First time on the site and would love some help on this project I've been trying to put together. In a nutshell it's a foot scooter with an motor assist. The concept is a hybrid scooter, with the primamry power being your foot and secondary power being a small electric motor that kicks on with the push of a button. I don't have much experience with battery/motor/electrical/resistance calculations, so I'm a bit lost with puttting the parts together. Here are my questions...

The Facts:
person weight: 200
Goal: To be able to use a foot scooter (specifically, a Xootr http://www.xootr.com/kick-scooter_cruz.html) in it's normal foot powered capacity, but have an electric motor on board that acts as assist when already going a full speed. Ideally, would like to maintain 3-5 mph.


Questions:


1. If I'm only using the elctric motor as an assist, how big should the motor be? Here are the sizes I'm looking at...


http://www.allelectronics.com/make-a-store/item/DCM-130/24-VDC-135W-MOTOR-W/BELT-GEAR/1.html


or


http://www.allelectronics.com/make-a-store/item/DCM-1250/24-VDC-250W-MOTOR-11-TOOTH-SPROCKET/1.html


I'm starting to realize that the size of these motors will require a lot of battery power. So I'm looking for a nice compromise that keeps the battery weight low.




2. What's the best way to calculate battery life per motor (because the motor will only be an assist it doesn't need to last very long)?


3. Will I need any resistors between the batteries/chargers/switch?


4. I'll be using a slide design to move the motor on and off the back wheel, but I'm open to suggestions because I actually need a way to lock the motor in position. I'm currently thinking of just using my back foot to hold the motor in place.



Doable? or Am I knucking futs? Any input is appreciated.

 

1. If I'm only using the elctric motor as an assist, how big should the motor be?

I think I've seen small chainsaw or leaf blower motors used for this. Maybe 1/4hp is a place to start? That'd be 150-200 watts.

2. What's the best way to calculate battery life per motor (because the motor will only be an assist it doesn't need to last very long)?

Batteries are rated by their capacity in amp-hours. Multiply by the voltage to get watt-hours. Then cut it in half or more to allow for not running the battery completely down, and various other inefficiencies. So a 12v, 100 Ah battery will supply 1200 watt•hours, or 6 hours at 200W. More like 3 hours in real use.

3. Will I need any resistors between the batteries/chargers/switch?

Ideally, no, since all a resistor can do is burn off energy. But of course you'll need to make sure your motor and power supply are matched up.

4. I'll be using a slide design to move the motor on and off the back wheel, but I'm open to suggestions because I actually need a way to lock the motor in position. I'm currently thinking of just using my back foot to hold the motor in place.

I suggest a lot of research into how others have solved this. No sense reinventing your wheels.

Doable? or Am I knucking futs? Any input is appreciated.

Both, IMHO. I think you'll ultimately be disappointed at how much battery and motor weight is needed to get such a little bit of assist. But I also think you can make something that works fine, and isn't absurdly impractical.

 

Did you see this one? http://www.allelectronics.com/make-a-store/item/DCM-1350/24-VDC-350W-MOTOR-15-TOOTH-CLUTCH/1.html It has a roller clutch that lets it freewheel until power is applied to the motor. That way the motor could be fixed in place and only used when needed.