Arduino with SD Shield taped to back of bike.
I actually took the bike out to run some errands. One of the annoying things is that I’m using the current and speed control on the Cycle Analyst. It uses a PID loop to do this, and the defaults are tuned for a bike with less power than mine. That means it oscillates. If you look at the image below, this represents me running on flat ground with the throttle wide open. You’ll see the PID loop cranking the amps up and down and the speed oscillating about 3mph. I’m trying out Tableau so you should be able to play with the data.