I plan to do some riding this summer, and although my neighborhood is safe, I wanted a disc lock for when I'm in unfamiliar areas.
I saw a cheap disc lock on eBay; it's your typical disc lock, but includes a 120dB motion alarm.
The cost is only $7 US - but shipping to US/Canada is another $19!
It arrived today and I have checked it out. The lock seems reasonably sturdy - it won't stop a dedicated thief, but will slow down a casual one. The lock mounts easily on the front disc of my 250r. It can be locked by pressing down on the lock button, and uses a barrel-style key to unlock.
When you lock the carbide steel, the lock lets out a moderate chirp. You have 15 seconds before the alarm actually kicks in.
If the lock gets moved or bumped, it chirps a couple of times. You have only a couple of seconds to either stop moving the lock or unlock it before it starts letting out a loud siren wail.
I've just played with it a bit with my bike in the driveway, but it seems to work fine. It's not a fancy alarm, but if you just want some basic protection it fills the bill.
One caveat: the power comes from six LR-44 'watch-style' cells. The lock comes with a spare set, but when you eventually need new batteries, I wonder how much it will cost to pick up six more such cells..?
The vendor is here:
http://cgi.ebay.com/ebaymotors/BRAND...3A1|240%3A1318