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Motorising Honda Pulse scooter

Hello again, need advice on another modification project, although this one might be a little more "geared" toward this website (no pun intended, seriously). So somebody gave me this Honda Pulse scooter, which I researched and discovered that it was once a pump action scooter propelled by depressing a lever above the back wheel with one's foot. However, by the time it got to me, the #25 chain was rusted and broken. Still works great as a push action scooter. I happen to have a used (but funtional) 2600 RPM 400 watt 22ah Currie Electro Drive motor with a 9 tooth #25 sprocket. The sprocket fixed to the 7" diameter back wheel of the scooter has 13 teeth. I entered these figures into the gearing ratio calculator on this site, and, unless I entered something wrong, it would appear that this ratio is not suitable, as it calculated a 38.5 MPH top speed (which, due to other factors, is impossible with only 400 watts). So it looks like back to the drawing board with this one too? Also, can you tell me which batteries you sell that would be suitable for this motor? Much appreciated.

For a 24 Volt 400 Watt motor, we recommend a minimum battery pack capacity rating of 10Ah (two 12V 10Ah batteries). 

OK thank you. And as far as the gearing ratio, to allow the motor to run at 2600 RPM I'm guessing I would need the wheel sprocket to have about twice as many teeth as it currently has, maybe more. With the proper gearing ratio, what type of top speed should I have? I'm thinking 15-18 MPH?
I used the gear ratio calculator again and changed the wheel sprocket size to 30 teeth, which will top out about 16 MPH. Sounds about right to me. Attached is a picture of the wheel sprocket which I just discovered is not fixed. Space here is tight, so I'm not sure a 30 tooth #25 sprocket will fit, and that's assuming the original sprocket will even be removable/replaceable. I will now proceed to remove the back wheel and try to see if a sprocket replacement will be possible.
OK so I removed the back wheel and got the sprocket out. It appears to be permanently attached to the two inch long half axle, which has an inch diameter bearing on the outside and a groove that locks into a rod in the center of the wheel hub to drive the wheel (shown below). I'm not sure if or where I can find a replacement half axle with these dimensions and a larger sprocket. Or maybe there is a way to secure a larger sprocket over the original one? I will look into it. If neither option is possible, is there a way to secure a 4 tooth #25 sprocket attachment to the motor's 9 tooth sprocket? That sounds a bit ridiculous, but it would allow the motor to reach target RPM, I think.

For a 400 Watt motor, the top speed would be best geared at 16 MPH or less. Maybe a larger sprocket could be bolted or welded onto the existing one. I don't think that 4 tooth sprockets are made, and if they are then the motor's shaft would be too large for it and would prevent the chain from fully resting on the 4 tooth sprocket's teeth. 

Thanks for the feedback! A 30 or 32 tooth sprocket should be suitable, assuming I can manage to fix it to the original sprocket. There are very few hills where I ride, and when I do encounter a hill I will not use the motor, so I'd like to gear it for top speed rather than torque. 15-16 mph should be fast enough. Well I have some preparation to do now, like choosing the right battery pack and sprocket, fixing the brake and cutting or grinding some useless material from the frame so the bigger sprocket will fit. Thanks again for all the help, ESP support, I have some direction now but I may have more questions in the near future. At any rate, I will post significant progress and the end product if I am permitted to do so here and if this project gets off the ground.

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