Originally Posted by
Roody
The triggers respond to metal mass, not to magnetism. You need to get your rims over the buried coils. If a bicycle won't trigger it, it's defective--usually either improperly installed or adjusted wrong. Get it fixed or ignore it.
This has been discussed on BF many times. You'll often see grooves in the lane (often they are hexagonal..). The theory is that if your rim is placed over a portion of the hexagon, it should trip the light
http://auto.howstuffworks.com/question234.htm
It sometimes works for me...
Here's another thread that discusses it...