Thursday, December 29, 2011

numbers, letters and pictures...but what does that mean - Understanding tires Part 2

This is the 2nd of 5 posts about tires.
1 is about how important it is to understand tires and what to consider when buying them.
2 (this) is about the tire measures and proportions one can see in the tire walls.
3 is about tread design
4 is about compound and structure
5 is about maintenance and repair

When buying tires, there is a lot of information to consider. Starting with the car, the weather it's supposed to be driven, the way it's supposed to be driven and for how long. Most don't know this and trust their choice to what they think it's the car's manufacturer choice... however this is not always the case. Apart from a couple of cars out there, where the manufacturer creates a full spec tire on a venture with a chosen tire manufacturer (Honda, Lotus, Porsche and some others), the majority of them come fitted with standard tires chosen to fit the rims. 
Even when the manufacturer does create a full spec tire for the car, just how outdated will they be 8 years later? What if you change the rims? What if you change the suspension and setup a more aggressive camber? Or tune UP the car to increase power?
One can find hundreds of reasons to chose different tires. The choice however will mean you understand the numbers, letters and symbols on the tire's side-wall.
This article will explain exactly what those references mean, helping you understand and chose according to.

The tire side-wall contains all the information you need to know about your tires. 
Starting with the size measures:

225/45/ZR17 means exactly that the width of the tire has millimetres, and 45% of that measure in this case 45% of 225mm =  101,25 millimetres. 
It's very important to understand this last number when sizing your wheels. If you increase tire width, you MUST reduce the percentage in order to maintain height. If you increase you rim size, then you must decrease tire height to maintain total wheel diameter. Having the wrong sizes not only changes the car's power delivery, but also the odometer feedback, leading to inaccurate speed readings and mileage count.
The ZR means Z speed rating (Z being above 240km/h witch can be read in the chart bellow) and R for Radial Tire Construction. The Z is an old marking now depreciated, the actual speed reading comes in the next set of number/letter data.
The 17 is the Radius of the rims to fit, in this case 17 inch rims.
The 93Y is the Service Load Description. These numbers indicate the Load that each tire is engineered to withstand while the letters are the actual speed rating. 93 means 1433pounds (650 Kilograms) and the Y means the tire can rotate up to 186mph (300km/h).
Load IndexPoundsKilograms
Load IndexPoundsKilograms

Now for the position on the rims and the car: 
Today's tire tread design are normally made to channel water out of the tire's contact patch to the ground and avoid aqua-planing, This means that the tire may have a specific rotation direction. In this case, the rotation is clearly indicated.
Another tread design fact is the asymmetric design. If a tire has an asymmetric design, this means that the inside half of the tire is different from the outside half and it's design is done that way to help the tire handle cornering load better. Those cases have the OUTSIDE indicator.
In any one of these cases, switching car tires front to back MUST either be done always on the same side of the car, or if crossing, done in a tire-shop and with tire dismount/re-mount in order to respect the rotation and outside mounting of the tire.

The type of tire construction:

Although not really as issue any-longer, since today's tires are almost all radial and tubeless.
Still, these mean the Tubeless tire will have no air tube inside, hence in need of a perfect seal between the rim and the tire in order to maintain the air inside. This is assured by steel belts enclosed in the rubber contact belts with the rim, making it extremely rigid and assuring a permanent fit.
The Radial is the way the tire is built, meaning that it's composed from several parts "glued" radially together to form a full circle.

Wear, temperature and traction specs:
Tread-ware grade is a reference that explains how soft the rubber compound is, ans how it will wear when compared to a reference tire.  The reference tire scores 100, so a 240 tread-wear means that this tire will endure 2,4 times the reference tire's miles before wearing out. This is calculated buy the wear done in a 7200mile test against the Uniroyal reference tire. The harder the compound, the more it will last and the less it will grip, so when considering sports tires, don't expect high numbers from this, or get ready to pay for frequent car repairs for going off road often due to low grip ;)

Traction A means that under a test while the tire was dragged in a wet track, at 40mph without being allowed to rotate, it scored A out of a scale from AA, A, B and C, being AA the best compound. This grading, although related to the compound isn't meant to rate the tire's corner abilities, but rather it's ability to stop the car in a straight line braking on a wet track.
Temperature A means how good the tire is dissipating heat instead of allowing buildup (eventually leading to greasiness and/or catastrophically failure). The higher the car goes, the faster the tire will be compressing it's contact area and expanding the opposite side, building heat.
An A grade means the tire car drive continuously over 115mph and the tire will be able to dissipate fast enough to cope with it. The B rating is between 100mph to 115mph, and C from 85mph to 100mph.
Mind that aggressive cambers will mean that the tire will have to cope with an uneven inside deformation according to the outside one, this will increase heat build-up, rendering this rating useless.

The materials used in the construction of the tire: (to be detailed in the 4rth post)

Maximum specific load and working pressure figures:

The build Date:
This is a very important part. The 2511 numbers on the right are extremely important. They mean that this tire was manufacturer on the 25th week of 2011. Rubber compound degrades and fissures over time. A tire starts loosing it's properties after 3 years. Some countries have laws that prohibit a vendor from selling tires 6 years old, and most sellers recommend tire changes after 10years.
Still, is a sports car, it's not expected for a tire to live long, that means that you should NEVER have them over 6 years, as the compound will definitively not grip as well as intended and it will certainly have micro fissures, compromising integrity and making the tire tread shred under abuse.
My advice is to USE your tires... you'll enjoy your car better and keep your tires fresh.

Finally, the warnings: 
It's obvious that you should buy the tires for you rims and vice-versa...but its never a bad idea to explain that a tubeless tire with 17" steel belt, when stretched to 17,5 while mounting, the steel belt will be over it's elastic deformation limit and start plastic deformation, and worse in an uncontrolled stretching environment, allowing one part to stretch differently from the rest. Not only the tire will not return to it's previous size, but also the entire tire structure would be wrapped, creating vibration, twisting, heat build-up and eventually a catastrophic failure.

Hope you've enjoyed; next time I'll talk about the tread design.