Hydroplaning (road vehicle)
From Wikipedia, the free encyclopedia
Hydroplaning (aquaplaning) in a road vehicle occurs when a layer of water builds between the rubber tires of the vehicle and the road surface, preventing the vehicle from responding to control inputs. It becomes, in effect, an unpowered and unsteered sled.
Contents |
[edit] Causes of hydroplaning
The ability to control a vehicle depends entirely on the friction between the tire contact points and the road surface. Every vehicle function that changes direction or speed, from turning, to accelerating, to braking, depends on the interface of those small patches of rubber and the asphalt. The higher the friction, the greater the control.
An element between the tires and the road that reduces friction, then, will diminish control. If that element is nonfrictional, like water, the vehicle may lose control entirely.
The tread of a rubber tire is designed to remove water from beneath the tire, providing high friction with the road surface even in wet conditions. Hydroplaning occurs when a tire encounters more water than it can dissipate. Water pressure in front of the wheel forces a wedge of water under the leading edge of the tire, causing it to lift from the road. The tire then skates on a sheet of water with little, if any, friction, and loss of control results.
If all four tires hydroplane, the vehicle will slide until it either collides with an obstacle or until wheel road friction is regained.
The likelihood of hydroplaning increases with the speed of the vehicle and the depth of the water, and if the tire tread is worn, naturally low profile, or hampered by underinflation. Vehicle weight is an additional factor; lighter cars hydroplane more easily.
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 that effectively squeezes water out of the way. Speeds of 200 mph or more are necessary to achieve hydroplaning on narrow round-profile tires.
[edit] Cruise Control
Cruise control poses a special danger.
Hydroplaning requires great driver awareness, both of the inital condition and in skid recovery when traction returns. A great deal of information about the car comes from the throttle pedal. If the driver isn't touching it, he may not realize that two wheels have begun to hydroplane, and may not be able to act with sufficent speed to prevent the remaining two from doing so. Loss of control is then inevitable.
The action the driver must take is to disengage cruise control. This requires a button press or a tap of the brakes. The latter, if done without care, may upset the car. Both movements take additional time the driver may not have.
Contrary to internet lore, however, cruise control systems do not perpetuate hydroplaning. A car has no absolute sense of speed; it determines its speed by the rate of rotation of the drive wheels. If the drive wheels have no traction, the cruise control will throttle back the engine to whatever minimal power level would be required to rotate the tires at that speed if the car was suspended on a rack. Because this response is not immediate, the drive wheels may briefly spin faster until the system compensates. Regardless, the car will slow down.
[edit] Loss of traction in low water situations
Hydroplaning most often occurs when there are large volumes of water on a road surface. Even slight wetness on a road, however, can cause a car to lose traction. This effect differs from 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 contacting the road rolls over these miniature puddles, the rubber of the tire seals the edges of the pits. Because water does not easily compress, each pit essentially has a barrier over it that prevents the rubber from pressing into it. The result is a reduction in traction. A complete loss of control, however, is unlikely.
[edit] How to deal with hydroplaning
[edit] Car Response
What the driver experiences when a car hydroplanes depends on which wheels have lost traction and the direction of travel.
If the car is traveling straight, it may begin to feel slightly loose. If there was a high level of road feel in normal conditions, it may suddenly diminish. Small correctional control inputs will be ignored by the car.
If the drive wheels hydroplane, there may be a sudden audible rise in engine RPM and indicated speed as they begin to spin. This will occur only briefly if the car is in cruise control.
In a broad highway turn, if the front wheels lose traction, the car will suddenly begin to drift into an outer lane. If the rear wheels lose traction, the back of the car will begin to slew out sideways into a skid. If all four wheels hydroplane at once, the car will slide to an outer lane. When any or all of the wheels regain traction, there may be a sudden jerk in whatever direction that wheel is pointed.
[edit] Recovery
To recover, never turn the steering wheel of the car or apply the brakes. Either action could put the car into a skid from which recovery would be difficult or impossible.
Instead, with no change in steering input, ease pressure off the gas; control should then return. If braking is unavoidable, use light pumping actions on the brakes until hydroplaning has stopped.
[edit] Prevention
The best strategy remains not to hydroplane in the first place. The risk greatly increases above 60 MPH, and in areas with any apparent standing water. Those with performance vehicles with wide tires and summer tread should take additional precautions. Driving in a lower gear can help one determine if the car has begun to float.
Stability control systems like ESC and DSC should not replace proper driving technique. They rely on the same braking mechanism at the driver's disposal, which in turn depends on road contact. While stability control may help recovery from a skid when the vehicle slows enough to regain traction, they cannot prevent hydroplaning.
[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.
- Smart Motorist - Driving in the Rain
