i'ts my understanding that this little wheel spins around and gives you your indicated speed. but what does this run against ? i would think the chain but none of them ever look black ?
The white plastic cup fits over the bolt that holds the front sprocket in place. This turns the sensor.
I don't know about getting it down there but http://www.google.com/aclk?sa=l&ai=...?productid=30538&rct=j&q=speedohealer&cad=rja