I have very limited experience of electric bikes, just one conversion I did and my wife’s current ebike, both with hub motors - so I am a little confused by Andre’s post saying that the mid motor itself might have gears?
Two e-bikes already makes you an expert!
I'm talking about virtual gears, a control under your thumb or your forefinger as the case might be, which electronically sets up a higher current regime for the motor. For instance, on my Bafang bottom bracket motor I have a controller fascia fitted which came with a five-way switch which I put on the handlebars near my left hand. The switch performs many other functions besides giving extra juice when needed; it doesn't actually need the thumb throttle to work -- in fact the thumb throttle cuts out software gears, which is no problem for me because I use the thumb throttle only when I want instant full power regardless of battery drain on one particular hill which steepens towards the top and is crazy steep for the last five yards.
Okay, so why is the switch so urgently required?
See, the so-called pedelec is a profoundly perverse machine, deliberately made so by German legislators, than whom there is no stupider herd-influenced creature on earth. It works only while you pedal and then only to the extent that you pedal. So when you're ascending a hill and you naturally slow down your pedaling, as you go slower, just when you need it most, the electric bike gives you less and less and less juice. You wouldn't pay for a car that did this to you.
But if you inform yourself and shop smart and are ruthless about putting down dealers who try to set themselves up as adjuncts of the police by disconnecting the best features of the controllers (I spoke to one who proudly told me he drags a screwdriver over the relevant part of the PCB -- not a joke, real life experience), you can get a full kit including the controller I describe above which comes in the full kit of the Bafang BB and its controller. You can also buy the controller kit that I'm talking about separately in China and fit it yourself. I bought the whole lot of motor, controller and battery, plus a comprehensive fitting kit, together with a thumb throttle (not in the kit) from a dealer in England who has the correct attitude. There are interruptors for normal wire operated brakes in the standard kit, and the dealer sourced and supplied to me for very reasonable cost plug-in interruptors for my Magura rim hydraulic brakes, which were already in the controller software. You can actually program the particular controller I have with a laptop, but I read the list of what was already in the ex-factory programming, decided that everything I could possibly want was there, and didn't bother to buy the interface; I still haven't thought of any function I could want that isn't there. One function that I didn't then see a lot of use for but have used many times since was a walking pace setting, which lets you walk the bike up hills without effort if you're deep in conversion with non-electric companions; it also climbs stairs, and rolls up the ramp to the high pavement in front of my town house, and best of all for me, it rolls effortlessly up three little steps right in the doorway where there just isn't space for two panniers full of shopping and me at the same time. For me, this small boon is one of most agreeable features of the controller.
I'm not sure that all this is particularly relevant to Martin's installation, because I assume he wants to keep it basic because he's on deliberate learning curve (I too made my first installation as a deliberate learning curve). If you want more about my second installation, which has served well for many years, look up Bafang BB on this forum, and also the concept of the Coulomb which I have appropriated to explain why you need a humongous battery, and as an appropriate unit for the current available in the next second, which is more important on an electric bike than any other algorithm. (Ironically, at the other end of the scale, the current Rolls-Royce still hasn't got a revolutions counter. Instead it has a Power Reserve meter which works very much like the one on the controller Bafang supplies for e-bikes.)
