Leg drive: 6:1 hip from the chassis, 9:1 knee carried on the thigh

Hip motor in the chassis. Knee motor rides on the thigh and swings with it, which removes the hip–knee coupling entirely and costs almost nothing in swing inertia.

Drawn to scale at 1 mm = 1 px. The knee drive — motor, 42T/16T intermediate shaft and both belts — is entirely carried on the thigh, so the whole assembly swings as one rigid unit with the leg.

Why mounting it on the thigh is the right call

The coupling on the left is not a tuning problem, it is geometry: the belt is anchored to a shaft that does not move with the leg. Carrying the motor on the thigh removes it outright.

What it costs in swing inertia

Part	Mass	Radius from hip	Share of leg inertia
Wheel + wheel motor	550 g	460 mm	78%
Shank structure + tube	187 g	345 mm	15%
Knee pulley 48T	120 g	230 mm	4%
Thigh tube + structure	237 g	115 mm	2%
Knee motor on the thigh	430 g	45–95 mm	0.6%

Moving the knee motor onto the thigh raises swing inertia by 0.6%. Inertia goes as radius squared, and the wheel sits at 460 mm while the motor sits at under 100 mm. The wheel alone is 78% of the total. The usual reason to chassis-mount a leg motor simply does not apply here, so you get the simpler kinematics for free.

Numbers

Drive	Stages	Ratio	Pulley	Peak torque	Belt tension	Belt
Hip	14T → 84T	6:1	133.7 mm	10.8 Nm	166 N	9 mm
Knee stage 1	14T → 42T	3:1	66.9 mm	5.6 Nm	166 N	9 mm
Knee stage 2	16T → 48T	3:1	76.4 mm	15.7 Nm	436 N	15 mm

Watch the clash at the hip. The 84T hip pulley is 133.7 mm across, so anything bolted to the thigh within 67 mm of the hip axis fouls it. Either put the knee motor outboard of that, as drawn, or run it in a different plane alongside the hip pulley.

Stage 2 carries three times the motor torque — 436 N against 166 N in stage 1. Use 15 mm there; stage 1 can be 9 mm. Both GT3 5M, 5 mm pitch. Tension both belts with a slotted mount or an idler: an under-tensioned belt skips teeth at peak torque, which on a balancing machine means losing the joint exactly when it is catching itself.