Originally Posted by
Eric F
How much ferrous metal is on/in your bike? My road bike has very little (axles and bolts are about it).
All my bikes are either aluminum or titanium. Very little of it is ferrous.
This illustration from an article from the US Federal Highway Administration explains what is happening nicely. The wheels are the part of the bike that trips the light because they are closest to the loop. The vast majority of bicycle wheels are aluminum so having iron around isn’t really needed. The frame has little influence on the induction coil.
The black arrows represent the current flow in the loop wire and the induced current flow in the cycle wheels and frame. The white arrows represent the magnetic flux generated by the current flows. The result is a change in the loop frequency. The loop amplifier can measure this change. Note that it is the shorted turn that causes the reduction in loop inductance and thus the actuation. Source: Based on Figure 2-9, FHWA, Traffic Detector Handbook.