Re: Acts/ Dictates/ Mandates/ Mantle - Amy's Thread
Washington allows the use of chains from Nov 1 to Mar 1. It tears up the road if there isn't snow, but given you can only drive max 30 mph with them, it's not a long term solution. Snow tires is probably the way to go it you drive daily in conditions that require them. But what do I know, right?
It isn't really snow that demands chains or studs. It's ice, often ice under snow where you can't see it. Any all-season radial can handle two inches of fresh snow on a hard road, and more if you've got a high-ground-clearance vehicle. But when the snow is packed, the road surface isn't hard, the grade is quite steep, or the there is running water or ice under the snow, things change.
A tire must provide dry adhesion. It must provide adhesion with thin and heavy films of water. It must provide adhesion wet or dry when there is a little bit of oil on the road surface (from fresh bitumen or from engines idling above). It must drain standing water away when it hits standing water. It should get some adhesion both on cold ice and on warm, wet ice. It should have some adhesion on hard-packed snow, and its tread must cut into and grip both mud and looser snow (packing the snow to some degree). And of course there's slush ... .
It's possible for a tire to excel in some of these areas and stink in others. I've had such tires: a set of Pirelli's at a very nice price that were superb at everything except snow, and the OEM's on my present Chrysler, which were terrible in the rain. (I replaced them with a set of Gooyear Assurance for about $50 less a tire and got improvements everywhere--a much better balance between good and bad conditions. Still not cheap, but I feel they were worth what I paid. Probably lost about 1/2 mpg.)
In addition, tire manufacturers want the tread to last, because people don't want to replace tires every 15,000 miles any more. (Pure snow tires get a partial pass on this; when driven in the snow they don't get as much abrasion.) They try to make general-purpose tires (M+S radials) quiet. The automakers especially want quiet in their OEM tires because NVH (Noise, Vibration and Harshness) is a major sales issue. And the automakers want the tires to roll easily, to improve the CAFE numbers. (Note that the difference between 27mpg and 32mpg in my car can be nothing more than the quality of the bitumen/aggregate surface!)
The modern tire is a marvel in how well it meets all of these requirements, and of course the synergy between mechanical construction and the chemistry of the rubber mix is half High Art and half Black Art. (See the New Hacker's Dictionary for that last term--and related terms.)
Now here's a challenge: How does a tire work, structurally? How is the weight of the car transferred through the wheel and the parts of the tire to the road surface? This is a good one to chew on. I posed it to a fellow with a PhD in physics who'd worked in the field. He solved it overnight--I suspect on the ride home. Most people take a bit longer. I think I'll hold the answer for the new year, unless someone gets very warm.