Problems using a standard wind motor with 20" wheel

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Aug 6, 2022
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I know just enough about the winding of hub motors to be dangerous. It is my understanding that 26” wheels normally use a standard wind motor, but 20” wheels normally take a fast wind motor.

The G020 motor seems to work fine with the smaller 20” wheel. With the smaller wheel, the top speed is reduced from over 25mph down to just 20mph. However it seems to be quicker off the line, and is a great hill climber. Overall a good tradeoff for the type riding I normally do.

I was expecting similar performance from the Bafang CST motor. As I received this used, I don’t know how it was used (or abused) in the past. In comparison to the G020 is is still a good climber, but not as good as the G020. It has even a lower top end of maybe of 17mph. While I could live with this as my “off road” trike that seldom see’s speeds above 15mph, there is another issues.

While climbing at speeds below 7mph, the motor is nice and quiet. However, at over 8mph the motor emits a loud whine. It is loud enough that people can hear me far down the trail, and the noise is distracting from my ride enjoyment. I am thinking that the CST motor does not like to be pushed at higher rpms which is happening by using it with a smaller wheel. While the G020 motor also gets a little louder at higher speeds, it is nothing in comparison to the CST. In the case of the CST motor, maybe it should only be used with a 26” wheel.

So what to do? I have been able to find a new G020 motor, and I can get it for either a 20” or 26” rear wheel. My inclination is to get the proper 20” wheel model. This should provide more top end speed, and provide smoother power delivery for general riding. The plan would be to use this 20” motor on my group ride trike. I would move the current G020 motor to my “off road” trike where the better climbing and lower top end speed would make more sense.

While these seems good on paper, maybe I’m missing something. Any suggestions of comments would be welcome.
 
Make sure the noisy one isnt just a geared hub and if so, yes it is more noisy and max speed get harder to reach but geared motors would be perfect for hill complimbing.
If no gear set, you may check your hall sensors wiring. Some controller can fit sensorless motor but when i did try it, i was gettting more noise and less efficiency from the motor than when i correctly connect the hall sensors to the controller

At last, the motor may have something bad, a loose magnet or loose winding at some point....

For replacement keep in mind that torque is amps and speed is voltage.
Shunt mode a controller for more amps could help hill climbing but it can burn the controller or the motor if you go too far.
I would rather get a 26inch motor and increase voltage to have the lower kv torque and increase battery voltage to gain speed as long as the controller allow it...This way it wont rise much in temp wihile hill climbing but more top end will be possible.
Just going from 13s to 14s gives an extra 7.7% speed and it won't hurt anything. But it mean a batterie and charger replacement.
If you overvolt it too much you can blast the controller capacitor or the fet if it can handle it and there is also a maximum rpm for the motor balance and magnet glue to fall appart but they probably use same glue on 20'' and 26'' so i would not be afraid to get the 26'' to spin to the 20'' rpm.
 
Make sure the noisy one isnt just a geared hub and if so, yes it is more noisy and max speed get harder to reach but geared motors would be perfect for hill complimbing.
If no gear set, you may check your hall sensors wiring. Some controller can fit sensorless motor but when i did try it, i was gettting more noise and less efficiency from the motor than when i correctly connect the hall sensors to the controller

At last, the motor may have something bad, a loose magnet or loose winding at some point....

For replacement keep in mind that torque is amps and speed is voltage.
Shunt mode a controller for more amps could help hill climbing but it can burn the controller or the motor if you go too far.
I would rather get a 26inch motor and increase voltage to have the lower kv torque and increase battery voltage to gain speed as long as the controller allow it...This way it wont rise much in temp wihile hill climbing but more top end will be possible.
Just going from 13s to 14s gives an extra 7.7% speed and it won't hurt anything. But it mean a batterie and charger replacement.
If you overvolt it too much you can blast the controller capacitor or the fet if it can handle it and there is also a maximum rpm for the motor balance and magnet glue to fall appart but they probably use same glue on 20'' and 26'' so i would not be afraid to get the 26'' to spin to the 20'' rpm.
Thanks for the help. Perhaps a bit more information...

The standard wind Bafang G020 that I am happy with is a 48w 500w geared hub motor. It is mated to a KT 22a controller and a 17.5ah battery. (original Bafang controller was 18a) Even though the motor was taken out of a 26" wheel, it works great for me in a 20" wheel. Top speed is limited to about 2mph, which is fine with me. I much prefer the good climbing capabilities.

The standard wind Bafang CST that I am having issues with is also a 48w 500w geared hub motor. It is mated to a KT 22a controller and a 14.5ah battery. As mentioned it is nice and quiet up to about 7-8mph where it starts getting very noisy and will not go above 17mph.

Both motors are fitted to near identical recumbent trikes which have a 10 speed rear cassette and a triple crank.

Since I am happy with the G020, I makes sense to use replace the problematic CST motor with a standard wind G020. This way I will have identical motors that should perform the way I want. As an additional test, I will be replacing one of the 22a square wave controllers on one trike with a 25a sine wave controller to see it this makes the motor even quieter.
 
if both geared, can you distinguish if the extra noise is more electrical or mechanical? A missing shim, a bad gears or not enough grease may impact it ?
We have an industrial motor shop were i work and sometimes we have to glue with vernish some ''humming'' transformer and it make them a lot quieter. We dont work on BLDC but a loose coil could make frequency noise like a speaker and noise would increase with amperage.

My MXUS 750w with planetary gear is very noisy from day1. I never expected it could be abnormal but peoples know i have an ebike few seconds before i get near them. I'm rebuilding a BionX hub direct drive to compare it...

I do have a 22a controller on the mxus and just ordered a 26a sinewave but i'm not sure if the 22 is square or sinewave. I Read that a ZWS in the number would be Square and SVP would be sinewave but i need to get it out of the garage first to check this...
 
I have done more riding with both motors. Is the noise more electrical or mechanical? Kind of both with the CST motor. I generally ride in PAS level 1 and 2 (out of 5 levels).

The G020 motor makes more noise at higher speeds (RPM), but the level is acceptable. It also makes more noise the higher the PAS level. Again the noise level is acceptable. The noise is the typical gear noise you get with a geared hub motor. I will be trying out the sine wave motor as soon as I have a chance.

With the CST motor, the noise level is acceptable at low speeds, but as speed increases to moderate levels the gear noise and electrical noise both seem to increase dramatically. My uneducated thought is: (1) There may be a mechanical problem with the motor that shows up at higher RPMs. (2) The controller may not be very compatible with the CST motor. (3) the winding is slower than the G020 motor. There is plenty of low end power, but it fades rapidly at higher speeds and there is a lower top speed. (4) All of the above. I plan to test the motor with the old 18a Bafang controller and the new 25a sine wave controller to see what difference that makes, if any.
 
Geared motors are typically very noisy and inefficient in a 20" wheel.
I have not seen a single one designed with the appropriate gear ratios for smaller wheels.

An additional problem is that the gear friction is now much higher per RPM than it was designed for. When cruising on the flat ( which you're doing most of the time ), this shows itself as reduced efficiency. In some motors, this can cause the motor to overheat at low speeds. The MAC can handle this OK, but it's a big motor. Smaller motors can have bigger problems.

1716139007046.png

You can also run into issues with the electrical RPM being too high when you run fast windings with geared hubs. MACs have this problem, so previously, a modified infineon was sold that could handle the high eRPM. Few controllers can handle it. Controllers that can't handle the high eRPM will stutter somewhere north of 25mph.

In other words you're far out of optimum conditions for this motor. For a 20" wheel, a DD the size of a 9C 27mm or smaller is recommended.
 
It sounds like perhaps I got lucky with the Bafang G020 Motor. While maybe not optimum, it is still usable. This is definitely not the case with the CST motor.

It seems like if I'm going to spend money replacing the CST motor, I should not go with another geared hub motor. This means I should look at some other choices:

(1) Use a direct drive motor instead. I need to research this as it is my understanding that DD motors are much heavier, overheat on hills, and drain your battery quicker. Any one of these would be bad for my application.

(2) Change to a 26" wheel with a geared hub. I can do this on my ICE trike but it is not cheap, and the weight of a hub drive does not work well with the rear suspension. My last trike had this problem.

(3) Change to a 26" wheel with a mid drive. Again, I can do this on my ICE trike but it is not cheap. This would eliminate the weight problem of a hub drive and rear suspension though. This would be the most expensive option as it would require a new ear frame section, new rear wheel and tire, and the mid drive.

(4) Stay with a 20" wheel but switch to a mid drive. This would not impact the rear suspension. However the bigger issue here might be with getting the gear ratios I need. This might not be an issue as the mid drive is more powerful so I might be able to get by with taller gearing.
 
If you can go with a 24" wheel, then you can make pretty good use of a geared motor.

Shengyi SX2 is the current power:weight:efficiency ratio champion in that size. About 7lbs.

1716159137589.png
 
Bafang CST W/ 20" wheel...
View attachment 353191


I'm running the CST with a 48v battery and a 22a KT controller. I tried to do a simulation using as close of a controller as I could determine. The graph shows the results of both 20 and 26" wheels. The 20" (System A) results are far better than my real world results. On mine, the power drops off much sooner, and speed tops out at 17mph. I'm still thinking there may be issues with the motor it's self.
CST 20 v s 26 No2.png
 
I'd go with #1. But as they say,"Different strokes for different folks".
If I choose a DD motor, which one would you recommend that would work with my KT 22A controller? for general riding including hills, how many watts will I need?
 
Not to sway anyone, but I would never put 16 pounds in my wheel. The 500-700W rated, 7 pound Shengyi would be my pick because it has silent running helical gears. However, I'm not a fanboy of hub motors so my advice is irrelevant.

SX1-SX2 - Grin Kits - Product Info
 
I've ruled out direct drive motors due to weight. The Bafang G020 motor I have now is similar to the Shengyi in both weight and power output. Even it's lighter weight makes it self felt with the rear suspension. The more I research alternatives, the more I am leaning toward increasing the rear wheel size to 26". I can do this by purchasing a new rear subframe for my trike. The other option I am considering is a BBS02, but I'm not sure I would have issues similar to what I now have with a 20" rear wheel.
 
A controller that might work is the infineon types from MAC motor tech.
These controllers are rated for high eRPM, were designed for MACs ( high eRPM compared to most geared motors )
Can't guarantee it would work but it's your best shot until you start playing with VESCs ( high RPM, FOC, but maybe not noob recommendable )

Consider also some in between wheel sizes. There's 22" and 24". 22" has BMX/smooth tread tires and 18" motorcycle tires fit. 24" has a wide range of tires available.
 
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