fenrir :a failed robot in 12 hours
fenrir is a one-pound drum spinner: a printed chassis, an orange printed drum on a belt off a brushless motor, four foam wheels on printed pulleys, two small brushless drive motors, and a wedge up front. we built it in one night for trc metal meltdown on september 19, 2026.
it failed at the event. the cad left no room for wiring, and the motors did not fit because the tolerances were never fixed. the electrical side, which was my part, worked on the bench. i came out of it with real soldering experience.
01mechanical
the mechanical twin of the electrical blow-up: 78 parts in 19 groups. drag to turn it, scroll to zoom, slide to explode. hover or tap a part for its group; the legend below does the same from the keyboard. electronics, motors, battery, switch, led and wiring are left out on purpose, they are in the electrical blow-up below.
source: the v2 assembly step export from onshape, 2026-09-16, split into 80 named parts on 2026-09-22 · nodes.json · static render
02electrical
the whole harness laid out flat before it went into the chassis, with every part numbered. parts named from the order emails and the photo.
- repeat mini brushless 1106 mk4.1, drive motor, left. 3500 kv, 28.5:1, 3 mm shaft.
- repeat mini brushless 1106 mk4.1, drive motor, right. the only two drive motors in the record.
- repeat am32 dual brushless drive esc. the power board: battery in, 5 v bec out to the receiver, weapon esc off the bottom pads.
- radiomaster er4 receiver, elrs 2.4 ghz, four pwm channels. antenna trailing bottom-left.
- readytosky 45 a blheli_s weapon esc. no bec; signal and ground only to er4 ch3.
- repeat ant belt drive 2305, 2400 kv, weapon motor. three phase leads soldered direct.
- xt30 battery lead. to the 3s lipo, straight to the switch, no adapters in the power path.
- fingertech switch, inline in the battery + lead before the drive esc.
- power led lead. the fingertech red led goes on the end; not attached yet in this frame.
- signal and bec leads, am32 to er4. 5 v, ground, ch1, ch2.
03parts
what was actually ordered, from the confirmation emails.
| part | qty | order |
|---|---|---|
| repeat mini brushless 1106 mk4.1, 3 mm shaft, 28.5:1 (drive) | 2 | repeat robotics #13865 |
| repeat ant belt drive 2305, 2400 kv (weapon) | 2 (1 spare) | repeat robotics #13951 |
| readytosky 45 a blheli_s "beetle spinner" esc (weapon) | 1 | repeat robotics #13951 |
| am32 dual brushless drive esc | 1 | repeat robotics #13951 |
| ant spinner pulley 18t-3m-05 | 2 | repeat robotics #13951 |
| fingertech switch | 2 | repeat robotics #13951 |
| gold clamp hubs, 3 mm (2-pack) / 6 mm (2-pack) | 3 / 2 packs | repeat robotics #13951 |
| radiomaster pocket radio controller, elrs | 1 | rotor riot 141578 |
| radiomaster er4 2.4 ghz pwm receiver, elrs | 1 | rotor riot 141578 |
| shortsaver 2 smoke stopper | 1 | rotor riot 141578 |
| 18650 li-ion 3200 mah 2-pack (radio) | 1 | rotor riot 141578 |
| fingertech foam wheels thin, 1.25" × 0.50" | 4 pairs | palm beach bots #21961 |
| fingertech lightweight twist hubs thin, 3 mm × 0.50" | 3 pairs | palm beach bots #21961 |
| s3m timing belt 183 mm, 61 t, 4 mm | 4 | palm beach bots #21961 |
| mf63zz 3×6×2.5 mm flanged bearings | 6 pairs | palm beach bots #21961 |
| fingertech power led (red), twist hub key, mini power switch screw | 1 each | palm beach bots #21961 |
not in these orders (bought elsewhere by the team): the 3s lipo, charger, shafts, needle bearings, heat-set inserts, m3 hardware, gt2 belt, filament.
04the twelve hours
