My wife and I use Polar 720's with both cadence and speed. Magnet is on the timing chain side crank arms as are the sensors. I have separate sensors for captain and stoker. No crossed signals - both work flawlessly. The ability to download all the info from the Polar - for a ride is invaluable for training - not to mention pretty cool. I've had to replace the batteries in the units once in 3 years of use.