patrik5504 said:
I want to use the QS 205 3.5t motor, but it is not adapted to mid-drive, it is possible to give it a freewheel, as in the photo. The question is, can I use a single freewheel 16t to use the mid-drive motor? it will work?
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won't there be a problem with the motor overheating?
To know if it will work, or if it will overheat, you need a lot more information. A drawing (even a crude sketch) of how you want to setup your drive, including chain path, would help you and us see how your system needs to work.
What do you want the freewheel to do? It is normally used to keep the wheel from driving the pedals. Since the motor won't be in the wheel, you don't need it for that reason. It can be used to keep the pedals from driving the motor when the motor isn't used, if that's a mode you'll use. If you want to keep the wheel from driving the motor when motor not in use, put the freewheel at the wheel, just like normal.
As a side note: If you have it as a middrive, and you want the motor to spin your chain to the rear wheel on the right side, or to the pedals to then spin the chain, it won't work, because the freewheel will disengage as the motor spins forward and the chain will just sit there. If you flip the motor over the other way, so it drives the left side, then it would engage the chain, and only freewheel when the motor was not spinning fast enough to be faster than the chain being moved by teh wheel. However, the chain would then always be moving if the wheel is moving, and you would need a guard to prevent you or other things getting into the chain.
For the number of teeth: The 3.5t motor will spin really fast (vs a higher number of turns). a smaller front sprocket (like a 16T) vs a larger rear sprocket will convert that speed into torque. If you want to keep the higher speed, you need the same size or smaller sprocket at the rear. Smaller than 16T at the rear is hard on the chain and sprockets, and may skip under high loads, so it's better to use larger sprockets on both ends. Without knowing your intent and system setup, can't suggest actual sprocket sizes to use.
You'd need to know what your specific riding conditions will be, your terrain (slope of hills, etc), wind, road conditions, riding style, speed, weight of everything and you together, along with the intended controller and battery, as well as the gearing from the motor to the road and the wheel size, to see if the load on the motor and the power supplied to it would be greater than it's ability to handle.
You can see how this works in the http://ebikes.ca/tools/simulator.html