The Vehicle-Side Multipliers

The surface is only half the story. The handbook's own list — worn tyres, poor brakes, overloading — attacks the same f-budget from the vehicle's side.

Tread depth — a wet-weather organ. The legal minimum is 1.6 mm, but wet-grip performance degrades well before the tread is illegal. Tread exists to evacuate water from the contact patch; at 2 mm a tyre is legal and already measurably worse in rain than at 4 mm, with a lower aquaplaning threshold. Dry grip barely notices tread — it's the wet column where worn-but-legal tyres quietly move you down the friction ladder.

Pressure and heat. Under-inflation flexes the tyre's carcass, distorts the contact patch, and builds heat — and heat is what destroys tyres at highway temperatures. Wrong pressure in either direction shrinks effective grip. Check cold, and check before long runs.

Brake condition. Worn pads, aged fluid, or braking unevenly across the wheels all cut the deceleration your f-budget assumes. Poor brakes lengthen the braking stage directly — the handbook's point stands exactly as stated.

Load. More mass means more kinetic energy through the same brakes: longer stops and earlier fade. Plan for substantially increased braking distance when heavily loaded approximately 40% more for a heavy overload.