Why is narrower better on a winter tire?????
11-22-1999, 08:33 PM
#2
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Re: Why is narrower better on a winter tire?????
Narrower is better IF your priority is snow performance. If you're more interested in dry performance and just want M+S tires as insurance, then probably narrow is not better. But if you want to maximize the winter performance:
On a very slippery surface, a greater amount of weight per area (psi at contact patch) generates greater traction. Weight distributed over a greater area just leads to slippage. A wider tire generates less pressure per square inch than does a narrower tire. In addition, the narrower tire has a (relatively) longer, narrower contact patch that does a wider tire, which in turn creates a greater degree of directional stability. Tire design is also very important, as a snow tire will have a "high void" tread with lots of open space for shedding packed snow. It will also have lots of sipes, which also greatly improve traction.
Interestingly, someone here a few weeks ago posted the URL for the Pirelli Rally tire site. It's pretty impressive to see the tire sizes there - things like 135/90-15 ! And nobody drives in snow like those European rally folks.
On the S4, the car lists 225/45-17 as the standard tire size, and also lists 205.55-16 as the M+S size. On my previous A4 2.8, the stock size of 205/55-16, and the M+S size was listed as 195/65-15. A narrower, higher profile tire allows the same rolling radius, so speedo, etc. are unaffected. At the same time, the narrower tread allows greater pressure at the contact patch and improves traction and directional stability. As a further bonus, a narrower tire, particularly the 195 on an A4, may give enough clearance to allow the mounting of tire chains for those rare days when the road has become a rink.
I believe Tire Rack also has some info on their site about the whys and wherefores of snow tires, and that may also address the sizing issue.
DMoore
'00 S4
On a very slippery surface, a greater amount of weight per area (psi at contact patch) generates greater traction. Weight distributed over a greater area just leads to slippage. A wider tire generates less pressure per square inch than does a narrower tire. In addition, the narrower tire has a (relatively) longer, narrower contact patch that does a wider tire, which in turn creates a greater degree of directional stability. Tire design is also very important, as a snow tire will have a "high void" tread with lots of open space for shedding packed snow. It will also have lots of sipes, which also greatly improve traction.
Interestingly, someone here a few weeks ago posted the URL for the Pirelli Rally tire site. It's pretty impressive to see the tire sizes there - things like 135/90-15 ! And nobody drives in snow like those European rally folks.
On the S4, the car lists 225/45-17 as the standard tire size, and also lists 205.55-16 as the M+S size. On my previous A4 2.8, the stock size of 205/55-16, and the M+S size was listed as 195/65-15. A narrower, higher profile tire allows the same rolling radius, so speedo, etc. are unaffected. At the same time, the narrower tread allows greater pressure at the contact patch and improves traction and directional stability. As a further bonus, a narrower tire, particularly the 195 on an A4, may give enough clearance to allow the mounting of tire chains for those rare days when the road has become a rink.
I believe Tire Rack also has some info on their site about the whys and wherefores of snow tires, and that may also address the sizing issue.
DMoore
'00 S4
11-23-1999, 05:39 AM
#3
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Not quite true ...
I also wonder why a narrower tire is better in a snow tire, but greater pressure per square inch is not the reason. At a given tire pressure and vehicle weight, the contact patch area will be exactly the same size regardless of tire width. This is because the contact patch area times the tire pressure represents the total downforce of the tire against the ground. A narrower tire simply has a different SHAPE to the contact patch - narrower and longer rather than shorter and wider. So, the difference in snow performance must have something to do with this.
I'm thinking that the narrower tire allows a taller sidewall, possibly allowing for better a better "snowplowing" effect when the tire tries to slide laterally. Or, perhaps it makes it easier for snow/slush to move around the sides of the tire instead of for the tire to have to ride up onto the snow/slush. This is the reason why narrow tires are better at hydroplaning resistance.
I'm thinking that the narrower tire allows a taller sidewall, possibly allowing for better a better "snowplowing" effect when the tire tries to slide laterally. Or, perhaps it makes it easier for snow/slush to move around the sides of the tire instead of for the tire to have to ride up onto the snow/slush. This is the reason why narrow tires are better at hydroplaning resistance.
11-23-1999, 01:05 PM
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DMoore is right, you are wrong...
First, you're absolutely wrong about a narrower tire allowing more sidewall. Case in point, the 205/55 R 16 vs. the 225/50 R 16.
The 225 is wider, and has a lower *aspect ratio* but has the SAME height sidewall. For properly sized tires, the sidewall height should only depend on the size of the wheel (plus sizing = less sidewall).
Secondly, DMoore did talk about directional stability and narrowness, which I think you're just restating.
Finally, although the contact shape changes, so does the area. Wider tires tend to have a larger contact patch as well, which does not "dig" through the snow as well.
The reasons DMoore discussed are widely accepted as why narrower tires perform better in snow.
BTW, just for the record, the sizing for 16x7.5 R-28's for S4 snow tires is a seperate debate with other factors. For just a theoretical discussion of tire sizing, DMoore was right on, 100%.
-Rob
The 225 is wider, and has a lower *aspect ratio* but has the SAME height sidewall. For properly sized tires, the sidewall height should only depend on the size of the wheel (plus sizing = less sidewall).
Secondly, DMoore did talk about directional stability and narrowness, which I think you're just restating.
Finally, although the contact shape changes, so does the area. Wider tires tend to have a larger contact patch as well, which does not "dig" through the snow as well.
The reasons DMoore discussed are widely accepted as why narrower tires perform better in snow.
BTW, just for the record, the sizing for 16x7.5 R-28's for S4 snow tires is a seperate debate with other factors. For just a theoretical discussion of tire sizing, DMoore was right on, 100%.
-Rob
11-23-1999, 07:13 PM
#5
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Hey, hey...
In my posting, when I wrote taller sidewall, my language was chosen poorly and you are right to correct me. My typing got ahead of my thoughts. I fully understand that the actual dimension of the tire sidewall doesn't change with tire width. I can readily calculate that a 225/50 tire has a sidewall height of (225/25.4)*0.5 = 4.43 inches and that a 205/55 tire has an almost identical 4.44 inch sidewall. What I meant to express was that the taller aspect ratio of a narrower tire might allow for better lateral "snowplowing" of the snow because it would allow some lateral flex or "give" in the tire. I wasn't trying to state a fact with this, just throwing out an idea in the hope that any resulting dialogue might eventually answer this question properly. No need to take an attitude in responding. Seeking the truth isn't a contest.
When I was talking about the contact patch area, however, I was making a statement of fact and not speculating. It's called Newton's third law. The contact patch area is the sole part of the tire touching the ground and transmitting its share of the car's weight to the ground. How is the tire doing this? How is the force of the car being transmitted to the contact patch? Certainly not through the tire rubber that surrounds the contact patch, which is the only thing actually touching this patch.
The only thing, that is, except for the air pressure in the tire. This downward force is the contact patch area times the tire pressure. You can reduce the patch area by increasing the tire pressure, like you generally do with a snow tire. But changing the width alone doesn't do it. You just get a longer, narrower patch with the same area. Ever notice that the contact patch increases in size as you press down harder on a tire? Or that a tire with lower pressure has a larger contact patch area (hence the term "flat tire")? The patch is simply changing in response to trying to maintain a constant product of the patch area and the tire pressure.
When I was talking about the contact patch area, however, I was making a statement of fact and not speculating. It's called Newton's third law. The contact patch area is the sole part of the tire touching the ground and transmitting its share of the car's weight to the ground. How is the tire doing this? How is the force of the car being transmitted to the contact patch? Certainly not through the tire rubber that surrounds the contact patch, which is the only thing actually touching this patch.
The only thing, that is, except for the air pressure in the tire. This downward force is the contact patch area times the tire pressure. You can reduce the patch area by increasing the tire pressure, like you generally do with a snow tire. But changing the width alone doesn't do it. You just get a longer, narrower patch with the same area. Ever notice that the contact patch increases in size as you press down harder on a tire? Or that a tire with lower pressure has a larger contact patch area (hence the term "flat tire")? The patch is simply changing in response to trying to maintain a constant product of the patch area and the tire pressure.
11-25-1999, 04:03 PM
#6
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Correct on the contact patch...unneeded slam!
The issue is full of folk mystique. What confuses people is the automatic assumption that wider is better. It is better for lateral force, but just due to the shape change, not due to greater area (the area is the same). What confuses this issue is the difference in sidewall inserts, and contact patch design. Same design, same pressure=same area.
11-25-1999, 09:57 PM
#7
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What slam?
The only thing I sounded forceful about was the sidewall height, which he then admitted he was absolutely wrong about.
I also said wider tires *tend* to have a larger contact patch. This is true, because the wider a tire gets, usually the less void space (tread pattern) it has.
I also said wider tires *tend* to have a larger contact patch. This is true, because the wider a tire gets, usually the less void space (tread pattern) it has.
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11-26-1999, 02:39 PM
#8
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Translation and disagreement on the facts.
Tone doesn't come over well.
It read like a slam to me. And, no, tire void is not dependant on width as a rule. In a given tire I have found the opposite. The larger sizes come in either the same tread pattern with more voids, or an entirely different pattern (obviously since they hydroplane easier); the 300+ michelins for the viper, the pirelli direct for the front instead of the asm in wide sizes, and the widest Yok S's come to mind.
It read like a slam to me. And, no, tire void is not dependant on width as a rule. In a given tire I have found the opposite. The larger sizes come in either the same tread pattern with more voids, or an entirely different pattern (obviously since they hydroplane easier); the 300+ michelins for the viper, the pirelli direct for the front instead of the asm in wide sizes, and the widest Yok S's come to mind.
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