We have all felt the phenomena while driving on wet road when you lose control of your car. Sometimes it may just be for a second, in more severe cases it causes accidents; this phenomenon is called hydroplaning. Hydroplaning occurs when enough water pressure is built up in front of your tires that it pushes itself underneath the tires so your car is riding on water.
There are several different ways for this to happen: Dynamic, Viscous, and Reverted Rubber Hydroplaning. The two main types of hydroplaning that can be distinguished are Dynamic and Viscous hydroplaning. In general, both types of hydroplaning can occur at the same time. We will focus on dynamic hydroplaning here because it is the most important. When the water pressure reaches the weight of the car, it will force itself under the tires, causing you to lose steering and the usage of your brakes. Next, viscous hydroplaning can occur at a much slower speed than dynamic due to the combination of the viscous properties of water and the asphalt. Last, but certainly not least, we have reverted rubber hydroplaning. This kind of hydroplaning occurs when you “lock” up your brakes. The friction between the tires and asphalt heat up the rubber and cause it to flatten out or melt which acts as a seal to keep water from escaping underneath the tire. The sealed water is then heated by the rubber and turns to steam which lifts the tire off the ground.
The effects of hydroplaning are huge. The most well known situation where hydroplaning occurs is probably a automobile on a wet roadway. As a vehicle is traveling down a road, it hits a relatively deep puddle at a relatively fast ( 50MPH+) velocity and loses nearly all or all of its contact with the roadway due to the effects of the water pressure on the tire as discussed previously. In many cases, as the driver no longer has way to control the car, this results in an accident which can be severe due to the speed involved, as the vehicle will be incapable of slowing during hydroplaning. Further compounding the issue is the fact it is often difficult for a driver to perceive the depth of the water, and they will therefore assume they can drive through it and will not realize their mistake until it is too late.
A more specific example of the situation above is a motorcycle hitting a relatively deep puddle. In this case, the effects are worsened as compared to an automobile due to several factors, including the smaller number of contact patches (2 instead of 4), which leads to a higher chance of near-total loss of traction, and typical style of tire which has much less tread than the typical automobile tire.
A lesser known situation, however, is that of an airplane on a wet runway. In this case, the high speed involved means that the pressure of water required to have an effect is much lower than what is required in the case of an automobile. As such, hydroplaning has been the subject of much research by NASA in an attempt to both understand the relationship between the water pressure and the vehicle’s speed/tire pressure and come up with solutions to prevent it. This research, however, is not only relevant to the conditions NASA focused on but also the conditions you may encounter on the roadway.
An interesting part of this research is the so called “NASA Equation”, which has been used extensively since the time it was developed in 1962. It gives an estimate for the speed at which a tire will hydroplane, given the speed of vehicle and the tire pressure. The equation is shown as :
where \(v_p\) is the speed at which hydroplaning begins in knots and P is tire pressure in psi. This equation is under the assumption that either the tire is treadless/slick or that the water depth is greater than the depth of any tread on the tire, as nominally if the depth of water is less than the depth of the tread the tread will prevent hydroplaning. (Guo 2013) From this equation, you can see that as you increase the tire pressure the speed at which hydroplaning occurs increases.
In more modern research, an equation was also developed which integrates physical characteristics of the tire into the equation. By this new equation, it can be shown that as tire contact patch width decreases and length increases, the hydroplaning speed increases. These factors play a large part in the speed at which hydroplaning occurs, perhaps equal to the effect of tire pressure itself.