If treaded tires have more grip than tires with less tread, why are racing slicks so grippy?
In: Engineering
Racing slicks without tread are designed for completely dry, clean, flat conditions.
When a slick tire without tread encounters a puddle, it can cause a film of water, or spilled oil depending, to cling to the surface of the tire.
As the tire rotates downwards to contact the road, the film of water can’t quickly escape the space between the road and the tire due to the high speed. This causes *Hydroplaning*. In other words, the thin boundary layer of water clinging to the tires’ surface acts as a lubricant, which prevents the rubber of the tire actually touching the road, or at least significantly reducing the area of direct contact.
This can reduce the traction on a wet tire by up to 95%, which could lead to a crash.
In fact, the journal bearings on an engine crankshaft work in the same way as this. Using a thin film of oil pumped between the bearing and the journal surface of the crankshaft. Because the crackshaft journal and the inner raceway surface of the beading are machined and ground very smooth, during operatiom the crankshaft floats on a thin oil film. This cushions it from vibration and creates very low wear.
The purpose of the treads is to act as channels for water to flow through when the the tire contacts the road. This allows the high spots on the tread to contact the road, while the pressure of the contact forces water out through the groves add channels so it cannot form a film. This is a compromise, but you definitely want that on a consumer car.
This makes hydroplaning a lot less likely with treaded tires, although the traction in dry, clean conditions is less than slicks.
Likewise, sand, gravel, or dirt on the roadway can also reduce traction due to particles of sand or gravel acting like ball bearings. Slick tires are more prone to this problem. Again, grains of sand or gravel get caught temporarily in the grooves on treaded tires which keeps them from rolling relative to the tire and the ground. This means that with a treaded tire traction might actually be improved in wet weather due to sand on the road. As is often the case on most roads, there’s a fair amount of sand and dirt getting tracked around vs race tracks which tend to be swept and cleaned before races.
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