|Do you really need Winter Tires?||Back to Home Page|
Do winter tires actually provide better traction in all winter driving conditions? The key word there is "all", not just ice and packed snow. Canadian winter driving conditions include rain, fresh snow, and slush. Of all these conditions, slush presents the highest risk factor, and yet little information and test results are available for this hazard. The Finnish company Nokian does do slush testing on their test track every year, but it is primarly used to improve their own product, and I could not find any quantitative results.
The first theory to test is that vehicle manufactuers have tended to go to wider tires with a larger footprint, resulting in lower ground pressures. To accomplish this, we chose 2 vehicles that we had access to.
The basic input data with the initial results is here:
and The Dynamic input data and final results are here:
From these results, we can see:
2008 1988 Contact Patch Width 196.2 mm 149.9 mm Contact Patch Length 163.4 mm 147.3 mm Contact Patch Area 320.62 cm2 220.72 cm2 Average Ground Pressure 1.54 kg/cm2 1.73 kg/cm2
There is approximately 12% more ground pressure on the older vehicle. One must appreciate that this is not a true average, as the weight distribution will vary across the ground profile of the tire, but it does indicate that our theory was correct.
Now we will examine how this might impact the friction forces on the tire. Friction is defined by the formula:
F = C * N
where F is the frictional force required to move the object, N is normal force or the vertical load applied to the object, and C is the coefficent of friction or the fractional amount of the normal force needed to move the object. Generally the Static Force required to get the object moving is more than the Dynamic or sliding force required to keep it moving, and the coefficients will be different. For example the average coefficients listed for a rubber tire on dry pavement is 0.9 Static and 0.8 Dynamic. A rolling tire would be close to Static conditions, and a skidding tire would be close to Dynamic conditions. An interesting result of this equation is that in the case of static or sliding friction of hard surfaces, the friction is independent of the area of the surfaces. This is not intuitive, as we would normally perceive that the friction force on a one square inch object would be more than the friction force on a one square foot object supporting the same weight.
One would also think that this coefficient is always less than 1, but when we look up the sliding friction of rubber on glass, we get 2+. When we add other factors such as Molecular Adhesion, Surface Deformation, and Wear to the equation, the Coefficient changes.
Molecular Adhesion is the temporary bonding forces that exist when a flexible surface such as rubber is pressed against a hard surface such as glass or pavement. When those surfaces get wet or icy, those forces virtually disappear. That explains why the Coefficient of Friction between rubber and wet glass is much lower than rubber on dry glass (lab studies indicate by as much as a factor of 8). Since we don't often drive on glass, a more realistic observation would be:
The wide variance is caused by other factors such as Surface Deformation. A surface such as ice or packed snow may look smooth, but when we examine it under a microscope, we find that it is quite irregular, just like the bumpy asphalt surface. And that is where the ability of the rubber to conform to the shape of the road surface comes into play. Friction forces due to deformation (also called mechanical keying), provides most of the friction force between a tire and a wet surface. One way to increase mechanical keying is to increase the unit force applied between the surfaces. When we decrease the footprint by using narrower tires, we increase the ground pressure and subsequently the mechanical keying available. This is demonstrated in the following figure, where the actual contact area is increased by decreasing the footprint.
The other way to increase the contact area is to use a softer rubber. The softer rubber will maintain it's flexability at lower temperatures. Whether or not it is the magical number of 7 degrees Celcius is neither here not there. If softer rubber will work better at lower temperatures, why not use softer rubber tires all year round? Unfortunately, softer rubber also increases the rolling resistance and wears faster in warm weather conditions. And will the softer rubber actually help in deep snow or slush?
In deep snow, the only thing that really helps is an aggressive tread pattern and lots of unit download force. The tire must compress the snow into the pattern of the tread, and then use the irregularities in the tread design to provide the grip between the tire and the newly formed road surface. To a degree, a narrower and longer contact area (bigger tire diameter) will work better than a wide contact area of the same total area, as it is better able to compress the snow.
Before we look at slush, lets first look at hydroplaning, because slush is a combination of snow and water. As a tire goes through water, it channels some of the water through its circumferential grooves, but also pushes a wave of water in front of it. If the water is deep enough and the speeds are high enough, it will get "over-the-hump" like a boat in deep water. A river boat has a wide flat bottom that allows it to get "over-the-hump" at a much lower speed than a deep hulled boat. The same is true for a wide tire with low down force. It will hydroplane at a lower speed, actually riding on a cushion of water.
Slush is snow that has liquid water entrained in the snow crystals. As a tire goes through slush, it attempts to squeeze the water out. Under these conditions, a tire will plane at a much lower speed than with just plain water, and softer rubber isn't going to help much because the tire is no longer in contact with the road surface. Once again, narrower tires with large circumferential grooves will handle this task better. Cars will normally hydroplane at speeds of 85+ kmph, but slushplaning is know to occur at speeds as low as 50 kmph. Large trucks handle slush much better than cars because of the huge unit downforce that they present. The tires are larger, and there is more of them, but they also carry a lot more weight. Where the pressure in car tires is between 25 and 35 psi, truck tires carry pressures from 75 to 220 psi, and all that extra ground force allows them to more easily squeeze the water out.
Wear is a force that is created as the rubber slides over a rough surface, and is not a major factor in winter driving. But in sliding or Dynamic friction, there is possibly another factor at play called Viscoelasticity. Viscoelasticity is a measure of how quickly the rubber rebounds after deformation, and is taken advantage of in racing tires. I could not find any information about whether or not winter tires try to do the same, but it might be a factor.
So it appears that if you are going to be driving a modern vehicle on icy or packed snow surfaces, you are better off using winter tires with the softer rubber. And the same width winter tires may not actually provide any better protection than your normal tires under certain conditions such as slush. You would be better off to use narrower rims with narrower snow tires and save yourself the expense of having to change the tires on your existing rims and re-balance them. When I owned a Honda CRX, it was equipped with low profile tires on 15" rims. It hydroplaned on me a couple of times, and when it snowed, the tires behaved like a set of skis. I discovered that the cars were shipped from the factory on standard 14" steel rims and higher profile tires, and were replaced with alloy 15" rims and low profile tires before they hit the showroom floor. So I purchased four of the rims at a decent price, saving the dealership the cost of shipping them back to the factory. The higher profile winter tires provided considerably better traction in winter driving conditions.
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