Review: Garmin GSC10

After almost a month with the Garmin Edge 500, I noticed that even the high sensitivity GPS chip tended to have odd spikes. For instance, it claimed that we hit 55 mph on Skyline, when my same bike computer said we did not exceed 45mph. On my single, I would not care, since I'm hardly the kind of person who sets speed records anyway, but on the tandem, we've hit freeway speeds, so I found myself wanting some precision, especially for the upcoming Tour of the German Speaking Alps.

Now, even regular bike computers don't stay accurate because of tire inflation variation, air temperature, and just plain inaccuracies while calibrating the wheel size. However, the Garmin GSC 10 Speed/Cadence Bike Sensor does the right thing, which is to calibrate the wheel size using the GPS unit, and then use the spinning wheel as a check against GPS jitter. On top of this you get a cadence sensor as well, which produces nice charts telling me that keeping my cadence on the tandem up on Redwood Gulch would be a problem. [Update: Our Mt. Charlie Ride showed us doing 79.1mph. I don't find that believable at all, so there's still some jitter!]

Mounting and setting up the unit was a cinch. The unit cleverly has both the cadence and the speed magnet arm on the same sensor. The arm is designed in such a way that you can twist it up and down without needing a screw driver to loosen it up. This is a big feature, since if things get twisted around on a ride somewhere you can fix it without getting out the tool. Checking to make sure that you got everything lined up is straight forward: you push a reset button and the LED blinks every time a magnet crosses the sensor arm, so you can tell whether or not you've got it right. The unit uses CR2032 batteries Pack, which I had a stash of back when I had an operational Sigma MHR 2006. A search on the web indicates that the battery should last about 2500 miles, which sounds kind of low to me, but given that everything is wireless is perhaps understandable.

My biggest complaint is that mounting the unit uses zip-ties, instead of rubber bands or something less permanent. My experience is that zip-ties are prone to breaking off from fatigue, and of course getting snapped through abuse, and I'm unlikely to carry zip ties on tour. The battery cover is also in an awkward place and might require removing the rear wheel to replace. No big deal if you're not a cycle tourist, but still a pain.

All in all, it's a nice package, and I'm even tempted to get one for my single. I'll probably wait to see whether this thing survives the upcoming tour before I buy a second one for the single bike. Otherwise, first impressions are: recommended.
[Update: I just switched the battery, and it's been about 1500 mile per CR 2032]