So moved my 500w Bafang geared hub motor from my old 26" rear wheels trike to me new 20" rear wheeled trike. I installed everything including the KT display and controller from the old trike on the new one. I changed the speed setting from a 26" to a 20" wheel, and everything works fine. The speedometer is very accurate. However the motor cuts off at about 17mph now. On the old 26" wheeled trike it would keep going up to at least 24mph (the limit of my gearing). Is there any way to get the speed up over 20mph?
As was noted in the first response to your thread, the speed will be proportional between the wheel sizes.
If you decreased the size by 20:26 then the new speed will be that ratio x 24. So 20 / 26 = 0.7692307692307692 X 24 = 18.46153846153846. Since your speed is slower than that, probably your old 26" tires were "fatter" than 26" and your new 20" tires are "skinny" so they're actually 20" or less.
To make it go faster, you would need to do any of the following:
-- put the motor into the same size wheel it was originally in
-- replace the battery with one that is proportionally higher in voltage (1/0.7692307692307692 at minimum) than what you have now, and if your present controller/display aren't rated for that, then also replace those with ones rated for at least that voltage. Note that if you pick a battery that is only that high in voltage when it's fully charged, you'll only get the speed you want at the start of a ride. So pick one that is still that high a voltage when it is empty, so you'll get that speed even at the end of the ride.
-- take the motor out of the wheel and build a middrive with it that is geared with the right ratio to the wheel to give the speed you want.
-- an option that will give some higher speed is to replace the controller with an FOC controller that offers Field Weakening (Phaserunner, VESC, etc), then tune the controller to work with your motor and system, then experiment with FW in it until it goes as fast (while riding) as it can get--that may not be much faster, or it might be enough for what you want. Note that this will heat the motor more, and use your battery faster, the entire time you are riding at a speed that requires FW. How much more and how much faster you won't know till you're doing it, but it could be enough to overheat the motor.
I was told by the retailer that there is no difference in the hub motors no matter what size physical wheel you choose. Is this True for the Bafang 500w rear geared hub motor kits typically sold on Amazon?
There are different windings available for some motors, but documentation on what speed that gives you in any particular wheel at any particular voltage is hard to come by. See the motor simulator at ebikes.ca for a few documented motors and a way to see what any particular motor / system should be able to do under specific riding conditions.
You can at least guess the motor speed you'd actually get for something you find on the web by taking the speed it says it goes in the wheelsize it's provided in at the battery voltage it's sold for, and taking the wheelsize and battery voltage you are using and creating a ratio with the ad's values, and using that ratio to see what speed it would go with your values.
It was not to change to a middrive, it is to move your hubmotor out of the wheel and into the frame *as* a middrive (there are a number of ways of doing it but it will take significant DIY to make any of them happen).
