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Originally Posted by Don Coffey
This can't be right. The whole idea of a wider wheel is to increase the size of the contact patch therefore increasing traction, whether it be cornering, braking or accelerating, the wide wheel will always out perform the narrower wheel.
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Well, it is right. The wider wheel simply can't have a bigger contact patch area, unless you lower the tire pressure or increase the vehicle weight. What the wide wheel does have is a different shape for the contact patch, wider (and shorter around the tire circumference) than the narrow wheel.
To increase the contact patch area, look to off road drivers in sand, or drag racers with very soft slicks. Very low pressures, with resulting larger contact patch areas.
The wide tire won't have any more traction than the narrow tire, if all other things are equal. Fortunately, all other things are not equal. The wider tire will usually have a softer compound, or multiple compounds. Soft compounds cause faster wear, and give more traction. The wide tire will also have a shorter sidewall, which will flex less, meaning that steering inputs are transmitted more directly, resulting in more precise handling. This is without considering all of the science around friction related to deformation of the tire at the contact patch, which is related to the shape of the contact patch.
Why not just put a soft compound on a tall tire? Because it won't last, the sidewall flex will lead to early failure.
With snow tires, the wider tire, having a wider contact patch (of the same area), has less of the rim circumference on the contact patch. That means that not as many traction lugs on the edge of the tire, which traditional snow tires relied on, are engaged at the same time (there are fewer on the ground at any moment). That is where the common understanding that snow tires should be narrow comes from. There is a reason for it, it just doesn't have anything to do with pushing down more into the snow.
Modern snow tires with winter rubber compounds are designed to get traction at all of the sipes on the tire, not just from the lugs around the shoulder of the tire. That is why it is less critical now to have narrower tires. It does still help, but if you think back 5 or 10 years, there was no such thing as an 18" snow tire. There are lots available now.
Interesting subject. Tire design is moving very quickly, much more quickly than vehicle design.
Jeff