Slick Tires, What About Rain?!
For years, race car drivers have known that slick tires offer the most grip, best performance, and fastest times. Now, sixty years after those first slicks rolled onto the drag strip, bicycle tires are finally coming around. In this post, we'll cover the benefits of a tread-free tire, address the common concerns, and hopefully get you rolling even faster than before!
Better Grip
Ideally, your tires are the only part of your bike in contact with the ground. All of your acceleration, braking, and cornering forces are transferred to the tarmac via the tiny patch of tire where the rubber meets the road, also known as your contact patch. When the forces acting on your tire exceed the friction between the tire and the asphalt, you lose traction, slip, and that's no good.
So how do we maximize grip? By maximizing the amount of tire/road contact. Picture it this way, the coefficient of friction between your tire and the street is constant whether you have giant 2.5"-wide tires, or skinny 19mm ones (assuming they're made of the same compound) but, because the larger tire will have a larger contact patch, your cornering/acceleration/braking forces are distributed over a wider area, reducing the likelihood that they'll exceed the friction between the two surfaces. Large tires improve traction by benefitting from that larger contact patch, slick tires improve traction by making sure every inch of their contact patch is being used effectively.
On a treaded tire, all those little grooves and channels that seem to hint at grip, actually hinder it because they're not touching the tarmac. Just think, if a tire's surface is 10% tread, it'll have a 10% smaller contact patch than a slick tire of the same size.
Lower Rolling Resistance
Treaded tires also suffer from that tread deforming under the weight of the bike, rider, and the forces acting on them. When weight is put on a tire it deforms, changing in shape but not volume. On a treaded tire, those "tread voids" give the tire room to warp and, because the rubber "springs back" with less energy as it leaves the road, you're losing efficiency to rolling resistance. Slick tires deform less due to their lack of treads, and so you experience less rolling resistance as well!
What about the Wet?
First thing's first: yes, water makes things more slippery, that's pretty much universally true. But, water won't make a slick tire more slippery than a treaded tire made of the same compound. In fact, because of the slick's larger contact patch, it's more likely to maintain grip in the wet than its treaded counterpart because, while the water lowers the coefficient of friction, it still benefits from the larger contact patch distributing those forces over a wider area.
But what about hydroplaning? Pages of DMV handbooks and hours of driver's ed. videos hammered into our heads the horrors of operating a car in the rain. "Water can't compress", they warned. If you're moving too fast, you'll "trap" the water beneath your tires and ride along the top of it - out of control. That's why car tires have those deep channels for diverting water from underneath them. But what about on a bike?
There's a few reasons you don't have to worry. First, the shape of a car tire and the shape of a bike tire are dramatically different. A car tire is wide and always rolls vertically, resulting in a contact patch that looks like a large, rounded-rectangle. A bicycle tire is narrow and, because bikes lean to turn, it has to be round. That results in bike tires having a contact patch that looks more like a narrow oval. This oval shape is much better at "pushing" water out to the sides of the tire, and actually makes it fairly impossible to get enough water underneath the wheel to cause hydroplaning.
But what if you could "trap" the water under there? You'd still be going too slowly. Bike tires run at much higher pressures than car tires (a typical bike tire will be inflated to 70-100 PSI, where a typical car tire is usually 30-35 PSI). Those higher pressures not only do a better job of "pushing" the water out from under the tire, but we can actually calculate the speed needed to hydroplane based on tire pressure (thanks to NASA, the FAA, and the crazy things aerospace tires have to contend with). The speed at which hydroplaning becomes a concern is 9 times the square root of the tire pressure in PSI. So, in our typical car at 35 PSI, hydroplaning can occur at 54mph. On our bike we'd need to be going 75mph (at 70 PSI) all the way up to 90mph (at 100 PSI), speeds you likely won't be hitting anytime soon (and definitely not something you should be attempting in the rain). The point is, you don't need to worry about hydroplaning.
Conclusion
Slick tires rock! From improving grip to reducing rolling resistance, they're more effective than treaded options on dry pavement, and those same characteristics make them better in the wet as well. It can be a hard sell to the uninformed, which is why you'll see so many bike tires with "vanity treads" - small, shallow treads that mimic a car's tire, but that are clearly too small to provide any actual benefit (beyond making a nervous customer think they look "grippy"). But, those vanity treads still cost you valuable contact with the road, and now you know slicks are the way to go. So, what're you waiting for? Your rides can only get better!