Well, its obviously speed limiting to the 21 MPH. You are not going to get more out of the motor, unless you are able to set the internal limit higher. (and that may be the max, depending what the amps of the controller are rated at.) Which model of SDuro did you buy? (there were 6 in 2016). Also your top speed is going to be dependent upon voltage, gear ratios, and watt output. For example, to consistently get to 28 mph, you'll need at least 500 watts (preferably 750 watts), and 48 volts, but even then, 28 mph is a reach for most 500 watt motors. No way are any of the mid-drives that Haibike makes going to get you to 35 mph. Sduro's are more about torque, and hill climbing, not top end speed. You chose the wrong bike if you want higher top speed. And no one is going to legally be able to sell you anything using just the throttle, that goes above 20 mph. You could probably get to 35 mph with pedal assist, but its going to take more wattage than is legally permitted on the streets, which is 750 watt max. You're really talking about something closer to an electric motor-cycle to hit 35 mph. Luna cycle could probably custom build you an ebike that has 2500 watts worth of motor, and can hit 35 mph. But it would not be street legal, and would require to be registered as a motor cycle, and you need proof of insurance, or just ride on private property. I doubt any manufacturer in their right mind, would want the liability of building you anything above 28 mph, for street purposes, unless it gets registered as a motorized vehicle.
