The new Super Touring rules regarding section width came about because NASA tech inspectors were finding numerous tires — not just one brand or one model — where the width listed on the sidewall didn’t reflect the actual width of the tire. A tire advertised as 205, for example, might measure 225 mm, which is kind of ironic because of how closely the tire industry is regulated.
That created a loophole within the rules, and if a competitor knew which tires were wider than their advertised section width, it created an unfair advantage and the potential for higher costs to racers. What’s more, competitors also were using wheels much wider than recommended for the tires they were running, like mounting a 205 on a 9-inch rim. That expanded the contact patch and stiffened the sidewalls to make the tires more responsive, but it also could be unsafe.
“Our rules were written to have a component included within them be tire size. What we were finding is that some of the tires that existed essentially didn’t comply with the way our rules were written, and we needed to come up with a different method for evening the playing field,” said NASA Vice President Jeremy Croiset. “That’s realistically why we came up with our own section width measurement tool. We believe this new system represents a much more consistent and fair way to effectively segregate tire size within our rules structure.”
To enforce the rules, NASA built four “go-no-go” section width tools, which all regions now have. According to the ST1-4 rules, vehicles with a minimum competition weight greater than 3,100 pounds must use the 282 mm section width tool. For cars less than 3,100 pounds, the 266 mm tool will be used.
For ST5, cars with competition weight between 2,400 and 2,749 pounds, the 257 mm tool will be used. Cars heavier than 2,749 pounds use the 266 mm tool. Cars lighter than 2,400 pounds use the 226 mm tool. Competitors can order their own tools on the DriveNASA.com website or print their own template from a PDF available on NASAProRacing.com so they can check their tires before they get tech-inspected at the track.
With the new system, all brands of tires are back on equal footing because it places more emphasis on construction and compound and less on width. The new system also discourages people from using wheel/tire combinations that are not based on a manufacturer’s recommended wheel widths.
“It is set up to automatically adjust for the depth, just in the way we cut the tools,” said Super Touring National Director Greg Greenbaum. “So, if you take the tool and put it over the tread and if the tool fits over the tread and when you let go of the tool, the tool can fall off, it’s legal. If you have to jam it onto the section width to the point that the tire doesn’t fall, then it’s a no-go.” – Brett Becker
How to Read a Tire Sidewall
It’s been a common belief that the tire industry has few guidelines to follow in regard to sizing, construction and so on. Given the new Super Touring and Time Trial tires rules and the coming change to the PTE-F/TTE-F classes, I thought providing the actual standards and information would be of assistance to all drivers.
Tire construction consists of more than 200 unique components where the slightest change in the build instructions has a dramatic result on the performance characteristics. This is not limited to compound, which among us amateur racers, is most often thought of as the end-all, be-all.
The tire industry is regulated worldwide. Simple things like standardized wheel rim and tire sizing didn’t happen by accident. Dating to 1903, the Tire and Rim Association was formed to regulate and standardize clincher rim dimensions and tolerances so that tires and rims would be interchangeable. This also was the genesis of stamping data on a wheel to assist with use of the recommended tire size.
Today the TRA compiles a book each year that contains all TRA standards, which include designation (tire size), load rating and dimensions, rim contour and dimension, valves and more.
So, let’s dive into what all those markings on the side of your tires mean. For this discussion, we’ll use the tire size I race on: P205-50-ZR15 84W.
Most tires begin with a letter that identifies the type of vehicle they should be used on. Racecars that use DOT-rated tires lead off with the letter “P,” which stands for “P-Metric” as fitted to passenger cars. Your trailer and heavy-duty tow vehicle tires should lead off with the letters “LT” because this designates them as tires capable of carrying heavy cargo and large trailers.
The three numerals that follow the P identify the section width — cross section — of the tire in millimeters. This is where the new NASA section width rules come into play (see sidebar). Thus a 205 tire indicates it is 205 mm at the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified-width wheel. This is one of the common misunderstandings when we measure our tires. We’ve all simply tried to measure tread width on whatever wheel we use or even unmounted.
Sidewall Aspect Ratio
This figure is a typically a two-digit number that identifies the tire’s profile or aspect ratio. What is aspect ratio? This is the tire’s sidewall height, from rim to tread as percentage of its width. Thus, our example of 50 is 50 percent of the width of 205. The math: 205mm/25.4 (25.4 is one centimeter in inches) = 8.07 inches. Fifty percent of 8.07 inches is 4.03 inches.
If my tire were a 205-70-15 the sidewall would be 5.64 inches tall, 1.6 inches taller. For one more example, let’s use a 225-50-15. 225/25.4 = 8.85. Fifty percent of 8.85 inches is 4.42 inches, .395 inch taller than the 205-50-15. Now you know why percentages are used. The variables are endless.
A letter, “R” in the case of radials, before the wheel diameter number, identifies that the tire has radial construction of the body plies. They radiate out from an imaginary center of the tire. Fully 98 percent of all tires sold today are radials. A “D” would identify the tire as having internal plies that crisscross on a diagonal. This is the commonly known bias ply tire used in some truck and spare tire applications. Run-flat tires use the letter “F” to designate they are self-supporting.
The only speed rating in the tire size today is the letter “Z” and it precedes the construction identifier, “R” in this case, as in “ZR15.” All other speed ratings are covered in the service description.
Tire and Wheel Diameter
The next two numbers indicate wheel diameter. Wheels from 8 to 28 inches are called “inch rims.” These are the most common sizes for cars, SUVs and light trucks.
Your truck or trailer may have rims in the half-inch category, 16.5LT for example that were popular on Ford vans for ages. Some trucks and trailers use everything from 14.5-inch to 19.5-inch rims.
And finally, how can we forget metric rims, 365, 390 and 415, expressly made for Michelin’s TRX line of tires that saw limited use on a variety of cars such as Ford Mustangs, BMWs, Peugeots and others in the ‘70s and 80s. They are still in production today!
In our example, 84W represents the tire’s service description. This is the load index and speed rating, except for Z rated tires since 1991. The first two digits represent the load index, the letter the speed rating. Our sample tire or 84 = 1,102 pounds load rating, the speed rating of W=168 mph. Now, you might be thinking, ‘Wait, doesn’t Z rating mean speed in excess of 149 mph?’ It does, but as the speed of cars increased, there was no delineation of how far above 149 mph the tires were rated, so along came the W = 168 mph, and Y = 186 mph ratings. If the Y is in parentheses, like this (99Y), then the tire has been tested in excess of 186 mph.
UTQG: Uniform Tire Quality Grading
Tread wear is the one we all care about! How sticky are my tires? The UTQG is a set of standards for passenger car tires that measures tread wear, temperature resistance and traction. Our sample tire is a 40 BA. The number is a comparative rating based on wear under controlled conditions on a government test track. What does that mean?
A lower numerical rating indicates a higher coefficient of friction and thus shorter stopping distances. Softer, stickier tires, for example, 40 as opposed to 80, wear out faster. The first letter, B in our sample, is the traction grade. It is a rating based on the tire’s ability to stop on wet pavement under controlled conditions. Now you know why your sticky race tires aren’t so hot in the rain. It’s not just because of the lack of tread. It’s the tread’s ability to grip wet pavement. The final letter, A in our sample is the temperature grade. There are three levels, A, B and C. Every DOT racing tire I reviewed for this story has a temperature grade of A, meaning they are able to dissipate heat up to a maximum speed greater than 115 mph.