Winter Tires 101

Winter Tires 101

by on with No Comments

Cold weather means it’s time to think about Winter Tires.

 

This article will explain how a winter tires work, and will hopefully help you decide if they are the right choice for you. As an added bonus, we might even talk a little about the mystical “All-Season” tire at the end.

A winter tire has a number of unique weapons to defeat the evil snow, ice and cold. Let’s talk about each individually.

 

  • Tire Studs

  • Tread Compound

  • Tread Depth

  • Siping

  • Bite Particles

  • Hydrophilic Coatings and Multi-Cell Technology

 

 

Tire Studs

Studded Tire

Tire studs are the most obvious way to grab the ice and function the same way as sports cleats. Hard studs are inserted into the rubber of the tread and dig into ice. Studs are primarily beneficial on ice rather than snow. Studs should only be used if you absolutely need them, since they are rough, noisy and can damage roads and surfaces. They will also reduce your traction on a dry road, since they cause a reduction of contact area with rubber and road. Snow chains are a temporary alternative to tire studs, but reduce the speed you can drive. Note: you can buy stud kits and install them yourself!

 

Tread Compound

 

Tread Compound is more important to a tire’s performance than people tend to realize. Rubber does not have constant properties, and rubber’s grip, shape and hardness depend on temperature. Rubber behaves like a polymer in many ways and has a glass transition temperature, which is the point where a polymer goes from soft to hard and brittle in just a few degrees. Typical summer tires tend to have glass transition temperatures at 40-45 degrees F, which causes traction to dramatically drop below these temperatures.

Tires are primarily made from polybutadiene, however various grades and blends can be used. Specifically, the tread, sidewalls, and rubber under the tread typically have different blends because different qualities are desired. Dropping below the Tg of the tread will reduce grip, while dropping below the Tg of the sidewall will reduce the tire’s ability to deflect and flex, and could lead to cracking. It could also reduce the tire’s pressure rating and weight capacity.

Winter tires use a tread compound with a low glass transition temperature to avoid these problems. Winter tires also use softer tread compound to increase the contact area of the tire with the road. Softer tread compound also allows the tire to deflect around small imperfections on the roads/snow/ice surface, providing additional grip.

Fun Fact

The glass transition temperatures (Tg) of the O-rings in the Challenger space shuttle’s solid rocket booster are largely credited for causing the disaster. They became brittle on an unusually cold Florida day and likely failed.

During the Rogers Commission hearings in 1986, Richard Feynman placed a small O-ring into a glass of ice water. When he removed the O-ring from the glass, he shocked many people by breaking the O-ring with little effort.

Don’t say we didn’t warn you!

 

Tread Depth

Winter tires are manufactured with deeper tread depth. This provides additional tire life, although it isn’t the primary purpose. Deeper tread depth allows the tire to perform better when water, slush and thin snow is present on the road surface by dispersing the snow towards the sides of the tire.  The deeper tread is also designed to intentionally allow dry snow to pack into the tread. Packed Snow-on-snow traction is relatively good, and allowing the tire to maintain packed snow in the tread allows improved traction and snow-on-snow friction in deep snow.

You want me to PROVE that snow-on snow-traction is good? OK Fine. The freezing point and melting point of ice is relative to temperature AND pressure. By increasing the pressure on ice, you can lower its freezing point, which causes it to phase change to liquid. This happens every time the tire rolls underneath the tread. The weight of the vehicle can lower the freezing point of water by over 20 degrees F. When the ice/snow melts, liquid from the tire touches liquid from the road. After the tire rolls further, the weight is reduced and the water immediately freezes back into ice (assuming its cold enough outside), except now there is an “ice bond” between the tire and road. This is also why you can make a snowball by compressing the snow in your hands. If it isn’t cold enough for ice to reform quickly, multi-cell technology discussed below is your friend.

Observe the phase diagram of water:

Water phase diagram

Hopefully it goes without saying that as the tire wears, and the tread depth is reduced, that the advantages of this design characteristic are diminished.

 

Siping

 Siping (that’s fun to say). Siping can be seen as the tiny slits in each individual tread block. Siping allows the individual tread blocks to deflect as the tire rolls and corners, which provides small biting edges against snow and ice. These sipes tend to have a Z or zig-zag pattern, which provides a biting edge both laterally and longitudinally. You’ll never see siping on summer tires, because siping also reduces the tires rigidity in a corner and reduces handling. If there is one thing that is true about cars, it’s that everything comes with compromise.

 

Bite Particles

Bite particles are solid and abrasive or hard particles that the tire manufacturer can imbed into the tread compound. As the tire wears the particles are exposed and improve traction on ice. These are less helpful on snow or wet roads, but are a great compromise to tire studs. Because they are hard particles, they are not as sticky as rubber and will reduce traction on a dry road, so you don’t want to see these in the summer.

Multi-Cell Technology

Bridgestone’s Blizzak winter tires are a great example of multi-cell technology. The tire features two separate tread compounds, with the outer edges employing “Multi-cell technology. The rubber is formed with thousands of small pores and are exposed as the tire wears. These pores provide thousands of miniature biting edges on the roads surface. They also provide an area for water to wick as water is formed when the tire rolls on ice, which is discussed in “Tread Depth” above. As we all know, a water layer on top of ice is VERY slippery.

 

All-Season Tires

So what makes these so special? Why doesn’t everyone simply use an all-season tire if they can perform “in all seasons” or year round? Basically, all season tires are the king of nothing and the loser at everything. That is to say, they don’t perform as well as a winter tire in the winter, or a summer tire in the summer, and yet still have many of the disadvantages of each. All-season tires typically have some siping, which improves snow/ice traction in the winter, but also reduces precision and handling in the summer. They have a soft rubber compound for the winter, which makes them wear faster in the summer and feel ambiguous in the corners. They have a softer sidewall to increase the tires contact patch area, which makes them comfortable, and handle poorly. If you really don’t care about performance, these might be OK to use year-round, but that’s not our choice.

 

Our recommendation is that if you live in a norther region where temperatures typically drop below 40 degrees and snow is likely, get a set of winter tires and winter rims. A set of 4 basic steel rims is usually only a couple hundred bucks brand new. Buy a set of winter tires and put them on the cheap steel rims, and swap them out as the seasons change. This has the added advantage of keeping the salt and rocks off of your expensive alloy’s, and will make sure you get maximum performance year-round.

 

LEFTLANEBRAIN