Gearing & wheels
Gear ratio
Section titled “Gear ratio”Gear ratio converts high-rpm/low-torque motor output into wheel speed and torque. See the formulas in Power, torque & gearing.
- Too tall (low ratio): high top speed, weak push, motor stalls and overheats.
- Too short (high ratio): strong push wasted as wheelspin, low top speed.
- Aim for the tires to break traction near full motor torque.
Beetleweight drive ratios commonly fall between 20:1 and 60:1 depending on motor Kv and battery voltage. See Team Run Amok’s guide to optimum gear-ratio selection.1
Wheel diameter
Section titled “Wheel diameter”| Larger wheels | Smaller wheels |
|---|---|
| Higher speed at a given rpm | More torque at the contact patch |
| Climb debris and arena seams | Lower CG, harder to high-center |
| Raise ground clearance | Package more easily inside armor |
30–50 mm is the usual beetleweight range. Larger wheels that protrude past the armor are vulnerable to spinners; recessed or partially shrouded wheels survive longer.
Wheel type
Section titled “Wheel type”Combat wheels need traction, durability, and low weight at once. The common options today:2
| Type | How | Notes |
|---|---|---|
| Custom-cast polyurethane | Cast resin (Smooth-On Vytaflex, 30–50A; down to 20A for grip) around a 3D-printed hub | The current default — best traction and durability, any size/shape, hub features integrated. See below. |
| LEGO tire on a printed hub | LEGO “solid” or “ZR” tire glued to a TPU hub | Cheap, tough, good traction; harder than cast PU so it wears less — favoured for tangent drive. |
| Foam | Neoprene/EVA disc on a hub (Fingertech) | Very light, great shock absorption, but shreds when hit and has low grip unless coated with contact cement + liquid latex. Was the default; now less common. |
| Off-the-shelf | Banebots compliant wheels are about the only viable one | Heavier and more fragile than custom; needs a set-screw or clamp hub (never press-fit) and wheel armor. |
| Cleats | Thin spiked titanium/steel disc that digs into a wood floor | Highest traction on the right wood, but fragile and floor-dependent. Do not work on steel floors. Usually backed by a normal wheel. |
Control bots and pushers run the softest, grippiest wheel the durability allows; spinners accept a harder, tougher wheel. On steel-floor events, downforce magnets do more for traction than wheel choice.2
Casting polyurethane wheels
Section titled “Casting polyurethane wheels”3D-print a hub with a retaining feature (a gyroid-infill ring is the popular method), seat it in a printed mould with a centring pin, and pour equal parts (by weight) of Vytaflex A and B. Working time is ~10 minutes; cure overnight, or ~16 h, or accelerate at 60 °C in a filament dryer. Use Vaseline or a proper mould release, and colour with mica powder — most dyes interfere with the cure.2
Hub and axle
Section titled “Hub and axle”- Screw/clamp hub on a motor shaft (e.g. Repeat screw hub) or hex hub / hex shaft — standard on combat drive units; secure, unlike a press fit.
- Set-screw on a D-shaft — simple, can round off under shock.
- Bushing on a dead axle for indirect drive: a printed hub running on a brass or lubricated-nylon bushing around the axle; low-rpm plastic wheels can even run directly on a greased axle.
- Support a live axle on two bearings; a single-shear motor output shaft bends when the wheel is hit from the side. Protect it with a stub or armored pocket.3
References
Section titled “References”Footnotes
Section titled “Footnotes”-
Team Run Amok — “Optimum Gear Ratio Selection” and the Tentacle drivetrain calculator. ↩
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Ping, L., with L. Skotiniotis, I. Talbert & D. Tran. Beetleweights for Dummies (Fall 2025 revision) — builder-maintained beginner guide distributed via the NHRL Discord. Sections: Wheels & Hubs; Appendix I (Casting Custom Wheels). ↩ ↩2 ↩3
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Meggiolaro, M. A. RioBotz Combat Robot Tutorial, v2.0, PUC-Rio, 2009 (CC BY-NC-ND) — wheels, hubs, and axle support. ↩