Ebike lack of top speed

Joined
Oct 3, 2018
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I just finished my diy ebike, with a 36V 300W hub motor and a 48V controller / battery pack. The controller's max. amp is 18A, but I found out it was only 14A, no matter what I did, it topped out at 14. My max. speed is 47-48 km/h which is pretty nice from 14A. I did the shunt mod, because why not, I was surprised, it draws 24A now and gets up to speed faster, but over 40 km/h, there's no benefit, the top speed is the same and the amps go back to 14A at top speed, but I though I could go faster with more amps, but now only the peak amp is increased. I'm wondering why can't I go faster, because the no load speed of the motor is over 60km/h, so I guess even if I do the shunt mod, the controller is still a 350W controller. Or maybe it's because of the hub motor? Anyway, the motor and the controller wasn't even warm, so at least the heat is not a problem (yet).
 
Tiny motor tiny controller. Try a QS motor like 205 or 273 and a real controller like a ASI BAC 4000. Not only will your need for speed be satisfied you will have a nice torque request throttle as opposed to a POS speed request.

For that setup you should be pleased with the speed you've already got out of it.
 
It's actually a large motor, the wheel weights 7 kg. I took it from an Ebike used by old people and it was limited to 25 km/h (that's the law), and it was laced in a 22" rim. So it goes faster in a larger wheel for sure and me with the bike weights around 85 kg, so that's why can I go that fast. I think the shunt mod won't affect the output over a desired RPM that's why I can't go faster. One thing I can test is what happens if I increase the air resistance, will it go slowe or will it go with the same speed with more amps?
 
The combination of motor that produces low RPM per volt + low voltage + smaller wheel than stock = low top speed, no matter how many amps you throw at it.
 
Sounds like your motor has reached the point where no amount of amps added will increase its speed. If so, then I think that your only two options for that motor/wheel combo are to increase the voltage, or use a motor controller that enables field weakening.
 
Did find a 300 watt motor to demonstrate controller Amp effect.

1st - oem @36V

300w.jpg

Pushed to 26" wheels and 48V with 18 - 24A comparison (near 500w output!)


Motor Simulator


Electric motors can draw maximum amps at 0 rpm.
Amp draw reduces till near 0 at max unloaded motor speed.
This is because an electric motor is also an electric generator!
Near 47kph, the motor is using 48v to generate near 48v, resulting in minimal Amp draw and torque production.
Motor draws maximum controller amps from 0 rpm till it matches the motor ability to draw that number of amps.
 
Something we should have suggested earlier is to elevate the motorized wheel and see how fast it will go "unloaded". That will give you a baseline for what is potentially possible by adding more amps. My guess is that the unloaded speed is pretty close to what you are currently seeing while riding.
 
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