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[Winston Wolf]

With winter shortly arriving, I have been looking at getting my snow tires. I have searched all the major tire manufacturers, but can not seem to get the sizes needed. I want to keep the same tire sizes as stock, 225/40 18 front and 255/35 18 rears. Has anyone found a setup that works? Either Runflat or not?

[Orb]

Originally Posted by Winston Wolf

With winter shortly arriving, I have been looking at getting my snow tires. I have searched all the major tire manufacturers, but can not seem to get the sizes needed. I want to keep the same tire sizes as stock, 225/40 18 front and 255/35 18 rears. Has anyone found a setup that works? Either Runflat or not?

Pirelli Winter 240 Sottozero have these sizes.

Orb

[scollins]

Originally Posted by Orb

Pirelli Winter 240 Sottozero have these sizes.

Orb

Which happen to be runflats in the 225/40-18 and 255/35-18 sizes. Also, the Blizzak LM-25 is avaibable in 225/40-18 and 255/35-18 sizes, but are not runflats.

Both options at Tire Rack....

[E91_E88_E21]

Tyre-Fabrikation: Hankook
Typ: W300 XL
ReifenDimension front: 225/40 R18 92V
ReifenDimension rear : 255/35 R18 94V

thats what i can tell you

[Orb]

Originally Posted by scollins

Which happen to be runflats in the 225/40-18 and 255/35-18 sizes. Also, the Blizzak LM-25 is avaibable in 225/40-18 and 255/35-18 sizes, but are not runflats.

Both options at Tire Rack....

You assumed Tirerack has full listing for all tires but they don’t in a lot ofcases. Always go to the vendor website for sizes. The Sottozero are available in non RFT as you would expect. RFT are not the norm (OEM only) so there is alway non RFT version. The Sottozero II is out this month as well.

Orb

[nicknaz]

OP you get better snow traction with narrower tires. I think you should go with 225 width all around. You'll also potentially keep your tire cost down since you'll be able to rotate the tires

[Orb]

Originally Posted by Nick Nazareno

OP you get better snow traction with narrower tires. I think you should go with 225 width all around. You'll also potentially keep your tire cost down since you'll be able to rotate the tires

You do know what you said about traction is a myth....please explain how this is so.

Orb

[rmorin49]

Originally Posted by Orb

You do know what you said about traction is a myth....please explain how this is so.

Orb

I, too, have read this many times. Is it true? Don't know from personal experience but the theory is that the narrow tire will have more bite as it will sink into the snow more. On ice, perhaps there would not be any difference, in fact perhaps a wider tire would grip better.

I'd be interested in seeing some data.

[BMdblU]

Originally Posted by Orb

You do know what you said about traction is a myth....please explain how this is so.

Orb

A wider tire will 'float' on the snow, where as a narrow tire will cut through it and deliver better traction.

[Orb]

Originally Posted by rmorin49

I, too, have read this many times. Is it true? Don't know from personal experience but the theory is that the narrow tire will have more bite as it will sink into the snow more. On ice, perhaps there would not be any difference, in fact perhaps a wider tire would grip better.

I'd be interested in seeing some data.

It is true because traction has nothing to do with pushing through the snow. The only thing matter is what the tire is contacting to and it does not mater if the snow is 3 mm or 20 mm deep. Traction is function of your normal forces, friction and contact area. We all know that a wider tire has more traction so why would this you with snow. I will wait for someone to prove this myth. One should understand there are benefits to wider and narrow tires but it comes down tire design. Maybe 20 years ago this might have been a different discussion but it is no longer the case. There is a reason why the Sottozero are made OEM nominal sizes.

Orb

[nicknaz]

Originally Posted by Orb

You do know what you said about traction is a myth....please explain how this is so.

Orb

Orb,

Unfortunately, I don't know enough about tire tread design to discuss the underlying physics in detail with you.

My understanding from shopping for tires while living in Canada is that the narrower tire focuses the vehicle's weight on a smaller area, allowing the tread to perform this evacuation of snow/slush more efficiently (ie if the tread design and vehicle weight are held constant, the narrow tire will evacuate the snow/slush more efficiently.

I am having a hard time finding an online resource that outlines the physics in more detail. If this phenomena is incorrect, as you are suggesting, it is certainly very well propagated (links below)

On my old cars (integra, focus) I've tried both 185 width and 195 width tire of the same type and the 185 tire did 'feel' like it gave more traction

http://www.tirerack.com/winter/tech/...jsp?techid=126

"A wide, low profile or large tire has to "plow" a wide path through snow which causes more resistance. The narrower the tire, the easier you can get through snow."

I did a quick search on the internet and many other reputable sites are posting similar advice

http://www.canadiandriver.com/winter...nter_tires.htm

"How can a narrower tire improve road grip? The rule of thumb from experts is that a narrower tire will cut through rain, snow and slush and bear down on the road better than a wider tire, which will tend to float up and over, losing grip in the process."

http://editorial.autos.msn.com/artic...umentid=434745

"Contrary to conventional thinking for dry pavement conditions, on snow and ice narrower tires tend to provide better traction"

[Nikolas]

The narrower tire concept is true, but only for driving through fresh snow. The narrower tire does not get as much resistance. However for driving on packed snow and ice the goal is to get as much siped surface area as you can. The more surface area of traction you have the better. Since most of the time in areas that receive snow you are actually driving on packed snow and ice, do not worry about the 255 width in the rear as it will help you, and since the fronts are doing the cutting in the fresh snow, I don't think the added width in the rear will make much difference at all, other than providing you with more traction on the packed snow and ice surface.

[Orb]

Originally Posted by Nick Nazareno

Orb,

Unfortunately, I don't know enough about tire tread design to discuss the underlying physics in detail with you.

My understanding from shopping for tires while living in Canada is that the narrower tire focuses the vehicle's weight on a smaller area, allowing the tread to perform this evacuation of snow/slush more efficiently (ie if the tread design and vehicle weight are held constant, the narrow tire will evacuate the snow/slush more efficiently.

I am having a hard time finding an online resource that outlines the physics in more detail. If this phenomena is incorrect, as you are suggesting, it is certainly very well propagated (links below)

On my old cars (integra, focus) I've tried both 185 width and 195 width tire of the same type and the 185 tire did 'feel' like it gave more traction

http://www.tirerack.com/winter/tech/...jsp?techid=126

"A wide, low profile or large tire has to "plow" a wide path through snow which causes more resistance. The narrower the tire, the easier you can get through snow."

I did a quick search on the internet and many other reputable sites are posting similar advice

http://www.canadiandriver.com/winter...nter_tires.htm

"How can a narrower tire improve road grip? The rule of thumb from experts is that a narrower tire will cut through rain, snow and slush and bear down on the road better than a wider tire, which will tend to float up and over, losing grip in the process."

http://editorial.autos.msn.com/artic...umentid=434745

"Contrary to conventional thinking for dry pavement conditions, on snow and ice narrower tires tend to provide better traction"

I hope this link helps a bit but it is still far too simple. The tire friction is very non linear. The assumption above is one of stating a tire design it not working but this is not the case is it?

http://hyperphysics.phy-astr.gsu.edu/hbase/frict3.html

Orb

[Nikolas]

Orb that link does not take into consideration the force that unpacked snow applies to the tires in front of the tire as the tire "plows" through snow. This is why a narrower tire actually does perform better in deep unplowed snow. This is also why you see those mud bog tractors with really narrow tires. However, on packed snow and ice, Orb is 100% correct. I have tried this first hand and the reality is the more surface area you can distribute across the surface will result in more traction.

[Orb]

Originally Posted by Nikolas

Orb that link does not take into consideration the force that unpacked snow applies to the tires in front of the tire as the tire "plows" through snow. This is why a narrower tire actually does perform better in deep unplowed snow. This is also why you see those mud bog tractors with really narrow tires. However, on packed snow and ice, Orb is 100% correct. I have tried this first hand and the reality is the more surface area you can distribute across the surface will result in more traction.

Okay, your understanding of basic physics is not so good. Everything you stated has absolute nothing to do with traction at all. The deep snow is nothing more than a resistance for a wider tire; hence, the wider tire will require more power to maintain the same speed. The assumption that this is anything more can’t be supported.

Things that do change are the normal force when you accelerate, brake or turn a corner. Friction is the key thing here as it changes drastically with changes to the normal force. You might be making the assumption that pressure changes a lot. The equation is (Pressure = Normal Force/Area) so if 265/30 x19 tire had 10 mm wide contact patch and 900 lb load on the tire the then pressure would be (219 PSI = 900 lbf /4.106 in^2) and same condition for a 225/30 x19 would be (259 PSI = 900 lbf / 3.487 in^2). Do you think the pressure will make a big difference?

Both wide and narrow tires can provide good traction but it based on the tire design and has everything to do with the friction. The components of the friction created by the tire are its compound and mechanical features. If you have a tire that is to narrow and the normal force get to high (from acceleration) you can rapidly lose friction so you have very little traction…guess why your tires spin in the dry, wet or snow. A similar argument can be made for tires that are two wide. It all a balance and it comes down to the tire design in the end.

Orb

[Nikolas]

Actually Orb my understanding of basic physics is pretty well based in my education - I am a licensed architect. That aside, I guess I missed the boat on how resistance will not have an impact on traction.

What you are not seeing is that snow can produce a load on the object that is passing through it. This load can vary based upon the water content of the snow. This is why it is very easy to drive through very light snow and then with the exact same car driving through heavy wet snow - not so easy. Everything you are stating above is assuming a static situation where the traction is based upon the contact patch only. In fresh snow you are dealing with more than the contact patch.

If you think that resitance has nothing to do with traction, try driving your car across a frozen lake at speed with a 40 mph cross wind. I have done this in Maine. Trust me that force has a lot more to do with your traction than you realize.

If you read my post you will see that I agree with the concept that wider tires in packed snow and ice do work better. In deep snow, you are 100% incorrect. As I have stated I see this every year and have tried many different setups. I actuall go for a wider winter setup now because most of the time I am driving on packed snow and ice. If it were always fresh snow, a) I would have narrow tires & b) I would be one happy mofo during ski season.

[scollins]

In the grand scheme of things though, for the conditions most people are likely to encounter, the difference in width between 225 and 255 is pretty inconsequential. The 255 is only 13% wider than the 225 (or a 225 is 11.7% narrower than a 255), which isn't that large a difference. Now, if we were going from a 255 down to a 195, different story.

This debate has raged in the off-road community for years. Is it better to go with "pizza cutters" or wider tires for various off-road conditions, especially snow and mud? Most have gone with wider tires, even in snow. Of course, they air way down (around 10 PSI) and try to float on top of the snow as much as possible. Not something we are going to do in the BMW.

And frankly, if the snow is deep enough that the tire width makes that much of a difference, then the 3 series is totally the wrong car for the conditions. An SUV or 4x4 truck should be used in those instances.

[Nikolas]

Originally Posted by scollins

In the grand scheme of things though, for the conditions most people are likely to encounter, the difference in width between 225 and 255 is pretty inconsequential. The 255 is only 13% wider than the 225 (or a 225 is 11.7% narrower than a 255), which isn't that large a difference. Now, if we were going from a 255 down to a 195, different story.

This debate has raged in the off-road community for years. Is it better to go with "pizza cutters" or wider tires for various off-road conditions, especially snow and mud? Most have gone with wider tires, even in snow. Of course, they air way down (around 10 PSI) and try to float on top of the snow as much as possible. Not something we are going to do in the BMW.

And frankly, if the snow is deep enough that the tire width makes that much of a difference, then the 3 series is totally the wrong car for the conditions. An SUV or 4x4 truck should be used in those instances.

Agreed, but the consistency of the snow has so much to do with the ability to pass through it. I've had my car stuck in 6" of Sierra Cement and in the same car drove freely through 18" of light dry powder.

[Orb]

Originally Posted by Nikolas

Actually Orb my understanding of basic physics is pretty well based in my education - I am a licensed architect. That aside, I guess I missed the boat on how resistance will not have an impact on traction.

What you are not seeing is that snow can produce a load on the object that is passing through it. This load can vary based upon the water content of the snow. This is why it is very easy to drive through very light snow and then with the exact same car driving through heavy wet snow - not so easy. Everything you are stating above is assuming a static situation where the traction is based upon the contact patch only. In fresh snow you are dealing with more than the contact patch.

If you think that resitance has nothing to do with traction, try driving your car across a frozen lake at speed with a 40 mph cross wind. I have done this in Maine. Trust me that force has a lot more to do with your traction than you realize.

If you read my post you will see that I agree with the concept that wider tires in packed snow and ice do work better. In deep snow, you are 100% incorrect. As I have stated I see this every year and have tried many different setups. I actuall go for a wider winter setup now because most of the time I am driving on packed snow and ice. If it were always fresh snow, a) I would have narrow tires & b) I would be one happy mofo during ski season.

I understand the water content in the snow and the other variable as well. The assumption you’re making is the tire is a ridged structure. The tire acts as a spring on the component forces you are talking about so the energy is conserved. Now you have to consider the width vs depth frontal area were the snow would make normal contact for a narrow or wider contact….which is the greater evil. The assumption of saying there is multiple contact patches would not be a good one at all!! We will see some affect to the normal force to a very small degree. You have to realize tire friction is really the driving factor not the small details you’re pointing out in slight changes in normal force.

I’m not assuming the problem is static. I’m looking at this from dynamic point of view. This is not a sliding friction problem. The problem turns into a time step free body diagram. We can assume the time frame would be 1^-6 seconds. Computationally, we can solve this problem at this time step with variable transient conditions so it is dynamic. We should be clear on this.

The cross wind effect traction as it an external force but doesn’t really have much to do with narrow tires have better traction in snow. We are getting off topic.

I am sure you tiring to point out certain vehicle dynamics that would be unfavourable with a wide tire in deep snow. In that case I wouldn’t disagree but not in regards with traction.

Orb

[Nikolas]

We are probably beating a dead horse here. I agree with you in regards to tire width and traction. The main point I am saying is that in deep snow where the snow depth exerts pressure against the front of the tire, a wider tire will not be able to pass through as easily as a narrow tire. While those forces may seem minor they are actually greater than one would expect. I can drive through 18 inches of light powder no problem. If that same snow were heavy wet snow, the car would not move. Clearly that impact is overpowering the traction capability of the tires. In this case the increased traction of the contact patch is not the issue. In this case decreasing the area of resistance would make the bigger difference in allowing the car to move.

At any rate, I go with "the wider the better for snows" as most of the time my winter driving is on packed snow. There are occasions where I have to drive in the fresh snow but they are not as often as the other conditions. Then I will usually take our Acura MDX.

[Orb]

Originally Posted by Nikolas

We are probably beating a dead horse here. I agree with you in regards to tire width and traction. The main point I am saying is that in deep snow where the snow depth exerts pressure against the front of the tire, a wider tire will not be able to pass through as easily as a narrow tire. While those forces may seem minor they are actually greater than one would expect. I can drive through 18 inches of light powder no problem. If that same snow were heavy wet snow, the car would not move. Clearly that impact is overpowering the traction capability of the tires. In this case the increased traction of the contact patch is not the issue. In this case decreasing the area of resistance would make the bigger difference in allowing the car to move.

At any rate, I go with "the wider the better for snows" as most of the time my winter driving is on packed snow. There are occasions where I have to drive in the fresh snow but they are not as often as the other conditions. Then I will usually take our Acura MDX.

We don’t have to agree but in mechanical engineering the key elements for this problem would be:

• Containment displacement drag: The frontal area of the tire in relation to snow depth, density, area…ect which is treated as a drag force. There is no influence on the normal force.
• Compaction rolling resistance: The vertical force is treated as rolling resistance. The horizontal component is treated as a drag force and again snow depth, density, area…ect applies.
• Rolling resistance: normal force reaction in relation to deformed wheel radius. Does affect the normal force.

I’m sure if you stepped through a free body diagram you draw a different conclusion, but honestly, it is not a straight forward problem. I do have the eqauations.

Will agree not to agree……so enough for now.

Orb

[Nikolas]

Good enough. I am speaking from my experience in driving in both deep snow of different types and packed snow. Where I live we get anywhere from 300-450 inches of snow every season with higher amounts above 7000 feet. The setup I use is working perfectly for my needs.