PT's Wheels and Tires FAQ

[Ry]

Originally posted in this thread: http://www.altimas.net/forum/showthr...threadid=76811

Originally posted by PT
OMG. Suspension/Tire noobs all over the place! Lemme help you guys out. I'll start with what all the numbers on the tires mean, and then I'll go into how all the mumbo-jumbo affects the handling characteristics of the tire.

Example Tire Size: 225/45ZR17 Michelin Pilot Sport

The "225" is the tire's Section Width in millimeters, at it's widest point. Many people mistakenly believe that this number measures the width of the contact patch (the part that actually touches the ground). In reality, this number is usually measured about halfway up the sidewall, as all/most tires tend to bulge out around the sidewall. So for our example, a 225 tire is about 225mm wide, at it's widest point.

The "45" number is not called the profile. It is called the Aspect Ratio. It is a measure of the sidewall height as a percentage of the Section Width. So for our example, the tire has an Aspect Ratio of 45. In real numbers, this means that the sidewall height of this tire is (225mm x 0.45) = 101.25mm = 10.13 cm = 3.99 inches. Many people automatically assume that a lower Aspect Ratio will lead to a shorter sidewall... Not true.

As a second example, take a 335/30ZR18 tire (OEM Dodge Viper Rear). This particular Tire has a much smaller Aspect Ratio. But do the math, and you end up with a 3.95 inch sidewall height. So this tire with a 30 Aspect Ratio has almost exactly the same sidwall height as a tire with a 45 Aspect Ratio.

The "17" is the rim diameter. You'll see some people (the ones who know what they're talking about) on this board post their rim sizes as 17x7. The second number is the rim width. This usually determines the range of tires that will fit on that rim. For instance, I have 17x7 rims with 225/45ZR17's. This is the largest tire I can fit n a 7" width rim. If I got 235 tires onto these 7" rims, the tires may fit, and would probably be fine for street use, but they would probably separate from the rim when I'm at the track. Yes. That would be a bad thing.

As for the size of the contact patch, the Aspect Ratio will give you a rough idea of how big the contact patch is, but this will change dramatically depending on many factors, some of which include Aspect Ratio, rim width, tread depth, and is even affected by the manufacturer. A 225/45ZR17 from Michelin, I know for a fact, has a larger contact patch than a 225/45ZR17 from Toyo.

Just because it's a "225" tire, does not mean that it will offer more grip than a "215" tire. A 225/45ZR17 tire can actually have a smaller contact patch than a 215/35ZR17. Again, it depends on the manufacturer, the rim width that you're mounting the tire on, and a host of other things. And just because you have a larger contact patch, does not always mean that your car will handle better or have more grip.

Now on to performance/handling... on the side of your tire, you will find a Treadwear Rating, a Speed Rating, Traction Rating, and a Temperature Rating.

Treadwear gives you a very rough idea of how long your tires will last and how much grip they have... A high treadwear rating, usually means a longer treadlife... most OEM tires (excluding performance cars) have very high treadwear ratings, usually north of 300. Lower treadwear ratings usually means more grip (but not always). My OEM '99 tires have a treadwear of 400. Comparatively, my Pilot Sports have a treadwear of 220. I've seen Racing Slicks that have a treadwear rating of 0. Though not all racing tires are as extreme, all the good slicks have treadwear ratings of less than 100.

Speed Ratings will tell you how fast the tire can safely rotate, before the manufacturer doesn't claim their safety. Most performace Tires are "Z" rated, and it shows up in the tire size, as in 225/45ZR17. As an alternative W or Y rated tires are acceptible, as most cars have electronic speed limiters that cap the cars at ~150mph.

P - 93 MPH
Q - 99 MPH
S - 112 MPH
T - 118 MPH
U - 124 MPH
H - 130 MPH
V - 149 MPH
W - 168 MPH
Y - 186 MPH
Z - 149+ MPH

The Traction Rating represents straight-a-head wet braking traction. This doesn't say anything about dry traction. You will see a letter A, B, or C with A being the highest. In 1997 a new top rating of "AA" has been introduced to indicate even greater wet braking traction. My Pilot Sports have a Traction Rating of AA.

The Temperature Ratings are also a series of letters. Temperature resistance is graded A, B or C. It represents the tire's resistance to the heat generated by running at high speed. Street tires, unlike racing tires, will actually lose grip as they heat up. The faster/harder you drive, the more tread degredation you will get. If you go to a track day on Street Tires, you will understand what I mean. Grade C is the minimum level of performance for all passenger car tires as set under Federal Motor Vehicle Safety Standards.

Sorry for the long post. Hope this helps... if any of you have questions that didn't get answered, feel free to IM me.