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The above is my daughter's tricycle that I plan to make run off some spare Lithium-ion 18650s. I was just thinking of mounting a small DC motor to where I am pointing, and it will use a small tyre to make contact with the wheel, and rotate it.

I have a few questions / concerns: 1) How do I calculate the Power (Watt) and/or Torque required from the motor, given the total weight of the tri-cycle, motor, and my daughter... in order to make it go at 5Kmph on a flat surface?

2) Both the rear wheels are free rotating independent of each other. So, if I rotate only one of them using the motor, will that push the cycle straight and forward, or in circles/curved path?

3) If she pedals instead of using the electric mode, I am guessing it will charge the battery, although I doubt she'd pedal fast and enough to overcharge it. Would I need to do something to prevent this?

4) Is there anything else that I haven't considered?

Considering the tricycle will not be used for more than an hour in a day, I can make a relatively small battery pack with less mAh capacity.

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    $\begingroup$ You won't get anywhere near stable 5km/h if you go with torque and power. Get a motor that produces a fixed RPM as needed, maintaining it with little regard for torque. Otherwise your daughter will be pushing the bike up the driveway and speed up uncontrollably when driving down a 5-degree slope. $\endgroup$
    – SF.
    Mar 1 '17 at 13:43
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    $\begingroup$ One I can think up out of top of my head would be car window mechanism (along with its gearbox). Usually what you want will be a common motor together with a gearbox, that limits own RPM by torque/friction of the gearbox, producing so much torque on output axis the final load is moot. $\endgroup$
    – SF.
    Mar 1 '17 at 14:13
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    $\begingroup$ Yes, PWM based regulator would work okay, providing you add something to read the RPM as well (rotary encoder?) - that considerably increases complexity and cost though. $\endgroup$
    – SF.
    Mar 1 '17 at 21:14
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    $\begingroup$ If you don't add a braking mechanism, you and your daughter will BeVerySad(TM) . At the very least, make the power switch some sort of lever on the steering handle (that looks like a bicycle brake) so any time the operator releases the lever, the motor shuts off. $\endgroup$ Mar 2 '17 at 14:37
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    $\begingroup$ @CarlWitthoft I was just thinking of keeping the speed really slow, most likely slower than what she'd be able to pedal. The front pedal and wheel driving mechanism is similar to a fixed gear bike, so once the push button is released, she'd need to stiffen up her legs to stop the trike. An alternative would be to somehow make the motor turn in reverse for a second or two. i.e. the switch either runs the trike or brings it to a halt. Perhaps, I can make the switch charge a capacitor that releases a reverse polarity charge when the main switch is released. $\endgroup$ Mar 2 '17 at 20:30
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  1. To calculate required torque, we are going to need a lot more information. Probably easiest to just pick a power like say 100 Watts. Then calculate the motor rpm you need based on the tire diameter you select.
  2. Powering one is actually better. If you powered both wheels you would need a differential. The steering of the front wheel should easily overpower the offset driving wheel.
  3. Don't mess around with charging the battery. Just install a double pull double throw (DPDT) switch (if using a dc motor) to give you forward and reverse options. Then install a on/off momentary push button.
  4. Design the motor interface such that fingers wont be pinched (remember kids are always trying to get injured). A tire type drive might not be your best option. Getting a gearbox that can direct drive the wheel (might have to modify its axle) might be a better option.

It is going to be a pretty large project when you get into it, so feel free to post new questions specific to gearing, battery calculations, etc. Good Luck!

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  • $\begingroup$ All great suggestions. Just a quick follow up. Would buying and including this PWM Controller in my circuit keep the motor RPM at a fixed level, regardless of the load... i.e. going slightly uphill or downhill? $\endgroup$ Mar 1 '17 at 21:12
  • $\begingroup$ Yes, a frequency controller on a brushless DC motor will set the rpm independent of torque. Remember that you want to set your rpm such that you are getting majority of the rated power out if your motor (i.e. make sure your gearbox is sized correctly) $\endgroup$
    – ericnutsch
    Mar 1 '17 at 21:16

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