you know as I have fixed 10,000 bikes , im very concerned with this idea that shimano can't provide the correct ratios. Just wrong!
The issue with the Shimano HG-61 cassette is that it doesn't come with an 11T sprocket, and the problem with Shimano HG-41 is that it doesn't come with a 36T sprocket.
The other issue when a bicycle is motorized is that the first 3 gears with a (standard) 4 tooth difference between adjacent sprockets is that the ratio is too short between those gears.
I spent a good deal of time messing around with sprocket sizes to come up with a consistent speed difference between selected gears, giving an 8 tooth count between first and second gear, a 5 tooth difference between second and third gear, a four tooth difference between third and fourth gear, a 3 tooth difference between fifth and sixth gear, a two tooth difference between sixth and 7th gear and a single tooth difference between seventh, eighth and ninth gears.
Using standard cassette ratios works out completely wrong, when applied to a motorized bicycle application.
I will agree that an 8 speed (cassette) system is an optimal choice, because you get the vital 11T sprocket with the option of installing the HG-61's 36T sprocket in place of the conventional 34T sprocket.
That doesn't sound like any meaningful difference, but when you have the optional SickBikeParts 24T sprocket installed on the front chainwheel, it gives you that little bit of extra hill climbing ability, or extra pulling power should you be hauling heavy loads.
The other advantage of an 8 speed system is that an 8 speed chain is half the price of a nine speed chain, yet eight speed chain lasts equally as long as a nine speed.