Winter Tire Sipes
To better decide what kind of a tire you need, it’s wise to understand how traction works.
We would all like a tire that grips well year-round in all conditions, wears well and is inexpensive. That tire does not exist because many of the things that we want in one tire are engineering contradictions.
On a microscopic level, a tire is not smooth. The tread face looks like the Rocky Mountains – sharp, jagged peaks and deep valleys between them create a hostile-looking surface.
That’s good because the road surface is not smooth, either – it mimics the contours of the tire tread. These two rough surfaces interlock, the tire rotates and there is grip to move the car forward.
And to ensure that the rubber fills in the road surface, it needs to be soft.That means the ideal tire is just a big, black, soft rubber ball.
But wait – we want the tire to last, so we cannot make it as soft as is ideal. That would be a racing slick: extremely grippy for a very short life cycle. So a street tire needs rubber that is harder than ideal.
Now our ideal tire is a big black ball of semi-harder rubber. But what if it rains?
We better have some grooves cut into the tire. Every groove is a void area where no rubber touches the road. Less rubber touching the road means less grip in dry conditions but more room for rainwater to escape from under the moving tire.
If too much water collects under the front of the tire, it will lift the car (hydroplane) and we can lose control. Since every tire has a hydroplaning point, every tread pattern is a compromise between wet and dry grip. So our ideal tire is now a ball of black, medium-hard rubber with tread grooves cut into it.
We want the tire to grip in both summer and winter. Now we have a big problem. Rubber compounds harden as the temperature drops and hard rubber does not grip.
We can create a compound that stays soft in freezing weather but it will wear quickly as the thermometer climbs. Which compound is the best for our needs?
Obviously, our ideal winter tire is now a compound that stays soft as temperatures plummet.
Here we reach another split on the road to the ideal winter tire: what do we worry about more, snow or ice? Each of these conditions require tires that are quite different.
Ice grip requires lots of rubber touching the road, like a racing slick made of winter rubber compound.
This slick would have tens of thousands of tiny slits called sipes cut into it. Sipes are not tread blocks but rather just cuts in the tread rubber. The Pirelli Winter Carving tire has more than 120 metres of sipes in just one tire.
Sipes are the key component in ice grip. Even the coldest ice releases a film of water when weight is put on it and this film acts as a lubricant between the tire and the ice.
Yokohama Tire research has shown as little as 10 one-thousandths of a millimetre can cause micro-hydroplaning on ice.
Yokohama further concluded that this condition is most common at temperatures from —6 C to 0 C.
It is the job of the sipes to open as they come in contact with the ice to allow the water a hiding place, thus drying the ice for traction.
Snow grip requires as many sharp edges as possible, so the more tread blocks we create, the more grip we get. The edges of the blocks bite into snow, push it aside and crush it until we get to a hard enough compacted layer for grip.
If the snow is too soft, the tire compresses it in between the tread blocks. Compression creates water and the wet layer of snow in the tire binds to the snow on the ground and gets traction that way.
The ideal winter tire is now soft rubber with lots of tiny, sharp-edged tread blocks covered in sipes. But again, these unsupported bits of rubber will make the car feel as if the tires are made of Jell-O.
So we need to reduce the tread block count and the sipe count to something that will drive on pavement without shaking our fillings out and tearing itself apart.
Multicell Ice Compounds
Now the engineers need to find a balance between sipes and blocks for the ideal winter tire.
Enter the Bridgestone Blizzak. It uses air pockets in the tire rubber to do the work of ice sipes. It’s a great concept and the public has responded by buying more than 100 million Blizzaks worldwide since 1988.
The concept has been improved many times over the years, air pockets have changed shape, grit bits have been added – but the basic principle is still the same.
There are millions of microscopic air bubbles in the top 55 per cent of the tread rubber. These are exposed layer by layer as the rubber wears. The little pockets act as hiding cups for the water layer on the ice.
Its only shortcomings: the tread depth cannot be totally filled with these air pockets; and a high-speed rated tire is impossible to build – it would become too squishy.
Squish and squirm cause heat as does speed, so Bridgestone has settled for 55 per cent tread depth of air pockets augmented by proprietary grit particles in each. After 55 per cent tread depth, the Blizzak becomes a regular winter tire.
Yokohama has solved this last issue in its IG20 Ice Guard tire. It has given its air pockets a hard resin shell, which contains absorptive carbon flakes.
These hard little balls are mixed into the tread mix from top to bottom. While intact, they are a stable part of the compound.
As tread wears, each new layer of air pockets is exposed and goes to work. The tire is more rigid since only some pockets are open. The carbon flakes wick away moisture.
The air pockets allow the remaining water to hide, and the rubber has relatively dry ice to grip. Though seemingly having solved the problem of tread squirm, Yokohama has not made a high-speed version of the Ice Guard tire.
Michelin has joined the air pocket line with its X-Ice Xi2. It uses vertical tunnel-like tubes in the tread blocks as a water-escape route.
Balanced Ice and Snow Grip
This is the valley where engineers walk tightropes. Which way to skew the product, ice or snow?
To top it off, there is a huge demand for speed rated (H to W) winter tires for sports cars and luxury sedans. For these applications, any hint of multicell is out. There is a Blizzak tire in this market segment but it is Blizzak in name only. It does not use multicell technology.
Companies such as Pirelli, Goodyear, Toyo and Nokian attack the problems in different ways to achieve the same end: grip without much squirm. They achieve this through innovative sipes that open to do their job but slam shut when under cornering load; tiny support tabs between tread blocks; using more than one rubber compound – the list is long. And these tires do their job – they cannot compete with multicell compounds on sheer ice, but work superbly on slush, snow and pavement.
I have lapped on a track riding on Pirelli, Goodyear, Michelin and Nokian winter V-rated winter tires. They all cut through slush, snow, soft or hard and found grip at World Championship Rally car speeds.
Most surprisingly, they ran at speeds not far off summer tires on dry, cold pavement.
The Pirelli Sottozero, Michelin Pilot Alpin PA3, Goodyear Eagle Ultra Grip GW3, and the Nokian WR are the best of these high performers. The Nokian WR is unusual because it can be left on the vehicle year round. It and the Yokohama W.drive are truly all-weather tires.
Choose by Appearance
When you want ice grip over all else, start with a multicell compound, then look for sipes. The more the merrier.
Snow grip is achieved by blocks with as many sharp edges as possible. A five-sided tread block has more edges than a four-sided one. A tread block which is jagged instead of straight is better. Tread blocks on the edge of the tire should stand out sharply.
And, of course, the tire must have the Transport Canada severe snow rating: a triangular pictograph with a mountain and snowflake inside.