Depending how long you ride for and how frequently, and if it's just for backup purposes, you could potentially use the Spy Gear Panosphere with a large MicroSD card.

http://www.amazon.com/Spy-Gear-Panosphere-360-Degree-Cam/dp/...

I have two of them and while the native recording is very fish-eye, the software that comes with the unit functions fine in focusing on a specific point.

With a 32 GB card and full charge, I think I've gotten about 45 minutes of recording. Might not be good for daily use, but for recreational outings it might be usable with some customization (e.g. super glue a mount to it). I've got one mounted on a swivel clip from a clip-on guitar tuner that broke and it's been a neat tool.

Edit: Example footage of using the camera: https://www.youtube.com/watch?v=cBpEGw3tTZ8