March 15, 2026
Superconductors vs spaghetti tendons
Electric motor scaling laws and inertia in robot actuators
Big motor or big gears? Robot nerds go to war
TLDR: A new explainer shows how motor size and gear choices affect how “heavy” a joint feels. Comments erupt into a showdown over superconductors, capstan-string tricks, and whether heavy gearing kills touch—turning a math lesson into a brawl over how robots should move.
Robotics nerds just got a brain‑teaser: three motor setups that all make the same torque, but which one “feels” lightest at the joint? The article breaks down motor size rules and introduces a size‑fair score, showing most off‑the‑shelf units cluster around a normalized performance of 11–15, with squat motors lagging. It explains how changing length and radius scales torque, heat, and inertia—the “reflected inertia” is how much the motor’s spin fights your movement through gears.
The comments, though, turn it into a science fair cage match. pfdietz dreams of cryogenic bots and room‑temp superconductors so motors waste zero power. hinkley waves a wand at Aaed Musa’s hypnotic capstan drive, plus a sideways‑stepping quadruped—string‑powered sorcery. Animats revives the eternal gear war: too much gearing and you can’t feel or back‑drive; too little and the motors get chunky. brcmthrowaway declares soft robots the real future, while numpad0 wants to “just blast solenoids” and route motion with tendons. Memes fly: “superconductors when?”, “give that robot violin strings,” and “gearheads vs goo‑lords.” In short, a calm, mathy explainer detonated into a street fight over whether robots should be quiet muscle cars, violin‑string tricksters, or gooey gymnasts.
Key Points
- •Reflected inertia is defined as rotor inertia multiplied by the square of the gear ratio and is central to comparing actuator architectures.
- •Under constant winding current density and fixed radial thickness, increasing motor length scales torque, rotor inertia, and resistive dissipation linearly.
- •The standard motor constant (Km) is size-dependent; the article proposes a normalized FoM by dividing by motor mass and radius to compare motors across sizes.
- •The normalized FoM applies to both rotary and linear motors and can be related to material properties, suggesting theoretical upper bounds for Lorentz-force actuators.
- •Empirical data from TQ frameless motors (16 g to 1.07 kg) show normalized FoM clustering around 11–15, with lower values for short stack lengths due to end effects and increased end-turn resistance.