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How to Read a Sidewall
Tire Size
Example P205/55R16 91W
- P identifies your tire as a Passenger Tire. The P stands for PMetric. If your tire size starts with LT rather than a P than it identifies the tire as a light truck tire.
- 205 identifies the tire section width, which is the measurement of the tire from sidewall to sidewall in millimeters. This measurement varies depending on the rim to which it is fitted.
(There are 25.4 millimeters per 1 inch.) - 55 is the two-figure aspect ratio. This percentage compares the tires section height with the tires section width. For example, this aspect ratio of 55 means that the tires section height is 55% of the tires section width.
- R indicates the construction used within the tires casing. R stands for radial construction. B means belted bias and D stands for diagonal bias construction.
- 16 The last dimension listed in the size is the diameter of the wheel rim which is most often measured in inches.
Load Index and Speed Rating
- 91 The load index and speed rating, or service description are the numbers that follow the tire size.
The load index tells you how much weight the tire can support when properly inflated. Load indices range from 74 - 150 for passenger tires with each numeric value corresponding to a certain carrying capacity. The carrying capacity for each value can be found on a load index chart. On each U.S. passenger car tire, the load limit is listed in pounds. European tires have the load limit listed in kilograms and sometimes pounds. - W Speed ratings are represented by letters ranging from A to Z. Each letter coincides to the maximum speed a tire can sustain under its recommended load capacity. For instance, S is equivalent to a max speed of 112 mph. Even though a tire can perform at this speed, we do not advocate exceeding legal speed limits.
| Speed Symbol | Speed (km/h) | Speed (mph) |
| A1 | 5 | 3 |
| A2 | 10 | 6 |
| A3 | 15 | 9 |
| A4 | 20 | 12 |
| A5 | 25 | 16 |
| A6 | 30 | 19 |
| A7 | 35 | 22 |
| A8 | 40 | 25 |
| B | 50 | 31 |
| C | 60 | 37 |
| D | 65 | 40 |
| E | 70 | 43 |
| F | 80 | 50 |
| G | 90 | 56 |
| J | 100 | 62 |
| K | 110 | 68 |
| L | 120 | 75 |
| M | 130 | 81 |
| N | 140 | 87 |
| P | 150 | 94 |
| Q | 160 | 100 |
| R | 170 | 106 |
| S | 180 | 112 |
| T | 190 | 118 |
| U | 200 | 124 |
| H | 210 | 130 |
| V | 240 | 149 |
| W | 270 | 168 |
| Y | 300 | 186 |
| (Y) | Above 300 | Above 186 |
DOT Serial Number
The "DOT" symbol certifies the tire manufacturer's compliance with the U.S. Department of Transportation tire safety standards.
Below is a description of the serial number. Starting with the year 2000, four numbers are used for the Date of Manufactuer, first two numbers identify the week and the last two numbers identify the year of manufacture.
Prior to year 2000 three numbers are used for the Date of manufacture, first two numbers identify the week and the last number identifies the year of manufacture. To identify tires manufactured in the 90's a decade symbol (a triangle on its side) is located at the end of the DOT serial number.
How to Build a Radial Tire
Tires are not just round and black they are sophisticated products that can take years of research and development to produce. If you have ever wondered how tires are made, the following is a roadmap for the construction of a radial tire:
Start with Rubber and Additives
Tire construction starts when raw chemical additives such as sulfur, carbon black and solvents are combined with natural and synthetic rubber. The process takes place in a large machine called a banbury.
In addition to mixing and grinding, the banbury heats the rubber to make it workable in preparation for further applications. The raw product emerges in the form of long, flat bands of rubber, which are then worked in rolling mills.
Six Main Components
It takes several machines to shape the rubber into the individual components of the tire: tread, ply, belts, beads, sidewalls, and innerliner.
- The tread rubber is extruded through a tuber, then measured, cooled and cut into precise lengths.
- Sidewalls are also extruded through tubers, along with the white rubber for a white sidewall or white lettered tire if required.
- The ply is produced in a calender mill, which combines thin sheets of rubber with nylon or polyester cord fabrics. The large sheets are cut to width, rolled and transported to the assembly area where all the components will come together.
- At the same time as the raw rubber is transformed into the tread and plies, the creel room equips the tire with its basic strength. Fine steel wire goes into the manufacture of belts for the steel-belted radial tire. Rubber from the mills and steel from the creel room are molded together into wide flat sheets, cut on the bias, rolled, and moved to the tire-building machine.
- The innerliner is a impermeable layer of rubber on the inside of a tire which creates a airtight chamber when fitted to the vehicle wheel. This layer eliminates the need for a innertube.
- The last major component of the tire is the bead. The beads are created out of wrapped steel wire, covered with rubber and formed into hoops. The bead anchors the fabric plies of the tire and seats the tire firmly on the wheel.
The Green Tire
The six components (tread, ply, belts, sidewalls, liner and beads) come together on the tire-building machine. These six components are assembled into what is known as an uncured, or green, tire in two stages.
- The carcass of the tire, including beads, plies, sidewalls and liner, is constructed on one side of the machine.
- The tread and the underlying belts are assembled next to the carcass on the other side of the machine.
The two subassemblies are then joined together and the result is a green tire.
Vulcanization
The next phase is vulcanization, the molecular transformation of the soft, gummy green tire into the tough, and longwearing, modern passenger tire. The green tire is placed in a curing mold and is subjected to intense pressure and high heat internally and externally for a specified period of time. Simultaneously, the tread pattern is imprinted onto the rubber. When it comes from the mold, the tire is ready for final finish and inspection.
Final Finish and Inspection
For showroom quality, any excess rubber is trimmed off the cured tire. Every tire is thoroughly inspected. The tire then undergoes various uniformity checks to assess ride and comfort quality. Once the tires have passed all the checks and inspections, they are sent to the distribution warehouse for shipment.
