I have been interested in using brushless motors for a mini sumo. After a great deal of frustration with trying to find pertinent online information I decided it was time to just try something to start to gain some knowledge. I chose a BL system for 1:16th / 1:18th scale RC cars called the Mongoose Micro Pro. It is a sensored motor so the low speed should not cog. It came with what they call a Three-80 motor. I think it’s a 380 sized motor. It has 13.5 turns, and is a 6000kv motor. They tell me this means it has an unloaded RPM of the battery voltage times the motors kv value. 6000 x 7.4 = 44,400 rpm. This is a scary number, but I know there will be a decent load. So I order up two. One arrives and the other is back ordered!
So my first impression is it’s small light and looks to be durable. The thing weighs in at .23lbs, so 2 of them is about 40% of the weight of the bot. This is sumo so there’s no weapon to deal with, just a scoop. It will be very close but I think it can work. I hope so. lol
The second impression is not so good. One of the first things I read are.
Never free-rev the motor. Never free-rev your system to “see how it runs,” or you may not get to drive it again!
So apparently it can cause damage to the rotor and or the ESC.
Adjust motor for proper gear mesh. Make sure the pinion/spur gear mesh is not too tight! If gear mesh is too tight, motor shaft breakage can occur.
Sounds pretty simple…
Motor operating temperature is the only way to properly set the vehicle gearing. The motor should be 160 to 175 degrees max at end of run! Change the gearing to avoid overheating. They then provide a list of cars and what to change the pinion to. That’s no help!
So I don’t have any idea how much gearing I need, but was thinking 45:1 would be awesome. If I can find a suitable gear train. 30:1 would probably be a lot easier.
So I guess there is no way to just start poking away at it. You must have a rolling chassis before any real testing can be started. Anyone else have any interest or experience with this?
I looked into the brushless option for a 3kg sumo and found a few problems that persuaded me to avoid this for now. The main issue I found was that most brushless systems are designed for RC cars or planes and for continuous use or primarily in one direction. Bi-directional brushless electronic speed controllers were completely unavailable or very hard to find for the longest time and even though there are now options available, most are directionally biased and don’t provide symmetrical control. For a combat robot or sumo robot this is usually unacceptable.
The next problem I ran into was lack of gearbox options. Most brushless motors spin crazy fast and are either mated with integrated RC car gearboxes or run without a gearbox to spin an airplane prop. There are gearbox options available but the severely limited selection made it difficult to find an acceptable motor option for the robot that provided the right speed, power, and power needs in the allowed volume.
At this point I started to seriously reconsider going brushless and reflected on the benefits I was hoping to gain using these motors instead of brushed. Brushless motors are very popular because they provide more power and speed with less current draw and in a smaller, lighter package than an equivalent brushed motor. For sumo robotics however run time and top speed are usually not a concern because the ring is small and the matches are short. Brushless would still provide more power but most sumo robots these days never stall their current motors in a pushing match so more power wouldn’t gain you much there either.
In the end I decided that Brushless motors would be the best option for a sumo project but the enhanced performance they provide would not be worth the added cost and extra design time. I still think this is a worthwhile topic to pursue but maybe in a few more years after the brushless product market continues to grow.
Best of luck though! I look forward to hearing about your findings and test results.
Yeah I have sidelined it for now. Even though the motors I chose didn’t have many of the issues you mentioned, they were on the heavy side and the gearbox search ultimately did the project in.
It has 13.5 turns, and is a 6000kv motor. They tell me this means it has an unloaded RPM of the battery voltage times the motors kv value. 6000 x 7.4 = 44,400 rpm. This is a scary number, but I know there will be a decent load. So I order up two. One arrives and the other is back ordered! 
That’s no help!