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Fat commutators allow for more surface area for the current to pass through. This allows a better connection and less resistance. The only downside would be a touch more weight on the armature and a touch more friction from the brushes. Both should be fairly negligible, though.

As for silver vs copper vs carbon, etc, I'm not sure on the specifics of each, but each has a relative conductivity, hardness, and price. For example, copper conducts well and is fairly cheap, but is soft, so it will wear much faster. Silver has a higher conductivity and is harder, but is also more expensive.

Open endball offers more ventilation to the brushes and thus hopefully keeps them cooler. Since there is some resistance in the brushes and commutator, there is also heat generated there. Open endball, however, allows more dirt and the like to enter the can if it's not protected.

The stronger the magnets, the stronger the pull towards the like poles, but also the harder it is to pull away from them. A motor works by flipping the magnetic polarity of the armature segments in sync with the rotation of the armature, so as always to attract them to the next magnet along the can. The stronger the magnets, the faster it will be attracted along. However, if your magnet is too strong, then once the armature section turns to the position of the magnet, it will "stick' there and not retain enough momentum to move slightly past it, flip polarity and start attracting the next magnet. In short, stronger is better, but after a point it can start to hamper instead of help.

Slotted armature is to help with weight. The lighter the armature, the faster it will spin. It also can help with heatsinking. The spiral armatures are designed so that the magnetic attraction is smoother as it passes around the can. Instead of one big bar that's constantly being jerked from magnet to magnet, it's now a gradual horizontal spiral. In theory, it provides a smoother rotation.

Quick change brushes are just that. You can change them without having to remove the motor and end bell. You'll pay more for them, though.

Brush material is different for the same reason I mentioned above. Different matierals have different conductivity (the higher the conductivity, the less resistance so the less power lost and less heat generated) and also different hardnesses.

The higher the number of turns, the less current your motor will draw and the slower it will rotate, but the more torque it will have, which is why crawlers use very high turn motors. The more turns, the faster and more current required. The gauge of wire need only be large enough to adequately handle the current you're drawing. You could always use a large gauge, but since the wire adds significant weight, the larger the gauge, the slower it will rotate. Generally, you should decide what top speed you want (at the expense of torque) and base your # of turns on that. Then choose the smallest gauge of wire that can adequately handle that current. Keep in mind, you can't disregard torque. If you do a very very low turn motor, it may go incredibly fast once it's going, but it might not have enough torque to even start from a stand-still, as the friction is much higher when the car is stopped than when it is already moving.

25 turns is very low. Even 30 is low. You might have a nice high top speed but you'll have very little torque and thus not much pickup off the line.

Also, you not only have to have the FETs to handle it (I'm not sure if 2*3 would handle 25 turns or not, as I've never looked into one...), you also have to have the batteries to handle it, that's a lot of sustained current draw. Your runtime will be quite short.

Since you're running 4 NiMH's, you're already losing out on >1V compared to alkalines. You would probably get much more benefit from upgrading to a set of LiPo's than you would from a motor change.

Wire gauge might slightly affect torque, but will have a larger effect on top speed. The higher the gauge, the more weight you have to turn around and at 10,000 or 20,000 RPM, a tiny bit of weight means a lot more effort.

I'm not that experienced with motor making, but I gather it's as much of an art as a science. Take something that you think should work and put it together. Measure it and run it and see how it performs. Then tweak it as desired to tune it.

True, it will have much more effect on acceleration than top speed I guess. But that's theoretically and only if the armature is perfectly balanced. If there's any imbalance, which there always is, however small, then the more weight you have, the more vibration you'll generate and the more friction, less top speed, etc.

No worries for keepin it up. The more you try and help other people, the more you learn yourself =) Or at least make it harder to forget what you already know...