Why? Because not only is the gap between the crank and the chainstay wider than the maximum reach of the cadence sensor arm (when it's mounted conventionally), but also because the (temporary) M590 cranks have hollow backs which would mean the cadence sensor magnet couldn't be ziptied into place without additional material to raise the magnet out of the recess.
How? I fitted the sensor unit under the chainstay, instead of above it, so that the cadence sensor arm extends back towards the rear wheel, instead of forwards towards the bottom bracket (I couldn't fit it above the chainstay and turn it around, because the speed sensor only tilts in one direction).
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| The sensor unit mounted backwards under the chainstay, showing the alignment of the speed sensor with the speed magnet on the wheel spoke, and of the cadence sensor arm with the eventual position of the cadence magnet (concentric with the hole in the crank arm). |
The cadence magnet attached itself to the inner end of the pedal spindle, but I was pretty sure magnetism alone wouldn't be enough to keep it there permanently. It couldn't be secured with a ziptie, because of course the pedal spindle would be in the way on the outboard side of the crank, so I popped the magnet out of its plastic holder, and used a short length of inner tube (with a small hole cut in it for the pedal spindle) instead.
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| The cadence magnet, looking lost in the depths of the hollow rear side of the crank arm. |
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| L to R: about 25mm of inner tube, with a slightly elongated hole offset a little towards one end; pedal; cadence sensor magnet; and the plastic holder and ziptie that would normally secure the magnet. |
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| The inner tube sleeve in position on the pedal spindle, ready to be fitted to the crank arm. |
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| Pedal fitted, with magnet attaching itself to the inboard end of the spindle. |
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| The inner tube sleeve pulled into position to secure the magnet. |
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| Outboard view of the pedal spindle and inner tube sleeve in place. |
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