N20 4WD Skid-Steer Rover Base

This is a small four-wheel-drive skid-steer rover base built around four N20 micro gearmotors with a 3 mm D-shaft. Each motor slides into a printed clamp that bolts under a 150 x 96 mm chassis plate. A pusher screw holds the motor in place, and a 56 mm wheel locks to the shaft with an M3 set screw and a captured nut, so you don't need to know the exact size of the D-flat. On top there's a cradle for a spot-welded 2S 18650 pack, and a deck on four legs carries a Raspberry Pi 4 (or an ESP32 on the M3 grid) plus a dual motor driver. At the front, a bumper plate with a 10 mm M3 grid takes an ultrasonic or ToF sensor. It's meant for anyone who wants a clean, fully printed rover to learn ESP32 or Pi robotics on, without starting from RC-car parts.

Everything prints flat without supports in PLA. The chassis plate is the reference for the whole kit. The motor clamps, wheels, battery cradle, deck and bumper all attach to its fixed M3 hole positions (I2, I5, I6, I7), so every part lines up to the same datum. The motor clamps hang under the plate and put each motor axle 12.2 mm below the underside, which with 56 mm wheels gives about 18 mm of ground clearance under the clamps.

Assembly order:

1. Press M3 heat-set inserts into the motor clamps (2 in the mounting face, 1 in the pusher boss) and into the foot of each deck leg.
2. Bolt the four motor clamps under the chassis plate with M3x8 screws from the top, with each outer clamp face flush with the plate side edge.
3. Slide an N20 motor into each tunnel from the inside until the gearbox face is flush with the outer clamp face. Snug the M3x6 pusher screw so it holds without crushing the motor, and route the wires through the plate wire slots.
4. Drop an M3 nut into each wheel hub slot, push the wheel onto the shaft leaving a 1.5 mm gap to the clamp, and tighten the M3 set screw onto the D-flat.
5. Bolt the battery cradle (M3x12 + nuts) and the sensor bumper (M3x12 + nuts) to the plate top. Strap the 2S pack in with hook-and-loop.
6. Mount the Pi 4 on the deck bosses (M2.5x10 + nuts) and the driver on the M3 grid, then screw the deck legs to the plate with M3x8 from underneath.

Tuning notes: wire the two motors on each side in parallel to one channel of a dual driver (e.g. TB6612FNG) for skid steering. Rubber bands or an O-ring stretched into the two tread grooves greatly improve grip on smooth floors. If your N20 gearbox is shorter or longer than the one this was designed for, the tunnel is open at both ends, so only the wire exit changes. If the motor is loose in the tunnel, a wrap of tape takes up the play.