Have you ever been in a car that drives through a deep puddle? You might have experienced a scary sensation – a sudden loss of contact between the tyres and the road.
This is called hydroplaning or aquaplaning, and it happens when a layer of water builds up in front of and under one, some, or all of the tyres, separating them from the road surface, and causing grip to all but disappear.
The advice on how to recover from hydroplaning is straightforward – lift off the accelerator pedal, brake slowly, and steer very gently in the direction you want to travel.
In most cases, this will be enough to help the tyres regain their grip. And after a few heart-stopping moments, you should be back on your way.
Hydroplaning isn’t just a safety issue that drivers have to worry about. Tyre manufacturers are obsessed with understanding the wet grip properties of their tires, and it shapes almost everything about their design approach.
Because tires are the only contact point between a vehicle and the road, they govern that vehicle’s behavior.
The effectiveness of breaking, acceleration and steering are all influenced by what happens at the contact patch – the small portion of tire rubber that directly meets the asphalt surface.
The grip a tyre offers depends both on the condition of the surface and the rubber itself.
The main thing to know about tyres is that it is a viscoelastic material, which means that it behaves somewhere between an elastic solid and a very thick, sticky fluid.
Each time tyre rubber meets a bump or dip in the road, it can deform and ‘flow’ over it, while clinging to the surface. This provides a frictional force and is the source of much of a tyre’s grip.
Anyone who watches Formula 1 will be familiar with ‘slicks’ – the wide, smooth tires that are the default choice for racing. Their rubber make the closest possible contact with the road, which provides the astonishing grip that these cars are famed for.
But there are lots of reasons that road cars aren’t fitted with slicks; for a start, they’re expensive and not particularly durable.
They also perform very, very badly on wet surfaces, so rain is an issue. Instead, the outer layer (or tread) of standard tyres is patterned, with a combination of raised ribs, angled blocks, deep grooves and narrow slits cut into them.
The job of these tread patterns is to remove water from the contact patch as quickly and efficiently as possible.
Typically, as the tyre rolls along, the slits splay out and suck water up off the ground, directing it into wide grooves that are cut around the tyre’s circumference.
From there, the water is channeled into lateral grooves that force it out the sides of the tyre and away from the contact patch.
All of this removal-and-redirect helps to minimize the amount of water that actually sits between the rubber and the road, and it is an amazingly efficient process.
Manufacturers Continental say that their road tyres “are capable of dispersing up to 30 litres of water a second [when the car is travelling] at 80 kilometres per hour.”
Even so, if there is sufficient surface water present that it can flood the tread blocks, a tyre can still experience hydroplaning.
Researchers from the University of Lyon and tyre manufacturers the Michelin Group have now found a way to visualize this process, and they hope that it will support the design of more efficient tread patterns.
They started with a specialized test track (one of 21) at the Michelin Technology Centre in central France. This track has a large glass panel embedded within it that allows high-speed cameras to capture images of the contact patch as a tyre is driven over it at different speeds.
For this work, published in AIP’s Physics of Fluids journal, the track was flooded with a layer of water 8 mm thick. This represents fairly extreme flooding, and so guarantees hydroplaning.
In a separate paper, the authors say that in practice, “99% of the time, a tyre encounters a water depth which is equal or below 1.0 mm.”
In many experiments (e.g. this one from some of the same authors), fluorescent dye can be mixed into this water, to improve the image contrast between it and the tyre contact patch.
Here, they used an additional technique. Called refraction Particle Image Velocimetry (r-PIV), it uses a sheet of laser light to measure the instantaneous speed of these tiny fluorescent particles – and therefore the water they’re suspended in – as they move through the channels of a treaded tyre.
What they found surprised them. In each of the wide, longitudinal grooves that go around the central circumference of the tyre, they saw two white filament-like features or columns within the water.
In the narrower longitudinal grooves closer to the sidewalls of the tyre, just one of these white columns was visible.
Speaking to AIP, study author Damien Cabut said, “This indicates the presence of a gaseous phase, possibly air bubbles or cavitation” within the tread patterns.
You can think of cavitation as very tiny cavities that continuously form and collapse in liquids that have been accelerated to high speeds. They’re common near propeller blades or in pumps, and they have major implications in how water behaves.
The bubble columns also weren’t perfectly symmetrical and parallel to the groove walls – counter-rotating swirls or vortices appeared at the junctions between the grooves and the lateral slits.
This is first time such flow behavior has been seen. The authors say that this might be due to “the impingement of small jets”, as water moves from the slits and into the grooves. Alternatively, it could be due “to some suction effects.”
Either way, these presence of these bubbles suggests that the fluid dynamics of hydroplaning might be far more complicated than we ever expected.
In addition to this ‘indentation’ form of grip, in dry conditions, tyre rubber can make an even more intimate form of contact. Called molecular adhesion, it involves chemical bonds between the tyre and road surface continuously forming, stretching and breaking as the tyre rolls along.