When i had my bike i bought a disk lock that had an alarm built into it. if you try to move the bike after you lock it, it will start beeping real fast and its unbelievably loud too, it cost me like 70 bux and you can buy them at www.lockitt.com. in my opinion thats the best protection you can have, especially if you leave your bike outside because regular alarms can be disabled by disconnecting the battery and with a regular disk lock you could just pick the bike up but as soon as they hear how loud this dam thing is they'll take off

 

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That sounds like a relatively cheap and good idea. Better make sure you keep fresh batteries in the shit!