N20 Twin-Track Mini Tank Chassis

This is a small skid-steer tank with rubberless print-in-place tracks. Two N20 gearmotors sit inside a printed hull. Each one turns a 12-tooth drive sprocket at the rear. Each track loops over an idler sprocket at the front that runs on two 624 bearings. The main complaint about printed tracked chassis is that the sprocket pitch doesn't match the track pitch. This kit fixes that by putting every link and tooth dimension into one contract: 12 mm pitch, 12 teeth, a 23.18 mm pitch radius, 28 links per track and 96 mm between axles. A slotted idler axle takes up any print tolerance. It's meant for hobby roboticists who want a continuous-track platform for an ESP32 or Arduino rover, a line follower or an FPV crawler.

The hull is an open tub. Inside it has a cradle for the two coaxial motors and four heat-set bosses for the top deck. Each motor's gearbox face sits against the inside of a side wall, and its shaft passes out through a vertical slot. A single clamp bar screws down over both motors. Outside the hull, the drive sprockets clamp onto the D-shafts with a captured M3 nut and grub screw. Each idler rides on an M4 bolt locked to the side wall. A keel skid rail on each side holds down the bottom run of track between the axles. Each track is four identical 7-link print-in-place strips, joined end to end with M3x8 screws. The top deck carries two 18650 cells and has a 10 mm M3 grid for the motor driver and controller.

Assembly order:

1. Press M3 heat-set inserts into the 4 deck bosses and 4 motor-clamp posts of the hull.
2. Seat both N20 motors in the cradle, gearbox face against the side wall and shaft through the wall slot, then screw down the motor clamp bar.
3. Bolt a skid rail to each side wall (M3x12 plus a nut inside the hull).
4. Push the drive sprockets onto the motor shafts. Line up the grub screw with the D-flat and tighten it.
5. Press two 624 bearings into each idler. Pass an M4x30 through the idler slot from inside, lock it with an M4 nut and washer outside the wall, fit the idler, and finish with a washer and an M4 nyloc snugged just short of binding.
6. Join 4 track strips into a loop around each sprocket pair with M3x8 screws at each join, then slide the idler bolt forward in its slot to tension the track and tighten the inside nut.
7. Wire the motors to the driver, fit the cells and electronics on the top deck, and screw the deck onto the hull.

Tuning notes: Print the track strips at 0.2 mm layers with at least 3 perimeters, then work every hinge loose before you fit them. The idler slot allows axle positions from x 47 to 50 mm. Leave about 3 mm of sag in the top run of track. Buy N20 motors with a 3 mm D-shaft at least 9 mm long. The sprocket bore reaches 7 mm past the hull wall. Motor length varies with gear ratio, so the cradle has no rear stop. The sprocket grub screw locates each motor along its shaft, and the clamp bar holds it down. For a stiffer track, use PETG.