Mecanum Wheel Set

What it is

A mecanum wheel is an ordinary wheel with a ring of free-spinning rollers bolted round its rim, each roller’s axle set at 45° to the wheel’s own. Bengt Ilon patented it in 1972 at the Swedish company Mecanum AB, which is where the name comes from, and warehouse robots have been quietly using it ever since.

The consequence of that 45° is the whole story. A normal wheel can push only along the direction it rolls. A mecanum wheel can push only along its roller’s axle — because the roller spins freely in every other direction and simply refuses to transmit force there. So one mecanum wheel’s ground force is pinned at 45° to the chassis, permanently, whatever the motor does.

One such wheel is useless. Four of them, in two left-handed and two right-handed pairs, give you four force vectors at alternating 45° angles, and the sum of four vectors can point anywhere on the floor and carry a twist. That is a holonomic base: three controllable degrees of freedom — forward, sideways, rotation — from four ordinary, un-steered motors.

Handedness, and why it is not optional

Mecanum wheels come in two mirror-image forms, usually sold as left (A) and right (B), or just marked L and R on the hub. A robot needs two of each, diagonally paired:

Diagonal corners share a hand. Buying four of the same wheel is a real and common mistake — a “set of four mecanum wheels” from a reputable seller contains two of each, but two singles ordered separately very often do not.

The X check

The reliable test needs no markings at all.

1. Put the robot on the bench, wheels down.
2. Stand over it and look straight down.
3. On each wheel, follow the line of the rollers you can see on top.
4. Those four lines should aim at the middle of the robot and draw an X.

If they draw a diamond — the O or rhombus configuration — swap two wheels diagonally and check again.

One subtlety that trips people up when they try to reason it out instead of looking: the rollers you see on top are mirrored from the ones touching the floor, because a roller at the top of the wheel is the same roller carried half a turn round. The rollers doing the pushing point the other way. It does not matter — the X you can see is still the right test, and it is the one everyone uses, precisely because you can see it.

Wiring four of them to an Arduino

Mecanum is a four-motor drivetrain, so a single L298N is not enough. Two dual H-bridges, or one four-channel board:

Two details that bite:

• Four PWM channels that share a timer will not all work. On an Uno there are six PWM pins across three timers, and the Servo library or tone() will steal one. A Mega or an ESP32 makes this a non-problem; on an Uno, check the timer map before you solder.
• Four encoders want four interrupt pins. An Uno has two. This is the single most common reason a mecanum build ends up on a Mega 2560 or an ESP32.

Minimal working code

The inverse kinematics is four lines. Everything else is plumbing.

// Mecanum inverse kinematics: a chassis command in, four wheel speeds out.
// x is forward, y is to the robot's LEFT, omega is counter-clockwise.
const float LX = 0.075;   // half the wheelbase, m
const float LY = 0.085;   // half the track, m
const float K  = LX + LY; // the only place geometry enters

void driveMecanum(float vx, float vy, float omega) {
  float fl = vx - vy - K * omega;
  float fr = vx + vy + K * omega;
  float rl = vx + vy - K * omega;
  float rr = vx - vy + K * omega;

  // Scale, do not clip. Clipping each wheel on its own keeps the speed and
  // throws the direction away, which is a bug you will chase for a week.
  float peak = max(max(fabs(fl), fabs(fr)), max(fabs(rl), fabs(rr)));
  if (peak > V_MAX) {
    float s = V_MAX / peak;
    fl *= s; fr *= s; rl *= s; rr *= s;
  }

  setWheel(FL, fl); setWheel(FR, fr);
  setWheel(RL, rl); setWheel(RR, rr);
}

setWheel() is where the velocity loop lives, if you have one. Open-loop it is just a PWM write, and open-loop mecanum drifts — see closing the loop on wheel velocity.

The numbers that matter

Three of them, and all three are geometry rather than anything you can buy your way out of.

That last row is what you are paying for. The first three are the bill.

Troubleshooting

Mecanum, or the alternatives?

The honest summary: a mecanum base is the right answer when the floor is flat and clean, space is tight, and the robot has to hold a heading while moving — a warehouse aisle, a competition field, a camera platform. It is the wrong answer when the robot has to push, climb, or know exactly where it is from wheels alone.

Where it is used

Drop these under a chassis and you are building the mecanum omnidirectional robot. Before you cut any metal, spend twenty minutes in the mecanum drive simulator — fitting a wheel backwards there costs nothing, and it is the failure you are most likely to build.