Grip on wet surfaces or the eternal struggle between tire and water
The tire’s grip on wet surfaces is a complex and dynamic phenomenon that involves two mechanisms: grip and hydroplaning.
First grip. The rubber’s contact with the road enables grip, even on wet roads.
Grip on a wet road is generated by the micro-slide between the rubber and the road. With its viscoelastic properties, the rubber will change its shape on the road’s bumps and generate forces that enable a vehicle to brake, accelerate or take a corner, even when the road is wet and covered by a thin layer of water. When the road is dry, another mechanism, adhesion, increases grip. Molecular connections exist between the rubber and the road, generating more force. Water’s first effect is to prevent adhesion and reduce available grip.
La gomme et l'adhérence
Secondly, hydroplaning can occur when a vehicle drives too fast on a wet surface.
The hydroplaning phenomenon is well known, particularly in sports like water skiing. This phenomenon occurs when, beyond a certain critical speed, the water pressure is enough for the skier to glide over its surface.
Water exerts the same lift force on the tire and therefore progressively reduces the contact area between the tire and the road.
Hydroplaning is often considered to be a binary process. That’s not true! Hydroplaning is a progressive phenomenon that, depending on the level of the water and the speed, gradually reduces the tire area in contact with the road. Often, hydroplaning does not occur on wet roads or only in low proportions.
The main factors that influence grip on wet roads are the road itself, the quality of the tire, weather conditions and speed. The road helps evacuate the water and thanks to the presence of micro rough patches, it helps the tire stick to the road through grip. When it rains, a worn road will therefore be more slippery than a new road.
The tire’s architecture and tread design also play a major role in evacuating the water between the tire and the road, offering the tire the best possible contact area with the road. The materials and tread making up this contact area enable the tire to stick to the road and generate grip.
Should we be worried about hydroplaning?
Based on VUFO* data, hydroplaning causes very few accidents (but quite a lot of concern!). Only 0.6% of wet road accidents are caused by hydroplaning.
The two main scenarios to remember are the following:
- When the road’s surface is wet (see the graph below), there is no hydroplaning whatever the speed, even if the grip decreases compared to a dry road.
- On a wet road (water level > 0 mm), hydroplaning occurs and its percentage increases with the speed of the vehicle and the water level. In situations with high water levels on the roads, most drivers adapt their speed.
The most unfavorable conditions for hydroplaning are:
- High water level (1 mm and above)
- High speed
- Low tire pressure
- Worn tires
This is why Michelin calls for testing tire resistance against hydroplaning in the most unfavorable conditions and therefore testing worn tires.
A design choice
For tires to perform well on wet surfaces, they must be able to generate a maximum amount of grip and evacuate a maximum amount of water to get the largest contact area possible with the road.
Tire performance results from the manufacturer’s design choices particularly for materials, tread and the shape of the contact area. The main technical challenge is to design a tire that guarantees a high level of performance, particularly in wet conditions, throughout its service life.
How can I prevent or reduce hydroplaning?
Start by checking the condition and pressure of your tires. Low tire pressure greatly increases the risk of hydroplaning. If your tire pressure is 30% below the recommended pressure, there is a real increase in the risk of hydroplaning. Finally, always adapt your speed to the weather and road conditions.
3 questions à
"Cyrille Roget, Director of technical and scientific communication for the Michelin Group"
1. How is Michelin working to minimize hydroplaning with its tires?
To ensure tire performance and safety on dry and wet surfaces, Michelin works simultaneously on three parameters: materials, tread pattern and the tire’s contact area with the road.
- Materials: we use a secret mixture of chemical components, natural and synthetic rubbers and additives to manufacture materials that enable the tread to grip the surface in all conditions.
- Tread: the blocks and grooves of rubber in contact with the road make this up. The grooves must contain water from the contact area and evacuate it towards the tire’s exterior. The rubber blocks must evenly spread the pressure and load over the entire contact area, and grip the road. Finally, the sipes and edges of the blocks will cut the water film, helping the tire to grip the road.
- The shape of the contact area: on a wet road, the shape of the contact area and in particular the shape of its leading edge affect water evacuation. A rounded leading edge will move the water to the sides and evacuate it towards the outside. A squarer, more rectangular leading edge will bring water forward, creating a wave effect.
2. New tires against worn tires... It’s not necessarily obvious which will perform the best, but it’s all about design, isn’t it?
That’s exactly right. Braking on wet surfaces is the main safety performance that degrades as the tire becomes worn. It should therefore be systematically tested on worn tires. One preconceived idea for consumers is that safety depends on the height of the tire tread. But the residual height of the tread is not relevant to measure the safety performance of a tire. All you need to know is that today on the market, some worn tires can brake just as well on wet surfaces as other new tires. Worn tire safety on wet surfaces stems from the manufacturer’s design choices. In no case can performance be guaranteed by tread height. This is why Michelin wants to test worn tires!
3. Is this why you are heavily insisting on the importance of testing worn tires in all conditions?
We are insisting on the importance of testing worn tires on wet surfaces because it is the principle performance that degrades as the tire becomes worn. Other performances improve with wear, particularly braking on dry surfaces, roll resistance and fuel consumption.