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.
Tread is useful in adverse driving conditions – rain, snow, ice, mud, gravel, off road conditions, etc. Tread allows for water and snow to displace, and sipes (the edges of the tread) mechanically dig into and grip the road material.
On a well paved surface in ideal conditions, tread serves no purpose.
Racing tires are made of a different compound than normal street tires. The rubber is much softer – in some cases, as soft as a pencil eraser, and when cold, they can be as sticky as duct tape, in my experience, or even more so. Racing tires also have an ideal operating temperature, so those sticky tires need to get hotter by almost 100F to get even sticker to provide maximum grip. Of course, these tires tend to wear out in short order – pit stops in NASCAR and F1, and up to 8 passes in top fuel drag racing – which I find astounding they last THAT long.
These tires are expensive, mostly because they’re made in such low volume, they don’t last, and are completely impractical for a road car.
You can buy summer performance tires for your sports car. I do. They’re a soft compound, much softer than a typical all-season tire, and they have light tread that is D.O.T. approved (and thus legal to be fitted onto a road car and driven on the street), but the tread is purposefully designed to wear off as fast as possible (all this to skirt the law). They produce maximum traction once they wear properly. I’ve never had a set last more than 5 months. I also drive it like I stole it, sometimes.
But take it from me, these tires are suicidal if it rained – yesterday. The high moisture content left on the road days after is enough to greatly reduce traction. I had a friend in a sports car eat it just trying to get out of his subdivision the day after rain. High humidity on a summer evening? Yeah, there’s condensation on the road that can kill you dead even at mundane speeds. You can hydroplane a car even at 10 mph, and ABS isn’t a silver bullet. These tires in the snow? Forget it. I was young, once, and both cheap and poor. A light dusting of flurries, and I was stranded at a stop sign with no traction or ability to move. I had a broom with me to sweep the snow from the front of my tires to get moving.
And that is why no-tread racing tires are called “slicks”. These tires are completely ineffective in any adverse condition, they were designed for one specific use case, and if you have summer tires on your car, and you’re out and about, and it rains, you’re stuck on the side of the road until it clears up, and then you drive home slowly and avoid traffic.
Little off topic but I remember in a Top Gear episode one of the boys is taking a crash course on driving a F1 car and a few times he lost grip, skidding around the corner, because he was driving too slow and the slicks required more friction to get warm enough to provide enough grip.. that was a memorable bit, I wouldn’t have thought speeding up would be the remedy for skidding around the corner
For a bit more detail on types of tread. Treaded tires for cars come it two main flavors. (Other than racing slicks)Those meant for rain and those meant for off-road and snowy conditions (yes these have differences but I’m grouping them together for simplicity. All weather tires generally have swooping tread patterns and some tread that runs straight. This is good for moving water out for between the tire and road. Off-road and snow tires generally look chunkier and have treads that zig-zag. Because these tires are likely to sink into the dirt or snow a bit the chunky treads provide much more surface area and “bite” into a soft surface.
Treaded tires have more grip *under certain conditions*, as do racing tires.
Flat tires = more surface area = more grip *in dry conditions*.
Treads are there to allow water and stuff to channel away from under the tire, so you aren’t hydroplaning all the time. Same deal with nonslip shoes and stuff, they have less grip on dry floors, but are there to keep you from slipping when wet, like when it rains, or there’s snow.
The rubber slicks are made out of is, if I’m remembering correctly, softer and a more malleable, which also helps they grip to the track better, though may be what contributes to the tires getting eaten up so badly.
racing slicks are made for dry, flat track surfaces where there’s more rubber contacting the pavement. for normal driving conditions that may include rain or snow, you need tread so the tire will grip better and not ride on a film of water. the grooves on normal tires let the water run through and keeps the tire contacting the road surface.
Treads don’t have more grip in dry conditions. The treads are there to give water somewhere to go (which is why Formula 1 cars go to treads in the rain). Slicks are grippy on dry, clear tracks.
They have some downsides. Any water or gravel on the road and your grip is screwed. If the tires are cold your grip is screwed. Perfect asphalt with warm slicks and you’re stuck to the road though!
Against a smooth, hard surface racing slicks work great. The second they get wet or dirty they’re trash.
The point of tread is to move water, sand, mud, and other debris out from under the tire so it can make good contact with the road.
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