Hi and
to the forum.
The Smart Pie is much smaller and lighter than the Magic Pie and is ideal to provide additional assistance on level roads and slight inclines, but it is not really powerful enough to climb steeper hills without a reasonable amount of pedal power being supplied by the rider.
The Magic Pie is much larger and heavier, therefore it can deliver a lot more power and produce considerably more torque than the smaller Smart Pie, and I would definitely recommend fitting a rear MPIII or MP4 for mountain bike use.
The MP4 is equipped with the latest vector controlled "Sine Wave" controller which makes it a lot quieter and more responsive than the MPIII and it also produces more torque.
The MPIII produces a "pinging" noise which increases with load whereas the MP4 produces no discernible load related noise when power is applied, even at full throttle!
The regenerative braking force is also much stronger on the Magic Pies than it is on the Smart Pies.
If you have an aluminium frame, you will need to fit some suitable torque arms to provide a secure fixing for the axle which is able to withstand the torque reaction under load.
Alan
