Help optimizing the spin rate on my custom gearbox build

Check your motor's internal resistance. Sometimes the motor itself struggles to maintain a constant speed under load, regardless of the coupling.

Have you considered adding a flywheel? A properly sized flywheel acts as a massive inertia damper, smoothing out those load-induced RPM dips dramatically. It's the classic solution for consistent high-speed spinning.

Try increasing the stiffness of your mounting points. Flexing mounts can introduce vibrational losses that make the spin feel erratic. Use rigid brackets.

What kind of load are you simulating? The load itself might be the source of the fluctuation, not the gearbox. If the load is intermittent, the spin will naturally vary.

Long response. You mentioned low-voltage, high-speed. This combination is tricky because the motor's torque curve changes rapidly. If the load suddenly increases, the voltage drop can cause a massive RPM drop, which then causes the motor to overshoot when the load decreases. You might need a dedicated speed controller or a voltage regulator that can handle the transient load demands. Also, ensure your power supply can deliver peak current, not just nominal current. Check your wiring gauge too; resistance in the wires can cause voltage sag under high load, making the motor think it's running on less power than it actually is. This is a very common oversight.

Are you using a belt drive? If so, tension is everything. Too loose, and it slips; too tight, and it adds drag. Find the sweet spot.

If you are using bearings, check the radial clearance. Sometimes, even if they look fine, microscopic debris or improper seating can cause binding that only shows up under high-speed, variable load conditions. A professional bearing check might be warranted.