All About: Tires

Contents:

• TIRE CONSTRUCTION
• SPEED RATING
• LOAD INDEX
• PROPER INFLATION
• MANUFACTURERS

READING THE SIDEWALL:

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

• Passenger car tires:
  • P-metric is the U.S. version of a metric tire-sizing system, for example: P215/65R15
    • P stands for passenger.
    • 215 represents the 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.
    • 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.
• Light Truck tires:
  • Light Truck tires in the U.S. tire-sizing system, for example: LT235/85R16
    • LT stands for Light Truck
    • 235 represents the width of the tire in millimeters.
    • 85 is aspect ratio; the ratio of height to width; this tire's height is 65% of it's width.
    • 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
    • "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.

TIRE CONSTRUCTION:

• RADIAL TIRES
  • The body cords run across the tire perpendicular to the beads. Radial tires have belt plies which run nearly circumferentially around the tire, under the tread, to stabilize the tread, to strengthen the tread area and add flexibility to the sidewall. By restricting tread movement during contact with the road, the belt plies improve tread life. Radial tires are usually constructed with polyester cords, which are softer and resist flat-spotting common in nylon cord. A radial tire usually has a squared off tread face and a distinctive sidewall bulge at the tire contact point, even with proper inflation.
• DIAGONAL (BIAS-PLY) TIRES
  • These tires have two, or more body plies which cross at an angle to the centerline of the tread and which strengthen both the sidewall and the tread area. Bias ply tires will always have an even number of body plies because of this. Bias ply tires are usually constructed from nylon cord, which is stronger than polyester cord. Nylon has an unfortunate tendency to take a set when statically loaded, especially when cold, resulting in the "square tire" syndrome. A bias ply tire usually has a rounded tread face and even at low pressures will not have any sidewall bulge. At extremely low inflation, a bias ply sidewall will wrinkle, similar to a drag racing slick.
• 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.

SPEED RATING:

LOAD INDEX:

The load index refers to the load carrying capacity of a tire, or how much weight a tire can support. For example, if a tire has a load index of 89, it can support 1,279 pounds (from chart, below) at maximum air pressure. Multiply that by four (4 x 1,279 = 5,116 pounds) to get your maximum load carrying capacity. More correctly, you should take into account the front to rear weight distribution of the vehicle.

Note: It is not recommended to install tires with a lower load index than what came on your car from the factory.

PROPER INFLATION:

There are several issues dealing with tire inflation. Among them are proper inflation for both on-road and off-road use.

On-Road:

Every vehicle should have a manufacturer recommended tire inflation value, usually on a sticker on the driver's side door jamb. This figure is determined by the manufacturer based upon the vehicles stock weight distribution, wheel and tire size. This is probably the best value to use if it applies. However, if you have changed wheels, tires, or weight significantly, this number may not be appropriate.

Tires, too, come with manufacturer-specified inflation specifications. These, however, are not vehicle specific, but rather refer to the maximum inflation pressure the tire can handle in relation to its maximum load carrying capacity. For example, assume you have a light truck tire with a 2500 pound maximum load rating at 50 PSI air pressure. Lets say there are four of these tires mounted on a 5000 pound vehicle (with 50/50 weight distribution), so the per-tire load is 1250 pounds (5000/4). Clearly, the tire is nowhere near its maximum load, in fact it is at 1/2 load in this case. A case could be made for inflating the tire to 1/2 its maximum pressure (25 PSI in this case) based upon the load on the tire.

Another school of thought is that you should inflate the tire such that it has uniform tread contact with the road. This can be determined in a number of ways. The easiest is to try to slide a thin card under the edge of the tread. Inflate the tire until you can just get the card under the edge a little bit. A more involved check is to place a chalk line across the tread face, drive a short distance straight ahead on a smooth surface and then observe the chalk line. You are looking for it to be evenly worn off the tread. One of the most accurate (and complicated) methods is to measure tread temperatures right after a high speed run. Even temps. across the tread indicate proper inflation.

Off-Road:

Just like on-road, there are several schools of thought on choosing the correct off-road tire pressure. Off-road, there are many more variables, such as the type of terrain, the tire and wheel construction which determine the type of problem you are trying to solve. The following solutions should work for 15 and 16" rims with safety beads. Note many 16.5" rims lack safety beads and running lowered pressures is risky. Rims with beadlocks are an entirely different issue.

Anyway, why do you want to lower your tire pressure off-road? Several reasons come to mind:

• It softens the tire, making it more flexible to absorb bumps.
• It increases the contact patch with the ground, increasing traction and flotation.

  So, how do you go about picking a pressure to run off-road?

1. One rule of thumb is 1/4 of the tires maximum inflation pressure. In the above case of a 50 psi max, 12.5 should be a safe lower limit of off-road pressure.
2. A more sophisticated method is to measure the height of the sidewall (from ground to rim) at normal street pressure, then lower the air pressure until your sidewall height drops to 75% of the street height. I tried this with my 33x9.50 BFG M/T tires and found this occurred at 12 psi, which is very close to the 1/4 of max pressure rule.
3. A third method is to pick a pressure where the tires start to bulge out (assuming radial construction) and beginning to get soft. For me, this happens in the 15-18 psi range.

   In any event, you want to pick a pressure that is low enough to handle the terrain, but high enough to protect the wheel and tire as well as preventing the loss of the bead.

   Now for some terrain-specific observations:

• On hard surfaces, lower is not always better. I notice my BFGs begin to get slippery below about 14 psi. I think this is because the tread starts to buckle (33x9.50 mounted on 6" rims) and squirm, reducing traction. Also, below about 15 psi, if a tire is on a rock, it will deflect nearly to the rim, which can lead to sidewall punctures. If you'll be going any distance at higher speeds, then keep the pressures higher. I'll choose 18 or 20 psi if pavement driving is anticipated, otherwise 15. Also, you should correctly observe that as the pressure drops, so does your ground clearance. If clearance is more important, keep the pressure on the high side.
• On soft surfaces like sand and snow, lower is better.
  • In fact, I follow these rules in the deep stuff:
    • Air down, way down, then air down some more. Then,
      • if your engine overheats, air down.
      • if your tires dig in, air down.
      • if you get stuck, air down.
    • After running for a while, check the air pressure.
      • Aired-down tires tend to heat up and raise the internal pressure, if this happens, air down.
    • If your tire looses a bead, you've aired down just a bit too much :-)
  • So how low is low?
    • I've been to 6 psi in my 33x9.50 BFGs in the snow. At 10, I was digging in going downhill, at 6, they just floated on top.
    • I've been really low in my 33x15.50 Swampers. How low? I don't know, my digital gauge reads down to 2.5 psi, then it says 0.0 (its supposed to go to 1.0, but I guess the mfg. lied). So, I estimate the tires are running 1-2 psi.

  Anyway, I think of tire pressure kind of like cross country ski wax. You have to know your tires and vehicle, read the terrain and then choose an air pressure to run. If you err on the high side, you can always go lower if needed. Of course if you have on-board air, its no problem either way.

More information:

• Harry Lewellyn's article on "Tire Pressures for 4-Wheeling" is an excellent primer on the subject.

MANUFACTURERS:

Below are links to tire manufacturers web pages I've found.

• B.F. Goodrich Tires
• Dunlop
• Goodyear Tire and Rubber
• Interco <Not really Interco's page>
• Michelin
• Nokian Tyres
• Toyo Tires
• Uniroyal
• Yokohama