I tend to agree with your friends that an 11 tooth motor sprocket should probably be the largest size used. I ran a gear ratio calculation using an 11 tooth motor sprocket and got a 19.74 MPH top speed.
The 24 Volt 450 Watt gear motor is used with a 15 MPH top speed by most electric bicycle manufacturers, however, they are playing it safe for warranty purposes and many people have successfully used the same motor with top speeds of 20 MPH.
A top speed that is over 20 MPH is really pushing the limits of a 450 Watt motor though. This assumes that the bike will not be pedaled while the motor is on though. If you plan to pedal the bike along with using the motor when the bike is in its lowest gear, then a 13 tooth motor sprocket should be fine. With a 13 tooth motor sprocket and the rear cassette in its lowest 14 tooth gear, you would need to remember to switch to a higher cassette gear when not pedaling or going downhill though, otherwise, the motor could be lugged which may overheat the motor or controller.
Ok i just got myself a new motor for my first e-bike conversion. A brand new spanking clean MY1018 24v450w stock with a 9T gear.
My question is how hi can i take the size of that gear before damage to the motor.
Now for the plan: the motor is installed on the right side of the bike in place of the padals. the pedal board is moved on the left side and linked to a freewheel on the left side of the 20" wheel.
the motor will power the original back wheel casette a 14to 28T. Some friends tell me not to go higher than an11T but could i go as high as a 13Tor bigger?