Im no electric whiz, but just a few things to add which maybe helpful as they were to me..

Well Garrys right u can rotate the magnets to sort the speed prob out.

The Sidewinder will attualy work at 14.4v
Just make sure the batteries can give the current needed so the voltage does not drop too far.. i think i attualy got my sidewinder to work down to 12.5v...
Well im going to run 2x bosch 12vs on 14.4v on my sidewinder.

There is a mod you can do to the sidewinder to run at 12v, but it by-passes a few safetys which then means if you have a dodgy battery, it can take out the brain side of things.. but run it at 14.4v and your cool - only extra few cells. Granted if your using SLAs your stuffed.

But Running the bosch 400 watt motors at 14.4v is no big who har... even running the 12v 400 watt motors at 18v on a sidewinder would be a decent wattage and keep the sidewinders happy.

Oh the other thing - yeah its a good idea to have a bigger battery pack eg 36v but use the Radio kit using dual rates or end point ajustments etc to limit the voltage to 24v or something - but be warned you do put more stress on the speed controller doing that than just using the correct voltage battery... speed controllers will get very warm fast doing that. I did it and things do get warm fast, i was advised against it.

Mr Stu