Friday, December 19, 2008

Studded Snow Tires

Studded Snow Tires

I've reading up on studded snow tires at the moment, primarily because of concern about ice. (You know why, of course.) It seems that there aren't a lot of choices in the Jaguar's standard size: 225/45-R17. The Pirelli Winter Carving gets enormous numbers of positive comments (and a few negative ones, as well), but those who specifically said that they ordered the Pirelli Winter Carving with studs seemed quite happy--and many of these were in Canada, and one especially notable comment from North Pole, Alaska.

The initial guess of the body shop was that they were going to need to replace the Jaguar's wheels because they were scratched up--and perhaps I could buy them cheap, and use them with studded tires in winter. I thought that this was a bit much, and I guess after reconsideration, they decided that the scratches could be polished out and refinished.

There are two reasons why you might buy winter wheels, instead of having a tire store swap the studdable and all-season tires every December 15 and April 1. One is that it typically costs around $15 per tire around here every time you get a tire changed. Buying a spare set of wheels for the Jaguar would cost about $280 (including freight). It would only take a bit more than two years ($15 * 4 * twice a year) to pay for the wheels. The other advantage is that after you have the tire store swap the tires--you then have to put the other tires in the back seat, and take them back home. At least with a set of winter wheels & tires, you just need to jack up the car to do the swap.

Unfortunately, by the time you get wheels and tires--it's almost $1000! Gasp! On the other hand, that's not so bad compared to $250 deductible every time you slide off an icy road--and hope that you don't end up in the hospital.

UPDATE: I have found a variety of discussions of the merits of studded tires vs. snow tires vs. all season tires scattered about the web. here's one that seems to have lots of information--but to be honest, I find it so oddly organized that I can't find what I looking for--the relative braking capabilities of studded tires vs. snow tires. The Washington State Department of Transportation web site reminds us that studs, as wonderful as they are on ice, carry a cost:
On untreated icy roads at or near freezing (32ºF) studded tires do provide some measure of improved stopping ability, but on a statewide average these (glare ice) road conditions occur less than 1 percent of the time in Washington. It is anticipated that the frequency of these events will continue to decrease as WSDOT continues to implement proactive snow and ice control practices.

WSDOT is particularly concerned about the use of studded tires in areas where motorists are exposed more to wet conditions than icy or glazed road conditions. WSDOT wants to make sure motorists are aware of the safety issues regarding studded tire performance in wet conditions.

Under wet driving conditions the stopping ability of vehicles equipped with studded tires is actually reduced. Tire studs reduce the full contact between a tire’s rubber compound and the pavement. Research on studded tires consistently shows that vehicles equipped with studded tires require a longer stopping distance on wet or dry pavement than do vehicles equipped with standard tires.

Also an issue for WSDOT is the accelerated pavement damage done to roadways by studded tires. The abrasion on pavement surfaces caused by studded tires wears down pavement at a much greater rate than do other types of tires.
This article discusses an Alaska government study of Blizzaks (a non-studded snow tire) relative to studded and all-season tires:
Stopping. In one test on packed snow, the Blizzaks shortened stopping distances from 25 miles per hour (mph) by as much as 33 percent. On the average, the Blizzak tires performed on packed snow only slightly better than studded tires and non-studded all-season tires. On the big pickups, the Blizzaks actually yielded longer stops.

Under icy conditions, the Blizzaks reduced stopping distances by an average of 8 percent over all-season tires. Studded tires reduced stopping distances by about 17 percent under icy conditions when compared with all-seasoned tires. Average stopping distances from 25 mph were 106 feet for studded tires, 118 feet for Blizzak tires, and 128 feet for all-season tires. These distances were about three times farther than stopping distances on packed snow and 7 to 10 times farther than stopping distances on bare pavement.

In one test near Fairbanks involving a sedan, a station wagon, and a van under generally icy road conditions (as opposed to the glare ice of a Zamboni-smoothed frozen lake, where studded tires performed best), the Blizzaks brought heavy rear-drive vehicles to a stop from 40 mph in 121 feet, as compared with 141 feet for studded tires and 179 feet for all-season tires.

On bare pavement, the Blizzaks showed a 2-35 percent stopping-distance advantage over studded tires. In a test with one full-sized sedan, the studded tires had stopping distances more than 40 percent longer than the Blizzaks or the all-season tires. In some bare-pavement stopping tests, the all-season tires were marginally superior, and in other tests the Blizzak tires excelled.

What all of these are saying is that if you have glare ice (like what I slid the Jaguar on the other day), studded tires are an advantage--but the difference between 106 feet (studs) and 128 feet (all season) is not as dramatic as I had hoped. What this tells me is that tires that meet the new Severe Winter Traction Standard are probably going to be only about 10-15% longer stopping distances than studded tires on glare ice, and about all season tires will only be about 25% longer stopping distance than studded tires. It also tells me that under the far more common bad weather conditions, such as packed snow, loose snow, rain--and even the occasional dry road!--the studded tires are inferior.

If you reach a point where studs might be an advantage, you need to recognize this before you start sliding. (And that's the mistake I made a few days ago. I did not recognize that this was ice, and very slick ice at that.) And if things are that bad, you either need to wait for conditions to improve, or put on chains.

I notice that Oregon's chain law allows use of Severe Winter Traction Standard tires on vehicles under 10,000 pounds GVWR instead of chains:
If your vehicle is rated 10,000 pounds GVW or less and is not towing you must use chains or traction tires.
There is one set of conditions where a passenger car might be required to use chains:
However, in very bad winter road conditions all vehicles may be required to use chains regardless of the type of vehicle or type of tire being used (this is known as a conditional road closure).
I really hate putting chains on--which is why I have been looking at studded tires and snow tires. I have some very unpleasant memories of putting chains on with my father when I was young--perhaps aggravated by being young, and not having adequate gloves at the time. My wife and I have put them on our Equinox once, and it wasn't much better. My experience in the Sierra Nevadas is that many people put on chains prematurely, well before there is a need for them to improve braking. But before I head over to Bend again, I need to verify that I have the right size of chains for the Jaguar.

UPDATE 2: This report over at indicates that the best snow tires are now better than studs even on glare ice--in this case, a skating rink!

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