Someone posted a link to an article recently about tripping the sensors, which obviously no-one read. The gist of the article was that the metal does not have to be ferrous, (alloy rims will work) but must form an electrically complete loop (ie its the wheels, not the frame that counts) and will function best when placed with its plane parallel to and on top of one of the cuts that form the loop. The article also said that some detectors do need to have their sensitivity adjusted to detect bikes.