I Have a 48V 350W Bewo. It comes with a choice of chain wheels a 48T or a 44T which I chose as getting up hills is of more importance to me than top speed. I don't know if there is a graph or table showing at what rpm it is most efficient,but through my own personal experience,I would have preferred a smaller chainring.
It is not possible to put on a smaller chainring because if you could the chain would hit off of the casing.
My bike has 700c tyres and a 33t back cog and taking into account that I am about 95kg ,obese but reasonably active,would have much preferred a smaller chain ring had it been possible.
The Bosch motors have very small chain rings which allows them to spin up to high revs easier and in my opinion makes them more efficient at climbing hills.
Don't get me wrong,should I turn up the power on the flatter areas,it goes like the wind,but even in my lowest gear I can do 16 or 18mph, but that to me tells a story of being too highly geared.
If I were to change all my gearing to a 10/11 cassette it would be possible to fit a 12- 44T and that would be better as it would be 1:1 with the crank,however that would be quite an upheaval and expense.
I have tried the bike with a nexus 8 hub, but,however the same applies.
My objective is to be able to go up steep hills with a reasonable cadence i.e not pumping pedals while standing on them and my impression is that the lack of interchangeable chain rings detracts from this otherwise good product
