I have salvaged quite some motors from old printers and used them for various projects, but these usually use inferior ferrite magnets (Like those flexible fridge magnets). They are not very strong so the motor strength is limited.
An easy way to increase the torque of your motor is to replace the magnets with neodymium magnets.
Step 1: Figure Out How to Take Off the Rotor
This is probably the hardest step, because every motor is different you might have to fugure out how to disasseble the motor youreself. I have encountered two ways the axle is fixed, they are both pressure fitted. The first one you can just take a hammer and hit the axle out. On the second the top part of the axle is wider so it won't come out. What I did was grind off the top with a dremel and hit it out with a hammer the same way I did with the first.
Step 2: Removing the Magnet.
Now you have to remove the magnet inside the motor. The motor has multiple poles, the original motor has 8 poles, you can increase the number of poles to increase the torque but it will decrease in RMP. If you want more speed you can decrease the amount of magnets. You can test how many poles your motor has by sticking magnets on the ferric material.
I removed the magnet by getting a screwdriver behind it and just prying it out. afterwards I sanded it. (sorry, I don't have pictures of this)
Step 3: Inserting Neodymium Magnets
As you can see the thickness of magnet also varies between motors you want to have your magnets as close to the rotor as possible to decrease the magnetic resistance in the air.
I bought my magnets for the first motor at a thickness of 1mm but I can probably fit 2mm ones in this new motor.
The magnets I bought from Aliexpress:
I actually would not recommend this seller because some of the magnets were cipped and are generally of bad quality. (But they are cheap :)
Your magnets will probably not fit snugly beside eachother so you will have a make some spacers I just used some matchsticks and cut them to shape to fit. in every gap there is about one and a half matchstick.
To secure the magnets I would recommend you glue them in place. I have just put some double sided tape on the back but this will probably not last long.
Step 4: Putting the Motor Back Together
This can be quite fiddly because the magnets will attract the stator and it can be a challenge to get the axle and stator to match up. I put transparent tape on the inside of the magnets to keep them from scraping against the stator. once you have the axle in the hole of the stator you can just hit the axle a few times with a hammer.
Done! Upgrade complete :)
Step 5: ESC Upgrade Help?
Next I want to upgrade a cheap speed controller to work with up to 30 volts they normally only work with 3S batteries (12v) but all the components are rated for 30 volts or up so it should be possible (Probably just by changing some resistor values)
If anyone can help me with this, that would be greatly appreciated.
For instance if you have schematics of a 6S ESC that uses an atmega8, please post them!
(The ESC here is a standard 30A esc that you can get for about 4 dollars of Ebay)