Car Aquaplaning (hydroplaning) Explained
Aquaplaning or hydroplaning by the tires of a road vehicle, aircraft or roller coaster occurs when a layer of water builds between the wheels of the vehicle and the road surface, leading to a loss of traction that prevents the vehicle from responding to control inputs. If it occurs to all wheels simultaneously, the vehicle becomes, in effect, an uncontrolled sled. Aquaplaning is a different phenomenon from water on the roadway merely acting as a lubricant .Traction is diminished on wet pavement even when aquaplaning is not occurring.
Every vehicle function that changes direction or speed relies on friction between the tires and the road surface. The grooves of a rubber tire are designed to disperse water from beneath the tire, providing high friction even in wet conditions. Aquaplaning 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, direct road contact, and loss of control results. If multiple tires aquaplane, the vehicle may lose directional control and slide until it either collides with an obstacle, or slows enough that one or more tires contact the road again and friction is regained.
The risk of aquaplaning increases with the depth of standing water and the sensitivity of a vehicle to that water depth.
Water depth factors
- Depth of compacted wheel tracks and longitudinal depressions
- Heavy vehicles can cause ruts in the pavement over time that allow water to pool, negatively impacting draining.
- Pavement micro and macrotexture
- Concrete can be preferable to hotmix asphalt because it offers better resistance to rut formation, though this depends on the age of the surface and the construction techniques employed while paving. Concrete also requires special attention to ensure that it has sufficient texture.
- Pavement cross slope and grade
- Cross slope is the extent to which the cross-section of a road resembles an upturned U. Higher cross slopes allow water to drain more easily. Grade is the steepness of the road at a particular point, which affects both drainage and the weight of the vehicle. Vehicles are less likely to aquaplane while traveling uphill, and far more likely to do so at the trough of two connected hills where water tends to pool. The resultant of cross slope and grade is called drainage gradient or "resulting grade". Most road design manuals world wide require that the drainage gradient in all road sections must exceed 0.5%, in order to avoid a thick water film during and after rainfall. Areas where the drainage gradient may fall below the minimum limit 0.5% are found at the entrance and exit of banked outer curves. These hot spots are typically less than 1% of the road length, but a large share of all skid crashes occur there. One method for the road designer to reduce the crash risk is to move the cross slope transition from the outer curve and to a straight road section, where lateral forces are lower. If possible, the cross slope transition should be placed in a slight up- or downgrade, thereby avoiding that the drainage gradient drops to zero. The UK road design manual actually calls for placing the cross slope transition in an artificially created slope, if needed. In some cases, permeable asphalt or concrete can be used to improve drainage in the cross slope transitions.
- Width of pavement
- Wider roads require a higher cross slope to achieve the same degree of drainage.
- Roadway curvature
- Rainfall intensity and duration
Vehicle sensitivity factors
- The driver's speed, acceleration, braking, and steering
- Tire tread wear
- Worn tires will aquaplane more easily for lack of tread depth. Half-worn treads result in aquaplaning about 3-4 mph (5–7 km/h) lower than with full-tread tires.
- Tire inflation pressure
- Underinflation can cause a tire to deflect inward, raising the tire center and preventing the tread from clearing water.
- Tire tread aspect ratio
- The longer and thinner the contact patch, the less likely a tire will aquaplane. tires that present the greatest risk are small in diameter and wide.
- Vehicle weight
- More weight on a properly inflated tire lengthens the contact patch, improving its aspect ratio. Weight can have the opposite effect if the tire is underinflated.
- Vehicle type
- Combination vehicles like semi-trailers are more likely to experience uneven aquaplaning caused by uneven weight distribution. An unloaded trailer will aquaplane sooner than the cab pulling it. Pickup trucks or SUVs towing trailers also present similar problems.
There is no precise equation to determine the speed at which a vehicle will aquaplane. Existing efforts have derived rules of thumb from empirical testing In general, cars aquaplane at speeds above 53 mph (72 km/h), where water ponds to a depth of at least 1/10 of an inch (2.5 mm) over a distance of 30 feet (9 meters) or more.
In motor vehicles
What the driver experiences when a vehicle aquaplanes depends on which wheels have lost traction and the direction of travel.
If the vehicle 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 have no effect.
If the drive wheels aquaplane, there may be a sudden audible rise in engine RPM and indicated speed as they begin to spin. In a broad highway turn, if the front wheels lose traction, the car will suddenly drift towards the outside of the bend. If the rear wheels lose traction, the back of the car will slew out sideways into a skid. If all four wheels aquaplane at once, the car will slide in a straight line, again towards the outside of the bend if in a turn. When any or all of the wheels regain traction, there may be a sudden jerk in whatever direction that wheel is pointed.
Control inputs tend to be counterproductive while aquaplaning. If the car is not in a turn, easing off the accelerator may slow it enough to regain traction. Steering inputs may put the car into a skid from which recovery would be difficult or impossible. If braking is unavoidable, the driver should do so smoothly and be prepared for instability.
If the rear wheels aquaplane and cause oversteer, the driver should steer in the direction of the skid until the rear tires regain traction, and then rapidly steer in the other direction to straighten the car.
Prevention by the driver
The best strategy is to avoid contributors to aquaplaning. Proper tire pressure, narrow and unworn tires, and reduced speeds from those judged suitably moderate in the dry will mitigate the risk of aquaplaning, as will avoidance of standing water.
Electronic stability control systems cannot replace defensive driving techniques and proper tire selection. These systems rely on selective wheel braking, which depends in turn on road contact. While stability control may help recovery from a skid when a vehicle slows enough to regain traction, it cannot prevent aquaplaning.
Because pooled water and changes in road conditions can require a smooth and timely reduction in speed, cruise control should not be used on wet or icy roads.