Wheels

Wheels and tires are extremely important in combat robotics. They're how your robot moves (translation), how much it can push (traction), and – because they usually stick out – one of the easiest parts for an opponent to attack. This page covers the common types of wheels, how to mount and protect them, and how to make your own. For the motors and gearing behind them, see Drive Systems.

Why wheels matter

Traction limits pushing power. More motor torque only helps until the tires start to slip. The most a robot can push is roughly the weight on its driven wheels multiplied by the tires' friction coefficient – about 0.8 for rubber on a typical painted wood or steel arena floor.[1]
Translation – wheel size and gearing set your top speed and acceleration (see Drive Systems).
Easily attacked – exposed wheels are a favorite target for spinners. An opponent can follow a wedge that ends at the side of the robot "right down the side into the wheel."[2]

Types

Foam

Foam wheels are light, cheap and soak up impacts, which protects small gearmotors. When they're hit they usually just tear or lose a chunk, rather than breaking and leaving the robot stranded.[3]

FingerTech neoprene foam wheels (1–3 in diameter) stretch onto Twist Hubs.
Lite Flite and slimmer Lectra Lite foam wheels (made by Dave Brown Products, sold at Robot Marketplace) are a long-time favorite; FingerTech Lite Hubs press into them so you don't have to glue wheels to motor shafts.
Adding grip: coat foam with rubber cement and then liquid latex for more traction.[4] You can also cut your own wheels from EVA foam sheet with a hole saw.[5]

Molded (urethane, rubber and TPE)

Molded tires grip better than foam and last longer, but are heavier and pass more shock to the motor. Softer compounds (lower Shore A numbers, e.g. 20A) grip better; harder ones (40A–50A) resist tearing.

FingerTech Max Traction wheels (150 g class) use soft A20 urethane locked onto the hub so the tread can't peel off, and Sumo Wheels come in A20 or tougher A45 urethane.[6]
Repeat Robotics Rubber Flex Wheels (TPE, 20A–45A) for 1 lb and 3 lb robots.
BaneBots wheels (T40, T61 and T81 series) come in color-coded hardnesses from 10A to 60A, and are popular from 1 lb up to 30 lb.
Just Cuz Robotics sells cast urethane wheels for 3 lb robots.
Colson Performa wheels are a classic for 12 lb robots and up.[7]
LEGO tires are surprisingly popular in small robots, on custom or printed hubs. Repeat Robotics sells common sizes for 1 lb and 3 lb robots.[8]

Doing your own molding

Many builders cast their own tires by pouring liquid urethane into a 3D printed mold around a printed hub. The basic steps:

Design a mold with a slight draft so the tire comes out, and a peg or recess that holds the hub in the center.
Give the hub hooks, ridges or channels so the tire locks on mechanically; roughen and clean it for better adhesion.
Use mold release on the mold, but never on the hub.
Mix and pour carefully to avoid bubbles, which can become weak spots, and hold the hub down so it can't float.
Let it cure fully (often overnight).

Casting urethanes such as Smooth-On VytaFlex and ReoFlex come in a range of hardnesses. Softer tires grip better but wear faster. Detailed guides: EVAC – wheel casting and Lieverloo – casting combat robot wheels. See also 3D Printing for printing the molds.

Hubs and mounting

How the wheel attaches to the motor is a common failure point:

Set screws are simple and adjustable, but tend to loosen. To help: grind a wide flat on the shaft, use a flat-point (not cup-point) set screw, and add blue threadlocker.[9]
Clamp hubs squeeze the shaft and hold well; some can be adjusted to slip on a big hit, protecting the gearbox.[10]
D-shaft press fits are common on small gearmotors such as N20s.
Hex shafts (common in larger robots) can't slip at all.
Don't rely on screw threads cut into soft plastic like TPU to hold a hub.
Support the shaft. Gearmotor output shafts are a common failure point. Where possible, support the wheel on both sides with a bearing, and trim shaft ends flush so a hit doesn't land on the end of the shaft.[11]

Size and ground clearance

Larger wheels can stop a robot from getting stuck on its back, and are the most common way to make a robot invertible – wheels that stick out above and below the chassis keep driving either way up.
Wheel size also sets your ground clearance and the height of your wedge or weapon. Too much clearance lets wedges get under you; too little and you'll get stuck on arena seams. For a 4-wheel beetleweight on a steel floor, 1/4" of clearance is plenty and 1/8" is probably fine.[12]

Traction and arena floors

Sticky tires grip best when clean but pick up dust and debris. Clean your tires between matches, and a cut tread can help shed grit.[13]
Arena floors vary (painted wood, plywood, steel). On steel floors, traction magnets add downforce where the rules allow them – they're not allowed in the Plastic class.

Protecting

Enclosed / guards: put the wheels inside the chassis or behind guards, so only the bottom of the tire is exposed. Extend wedges and armor past the wheels.
Inversion: if the wheels stick out top and bottom, the robot can still drive after being flipped (see Self-Righting).
Ablation: foam wheels and sacrificial tires can take damage and keep working; carry spares and swap them between matches.

Common failures

Tread peeling or slipping on the hub – use hubs with locking features.
Foam tearing or losing chunks – carry spares.
Hub slipping on the shaft – see Hubs and mounting above.
Bent or broken shafts from side impacts – support the shaft and protect the wheel.