Sunday, November 16, 2008

Beware Summer Tires

My Miata, Mia, has worn "summer" tires since I bought her. These tires provide good grip in warm weather, but when the air and road temperatures get down to 40 degrees Fahrenheit or less, the compound gets hard, and the grip fades. A lot. And if there is snow or ice on the road, these tires would likely lead me on some unplanned off-road excursions. Mia's so-called summer tires are, in truth, good for 3 seasons here in Maryland, not just the summer. Winter is another story. So, today I bolted on the winter wheels and tires I recently purchased.*

The tires are Dunlop SP Winter Sport M3 models in the 205/50R15 size. They are 10 mm narrower than the 215s summer tires, but it should be just about perfect for snow use. The tires are mounted on the famous** 949Racing 6UL wheels in the nickel finish and the 15x7.5+34 size. These wheels are the maximum width that Dunlop recommends for these tires; I bought wider-than-necessary wheels so that I would have the option of mounting wide---probably 225---R-compound tires on them later, if I wanted to make this car into a dedicated track toy.

You may have noticed that these wheels are much smaller in diameter---2 inches smaller---than the stock Mazdaspeed wheels. This change saves significant unsprung weight, which should improve the function of the suspension, but with the possible cost of increased transverse sidewall flex when turning. With the slightly narrower tires, I've cut the weight of each corner from 39.1 pounds down to 31.1. That's a huge weight savings.

Below is a photo of Mia with her new shoes. You can see that the outside diameter of the tires is a bit smaller, 8 mm in fact. That reduction sadly exacerbates theMia's unsightly wheel-well gap, and it should also affect the effective gearing slightly.

Below is a photo of the front left of the car, showing the pleasant nickel finish on the 6ULs. Also visible in this image are the decals I've added to the vehicle. They are, from front to back, a traffic cone (signifying autocross), Summit Point's Main Circuit, Thunderbolt Circuit at New Jersey Motorsports Park, and a "Christmas Tree" (representing drag racing). I purchased these decals from Levant Goken at, and I'd recommend that site to anyone looking for something like these.

This third photo shows one minor cosmetic benefit of the smaller wheels. With them mounted, my brake rotors don't seem comically undersized anymore.

* You might be wondering why I didn't simply put all-season tires on my wheels when I bought new rubber this spring. Well, all-season tires are, by nature, a compromise between warm- and cold-whether performance. By having separate 3-season and winter tires, I can have more-or-less maximum possible traction in any weather ranging from excruciatingly hot to light-ish snow. If we somehow get heavy snow here, I'm unlikely to be interested in hitting the roads, anyway. The downside to having separate setups is the additional cost associated with 2 sets of wheels and tires as well as the additional storage requirement.

** In Miata circles, at least. These wheels are designed specifically for small import cars with the 4x100 lug pattern, and they offer low weight and high strength at very modest prices. One of the guys in my club says it's foolish to buy anything else. They are also available in widths up to 9 inches, so they are quite popular with drivers of heavily modified Miatae.

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