M.L. Toys
M.L. Toys
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Need new motors? Grind a gearbox? Adding teeth to a pinion?
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By Grahamb89
Hi ive got a bmw i8 ride on, the 6v version. This came as either a 1wd 6v 7ah or 2wd 12v. Ive already put a 12v 20ah battery in and now id like to change the motor and add a second one but i have a couple of questions.

What rpm do people usually go for in a motor, looking at budget ebay ones to be honest and seen a lot of 30000rpm.

I know optimal speed is subjective so lets say if it did maybe 3mph on 6v now does 5 ish at a push i wouldn't want to faster than around 8 mph ish

Is there also a way to adjust speed, even if its just a high/low mode?

Thanks for your help!
User avatar
Hi/Lo mode is done by switching the motors between series and parallel configurations. Check out the Definitive Wiring Diagram thread for info on that.

I'm far from a motor expert but from what I understand, the higher RPM the less torque? Maybe? I know there is a thread or two all about motors as well.
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By Grahamb89
Great thanks, yeah the motor thing really confuses me, but after a quick google you're right it does seem to be the case that in electric motors if the power stays the same the higher the rpm of the motor the less torque, now bearing in mind its currently got enough torque to move with me on it :lol: it wouldn't hurt to lose a little torque in return for higher speed.
Sorry trying to wrap my head around this!

So generally torque in cars is hp x 5252 / rpm
So in an electric motor it makes sense that rpm increases torque decreases. But above all else just like my car I want more HP :lol: So what determines the HP? The voltage of the battery or the wattage of the motor, both?
User avatar
Higher voltage increases RPM of the motor. The gearing has a lot to do with it as well. Unfortunately I'm no mechanical engineer and provide little benefit to this thread. :lol:

In my mind, if you have a low tooth count gearbox (16T) and increase RPM you should gain speed and maintain decent torque but who the heck knows. Not I.
User avatar
By cadcoke5
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.

User avatar
By Grahamb89
That has really helped loads thanks! This way I should be able to use my current speed, gearbox ratio and circumference to find out my motors speed under current load and conditions. Then once I've got this compare it to the motors rated rpm, express this as a percentage difference then work backwards with a desired top speed in mind to choose a different motor.

At the moment it's running a 12v battery on a 6v motor, if all rpms are the same do I run:
1 x 12v motor
2 x 6v motors
2 x 12v motors ?
By Suburbancharlie77
Heck of an explanation there Joe! That's some serious knowledge not learned from the discovery channel!
Too bad that missing Intel is unavailable, as you have spelled out darn near all the remaining variables. That's awesome, thanks for that.

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