How do we manage to move and control a moving vehicle? Solely thanks to adhesion, which is the friction force that develops between the road and the tires. More simply: thanks to the tire's ability to grip the asphalt, "clinging" to the road surface and keeping the vehicle on track. More commonly, we talk about grip. The better the adhesion, the better the road holding.
Grip is not always identical for all four tires: it varies depending on the weight on each wheel. Under load – i.e., under stress – the tires deform, changing their contact patch, which is the contact surface between the tire and the asphalt. The contact patch is very small: it corresponds approximately to the palm of a hand. While driving, the driver's maneuvers and external forces acting on the vehicle periodically change the distribution of loads (i.e., weights) on the different wheels. And consequently, the contact patch also changes from time to time, as does the grip that each tire can develop.
The friction force of tires is measured by multiplying the vehicle's weight by a specific adhesion coefficient. This coefficient varies based on several elements. The main ones, related to the vehicle, are: tire compound, degree of wear, speed. But road conditions also affect grip: a completely smooth road surface would make adhesion difficult even for the most technologically advanced tire! Likewise, adhesion is reduced – sometimes drastically – in case of rain, snow, oil, gravel, or leaves.
If the adhesion coefficient decreases, wheel slippage and a (dangerous) loss of vehicle control can occur. How can we prevent this risk, as much as possible?
First tip: frequently check tire wear; by law, tires must be replaced when the tread depth drops to 1.6 millimeters. For safe driving, this value is too low: our School recommends replacing tires when the tread reaches 3 mm.
And again: always maintain a safe distance, remembering that in unfavorable conditions, the stopping distance increases, sometimes significantly.
Finally: speed! It should always be adapted to the conditions of the vehicle, the road, traffic, weather, and the driver. The legal speed limit can sometimes be too high; let's assess the situation and drive responsibly.
