of cars ...

Literally referring to motion across the surface of water, hydroplaning is a phenomenon often observed in cars driving quickly on wet roads just after it has started raining. At some critical balance between the weight of the car and its speed, the car tire loses molecular contact with the road and a thin film of water separates the two. Now the car is skimming on a (virtually) frictionless surface, making steering ineffective. One can quickly lose control of the vehicle.

of water on itself ...

Another interesting phenomenon of water, related to the physics that causes hydroplaning is the ability for beads of water to skim across a surface of a larger body of water without getting sucked in. If you've ever run a faucet into a full sink or a big pan of water, you may notice short-lived balls of water skittering across the surface away from the falling water. They exist long enough that you can be sure they're not bubbles. They move very fast and then vanish. In a way, these beads of water are hydroplaning as well. A thin cushion of air (optically determined to be on the order of nanometers) separates the bead from the water. This cushion is maintained as long as the water is moving quickly - similar to a piece of paper moving quickly and practically frictionlessly across a desk. Experiments have been done to test this hypothesis where the surface of water is pumped with a low amplitude vibration. This vibration maintains the separation between the surface and the bead, and drops of water have been maintained on the order of minutes.

Because water has such a high surface tension, it tends to minimize its surface area. Thus, small volumes of water will form spheres. The interaction for this surface to be disrupted require molecular contact between water on the bead surface and on the bulk surface. An alternate theory to the air-cushion hypothesis is that electrostatic repulsion maintains the separation while the bead is moving quickly. As it slows down, water can then reorient and allow cross body interactions, thereby assimiliting the bead into the bulk water.

Definitions

Aquaplaning is a very serious issue, leading to many road accidents and deaths each year, yet it is relatively easily avoided: slow down and change your tyres when the treads get too shallow.

First, there is a lot of confusion among motorists about the difference between wet grip and aquaplaning. Let me try to clarify:

Wet grip is the grip between the tyre and the road when the road surface is wetted, but the tyre remains in contact with the road, and the two can react against each other. Wet grip remains more or less constant at all speeds.

Aquaplaning performance is the ability of the tyre to clear standing water from beneath the tyre, so that the rubber is in direct contact with the (wetted) road surface. Aquaplaning performance declines markedly at higher speeds

The wet grip of a tyre is defined only by the rubber compound, and has nothing to do with the tread pattern, or depth of tread, except insofar as there is a different type of rubber underneath the main tread. So that when the entire tread is worn away, the compound in the sub-tread offers limited grip in the wet.

Aquaplaning performance of a tyre is defined almost entirely by the tread pattern and the depth of the tread grooves. It has nothing to do with the rubber from which the tyre is made.


Explanation

Aquaplaning is the condition in which the weight acting on the tyre (or half-shaft) is borne entirely on a film of water, with no direct contact between the tyre and the road surface. In this condition, the tyre is not in contact with the road surface and there is no exchange of lateral forces between tyre and road. Thus the tyre offers no grip at all in either the lateral (steering) or longitudinal (braking/accelerating) direction.

Once a tyre is aquaplaning, there is nothing to be done, except wait for the vehicle to slow down and for the tyre to regain contact with the road surface. It is essential for effective control of the vehicle that when the tyre does settle back onto the road surface, it is aligned with the direction of motion. If the tyre is pointing in a different direction from the motion of the vehicle, then any driver (even an expert) will have great difficulty in controlling the vehicle. When grip returns (usually very suddenly), the tyres will immediately force the vehicle to change direction, and this will frequently cause either the front or rear tyres into a lateral skid.

Aquaplaning happens because the ability of the tyre tread to pump water away from the contact patch is less than the amount of water it encounters during its forward motion.

A tyre tread pattern is designed to pump as much water as possible away from the contact patch. The deeper the tread grooves, the more water the tyre can cope with. However, if the grooves are shallower than the depth of water on the road, then it becomes increasingly likely that a film of water will build up underneath a tyre, eventually lifting the tyre entirely off the road surface, in much the same way that a hydrofoil can skim across the water on only a tiny bearing surface.

Tyre companies design the tread pattern using computers to model the flow of water under the tyre contact patch, and test them with high speed photography. They use a laboratory built underneath a test track. A camera is positioned under a glass plate built into in the road surface. The plate and surrounding road are covered in a defined depth of water. The car then drives through the puddle and over the plate at a pre-determined speed. The photographic apparatus is designed to trigger the flash and shutter at the instant the tyre is above the lens, producing an image of the tyre as it passes through the puddle. Fluorescein dye is mixed with the water to highlight the areas of rubber, air and liquid.

Using this technique, the researchers look at the amount of rubber actually in contact with the glass at various speeds and water depths.

At any given tread depth, the onset of aquaplaning depends on the forward speed and the water depth. It happens across a relatively narrow speed range. Up to a certain speed, the tyre is in good contact with the road, but as the speed increases, the tyre cannot cope with the inrushing water and the front part of the tyre starts to lift off the road surface. Eventually, as speed increases still further, the tyre lifts completely off the road surface and all control is lost.

How to recognise that you are aquaplaning

if you are driving along in the wet in a front wheel drive car, and the engine starts to rev hard, while the tyre noise decreases, then you are probably aquaplaning. In a rear wheel drive car, things are less obvious, because the engine note hardly changes, while the tyres go a bit quiet. Also, if you briefly tweak the steering wheel back and forth and there is no response, that is a sure sign that you are aquaplaning.

A rear aquaplane is harder to spot, especially in the front-wheel drive car, but potentially much more dangerous. In a rear-wheel drive car, the engine will start to rev hard, and you will have lift off the gas. Do not, under *any* circumstances try to steer the car--even to 'test' if you are aquaplaning. This will nearly always result in a rear-wheel skid, and could end up with the car doing a 180o or 360o spin (or something in between. Very nasty, especially on a crowded freeway).

This is one of the reasons why, if you have two good tyres and two poor tyres on your car, you should put the good tyres on the rear. If you do this, then you shoud never have a rear-wheel aquaplane.

How to deal with aquaplaning

If you were travelling in a straight line when the (front) tyres lifted off the road, the solution is easy: lift off the gas, and wait for the vehicle to settle back down onto the road. Do not for any reason make any large movements with the steering wheel. Try to avoid braking. Once the car is back under control, keep the speed low and look out for deep puddles on the road, where you may have to slow down even further.

If you were turning a corner, then you will have a bigger problem. The car massively under steers; that is to say, it fails to respond to the steering wheel, so the driver must turn the wheels to point in the direction the car is actually travelling. Do not apply the brakes. This might cause the rear wheels to break away as well as the front, which will lead to complete loss of control.

In a rear-wheel aquaplane, you more or less have to hope that the car continues in a straight line, or does not spin too far or too fast. Again, lift off the gas, and pray. Do not apply the brakes. This will break any residual contact the rear wheels have with the road, and might make the skid worse.

tyre companies test lateral aquaplaning by driving a car around a tight circular track, maybe 50 m in diameter. One part of this track is covered in deep (around 5mm) water, while the rest is simply wet with 1mm or so of water. The driver builds up speed until the car starts to aquaplane. If the rear tyres break away first, the resulting skid is very hard to control, but if the front tyres break away, it is a relatively easy to regain control of the vehicle by twisting the wheel in the opposite direction from the initial turning circle.

Final words of advice

Replace your tyres when the tread depth falls below 3mm

Try to make sure the rear tyres have more tread depth than the front ones

If you start to aquaplane, don't panic, just lift off the gas and wait for the car to settle down again

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