Hydroplaning (road vehicle)
From Wikipedia, the free encyclopedia
Hydroplaning (sometimes aquaplaning) in a road vehicle is an effect similar to planing in a boat. A layer of water between the rubber tires of a road vehicle and the road surface (or between airplane wheels and the runway) reduces the friction with the tires. This causes the vehicle to act like an unpowered and unsteered sled.
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[edit] Causes of hydroplaning
The engine provides power through the wheels to provide a controlled speed. The wheels also provide steering and breaking through the driver controlling the angle of the wheels relative to forward motion, and through the operation of the brakes. The tread in a rubber tire is designed to remove water from beneath the tire, providing high friction with the road surface even in wet conditions. This enables friction between the wheel and the road, allowing the wheel to rotate, and to provide rolling resistance, braking and steering power.
Wheeled vehicles are designed to operate properly when there is friction between the rotating wheel surface and the road. Any frictionless substance can force a vehicle to hydroplane, should the substance separate the tires from the road.
In a typical hydroplaning situation, increasing water pressure in front of the wheel means that the amount of water being dispersed by the tread is less than the amount being forced under the wheel. A wedge of water is forced under the tire which is lifted on a sheet of water.
The vehicle then loses braking, steering and power to the drive wheels because of loss of wheel contact with the road. The result is complete loss of normal control by the driver, and the vehicle will slide until it either collides with an obstacle or until wheel road friction is regained. The likelihood of hydroplaning increases if the speed of the vehicle is high, the vehicle is imbalanced, the tire is underinflated, has worn tread or the water is deep.
Hydroplaning can be especially dangerous while the car is in cruise control, as it tries to maintain the speed of the drivetrain. If the vehicle hydroplanes at that speed, then the cruise control will be trying to maintain the hydroplaning. Additionally, the lack of pedal feel increases the reaction time, allowing the time for the vehicle to advance the hydroplaning to all wheels so quickly that it may spin out of control before the driver can react. The problem with cruise control is that it tries to maintain the speed of your drivetrain regardless of tire to road contact. If you have traction control (or a posi rear or a solid shaft) the car will try to evenly maintain its set speed (transmission output) with varying degrees of success. If you have a typical drivetrain the engine output will go to the tire with the least friction and cause the vehicle to slow down while that tire alone spins. When the vehicle slows enough to regain traction the drivetrain will slow due to this resistance. The cruise control will then then increase the engine speed to regain the drivetrain cruise control setting. Depending on whether one or all of the tires lose grip this can cause the vehicle to veer into the larger traction side. The same problem can occur if the driver uses either gas or brake under uneven traction. When braking, the car will veer to the high traction side.
Two- or three-wheeled vehicles with round-profile tires, such as bicycles and motorcycles, virtually never suffer from hydroplaning in normal road use. The contact area with the road is a canoe-shaped patch which effectively squeezes water out of the way. Speeds of 200 mph or more are necessary to achieve hydroplaning on narrow round-profile tires.
[edit] Loss of traction in low water situations
Hydroplaning occurs when there are large volumes of water on a road surface. However, even slight wetness on a road can cause a car to lose traction. This effect, however, is different from what happens in hydroplaning.
Tires maintain traction on the road by using a mechanism called bulk friction, where the rubber of the tire pushes down into tiny pits in irregularities of the road surface. When a road becomes slightly wet, water can fill these pits such that the water tops them off without overflowing. As the narrow strip of tire that is in contact with the road rolls over these miniature puddles, the rubber of the tire seals the edges of the pits. By its nature, water can not be easily compressed, so each pit in essence then has a barrier over it preventing the rubber from pressing into it, thus reducing traction and leading to a reduction (but not necessarily complete loss) of control of the vehicle.
[edit] What to do when hydroplaning
When hydroplaning, you should never turn the steering wheel of the car or apply the brakes. Taking either action could cause the car to skid and possibly result in total loss of control.
Instead, keep travelling in a straight direction and ease the pressure on the gas; control should then return. If braking is unavoidable, light pumping actions on the brake should be used until hydroplaning has stopped.
[edit] References
B. N. J. Persson, U. Tartaglino, O. Albohr And E. Tosatti (2004). "Sealing is at the origin of rubber slipping on wet roads". Nature Materials 3 (7 November): 882–885.
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