Mecanum Wheel AGV Keeps Drifting? Five Causes, in the Order to Check Them

The AGV docks fine on Monday. By Thursday it is arriving twenty millimetres off, then fifty, and someone has started blaming the wheels. It is almost never the wheels. A Mecanum platform steers by making four wheels turn at four different speeds, so a drift is a speed disagreement showing up as a heading error. Replacing a wheel treats the symptom and leaves the cause in place.

Here are the five causes that account for most drift complaints, arranged by what to check first, plus the one test that tells you whether to look at the motors or at the floor.

Why Mecanum platforms drift at all

A Mecanum wheel carries a ring of rollers set at 45 degrees to the wheel axis. Each roller rolls forward and slides sideways at the same time, and the two components add up to a diagonal thrust. Four wheels, four diagonals, and the vector sum decides whether the platform goes forward, sideways, diagonally, or spins on the spot.

That is the whole mechanism, and it is also the weakness. The vector sum only points where you want it if all four thrusts are correct. If one wheel runs even slightly fast or slow, its diagonal changes length, the sum rotates, and the platform yaws. There is no steering linkage to absorb the error. On a differential-drive truck a small speed error is corrected by the casters and the geometry. On a Mecanum platform it goes straight into the heading.

So the first question is never "which wheel is bad". It is "which of the four is not keeping up".

The three-metre test that splits the problem in half

Before touching a wheel, run the platform empty down a marked straight line on the flattest floor you have, and measure the lateral deviation after three metres. Then load it to its normal working weight and repeat on the same line.

This one test saves the most time of anything in this article. It tells you whether to open the electrical cabinet or walk the aisle with a level.

Cause 1 — Four motors turning at four different speeds

This is the most common cause, and the least visible. Four motors of the same model still differ slightly in speed, and four drives of the same model still differ slightly in how they interpret a command. Add encoder signal interference and the differences grow.

What to check, in order:

1. Commanded speed versus actual speed. Read back each drive's actual speed at a steady command. If the drives report speeds that differ by more than a couple of percent, the platform will visibly drift over a few metres. That threshold is an industry rule of thumb, not a standard figure, so treat it as a starting point rather than a pass mark.
2. Drive parameters. Copy the parameter set from one drive to all four. A single mismatched acceleration ramp is enough to skew the heading during every start.
3. Encoder wiring and grounding. Route encoder cable away from motor power cable, and ground the shield at one end only. Interference on one encoder reads as a speed error on one wheel, which is exactly the pattern that produces a slow, consistent drift.
4. Wheel diameter. A worn or replaced wheel with a smaller effective diameter travels less per revolution. With four wheels commanded to the same rpm, the smaller one drags the platform toward its side.

Cause 2 — Rollers worn unevenly

Rollers are the wear part of a Mecanum wheel. The wheel does not fail, it loses diameter and grip at the contact patch, and the loss is never even across four corners. A docking zone that always approaches from the same direction wears one side of one wheel faster than the rest.

The practical rule: replace rollers as a set, never one wheel at a time. A new wheel next to three worn ones has a larger effective diameter, so it travels further per revolution and pushes the platform toward the worn side. That is the same failure you were trying to fix, now caused by the repair.

Check by standing the platform on a flat plate and measuring from the plate to a fixed point on each wheel hub. A difference across the four tells you how much diameter you have lost. Also look at the roller surfaces: flat spots, chipped edges, and a polished glaze all reduce grip before the diameter changes enough to measure easily.

Cause 3 — Wheels in the wrong position

Mecanum wheels come in left-hand and right-hand versions, usually marked A and B, because the rollers must lean in the correct direction relative to the platform. Swap two of them and the platform does not drift gently — it rotates or refuses to strafe.

Two layouts dominate:

• O-type: rollers face inward toward the platform centre. The thrust vectors converge, which concentrates force and gives the strongest drive.
• X-type: rollers face outward. Slightly better stability, slightly less concentrated thrust.

What matters is consistency. Pick one layout, mark the corners, and keep the same A/B arrangement on every truck in the fleet. A spare wheel fitted in a hurry is the usual way this gets broken, and the symptom looks like a sudden, total change in behaviour rather than a gradual drift.

Cause 4 — The floor is doing it

Mecanum wheels strafe by sliding their rollers sideways. Anything that changes friction changes the heading. This is the cause owners resist most, and it is often the real one.

Three floor conditions matter:

• Slope. A Mecanum platform is sensitive to cross-slope. Even a one-degree fall across an aisle pulls the platform downhill while the drive holds a straight command. That figure is a rule of thumb from field experience, not a specification.
• Steps and gaps. A step at a slab joint makes one wheel climb while the others roll, which yaws the platform. Grating slots and tile joints wide enough to catch a roller do the same thing intermittently, which is why the drift looks random.
• Contamination. Oil, water, frost, and fine dust all cut the friction the rollers need to push sideways. A platform that drifts only in one bay, only after the floor scrubber has been through, is telling you the cause outright.

Check with a straightedge and a level across the aisle, not along it. Cross-slope is the one that ruins tracking, and it is the one nobody measures.

Cause 5 — Load sitting off-centre

Four wheels share the load, but only if the centre of gravity sits between them. Shift the load toward one corner and that wheel carries more, its motor draws more current, and under a fixed speed command it lags. The platform steers toward the lagging wheel.

This shows up as drift that appears only with certain loads. Pallet positions, fork offset, and a battery moved during a retrofit all do it. If the empty run is straight and the loaded run drifts, and the floor is flat, weigh the corners or measure hub-to-floor height with the load on. The corner carrying more will sit lower.

Symptom to cause

Fixes that belong in the design, not in the maintenance log

Most of these causes can be designed out, and it is cheaper than living with a weekly alignment job.

Use encoders with enough resolution. A Hall-sensor-only setup gives coarse speed feedback. Incremental or absolute encoders give the drive something fine enough to hold four wheels together. Our Mecanum wheels are specified together with the drive package for this reason, not bought as a separate part.

Synchronise at the protocol level. Four independent drives running open loop will drift, no matter how precise the mechanics are. CANopen (CiA 402 drive profile) or EtherCAT lets one controller command all four axes on a common time base, which removes the accumulation error that open-loop control cannot see.

Specify a matched wheel set with a matched drive set. Roller wear is unavoidable, but buying wheels and drives from different suppliers means nobody owns the speed match. A matched set also stops you mixing connection types. The BM series uses keyway up to 12 inch and flange from 14 inch upward, which decides your shaft design before assembly, not after.

Keep a floor specification in the project documents. Flatness at joints, gap width relative to roller diameter, and a ban on oil and standing water are all cheap to write down at the design stage and expensive to discover during commissioning. The usual rule of thumb is that no gap should exceed a third of the roller diameter. The wheel you choose sets that limit. A BM20-500, at 8 inch diameter and 500 kg per wheel, has rollers of a known size, so the tolerable gap follows from the wheel rather than from a guess.

FAQ

Why does my Mecanum wheel AGV drift to one side?

Usually one of the four wheels is turning slower than the others, or its rollers have worn to a smaller effective diameter. Run the platform empty and loaded down the same straight line to see whether the cause is on the drive side or the load side before replacing anything.

Can I replace just one worn Mecanum wheel?

No. Replace rollers as a set. A new wheel beside three worn ones has a larger effective diameter, travels further per revolution, and pulls the platform toward the worn side.

Does floor slope really matter that much?

Yes, cross-slope especially. A Mecanum platform holds a heading by balancing four thrust vectors. A fall across the aisle adds a sideways force the drive has no way to detect unless it has position feedback.

Will a better controller fix the drift?

It fixes the speed-mismatch half of the problem, and only if the drives are synchronised on a common time base. It will not fix worn rollers, a wrong wheel position, or a sloped floor.

Conclusion

Drift on a Mecanum platform is a speed disagreement, and the platform has no steering gear to hide it. Run the three-metre test empty and loaded first. Then work through the five causes in order: four motors, roller wear, wheel position, floor, centre of gravity. Replace rollers as a set, keep one A/B layout across the fleet, and synchronise the drives at the protocol level.

Chasing a drift you cannot pin down? Send us the platform weight, wheel model, drive type, and a short video of the run, and we will tell you which of the five it looks like. Contact our engineering team.

Still choosing between drive types? Our Mecanum wheel vs steering drive unit comparison covers which one fits your aisle. For sizing the wheels themselves, see the Mecanum wheel buying guide.