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Designing the Hubris drivetrain

Printed planetary gearboxes, repurposed quadcopter motors, and a rocker-bogie suspension built from aluminium extrusions.

The Hubris drivetrain is a mixture of 3D printed parts, aluminium extrusions, and hardware left over from other projects. The design started at the wheels and worked its way inwards: first the in-hub planetary gearbox, then the rocker-bogie suspension, and finally the structure that ties it all together.

CAD view of the assembled six-wheel Hubris rover, with its body mounted above the orange suspension and yellow tyres.
The drivetrain and body together: printed parts and aluminium extrusions, ready to take on our version of Mars.

Starting with the wheels

The first thing I worked on was the in-hub planetary gearbox. The entire wheel hub is 3D printed, currently in PLA. That makes it convenient to prototype, although I don’t expect the PLA to survive long once exposed to our “Mars environment simulator”. The tyre is printed in TPU. The planet gears in each in-wheel gearbox run on standard 608 skateboard bearings.

Front view of the wheel with its cover removed, exposing three planet gears around a central sun gear inside the ring gear.
The in-hub planetary gearbox, with the cover removed to show the printed gears.

The motors are Turnigy Aerodrive SK3 outrunners, left over from a quadcopter project. That’s why the rover has only four driven wheels, despite having six wheels in total.

I’ve considered replacing them with six matching lower-KV motors. For now, though, the plan is to use field-oriented control (FOC) with two MKS XDrive boards and AS5600 sensors in a closed-loop configuration. I believe reusing the motors I already have is a more budget-efficient way to prove out the design.

Just because it’s going to “Mars” doesn’t mean it has to be gold plated.

Rear view of the wheel showing the outrunner motor inside the orange printed wheel mount.
The motor tucked behind the wheel, inside its printed mount.

Building out the rocker-bogie

Once I knew the wheel dimensions — about 130 mm in diameter — I could scale the rest of the design to fit and continue work on the rocker-bogie suspension.

The complete six-wheel rocker-bogie suspension without the rover body, showing printed joints connected by aluminium extrusions.
The rocker-bogie assembly, before adding the main body.

The suspension centres around a large 3D printed differential, mounted securely to the front of the main robot body. Almost all of the differential uses standard 608-sized skateboard bearings, but it also needed additional support from large 6812 bearings, shown in blue below.

Partially opened suspension differential showing the printed gears, aluminium extrusion shafts, and a large blue support bearing.
Inside the suspension differential. The large 6812 support bearings are shown in blue.

Extrusions, screws, and T-nuts

Both the rocker-bogie suspension and the body make heavy use of standard 2020, 2040, and 4040 aluminium extrusions. The printed parts connect everything together with an awful lot of M3 screws and T-nuts.

CAD view of the rover body frame, with aluminium extrusions, panels, and printed corner brackets secured by screws.
The main body, built around aluminium extrusions and printed brackets.

What’s next

I plan to redesign the differential’s centre gear and the bogie pivot to accommodate additional AS6500 position sensors. That will let us maintain an accurate picture of the suspension’s configuration as the rover moves across the terrain.

For now, the aim is to prove out the mechanical design with the parts we have. The paddock will no doubt have opinions about what needs to change.