NEMA17 Rocker-Bogie Mars Rover Chassis

This is a desktop Mars-rover chassis that uses the same rocker-bogie suspension as Curiosity. It has six wheels, and each one is driven directly by its own NEMA17 stepper, so there is no steering servo and no gearbox. On each side a rocker arm pivots on a 608 bearing, and a bogie arm pivots on a second 608 at the back of the rocker. A differential bar under the deck ties the two rockers together, so when one side climbs, the deck only tilts by the average of the two sides. All six wheels stay on the ground over obstacles about a wheel radius tall (35 mm). It's aimed at hobby roboticists who want a motorised rover rather than an RC-servo toy. Point to point, the wheels span about 160 mm front to back and about 280 mm across the outside of the wheels.

There are seven printed designs. The deck plate hangs from two pillow blocks. Each pillow block carries an M8 pivot bolt that runs through the 608 bearing in a rocker arm. The front motor bolts to the inner face of the rocker, and the middle and rear motors bolt to the inner face of the bogie arm. Each shaft passes out through its arm into a clamp-hub wheel. The rocker and bogie plates are the same through their thickness, so the left and right sides use the same print. You just mount the motors on the opposite face. The differential bar turns on a 625 bearing on an M5 bolt under the deck. Two short links connect the ends of the bar to an ear on each rocker, 30 mm below its pivot.

Assembly order:

1. Press a 608 bearing into each rocker arm and each bogie arm, and a 625 bearing into the differential bar.
2. Bolt the two pillow blocks onto the deck grid with M3x16 bolts and nuts.
3. Bolt the differential bar under the deck with an M5x25 bolt, with 3 mm of washers between the bar and the deck.
4. Bolt one NEMA17 to each rocker front pad and two to each bogie, using M3x12 bolts on the inner face.
5. Hang each bogie on the back of its rocker with an M8x35 bolt, 4 mm of washers and a nyloc nut.
6. Hang each rocker on its pillow block with an M8x40 bolt, a washer on each side of the bearing and a nyloc nut.
7. Connect the rocker ears to the ends of the differential bar with the two links, M3x25 bolts and nyloc nuts.
8. Push a wheel onto each 5 mm shaft, leaving a 1 mm gap to the arm, and tighten the M3 set screw onto the shaft flat.

Tuning notes: when the chassis sits level there is about 8 mm between the middle-motor housing and the rocker, so keep bogie pitch within about ±8°. A zip tie between the bogie and rocker makes a simple travel stop. Tighten the M8 nyloc nuts only until the pivots swing freely with no wobble. Add a drop of thread-lock to the wheel set screws. Six steppers at 1–1.5 A each need a 12 V pack of at least 3S and six step/dir drivers. Wire the three motors on each side in parallel from one step signal to get simple skid steering.