Velocidade de Hidroplanagem

Hydroplaning speed: NASA formula explained

Hydroplaning (or aquaplaning) is what happens when a film of water lifts the tire clean off the road. The classic NASA formula from Horne & Dreher (1963) puts the critical speed at V ≈ 10.35·√P in km/h, with P being the tire pressure in psi. Take a tire at 30 psi as an example: V ≈ 10.35·√30 ≈ 57 km/h. Once you go past that, the grooves can't shove water out of the way fast enough and the tire starts to skim. Worn tires give up far sooner. A bald tire, or one down to the TWI — Tread Wear Indicator mark of 1.6 mm (the legal floor in Brazil), can hydroplane at city speeds when the rain is heavy.

Applications

It shows up in driver education and driving schools, in highway accident forensics, in fleet safety training and in everyday tire maintenance calls. Race engineers lean on it too, when they're dialing in tire pressures for a wet stint.

FAQ

What should I do if I start hydroplaning? Ease off the accelerator, keep the wheel pointed straight, and resist the urge to stamp the brakes or jerk the car sideways. Wait until the tires bite again.

Does higher tire pressure really help? It does, as long as you stay inside the manufacturer's limit. More pressure pushes the critical speed up and keeps the contact patch from deforming as much. Stick to the value on the door sticker.

What's the role of the TWI? The TWI is a little raised bar sitting inside the groove at 1.6 mm. When the tread wears down level with it, the tire counts as legally finished, and it'll hydroplane at much lower speeds than a fresh one.