I already have a Garmin for speed and distance and elevation gain. But it requires an attachment to the bike for cadence. So what's an Accelerometer?

There is such a device that attaches to the shoe for joggers. My wife has one for her heart rate monitor. Like step-counters/pedometers that you wear on your belt, they are based on accelerometers. An accelerometer measures acceleration which is the rate of change of velocity.

Based on the circular and smooth nature of the pedaling motion, it might take a sophisticated and expensive one. The simplicity, reliability and low cost of attaching a magnetic-field sensor to the chain-stay is the way to go. Those who have problems with them might consider counselling.

As for GPS for speed/distance, not if you ride twisty single track or have large hills. My GPS receivers (I have two) run around 10% low in distance for mountain biking. That's true in the N Georgia mountains and the low rolling hills of N Florida.

Testing them on a two mile distance on flat suburban streets which involved only 6 turns, my gps, ATB cyclometer and Ford Odometer were all well within 1%.

I turn off the GPS on my HRM and use the cadence/distance/speed sensor instead. If I'm exploring a new trail, then Ill use GPS and rely on my cyclometer to correct my ride record. I also turn the GPS on for jogging/walking as it's accurate.

Al