TIRE CONSTRUCTION:

• DIAGONAL (BIAS-PLY) TIRES
• These tires have body plies which cross at a substantial angle to the centerline of the tread and which strengthens both the sidewall and the tread area. Bias ply tires will almost always have an even number of body plies because using an odd number of plies causes a pull. Bias ply tires are usually constructed from nylon cord. Nylon has a wonderful property in that it shrinks when heated, but it also has an unfortunate tendency to take a set when statically loaded, especially when cold, resulting in a flat spot, the "square tire" syndrome. A flat spot will typically work itself out at the tires heat up after a few miles of driving.
• A bias ply tire often won't have much sidewall bulge, even at low inflation pressures.
• At extremely low inflation pressures, a bias ply sidewall can wrinkle, similar to a drag racing slick.
• Another common feature of bias ply tires is that the tread can wrap around and up the sidewall of the tire in an
  uninterrupted fashion. Since there is no transition from a circumferential belt to radial sidewall (the bias ply runs from
  bead to bead) this will not cause flex-induced heating.

• RADIAL TIRES
• The body cords run across the tire perpendicular to the beads. Radial tires have belt plies which run about 15° to the circumferential direction, under the tread, to stiffen, stabilize and strengthen the tread area. This results in less tread movement during contact with the road, so the belt plies improve tread life. Radial tires are usually constructed with polyester cords, which resist flat-spotting common in nylon cord.
• A radial tire concentrated sidewall bulge at the tire contact point, even with proper inflation. It is difficult to determine if a radial tire is underinflated just by looking.
• While some radial tires may have "tread" on the sidewall, it usually has a break at the sidewall transition and the sidewall tread is very thin to minimize heat buildup from radial sidewall flexing.

• STEEL BELTS
• Either kind of tire can be constructed with additional belting under the tread, usually in the form of steel or aramid fibers for increased tread life and puncture resistance. The belts are always made of a bias ply material. In a radial tire, this causes a discontinuity where the sidewall and tread meet, thus the reason you never see radial tires with substantial sidewall tread. On a bias ply tire, the belting can help keep the tread face flatter to the road and since the tread and sidewall construction are the same, tread can be wrapped around and up the sidewalls for extra traction and protection.

• The average steel-belted radial tire is made up of:
• 28% Carbon Black
• 27% Synthetic Rubber
• 14% Natural Rubber
• 10% Steel Wire
• 10% Extender Oil
• 4% Organic Fiber
• 4% Other Petroleum Products
• 3% (S, ZnO, Ti02, etc...)

READING THE SIDEWALL:

Everything you ever wanted to know about a tire is written on the sidewall.

• Passenger car tires:
• P-metric is a metric tire-sizing system, for example: P215/65R15
• P stands for passenger
• 215 represents the section width of the tire in millimeters
• 65 is aspect ratio; the ratio of height to width; this tire's height is 65% of it's width
• If the aspect ratio is not listed, it is assumed to be the "standard aspect ratio" of 82%
• R means radial.
• B in place of the "R" means the tire is belted bias construction
• D in place of the "R" means diagonal bias construction
• 15 is the diameter of the wheel in inches
• XL/RF designates a, Extra Load or Reinforced tire
• In not specified, Standard Load is assumed and will usually not be labeled as such
• M+S designates a tire rated for mud and snow use
• Light Truck tires:
• Light Truck tires also use a metric tire-sizing system, for example: LT235/75R16
• LT stands for Light Truck
• 235 represents the section width of the tire in millimeters
• 85 is aspect ratio; the ratio of height to width; this tire's height is 75% of it's width
• If the aspect ratio is not listed, it is assumed to be the "standard aspect ratio" of 82%
• R means radial
• B in place of the "R" means the tire is belted bias construction
• D in place of the "R" means diagonal bias construction
• 16 is the diameter of the wheel in inches
• "LOAD RANGE D" identifies the load and inflation limits:
• A = 2 Ply Rating
• B = 4 Ply Rating = 35 psi max. load inflation
• C = 6 Ply Rating = 50 psi max. load inflation
• D = 8 Ply Rating = 65 psi max. load inflation
• E = 10 Ply Rating = 80 psi max. load inflation
• F = 12 Ply Rating = 95 psi max. load inflation
• G = 14 Ply Rating
• H = 16 Ply Rating
• J = 18 Ply Rating
• L = 20 Ply Rating
• M = 22 Ply Rating
• N = 24 Ply Rating
• XL= Extra Load
• REIN= Reinforced
• The Load Range rating is gradually replacing the older Ply Rating (as in 2PR) system
• Ply rating at one time indicated the actual number of bias (fabric) plies making up the tire, but with the new radial tires, you often only have 2 or perhaps 3 plies making up the tire carcass with some additional circumferential plies under the tread. So a tire labeled "8PR" or "6PR" will likely not have 8 or 6 plies in the tire, but the "8PR" tire will be stronger than the "6PR" tire.
• "RADIAL" indicates that the tire has a radial construction
• "MAX LOAD SINGLE 2623 lbs. at 65 psi COLD" indicates the maximum load rating of the tire and corresponding minimum cold inflation pressure when used in a dual configuration.
• Note that P and LT tires are often rated slightly different. If P-rated tire is used in an LT application, you need to de-rate the sidewall load capacity by 9% - 10% according to many tire makers (Firestone and Michelin, for example).
• M+S designates a tire rated for mud and snow use