What you need to know about wheel balancing, tyre wear, and the annual MOT test.
Key point: Wheel balancing is not tested during the MOT. However, the uneven tyre wear caused by prolonged imbalance (cupping, scalloping) can reduce tread depth below the 1.6mm legal minimum, which IS an MOT failure.
Minimum 1.6mm across the central three-quarters of the tread, around the entire circumference.
Cuts, bulges, exposed cords, or significant damage that compromises structural integrity.
Tyres on the same axle must be the same size and type. Mixing cross-ply and radial on the same axle is a failure.
If your car has TPMS (mandatory on cars registered after November 2014), a lit warning light is an MOT failure.
Unbalanced wheels cause the tyre to bounce slightly as it rotates rather than rolling smoothly. Over thousands of miles, this creates cupped or scalloped wear: a repeating pattern of dips and high spots around the tyre circumference.
The problem for the MOT is that cupping creates uneven tread depth. A tyre might read 3mm of tread at one point, but only 1.4mm in a cupped area just centimetres away. The MOT tester measures at multiple points, and the lowest reading counts.
This is how a tyre that looks fine to a casual glance can fail the MOT: the cupped areas drop below 1.6mm while the rest of the tread is still well above the legal limit.
Common scenario
A driver with 15,000 miles of unbalanced front wheels arrives for MOT. The tyres show 3mm average tread depth, well above the 1.6mm minimum. But the cupped wear has created dips at 1.3mm to 1.5mm in several spots. The MOT fails on tread depth despite the tyres appearing to have plenty of life left. The driver then needs both new tyres and a wheel balance, costing £250 to £500 when a £40 rebalance six months earlier would have prevented the cupping entirely.
Run your hand around each tyre
Feel for the wavy dips of cupping. If the tread surface is not smooth and even, the balance has been off.
Check tread depth at multiple points
Use a 20p coin or tread depth gauge. Measure at 3 to 4 points around the circumference. If readings vary by more than 1mm, cupping may be present.
Look for missing balance weights
Check the inside rim lip for clip-on or stick-on weights. Missing weights confirm the balance has been lost.
Listen for increased road noise
Cupped tyres create a distinctive droning or humming noise at speed that gets louder as the cupping deepens.
Cupped tyres have reduced contact patch in the dipped areas, reducing grip on wet roads. Stopping distances increase measurably.
Uneven tread depth from cupping increases braking distances, particularly on wet or cold surfaces where grip margins are already reduced.
The constant vibration from unbalanced wheels accelerates wear on wheel bearings, shock absorbers, and suspension bushes, creating further safety concerns.