Skylaw

I have often seen people discuss "hydroplaning" - some without really knowing what it is, and most without a useful way of knowing how to prevent it. I hope this information saves some lives.

Hydroplaning occurs when you enter water that is deeper than your tread depth, at a speed that causes the tire to lift off the road surface. When that happens, you are literally traveling on the water, not the road, much like a water skier (but with far less control). Tire design that allows water to escape the tread can help, but will not prevent, this problem. When you encounter the conditions, there is only one of the factors you can control: speed. Fortunately, the speed at which you will begin to hydroplane is predictable.

I learned the following formula in 1966, during Air Force pilot training, in "Flying Safety" magazine. It was used to predict the speed at which aircraft tires would hydroplane on takeoff or landing. I have tried it on my cars (with much lower pressure tires than the aircraft I flew) for years, and it works.

You will hydroplane when you enter water deeper than your tread depth, at a speed (in mph) equal to 9 times the square root of your tire pressure in psi.

Yes, I know. How useful is that for those not mathematically inclined? First, read the Tire Rack's superb discussion of tire pressure on their site. Since I run my tires at a cold pressure of 32 psi, their hot pressure is approximately 36. The square root of 36 is 6; 9x6 is 54. My hydroplane speed is 54 mph for that tire pressure. At that speed or any speed above that, my tires will hydroplane in water deeper than my tread depth. That's not much water.

Similarly, if you run low pressure tires, the square root of 25 (psi) is 5, and 9x5 yields a 45 mph hydroplane speed. For tire pressures between those, you can come fairly close with a straight-line extrapolation of approximately 1 mph per psi (Mathematicians know it's not a straight line; but to be safe, reduce the resut by 1 mph and you'll be on the safe side). I have not ever experienced a variation of more than 1 mph in hydroplaning speed, based on the formula.

Thus, for cold tire pressures between 21 and 32 psi (25 and 36 psi hot) the safe speed that will keep your tires on the road surface when you see puddled water is between 44 and 53 mph (higher if you run higher pressure tires), depending on your tire pressure. You can apply the basic formula for different pressures.

This calculation does not affect the loss of traction because of a wet road, especially one where oil has accumulated over a dry period. You still have to slow down for that; such loss of traction is not hydroplaning in the truest sense (some would argue that it is, because when the tire slides, rubber melts and the tire rides above the road surface on a thin, slippery layer of melted rubber). Still, the formula provided deals only with puddled water; and it works (I have tried it with high-pressure aircraft tires and all of my cars since 1966).

But if you try it, be sure to keep one side of the car out of the water - please!

Dvinn

Nice article, very informative.

I wonder if the formula will be different to tires designed more towards wet traction like the Dunlop sp9000 & sp5000 VS. summer tires like Michelin pilot sport?

Skylaw

Thanks-

I really don't know if the tread design would make much difference in the liftoff speed once the water depth exceeds tread depth. My suspicion is that newer wet traction designs might permit a slightly deeper puddle before the tire lifts off the road - but the liftoff speed would be the same.

This formula resulted from a very extensive analysis that I no longer have after nearly 40 years (it wasn't my work). It dealt with different types of aircraft tires, and I have found it acccurate for radial and bias ply auto tires, including wide-track (when they were popular). It is not dependent on vehicle weight (it applied to aircraft ranging in weight from about 3500 lb to 500,000 lb), and I have found it true for cars from 2800 to 4400 lb in my own experience.