The operation of vehicles is prohibited if:

Explanation

In both cases — both a missing wheel bolt and tyres with different tread patterns fitted on the same axle.

Wheel bolts are a shared assembly: the driving, braking and cornering forces are divided among all of them. With one bolt missing, the share of the rest increases, the wheel develops micro-movement at its seat, the holes go oval and the remaining bolts loosen one after another and then shear. The end of that chain is the wheel coming off while moving; that is why the law allows no tolerance for "only one bolt".

Different tread patterns on the same axle come back to the same "same axle, same type" logic. Two different tread patterns mean two different friction coefficients, especially on a wet surface. Under hard braking the wheel with more grip slows more, and the difference in force creates a torque about the vehicle's vertical axis that swings the car round — at precisely the moment the driver's whole attention is on the obstacle ahead.

The summary to memorise: on one axle, tyres must match in size, tread pattern, construction type and winter/summer type; and the wheel-fixing assembly must be complete. Both items in this question come from these two rules, so the comprehensive answer is correct.

• This item really is a defect but leaves the answer incomplete; mismatched tread patterns on one axle are independently prohibiting. • This is correct but incomplete too; a single missing wheel bolt is enough on its own to prohibit operation.

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