N20s are a great fit because they are compact, have a built-in gearbox for decent torque, and, most importantly, encoder-equipped versions allow for the closed-loop control these projects need . However, there's a mechanical reality you need to design around.

Here’s a quick look at how they've been used in real-world projects:

The Good: Why they work

• Built-in Encoders: For a self-balancing robot, you need to know exactly how fast the wheels are spinning to correct the tilt. The Hall-effect encoders available on many N20s (like the 6V 500RPM version) give you the feedback necessary for a PID loop to maintain balance .

• Size and Weight: They are tiny and light, which keeps the robot's inertia manageable. A heavy robot is harder to balance than a light one.

• Torque is Available: As we discussed in your previous thread, you just need to pick the right gear ratio. For a balancer, you generally want torque over speed.

The Catch: The "Backlash" Problem (Very Important)

This is the engineering detail that makes or breaks the build.
One builder noted that their N20 motors had "a considerable amount of play" (mechanical engineers call this backlash in the gears) . This means that when the motor tries to reverse direction to correct a fall, the gears have to "take up the slack" before they engage.

This causes a slight delay and a jerky motion, which can make the robot oscillate or struggle to find a stable balance point. High-end servos have tighter gear tolerances to minimize this, but N20s are mass-produced and will have some play.

Recommendation for your Build

Given your history with the 150/300 RPM versions being too weak, here’s the path forward:

1. Choose the Right Speed: Ignore the high-RPM motors. Look for an N20 in the 100 RPM to 500 RPM range at 6V. A 500 RPM motor (like the one in the commercial chassis) provides a good mix of responsiveness and torque . If you want more torque for a heavier robot, aim for something around 100-200 RPM.

2. Get the Encoder Version: You absolutely need the version with the magnetic encoder on the back. Without it, you're flying blind. The encoder lets your PID controller (like the one using an MPU6050 sensor) know how the robot is responding .

3. Account for Backlash in Software: When you write your PID loop, you might need to add a little deadband or tune it specifically to handle the mechanical slack in the gears. It won't be perfectly smooth like a direct-drive servo, but it will be powerful enough to balance.

TL;DR: Yes, they are good. Pick a sub-500 RPM version with an encoder, accept that there will be a tiny bit of mechanical slop, and you'll have a solid, compact balancer.