Just installed my winter tires ... Look at the difference!

[steelgray]

Yes, it must be a reson why rally drivers use narrower tires on snow than on tarmak.

[JCL]

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.

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

[fiber46is]

We should also remember that cars are getting bigger and bigger tires as standard and as an option you can have 1" or 2" bigger and then usually also wider tires. So, as car manufacturers are increasing the tire size tire manufacturers has to follow and provide options also to winter tires. Sure, tire technology has improved a lot in 10 years, but still you should remember that where man can't walk he is able to ski over, the wider tires the easier car starts sliding on snow instead of getting grip on road through the snow.

[JCL]

Snow tires don't grip the road, they grip the snow, if there is in fact snow under the tire.

[Don Coffey]

Originally Posted by JCL

The wider wheel simply can't have a bigger contact patch area, unless you lower the tire pressure or increase the vehicle weight.

Jeff, ok I understand where you are coming from now, however, regardless of tyre advancement, a narrower tyre will always be better in snow and this is simple because the narrower the tyre, the greater the weight per square inch and therefore the more the tyre will dig in to the snow. What we should also remember is that the x-drive will also do a lot to help keep the car moving regardless of the width of tyre, but having a more suitable tyre for the conditions will always be an advantage.

Don

[JCL]

Originally Posted by Don Coffey

...the narrower the tyre, the greater the weight per square inch

I tried to explain why this isn't true, above. See my previous posts. Perhaps another poster could chime in with a clearer explanation. Put simply, the pressure is the same on the inside of the tire and the outside. If you are running 32 psi, that is the ground pressure.

I do agree that narrower tires are often better in snow, even with recent tire advancements, but it doesn't have to do with pressure and pressing down through the snow. It has to do with a long and narrow contact patch, which results in having more lugs along the edge of the contact patch, engaging the snow. There is also a shorter front edge to the contact patch, and along that front edge the tire is plowing snow. A narrower tire therefore has to plow less snow, which results in less rolling resistance in deep snow.

A narrower snow tire won't always be better than a wider tire, because tires use different compounds and different designs of traction lugs, but it has some design advantages on its side.

Maybe we should go back to the ski analogy that another poster used. For the same person, with the same weight, and the same ski (thereby having the same ground pressure) does it sink in more or less if you turn it sideways (ignoring the effects of the edges for a moment)? No difference. Wide tires have a contact patch like a sideways ski. Narrow tires have a contact patch like a ski pointing straight ahead.

Anyone else want to take a shot at it?

[motordavid]

Originally Posted by JCL

I do agree that narrower tires are often better in snow, even with recent tire advancements, but it doesn't have to do with pressure and pressing down through the snow. It has to do with a long and narrow contact patch, which results in having more lugs along the edge of the contact patch, engaging the snow. There is also a shorter front edge to the contact patch, and along that front edge the tire is plowing snow. A narrower tire therefore has to plow less snow, which results in less rolling resistance in deep snow. ...

Jeff,
That explains the diff, in a nutshell, imo.
Br,mD

[Wagner]

I run the exact same setup! You do notice a huge difference in ride comfort but you lose a ton of traction.