In case of rain the wet road surface is undoubtedly an element of risk, capable of reducing the lateral grip when cornering and lengthening stopping distances. However, the most insidious enemy for the person behind the wheel is in fact aquaplaning, an even more devious phenomenon because it occurs suddenly and is able to neutralise even the most sophisticated electronic safety devices.
On both puddles and on ice
The effects on driving are similar to those generated by an icy surface, but while frost is only a threat during the winter months, aquaplaning can occur any time of the year. Owing to the poor maintenance on many sections of roads and motorways, often the road surface accumulates puddles that are deep enough to cancel the contact between the tyre tread and the asphalt surface. Modern tyres are designed to expel a considerable quantity of water during rolling, up to 30 litres per second if the vehicle is travelling at a speed of 80 km/h, but a series of variables could break this balance irreparably.
An accumulation of causes
Speed is the main cause. At speeds of up to 60-70 km/h, tyres are able to penetrate puddles without great difficulty, or in any case to compress and shift the "wedge" which is generated in front of the tread, without losing contact with the road surface. Above this limit, numerous factors come into play, ranging from the condition of the tyres to the characteristics of the vehicle.
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The delicate balance of tyres
First of all, the design of the tread is important, namely the presence of longitudinal and diagonal grooves to facilitate the expulsion of excess water; however it is clear that the effectiveness also depends on the state of wear, considering that with a depth of 1.6 mm (the limit set by the Highway Code), the effect is reduced by more than half. But it is also essential to respect the recommended inflation pressures, since any variation below or above this value could compromise that subtle balance, without forgetting that size does matter. In this case the rule is reversed with respect to what happens in ideal conditions: a narrow tyre breaks the surface of the puddle with greater ease compared to a tyre with a large cross-section.
A danger that gives no warning
In addition to the elements closely linked to the four tyres and to speed, other variables also come into play, ranging from the depth and extent of the puddle to the weight of the vehicle. A heavier car will be less likely to trigger aquaplaning phenomena, or at least these may occur at higher speeds. Whether triggered by just one of these factors, or a combination of critical conditions, the effect is immediate, without warning whatsoever, and it leads to an increase in the speed of revolution of the engine, combined with the total loss of control of the car. The action on the steering, brakes and accelerator does not have any effect, whether on straights or when cornering. Even the most sophisticated stability control devices are incapable of controlling the effect, since there is no contact between the road surface and the tyre tread.
The car floats out of control
In effect the vehicle floats on the puddle, continuing to travel with the inertia accumulated at the instant contact was lost. This means that when cornering the car will point towards the tangent, while on straights there is no guarantee that it will keep to the ideal direction. You should also consider that even the subsequent recovery of grip by a wheel after aquaplaning for a fraction of a second could turn into a potentially dangerous situation. Contact in fact is resumed suddenly, with a sudden change in direction on that side, not only in the case in which the steering wheel was turned in search of directionality.
Never underestimate tyres
The only system to avoid the risk of aquaplaning is prevention. The state of health of your tyres is as always fundamental. The pressure must be correct and the tread must be kept above the highest levels of wear permitted by the law, not below the threshold of 3 millimetres. Then you should consider speed, since at 130 km/h aquaplaning is virtually guaranteed in the presence of a 5 millimetre-deep puddle.
Violent steering corrections are prohibited
In the event of heavy rain, it is a good idea not only to reduce speed but also to stay in centre of the road, even if you are travelling on a motorway with permeable asphalt. The shape of the road in fact encourages water to build up on the sides, and thus also in the overtaking lane. If possible, you should follow the vehicle in front of you, keeping a safe distance, as this way the risk will be considerably reduced.
If prevention is not sufficient and one or more wheels begin aquaplaning, avoid abrupt steering correction manoeuvres. First of all, take your foot off the accelerator and do not act on either the steering wheel or the brake with force. For the duration of aquaplaning, which normally lasts mere fractions of a second, control inputs have no effect whatsoever, but may have absolutely unforeseeable effects as soon as the tyre grip gets back to normal.