Add more padding underneath the chainstay sensor mount. Aluminum vibrates more then the other materials and this can be casue for a failure for accurate data transfer. Let me know if you have any more questions.
My Powertap worked fine on a Cannondale aluminum frame. But I could never get it to work on my Softride Powerwing. I think the sensor might be very sensitive to the angle somehow and the since the Powerwing has no seat stays, the sensor was nearly level on the chain stay.
i agree with aus… i switched from carbon to Al and had problems. More padding under the pickup on the chainstay pretty much solved it. Also, a mag trainer has a lot more vibration than the road or even a fluid.
make sure the zip-ties aren’t too tight. If you pinch the wires the p-tap can give some pretty screwy readings, or just go blank.
I suggest using electrical tape instead of zip-ties.
If you are already using electrical tape, and/or the zip ties are not too tight- Remember that p-tap recalled all of the SL models earlier this year for rebuilds.