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Need new motors? Grind a gearbox? Adding teeth to a pinion?
By wnc1969
#150744
Hi All,

Forgive me if the answers to this type of question are somewhere else but I could not seem to find any . I am looking to understand what specs I am looking for in replacement motors for my power wheels.
My Current Setup:
Dune Racer with ECPU 24 Volt Easy ESC
- 775 1400Kv Motors 16T (purchased from ECPU)
- Running system at 24V 24ah

Jeep Hurricane with ECPU 24 Volt Easy ESC
- 775 1600Kv Motors 16T (purchased from ECPU)
- Running system at 24V 24ah

Everything is fine but I would like to purchase decent 775’s at a good price. Sending a link to me would be certainly be appreciated but what I am really looking for is to understand the motor specs and what it is that I am looking for in a motor. I have attempted to google this info but I cannot seem to get the details I need. I am looking for maximum torque over speed here. The 775's are fast enough for what I need, it's the torque to withstand the tough terrain since we are on a farm and the boys like to tear it up...

For example there are:
1: Torque considerations
2: Maximum RPM
3: Shaft Length
4: Shaft Diameter
5: Anything I missed?
6: Do all 775's come with a 5mm shaft? (or do I want a 3mm shaft?)
7: What it the correct shaft length to have on the 775 for a 7R gearbox?
8: Where can I get pinons? More Specifically 16T pinons?
9: What is the correct pitch for 7R GB pinons? I believe it is 32P please confirm

If someone could either point me to a good online resource or explain in detail the basics around motor specs for usage on BPRO's, that would be great. I want to learn...

I have gone on sites like banggood (and others) and there are literally too many choices of 775 motors with various specs.
Here is an example of a motor that to my mind should be good for my purpose. It lists as a 24V which to me seems great because I am running a 24V system. High Torque which I want due to the hills on my property etc. But what do I know...Feedback?

https://www.banggood.com/DC-24V-21000RP ... rehouse=CN
DC 24V 21000RPM High Speed Large Torque 775 Motor
Motor height: 67mm(without shaft)
Motor Diameter: 42mm
Output shaft diameter: 5mm
Output shaft length: 14mm
Mounting hole diameter: M4 screw
Diagonal mounting distance: about 29 mm
Voltage: 24V
Rated current:1.05A - Do I need to know what this means?
No-load current:1.55A - Do I need to know what this means?
Torque:0.09NM - What am I looking for here with respect to BPRO's?
Speed: 20000RPM - What is the sweet spot for RPM for BPRO's?
Net weight: about 418g

Thanks in advance!

William


User avatar
By Hammer-fm
#150752
This begs a good sticky response, but I won't try to do an entire post right now, nor am I a complete expert :lol:.

You are running a current-limited ESC, so your maximum torque is constrained by the product of the current limit * torque per amp generated by the motor.

In general, higher kV (RPM/volt) motors have lower the torque per amp of current. You can more easily see this from *complete* datasheets among a class of products. Unfortunately, the data you posted below is insufficient to properly evaluate the product, and they don't include a full datasheet anywhere in their listing. (note that datasheets are not always accurate, but let's pretend for a minute that these are).

Let's compare two motors:
#1: Mabuchi 8016 - [url]https://product.mabuchi-motor.com/detail.html?id=128[/url]. This is a 16-turn, 0.8mm winding 775 motor . The theoretical stall torque is 981mNm at 100A. We can express this as 9.81mN*m per amp, and if you had an ESC with a 50A limit, you'd theoretically generate a total of 440mN*m of torque between the connected motors (half the current would go to each, basically). Your top speed at 18V will be 18000 RPM vs. stock of ~15700 RPM, so it won't be much faster than stock. The real-wheel torque and speed depends on the gearing reduction in the gearbox, of course -- but if we keep that the same, we can compare to other motors.

#2: Johnson 683LP (don't have a ready web link, but this is what came from the Johnson catalog):

Manufacturer & Model Johnson HC683LG-001
Spec Voltage 12
No-Load Speed (RPM) 15694
No-Load Current (Amps) 1.18
Stall Current (Amps) 61.04
Stall Torque (mN*m) 400.13

This one has a theoretical torque per amp of 6.5mN*m/A. WIth the same 50A limit, you get only 325mN*m of motor torque. IF we use the same gearing (eg. 16T pinion), this will have quite a bit less torque than the 8016. However, at 18V you'd get 23,500 RPM, or 30% more speed than with the 8016 motor above.

You can make them "more equivalent" by modifying the gearing. If you ran the stock one with a 12T gear vs. using a 16T gear on the 8016, suddenly we change the effective torque and speed. In fact, you'd go from "effectively" 325mN*m to 433mN*m, and the speed would drop from 23.5k RPM "effective" to 17,625 RPM. When you compare them side-by-side, you see that the stock motor with a 12T gear is about the same performance as the 775 8016 motor with a 16T gear (though the stock motor will run hotter as it is a smaller case size).

The point is that -- within a certain tolerance -- the motors perform fairly similarly to each other. You can trade off torque-per-amp for speed, but there's not much in the way of magic bullets.

A "21000RPM @ 24V" motor is likely going to be torquier (per amp) than the ones you have (which are running 33-38k+ RPM at 24V). Likely 50+% more torque per amp, so they should be snappier off the line. You will give up a lot of speed -- perhaps too much (you'll be running only ~20-25% faster than the stock was, instead of ~2x). However, if that's the speed range you want, those would probably be a great fit. Slightly faster: The Mabuchi 8514 may be a decent fit and can be had from Banebots: [url]http://www.banebots.com/product/M7-RS775-18.html[/url]. It runs about 8.5mN*m/A on torque and ~1100kV -- it should be about 26k RPM at 24V.

Also, the Nichibo 7013F "$3 Jameco special" is about 1150kV: [url]https://www.jameco.com/z/775-7013F-R-Ni ... 10845.html [/url]. In my (somewhat crude) torque and speed characterization, it is fairly similar to the 8514 -- slightly faster and slightly less torquey. It has similar ventilation and has performed very well for me, especially if you get the "big vent" version (unfortunately there are two versions and Jameco ships whatever... I've purchased about 40 of these and they're 50/50. The small-vent ones perform just fine but I've destroyed the front bearing on a few of them -- not sure if it's a difference in the bearing itself, or they run hotter due to the small vent; the large ones are all still running).These may make a very good replacement for the 1600kV motors, although it may be similar enough to not warrant messing with the 1400kV ones. It should be ~15% more torque, and 15% less speed than the 1400kV, and ~30% different from the 1600 one.

Most people want the speed, but on bumpy/hilly surfaces it's not super useful to be able to go 10+mph, so you're probably thinking along the right lines by looking for a lower-speed option to gain back the torque, especially with a current-limited ESC.

(not sure why URL tagging isn't working...)
By wnc1969
#150767
Thanks Hammer-FM!
I really appreciate the thoroughness of your response. You mention quite a few calculations, may I ask where you learned this all? Or are there some basic formulas you can share here so that when I look at the various motor specs I can calculate at least some rudimentary stats?
I totally agree that this should be a sticky as well ;)

Thanks again
W
User avatar
By Hammer-fm
#150787
It's all Newtonian physics :lol: . I'm an engineer and although this isn't my particular field (my day job is working on designing test hardware for computer chips), it's a good use of the physics I studied in college and this is fairly easy to model closely enough for this type of work. I have had some amount of work in mechanical-related fields and Power Wheels are nice as I can use both my electrical-side skills (building motor controllers) and mechanical (gearbox/etc.)

You can look at my Power Wheels calculator/spreadsheet for more detailed information although it's not quite as user-friendly as it could be. I post about it a little here:

viewtopic.php?f=9&t=21804&hilit=spreadsheet#p149018

I have a newer version of the calculator - mostly motor additions:
https://drive.google.com/open?id=13doIo ... tz1qdg4Z8J

There is also some interesting information on the Vex Robotics website:
https://motors.vex.com

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