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Need new motors? Grind a gearbox? Adding teeth to a pinion?
By Wrenchman
I'm finally ready to start pulling some parts together for my scratch build project. I've decided to go with the dual motor/dead axle setup, but now I have some other questions that I'm seeking some guidance on...

1. Built-in Gear Reduction vs no built-in gear reduction. I'm not sure if I want to use two 24V 500W scooter motors with 11T/80T gears, or a 24V 350W gear reduction motor with 25T/47T gears (as are used on the Razor Dirt Quad). I'll be satisfied with a 10mph top speed, but it has to have enough torque to work ok on grass. It doesn't have to climb steep hills like a crawler, but it will need to be able to handle some gentle change in elevation here and there. I've checked out build threads on Treebeme's Frankenbeast and WesleyB82's Pinkenstein, and they both look like the gear reduction isn't necessary...but what would you recommend?

I expect the car to weigh as much or half again more as my daughter's stock Power Wheels Mustang. I have 10" pneumatic wheels for it, and will be buying double flanged hubs to mount sprockets to the drive wheels. I'm considering overvolting the 24V motors to 36V...but I don't know for sure how it will affect the speed and torque of the motors. Using a speed calculator, I figured that the gear reduction motor will give me around 6 or 7 mph, while the non-gear-reduction motor will give me about 10mph--both at the 24V rated max RPM. I've read that overvolting will boost the RPMs and therefore speed...but how much? And how fast will the motors toast out from the extra voltage?

2. Two motors, how many controllers? Can I run two motors off of one controller with a hall effect throttle pedal? I want to have reverse, in case that matters with the choice of controller.

3. Brakes. How effective are electric (shorted motors) brakes after the car goes over 5mph? Are they comparable to mechanical (cable or rod) actuated brakes?

4. Wiring? Has anyone made any wiring diagrams for setting up a build that uses scooter motors, a stock shifter (even just for forward and reverse), and the controller(s)? Would there be a problem with using a 2-1-REV shifter as a F-N-R shifter? Can a stock PW shifter hold up to 24V or 36V?

I'm sure that there are probably other questions that I'm not thinking of at the moment, but I'm hoping these are enough to get me started.

User avatar
By Hammer-fm
I haven't used those specific motors, but a few of the questions are easier to answer:
#1: Generally the no-load RPM is directly proportional to the applied voltage, so if you go from 24V to 36V you should expect:
(A) Stall current will be 50% higher
(B) No-load RPM will be 50% higher
(C) No-load current will be ~50% higher
(D) Peak power will be 2.25x the original (1.5x the voltage * 1.5x the current -> 1.5x the RPM * 1.5x the torque), assuming you don't have any current limiting in the motor controller happening at that point
(E) Peak power dissipation in the motor will also be 2.25x
(F) No-load power dissipation is also around 2.25x

Survivability depends a lot on how hard it gets pushed and what the thermal environment looks like. The scooter motors have quite a bit more mass than the stock power wheels motors but most of them don't have any forced-air ventilation. I'd expect sustained power dissipation to only be 2-3x the stock motors, but it will soak up a high-power event (like a full-throttle start from 0) much better. Most of the scooter motors have a higher efficiency, especially at light loads (lower RPM, less parasitic losses), and at 10mph you'll likely be running them fairly lightly loaded, at least on pavement/dirt/hills.
You can always buy controllers that are "36V ready", start with 24V, and add another battery when/if you need to.

#2: There should be no problem using one controller for both motors, but you'll want to make sure it has enough current drive capability to take advantage of both motors. A "350W" 24V motor will want 30+A at max power, so the controller should be capable of driving at least 60A to handle two. Buying a "1000W" controller won't help much, since that's a lot of the power increase is due to the higher voltage capability, which you wouldn't use anyway. If you're keeping the stock shifter then reverse can be handled through the shifter (I am using the stock shifter to handle forward/reverse on the F150 which runs 36V).

#3 - More effective at high speeds than low, but hard to keep "proportional". The problem is that the amount of slowdown is proportional to the current going through that, which -- when it's just a resistor or a shorted motor -- is directly proportional to the back EMF of the motor -- which is proportional to the speed of the motor. That means you get the most braking when it kicks in and you're going full speed, and it comes down to "almost nothing" when it slows/stops.

Mechanical brakes apply a torque proportional to the force on the brake pedal/cable, roughly regardless of speed (though in reality they have a higher friction at low speeds than high, given the same pad pressure).

#4 - I don't have a diagram but you may find something in the ESC subforum. On the F150 (running custom PWM controllers I built), I use the "high/low" switch as a throttle limiter (basically cuts the hall-effect pedal voltage to about half when in '1' or 'R'), and the forward/reverse is used for its original purpose. I'm not 100% certain you could make it F-N-R though, given how it operates mechanically.
As far as longevity of the stock shifter at 36V, I think this really depends on how much current you run through it more than the voltage. Any amount of soft-start is going to be an improvement over the standard on/off gas pedal, and you'll usually get that (soft start) with a standard ESC/scooter controller.

I haven't burned mine up yet, but they are technically rated at 16A or 30A (depending on the model). I did do the last vehicle (the Pink Rocket Mustang) using 40A (forward/reverse) and 80A (PWM controller vs. brake) relays. Part of that was just to reduce wiring run lengths though.
By Wrenchman
Thanks for all of that information , Hammer-fm! I like the idea of going with a 36v-ready controller and starting out with 24v. And knowing that a stock shifter will work without quickly burning to a crisp is encouraging, too. Have you used a scooter motor/chain drive setup on your projects? If so, what setup did you use and how did it perform on grass?

I'll check out the ESC forum like you recommended. I've never been much of an electrical guy, so I'm going to need as much information as I can get to be able to wire this thing together. Being a scratch-build, I don't have an existing circuit to start with.

Jawarren77, same here. I'm hoping this thing is in satisfactory running condition by summer.
User avatar
By Hammer-fm
Unfortunately I don't have any experience with actual scooter motors or chain drives; the setup I have is what one might call an "extreme" modification to the stock gearboxes: I remove 1st gear entirely and use two 775-motors to drive the 2nd gear. The thread is on the gearbox forum here. At 36V two such gearboxes gives me a theoretical max of about 1500W of net power and so far has been far more problem-free than I would have expected. My area doesn't have much in the way of grass besides just the park, and that's well-trimmed so I'm not sure that would be much of a comparison. The Mustang does fine going up fairly steep hills at excessive speed, and the truck did a stint at the local ATV park without any drama, including some decent rooster-tails in looser dirt, so I would think 6-8mph on grass wouldn't be too much of an effort with 2x 350W motors. Stock power wheels motors put out 160W each given ideal wiring (realistically probably 120W); at 18V this would likely hit 250-300W each, but they're not intended to run full power for very long.

As for the controllers: I'm an EE and so far I've built all of my motor controllers myself. I wouldn't generally recommend it -- by the time you get into the soft start, fast/slow shifterl imiter support, adjustable master speed limiters (for the younger drivers), relay control, etc., you have 60 components on a stripboard and hoping you didn't make a soldering mistake. Luckily there are lots of good motor controllers/ESCs out there. I'd recommend calling up Wes at EastCoastPowerup and talking to him about his experience with those motors.

I put together an Excel spreadsheet to help model various motor / gearing / vehicle / wheel combinations. It's not the most user-friendly (hey, I'm an engineer ... :lol: ), but if you want to try it, you're welcome to pull it from here: ... 90gJwhgL7q

A few example scenarios... note that this can also model current limits; it makes a big difference when the controller has a 60A limit and you're running two scooter motors in parallel.
spreadsheet comparo.gif
Example of spreadsheet modeling a few scenari
spreadsheet comparo.gif (47.13 KiB) Viewed 827 times
By Slowpoke
Running dual motors on one controller should not be a problem. I would get a ESC from Wes to make it as painless as possible. He has plug and play, and do it yourself kits. You should not have any problems climbing hills and you will see double digit speeds without overvolting.
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