RE: Redcat Sumo bad sounds no movement
First of all, get ready to get some experience disassembling and reassembling. If you are really paranoid about messing something up get a box with dividers, be organized, and take lots of pictures.
Second, What kind of sumo do you have?
For what I'm about to explain I am assuming the truggy or buggy.
I don't know much about the sumos, but looking at the exploded view from the manual they look similar to larger scales. I would take it apart and check the pinion and spur gears first. The pinion gear is connected to the motor and and spur is the gear it touches. After that there appears to be a shaft that goes through the center of the spur gear with smaller gears connected to the ends? Check that assembly. If neither of those are the problem, I fear that you have stripped one or both of your diffs. The diffs are the gear assemblys that connect to the shaft described above and deliver power in the direction of the wheels. The diffs are located between each set of wheels with shafts going to each individual wheel. Check to make sure the drive cups that connect to these shafts are not stripped somehow before checking the diff. The diffs are secured in a housing that will need to be removed to reach the main gear which can be disassembled by removing four screws on the gear to reach four small umbrella shaped gears.
I think I covered every possiblity but hopefully someone with one of these will chime in.
Sumo Gear package part # is 24017.
Sumo Diff assembly part # is 24020.
I think the gear package has enough to rebuild one diff and some extras.