Hello: I installed an Astrale 8 on my ride...the crank arm is concave on the side that faces the hanger set (Trek 5000 - Bontrager crank of course)...therefore, when I try to affix the magnet with the 2-sided (worthless) tape AND the plastic zip band, the magnet just "flops around" (won't sit flush doe to the concave crank backside). I can see it pulling toward the sensor (yes, it is passing the sensor in the correct spot), but it does not register cadence. Any ideas or experience? Thanks! - S