Reduction
Weight budget

The fight to stay under 3 lb.

A beetleweight has to weigh 3 lb (1,361 g) or less, and staying under that was one of the hardest parts of the build. The chart shows where the weight goes. Uncheck parts below to watch the total drop, or turn on the what-if parts to see what would put us over. Weights for the parts we made ourselves are estimates until we weigh them.

0 g0 lb
0 gHeadroom
$0Cost of selected parts
3 lb limit, 1,361 g
Planning range 1.0-1.6 kg

    Parts list

    Everything on the robot. S means the number came from the supplier and E means it's our estimate. Prices are USD before shipping and tax. Linked part names go to the store we bought from.

    Robot parts with dimensions, quantity, installed mass and cost. Checkboxes include or exclude each part from the weight budget.
    IncludedPartDimensionsQtyMassCost
    Selected total

    Not counted here because they don't ride on the robot: our FlySky FS-i6 transmitter ($57.99), the charger, and all our tools.

    Up close

    Views from the Inventor model with the outer parts hidden, so you can see what's underneath.

    CAD render with the top deck and shield hidden, showing the drive motors at each end, the belt-driven wheels and the white weapon carrier in the middle.
    With the deck and shield off: drive motors at each end, the weapon module in the middle.
    The green unibody frame on its own, with the carbon bottom deck and interior partitions visible inside.
    Just the frame. The green unibody holds everything, with partitions splitting it into bays.
    The center weapon module on its own: two white uprights with green curved ears, the dark blade on its hub motor between them, and a grey shield on top.
    The weapon module: blade, hub motor, carriers, green ears and the shield.

    Part drawings

    Outlines of individual parts taken from the CAD. Dimensions are each part's size in our model, in mm. They show the design, but they aren't fabrication drawings.

    Drawing of the green unibody frame: top projection 290 mm wide, plus isometric and side views. Envelope 290 by 136 by 36 mm.
    Green unibody, 290 × 136 × 36 mm
    Drawing of the rounded-lobe blade: 63.96 mm long, 32 mm tall, 4 mm thick, with a center bore and three bolt holes.
    Rounded-lobe blade, 64 × 32 mm, 4 mm thick
    Drawing of a slotted front fork tine: 113 mm long, 30 mm tall, about 3 mm thick, with a long slot for mounting.
    Slotted front fork, 113 × 30 mm
    Drawing of the wheel hub: 51 mm diameter, 16 mm wide, with a center hole and five holes around it.
    Wheel hub, Ø51 × 16 mm

    Stress test on the weapon carrier

    We ran a finite-element study on the white carrier that holds the weapon. We exported the part from Inventor, meshed it into about 13,500 tetrahedra, fixed the bottom face, and pushed down on the top with 20 N.

    Meshed white carrier with orange arrows pushing 20 N down on its top face and the bottom face fixed.
    01 The setup: 20 N down on the top face, bottom face held still.
    Stress color map of the carrier. Most of it is dark purple, with green and red around the axle hole. Peak 1.197 MPa.
    02 Stress. Warmer colors mean more stress, and most of it sits around the axle hole.
    Displacement color map of the carrier with the movement exaggerated 500 times. Maximum movement 0.0138 mm.
    03 How far it moves, exaggerated 500×. The real max is 0.014 mm.
    20 NLoad on the top face (about 2 kg pressing down)
    1.2 MPaHighest local stress, right at the edge of the axle hole
    0.014 mmMost the carrier moves under that load
    <1%Change in movement and overall stress when we refined the mesh

    Caveats: this is a simple static study with a generic plastic assumed, not an impact test or a strength rating. It doesn't tell us the robot survives a hit. It does show where the carrier works hardest, which is where we'll add material next time. We used Gmsh and scikit-fem and can share the scripts.

    Our CAD

    We modeled the robot in Autodesk Inventor and will share it with anyone who asks. A lot of the dimensions were estimated from photos of the real robot, so measure before you make anything from it.

    The CAD on this site is not our most up-to-date version. We keep improving the robot faster than we can update the site, so the files, renders and specs here are behind the real thing.

    Format
    STEP AP214, 1.8 MB
    Units
    mm
    Overall
    290 × 215 × 100 mm
    Body
    250 × 120 mm, 36 mm frame height
    Wheels
    Ø51 × 16 mm, 4 corners
    Origin
    Floor level, robot centerline
    Checks
    75 bodies, all sketches fully constrained, 168 part pairs checked for overlap
    Download CAD

    We can send the STEP file, and the native Inventor files (.ipt) if you use Inventor.

    Top view of the CAD: carbon deck with chevron vents, tines at the front, treads at both ends, and the steel shield in the center.
    Top view. The tines point up, toward the front of the robot.
    CAD with the top deck hidden, showing drive motors in each end, the electronics bays and the weapon module.
    With the top deck off, you can see the motors and electronics bays.
    Side profile of the hooked blade with a central axle hole and three bolt holes.
    The profile of our hooked blade.