Aquaplaning
Aquaplaning (also water slippery ) describes the floating of the tire on the water film of a wet road surface . In this case, a water wedge is pushed under the tire contact patch and leads to loss of grip . At the moment of aquaplaning, no management and braking forces can be transferred to the road. As a result, the vehicle may skid.
history
Compared to the history of the automobile, aquaplaning is a relatively new term. Aquaplaning was only discovered in the early 1960s during development work in the tire industry. As a result, the physical fundamentals were researched and countermeasures developed. The initially exotic technical term became increasingly known to the general public through specialist magazines, TV spots ( The 7th Sense ) and other media. Today, knowing and avoiding aquaplaning is an important part of driving school education. As a frequent cause of accidents, aquaplaning is very important in accident research .
Unless expressly stated otherwise, the following sections refer to passenger cars on the road.
Emergence
Aquaplaning occurs most frequently in passenger cars on the road .
Physical and conceptual demarcation
Aquaplaning is not to be confused with the lower friction on wet roads, which makes the road feel “slippery” or “smoother” than a dry road. Tires generally have a lower coefficient of friction on a wet road than on a dry road, which is noticeable, for example, in the extension of the braking distance and affects all types of road vehicles.
Due to the lower friction in wet conditions, the maximum possible lateral acceleration (cornering speed limit) also decreases.
The synonym slippery water is misleading insofar as the other types of slippery roads, such as. B. slippery snow or frost are features of the road, whereas aquaplaning is more of a driving condition that only occurs when several factors come together.
Driving dynamics
Aquaplaning occurs when the water on the road surface that is displaced by the rolling tire can no longer drain off quickly enough. The stagnant water slides under the tire like a wedge and separates it completely from the road. With the onset of aquaplaning, the friction between the tires and the road becomes very low, which means that steering or braking forces can hardly be transferred to the road.
From a physical point of view, the inertia of the draining water and the lack of drainage in the tire (negative profile worn or too low for the speed) work together.
In the case of a vehicle with two axles, the following cases can essentially be distinguished:
- A single wheel floats up: As a rule, a yaw moment arises around the vertical axis, which the driver can control by counter-steering . Control of the vehicle is retained. However, the vehicle is at the physical limit, so that further disruptions (for example strong crosswinds, evasive maneuvers) can lead to a complete loss of control over the vehicle.
The ESP can be activated when the vehicle turns considerably , including traction control, engine drag torque control and possibly braking a wheel in order to stabilize the direction of travel specified by the steering wheel. It is usually not possible to identify which of these mechanisms is active, since all ESP activities in the instrument cluster use the same warning symbol. Some vehicles have active steering (e.g. superimposed steering), which can then also intervene. - Both front wheels float up first: this is the easiest case because the vehicle drives straight ahead and the longitudinal axis of the vehicle is also held in the direction of travel by the rear axle. As soon as the aquaplaning phase ends, the driver regains control of the vehicle.
- All wheels float up at the same time: Here it depends on the center of gravity and the even distribution of the wheel forces how the vehicle behaves. Uniform forces and a center of gravity on the longitudinal axis of the vehicle lead to the same behavior as with floating front wheels.
Since there are probably deviations from the ideal distributions, the vehicle will turn more or less quickly from the direction of travel. There is a risk of loss of control if, at the end of the aquaplaning phase, the vehicle is either standing at right angles to the direction of travel or is turned so that it starts moving in a different direction and skids when the driver countersteers. - Both rear wheels float up first: Here the vehicle immediately begins to skid. Even if the aquaplaning is over, the vehicle is presumably in a position in which the driver can hardly regain control.
In addition to the recommendation to generally reduce the speed when it is wet, the driving dynamics aspects also result in the recommendation to use the tires with the best profile on the rear axle. If aquaplaning unexpectedly occurs, it starts on the front axle and the driver can best prepare for the dangers.
Influencing factors
Various factors are involved in the development of aquaplaning:
- Driving speed : the higher the vehicle speed , the greater the risk of aquaplaning; The vehicle speed is the only factor that the driver can immediately influence if there is a risk of aquaplaning.
- Thickness of the water film : The greater the height of the water film, the greater the risk of aquaplaning. The height of the water film on the road is the result of the dynamic equilibrium between the amount of falling water (precipitation) and the amount of water that can flow off the road.
- Type of roadway : The type and construction of the roadway affect the runoff of water, for example through the incline and curvature of the roadway. Water can accumulate in depressions or turning points of S-curves, road damage such as ruts or depressions lead to a locally strongly fluctuating thickness of the water film. Multi-lane roads lead to an increased build-up of the water film.
- Road surface : The choice of building material for the surface course and cavities can influence drainage. Little drainage on sealed surfaces increases the risk of aquaplaning. This aspect is taken into account when planning and building the roads.
- Tire tread depth : With the negative tread (proportion of the gaps between the tread blocks) and greater tread depth of the tire, the risk of aquaplaning decreases. The negative profile is mainly influenced by the tire manufacturer and the tread depth is influenced by the state of wear of the tire.
- Tire width : Wide tires are somewhat more prone to aquaplaning than normal or narrow tires. Thanks to sophisticated tread geometries, tire manufacturers have managed to keep the risk of aquaplaning within reasonable limits, even with wider tires. This only applies to a "proper" tread depth. It is very critical when wide tires with a small tread depth are used.
- Tire pressure : Insufficient tire pressure increases the risk of aquaplaning. When the tire pressure is low, the contact area between the tire and the road increases, while the "contact pressure" (force per area) between the tire and the road is reduced - a wedge of water can slide under the tire contact area more easily. Low tire pressure has another unfavorable effect: The tire contact area is slightly curved inwards. The lateral displacement of the water is made more difficult and the risk of aquaplaning increases.
- Tire make and type (summer / winter) : Tire tests show that the various tire manufacturers and makes perform quite differently when it comes to aquaplaning. The wet behavior - as described above - or the rubber compound has little influence, but the profile shape and the proportion of the negative profile does.
- Shock absorbers : worn shock absorbers can increase the risk of aquaplaning. Functioning shock absorbers ensure that the force with which a tire is pressed onto the road surface remains as uniform as possible in all driving situations. If this is not fulfilled, i.e. if this force is reduced even for a short time, aquaplaning can occur.
- Vehicle weight : The influence of vehicle weight on the risk of aquaplaning is relatively small. The unnecessary increase in the vehicle weight does not represent a sensible measure for reducing the risk of aquaplaning. A fully occupied and laden car only has a slightly lower risk of aquaplaning if the tire pressure has been increased accordingly.
Driving safety
Preventive measures
Aquaplaning can be avoided by the driver in a specific driving situation through driving style and driving speed. Regardless of a specific driving situation, the tire profile can be used to prevent it.
As explained in the Driving Dynamics section, the tires with the best drainage (if the tires are the same, those with the deepest profile) should be mounted on the rear axle:
- Especially for front-wheel drive cars with the same tire sizes on the front and rear axles, economy and safety can be combined if two new tires are mounted on the rear axle and the existing tires on the rear axle are replaced on the front axle.
- In vehicles with even tire wear between the front and rear axles, the advantage of the deeper profile on the rear axle can be realized by replacing the tires on the rear axle after half the useful mileage of the first set of tires. The aforementioned tire change strategy of the front-wheel drives can then be followed.
For vehicles with a low annual mileage, the first two tires should be changed a few years before the target maximum tire age is reached in order to avoid major differences between the age of the tires on both axles and thus the properties.
Experience has shown that narrower tires are less sensitive to aquaplaning and are more cost-effective; especially in winter operation, narrower tires can result in further advantages.
Countermeasures against aquaplaning
If aquaplaning occurs, dangers can be mitigated by various measures:
- Avoid braking: A braked wheel can lock up while floating. The wheel can no longer roll over the water wedge, which can extend the aquaplaning phase and thus the loss of control over the vehicle. If the locked wheel comes into contact with the ground again after aquaplaning, one-sided aquaplaning can lead to a yaw moment and cause the vehicle to swerve to the side.
- Avoid strong counter-steering during the aquaplaning phase. As soon as the wheels come back into contact with the ground, the vehicle could start moving in a different direction, which the driver intuitively responds to by making further corrections and thus causing the vehicle to skid.
- Disconnect your foot from the accelerator and, if possible, the power flow (depress the clutch, automatic in N), wait until the aquaplaning ends.
- Hold the steering wheel straight and only slightly counter-steer if you drift sideways.
Common errors
- “ABS, ESP and all-wheel drive help against aquaplaning.” It is true that ABS and all-wheel drive cannot prevent aquaplaning. If the vehicle floats up, the ESP can only help insofar as it throttles the engine output against the will of the driver. Only after the end of aquaplaning can ESP help maintain or regain driving stability.
- “There is no risk of aquaplaning for a motorcycle.” It is true that you can also have aquaplaning with a motorcycle and that this inevitably leads to a fall.
- “In heavy rain, you should always orientate yourself to the speed of the other road users.” It is true that the speed at which aquaplaning occurs can vary greatly from vehicle to vehicle.
- “Aquaplaning only occurs at speeds above 80 km / h.” It is true that when several unfavorable factors come together, aquaplaning can occur well below 80 km / h.
- “The best tires have to be on the front axle.” Occasionally, this is still widespread advice in the tire trade and especially recommended to owners of front-wheel drive. It is correct to always mount the best tires on the rear axle in terms of grip and drainage.
Effects on other types of vehicles
- Trucks and buses : Various sources state that heavy trucks cannot cause aquaplaning due to their high weight. The main reason is the lower speed compared to a car. The speed limits for coaches are higher, so that the risk of aquaplaning increases.
- Motorcycles : There are two properties of motorcycles that reduce the risk of aquaplaning: the smaller tire width compared to passenger cars and the round tire cross-section - this makes it easier for the water to be drained to the side. However, motorcycles also have properties that increase the risk of aquaplaning: The vehicle weight is much lower than that of cars. In addition, the front wheel is more at risk from aquaplaning than the rear wheel.
- Airplanes : Due to the high speed when landing on wet runways, aquaplaning can occur.
- Since the pilot has braking and steering options that do not require traction, other circumstances usually have to arise in order for a dangerous situation to arise, such as on Lufthansa flight 2904 .
Web links
Individual evidence
- ^ A b Karl-Heinz Dietsche: Kraftfahrtechnisches Taschenbuch . Vieweg + Teubner Verlag, Wiesbaden 2011, ISBN 978-3-8348-1440-1 , pp. 435 .
- ↑ Aquaplaning. on: kfz.net
- ^ From the Wikipedia list of aviation disasters 1976 to 1990 October 8 - Athens, Greece. A Swissair Douglas DC-8 shot over the runway in heavy rain and aquaplaning. 14 people died.
- ↑ Boeing slipped off the runway. on: spiegel.de , March 22, 2008.