RC motor torque.

I was looking around at RC motors for my bike and I noticed that maximum torque is not reached at maximum RPM's. I found a site that did testing and the maximum torque readings were coming in at over one half to almost one third of maximum RPM's. So to me this means if you are picking out a RC motor, you should be looking at two to three times the maximum RPM's you want, either for maximum acceleration or maximum speed. Is this what's playing out in the real world applications?

Here is the site for torques: http://www.rcguru.org/

Maximum torque is actually at zero RPM.

Maximum power should be at 50% of max RPM, IIRC.

There's a number of threads, mostly discussing transmission vs direct drive, that have all the math for all that stuff. (which I don't pretend to understand, so I'm just parroting what's there )

EDIT:
I do see that the page you link to seems to show (as expected) that torque is highest at lowest RPM, and lowest at highest RPM, and that power is highest around the middle RPMs (the ones that it isn't are probably limited by their controller somehow)?


So I think you may have confused the terms Torque and Power when reading their charts and gotten them swapped.


you can go to the motor simulator
http://www.ebikes.ca/tools/simulator.html
which is based on real tested data for the stuff listed there, and use a custom battery and controller using 0ohms and ridiculous current to eliminate those as limitations, and show just what the motor itself should do.

There is also a thread (and/or a wiki entry?) that has a table of motor parameters for such things here on ES, too.

Well, there is so much to know about what motor to use for what application. As far as the outrunners, I know that they are not low RPM motors, so from what you are saying, they seem to be low torque and get all their power from the higher RPM's. That is why they need such crazy gear ratios to get them to work with a bicycle, you need them spinning crazy fast to get them to move a bike. A ratio of 20:1 is probably a good starting point for a really low KV motor. 20 revolutions of the outrunner for every revolution of your 26" tire. That would give you a speed of 23 MPH with the outrunner spinning at 6,000 RPM.

Not all outrunners fit that rule...
DD hub motors are outrunners, they are low rpm (0-400rmp), and they work fine with a 1:1 ration to the wheel.
...it all depends on the detail design of the motor to suit its applicarion.
RC motors were not designed to propel Ebikes.

Hillhater said:

Not all outrunners fit that rule...
DD hub motors are outrunners, they are low rpm (0-400rmp), and they work fine with a 1:1 ration to the wheel.
...it all depends on the detail design of the motor to suit its applicarion.
RC motors were not designed to propel Ebikes.

Click to expand...

Are you saying a RC motor is a poor choice for a Ebike? They have a lot going for them. They make millions of them with all kinds of different parameters. Their small, powerfull, plentifull... Seem to work real good as a friction drive. Quite a few different design motors are being used for Ebike's. If you limit yourself to a motor that is designed for a Ebike, your leaving out all kinds of options.

Hub motors are designed for Ebike's but I feel that they are a big compromise, putting all that weight where you do not want it. Mid-drives are where you want to add the additional weight of a drive.

Sometimes an RC motor can be a practical choice for an EBike (ie, friction drive).
But generally they are not well suited, since in most applications they will require multistage reduction gearing which adds complexity, weight , size, power loss, and cost to any build
RC motors generally cannot be used to their max claimed output power on a Ebike, due to cooling requirements (designed to run in the airflow of a propeller) and are incredibly noisy when used near max power.
Good (reliable) RC motors are also quite expensive compared to purpose built hub motors, and have special requirements on their controllers.
If you want a mid drive, there are many commercial options ( Bafang etc) using quiet , proven, brushless motors...as well as the many custom built versions described in the "Non hub drive" threads on this forum.
It is also possible to use low rpm hub motor as a mid drive to avoid the complication of multistage reduction gear trains, and retain the efficiency and low nouse of a brushless hub. ( eg, Stokemonkey)

If you're set on using an RC motor, look up the Tangent threads, and just about any of Recumpence's builds.

I decided to try my hand at a RC build, here is my build: What I'm starting with: Giant LaFree bike. Comes with a transmission that has a ratio of 22:1 motor to the crank. From the crank (21 teeth) to the cassette on the rear wheel (34 teeth low, 13 teeth high). If I use the Top Speed and Gear Ratio Calculator off of http://www.electricscooterparts.com/mot ... ratio.html, I come out with a motor that turns 6290 RPM (85% of 200KV) for 13.7MPH in first gear. Seventh gear (seven speed cassette) speed at 6290 RPM would be just over 35.8MPH.

It's a unknown as to how efficient or how good the gearing will be for best climbing or speed. Depends on the voltage under load which determines your KV. At what RPM this motor is most efficient. The RC guys say 85% is a good number to start with for RPM. A guy ran a bunch of motors and found that best power was at 50% to 75% of KV RPM.



The motor I ended up buying is: GForce G180 Brushless Outrunner Motor (C6374-200KV)
Size: 62.8 x 68mm
Weight: 750g
Input Voltage: 4s-10s
Power: 3000W
It should be fairly close to correct MPH for the correct RPM of the motor at best RPM for power.
$85.00 for the motor, $45.00 for the speed controller and $9.00 for the servo tester (used as the throttle)

The options for other motors were just not as appealing. Higher cost, less selection, can not replace the motor shaft (important on this build as I need a splined input shaft solution), quite a bit of weight for the amount of power I want.

My parts arrived yesterday from Value Hobby. They seemed to have really good prices and shipping was only a few days. They had stuff in stock too, which HobbyKing seems to carry no stock now.

I hooked the motor up the the ESC (speed controller) and used a servo tester to energize it and test it. The motor is designed for 4S to 10S. I ran it at 4S just to make sure it works before I mod the shaft to a splined shaft. The speed control was very smooth, even in the real low RPM's. It was not noisy at all, well not under load anyways. The motor is real small for being a 3,000 watt motor. Probably one fifth the size and weight of a regular motor. I looking around for a cooling fan blade for it, don't know if that will be something I have to make or modify out of something else. I hear these things can be little toaster ovens.

Sorry I couldn't help.

Thud has a fan design for outrunners I think can be 3d printed, if you search his posts for it.

The torque constant (Kt) of any of our motors, regardless of type, in Nm per amp is 9.549 divided by the Kv in rpm/v.
Kt in Nm/A = 9.549/Kv . IOW all motors with the same Kv make the same torque per amp regardless of size. Kt and Kv don't really become useful unless you know current handling of the motor, and the important number there is the motor's Rm or phase-to-phase resistance.

Thanks, I looked at Thud's printed fan, it is looks really nice. Is there not a ready made fan out there for these motors?

deronmoped said:

Thanks, I looked at Thud's printed fan, it is looks really nice. Is there not a ready made fan out there for these motors?

Click to expand...

RC motors spend there life in RC planes or helis with lots of air passing through the motor so thats why there are no out the shelf fans.
I am using the Rotormax 50cc BLDC motor in my E-Bike but I am only runing it up to 3kw max which is around 1/2 its max power
so it says at a nice temperature.