of effort. For example, yesterday’s ride was pretty windy, so I tried to hold a steady 190-210W while riding into the wind, keeping my level of effort constant and accepting whatever speed that gave me. After a ride, my coach and I use the power data to plan my recovery time and to identify areas where I need more practice (e.g. climbing hills). Having inaccurate or dirty data makes both of these uses impossible.
The Stages power meter support FAQ suggests moving the watch around, but I haven’t tried that yet. My troubleshooting efforts so far have been limited to changing the battery in the Stages and making sure the Stages and Fenix both have the latest firmware. I’ll see what Garmin support has to say. Hopefully they have a magic fix; I have a very early-model Fenix 3 so maybe they’ve made some improvements since launch. Until then, I’ll keep recording each ride twice and keeping the cleanest data.