- Thu Jun 29, 2017 8:15 am
#146205
I am an engineer, though new to the world of ride-on toy gearmotors. I just joined the forum and was hoping to use these motors for some projects I am working one, that are not related to ride-on toys.

There are other important factors that have not been mentioned in this thread. The diameter of the wheel and the load.

I think you probably understand gear ratio. E.g. if the motor turns 20,000 RPM and your gear ratio is 100:1, then the output of the gearbox will be at 2,000 RPM.

To get the speed of the vehicle, from the above, you would also need to know the wheel diameter. From that you calculate the circumference of the wheel, which is Diameter timed Pi[3.14] So, a 6" diameter wheel has a 6x3.14=18.84" circumference. This is how far the wheel will take the vehicle for each revolution of the wheel.

So, if your motor turns 20,000 RPMs, and the gear ratio is 100:1, and the wheel is 6" diameter, the speed would be 2,000 x 18.84" = 37,680" per minute. That converts to 35.7 MPH !

However, there is something else we have not factored in yet. The motor specs are given as RPMs unloaded. The above calculation is the theoretical maximum assuming you have zero weight and zero friction. In the real world, it would be slowed down significantly by the load.

Unfortunately, the load is quite difficult to calculate. It would depend upon the surface, and the weight of the child. A surface of grass would be a much greater load than riding on a smooth surface.

Then, things only get messier. You can up the voltage and get more power, but there are limits. One of limits is how much heat builds up in the motor, and how fast it can dump that heat. The more a motor is slowed down by the load, the more current it draws, and therefore heat it generates.

Yet more difficulty comes from the fact that you don't have all the data for the motor. They provide a voltage and no-load RMP value. But, omit things like stall current and performance curves under various temperatures, etc. When I have to design something for industry, I rely on their sales engineers to guide me.

For the hobbyist on projects like modifying a ride-toy, it is not so much about engineering, but getting general guidance from people from places like this one, and trying to find an example that is similar to what you want to do.

-Joe