SparkFun Forums

[trueRF]

I'm looking to build a unique RC car with motors in the hubs of the wheels. The wheels I have now are about 2inches in diameter and about 3/4 an inch deep. That leaves me with about 1.5 inches diameter by 1.25 inches in depth (it's okay that the motor sticks out of the wheel some what. Something in the style of a pancake motor would be perfect. Larger diameter but short body length.

I've looked through polulu's stock of motors and didn't find quite what I was looking for there. They have some small and torquey motors and small and fast motors and fast and torquey motors that aren't so small. I'm looking for the trifecta.

There are some other types of motors around that I don't know a lot about and could fit the bill. The problem with finding a motor on a random hobby site is that they don't have all the data that polulu has. (this is a major oversight in my opinion) For instance there is a type of motor out there called an inrunner (typically used for RCplane propellers) which is very small and incredibly fast and they say that it doesn't have a lot of torque but how much is not a lot? For the current draw it seems like it ought to have a decent pull.

I've also thought of steppers, which I know have a lot of torque, but I've never seen how fast they can be. IIRC a stepper motor us used inside a hard drive so 7200rpm, but that motor isn't propelling a pound or two of circuitry and batteries.

I was hoping for some replies from RC hobbyists who would know about inrunners, or anyone else who found a motor somewhere that seemed like it fit my bill. For right now I'm not worried about current requirements or voltage. I can figure that out later.

[notexactly]

I have seen a video on YouTube of larger stepper motor (a standard NEMA one like you'd use on a 3D printer, IIRC) being driven up to ~9000 RPM using some particular kind of driver. I'm not sure a stepper would fit your requirements otherwise, though.

[waltr]

Any motor available will require a reduction gear drive to obtain the lower wheel speed and increased torque.

FYI: Hard drive motors that spin the platter use BLDC motors not steppers.

[trueRF]

Yeah, Since I've posted I've learned the error of my ways (with regard to the HDD motor)

I think I'm going to go ahead and get the inrunners (when I have some free money) and If I can't figure out a way to make them work on land. Well, I'll have to make them work in the air and build a quadcopter with them instead.

[waltr]

Inrunners are a bit heavy and lack torque for use in a Quadcopter.
Outrunners are much, much better for any aircraft due to power to weight.

[trueRF]

I'll have to ask around on a hobby forum about this. I'm still having trouble finding consistent numbers for motors. Even something easy to measure like "weight" and "length" is not consistently reported.

[trueRF]

I did some research this weekend. I think I might be able to pull this off with a stepper motor. Well 4 stepper motors. Originally I was leaning away from these because of the high constant current draw. But when you look at hobby grade performance motors they have substantial peak currents and even the minimum current draws are quite high. So there isn't a whole lot of benefit to using a hobby BLDC over a stepper.

Now I have some questions about step rates. Big Easy Driver supports a step rate of 1step/2uSec. If I'm reading/understanding right, which translates to something like 150k rpm. (based on a 200step motor running full steps.) That seems high. I'll have to do some more research on steppers and their drivers. I'm looking at BED because I can potentially run two smaller motors off of it vs buying 4 of the only marginally cheaper easy drivers. I may build my own board...I dunno yet