One of the more common questions I get asked is, "What's the best all-around tire?" Variations range from, "Most of the four-wheeling we do is in the mud, but I drive my truck back and forth to work, so it has to have good wear," to "My Jeep is a trail-only toy, so what's the best tire that works in all types of terrain from rocks to sand to mud? Should I go with a radial, a bias-ply, or even a bias-belted tire?"
Well, I have a standard answer: "There is no such thing as the perfect tire!" Everything in life is a compromise. With a tire, it's a combination of sidewall and tread strength, ride quality, tread design, wear, traction per terrain, and yes, even visual appearance-or as some would call it, the "cool factor." Something we also need to keep in mind is that there is a difference in the tire's strength, or resistance to damage, and its load capacity. The tire is not the weight carrier. The air inside the tire is what supports the weight, and this is not only in volume but in pressure. The tire itself is just the captive device for the air. Obviously, a tire has to have more strength to contain 65 psi than, say, 10 psi. So strength and load capacity somewhat go hand in hand, but are not always directly related.
But before we go any further, maybe we should clear up a few things on tire construction. The words "bias-ply," "bias-belted," and "radial" are all cool words to say around the campfire, but do we really understand what they mean? These terms refer to how the tire cords are positioned within the tire. Cords are nothing more than the reinforcing material and can be made from a variety of materials, such as polyester, rayon, nylon, steel, and Aramid fibers. (Early tires were made from cotton.) Each layer of material applied is referred to as a ply. The ply thickness is referred to as the "denier," which is actually a unit of weight equal to .05 grams per 450 meters.
Let's start with the bias-ply first. This design has been around from when the British first filled the tire with air in 1889. The plies are crisscrossed, one on top of the other, running continuously from one side to the other to form an angle mesh. The angle chosen is up to the tire manufacturer, but usually is either 30 or 45 degrees to the tread's centerline. The more plies added, the more strength is added to the entire tire from sidewall to tread, so a six-ply tire will have more strength than a four-ply tire will, or so it first appears. Somewhere along the line, some sort of strength-per-ply formula was established, so it became a common practice for a tire to have a four-ply sidewall but be listed as having six-ply strength. This seems to apply more to radials than bias-ply tires, but we will get to that later.
For years, this bias-ply was the standard construction for 4x4 tires as well as heavy-duty truck tires. The tires will flex well in the tread area as the sidewall flexes to conform to obstacles. However, the downfall is that even on the pavement, with highway pressures, the tire's tread tends to squirm around, reducing traction as well as tread life. At lower tire pressures, the additional heat buildup from flexing and plies rubbing against each other can actually cause the rubber to break its bond from the cords with resulting tire failure. Nylon seems to have solved that problem, as not only is it a stronger fiber than most of the synthetics, but it bonds to rubber quite well. However, nylon has a tendency to take a set, and produce that "thump-thump," out-of-round quality after it has been parked for a while, especially in cold climates, until it has warmed up. Polyester, while not quite as strong, doesn't take a cold set.
Bias-belteds: Some clever guy came up with the idea that the tread area could be stabilized by adding some belts of plies that would run around the circumference of the tire. This way, the sidewall plies could be reduced in number for better flex with less heat buildup, while the tread area could be not only stabilized for better traction, handling predictability, and tread wear, but also reinforced from puncture damage.
Radials: On this type of construction, the tire cords run directly across from one side to the other. This allows for much better flexibility without the plies rubbing against each other. Just like on the bias-belted, extra belts are applied to the tire's circumference. It's more work to build a radial tire, so they cost more. The advantage is that the tread remains quite stable and flat while the sidewall does the majority of the flexing. They're much less susceptible to heat buildup during highway use at off-highway pressures. However, there are a couple of weaknesses, the main one being sidewall strength. Generally, for proper function, only two sidewall plies are used, so on an RV-type tire, more rubber is added and/or the cord denier is high to improve strength. However, we now see more three-ply sidewall tires available. At lower pressures, a radial, especially on a relatively narrow rim, will offer a less stable feeling than, say, a bias-belted tire.
Often, steel cords are used for the belts and/or as many as four plies of belting are used for added tread strength. The drawback to this is the tread doesn't want to follow the contour of the terrain as well as a bias tire would.
I only hit on the high points of tire construction with the purpose of giving you a better understanding, and perhaps to encourage you to ask a few more questions the next time you're contemplating a tire purchase.