In the January issue of Tire Review, we talked about the primary role of a tire’s bead and sidewall. Now, we’re going to move to the belt package.
Let’s begin with some things you probably don’t know. For example, did you know that there are more parts in a tire’s belt system and carcass than there are in a V-8 engine?
Here’s another. Although it appears that a radial tire carries its load where the tire hits the ground, quite the opposite is true. Even though we can see that the sidewall is a bit deformed and must certainly be bearing the load at the 6 o’clock position, the tire and the weight it carries is actually hanging from the top of the hoop, at 12 o’clock. In fact, the tire is hanging from its belt system, the area of highest carcass tension.
OK, we admit it, this is abstract thinking, the kind of thing engineers love to talk about. But we don’t have time to waste, so let’s get back to some belt system functions we can all understand.
Pretend for a moment that you have X-ray vision, and you’re looking down through a tire. From an engineering aspect, you want to see how a tire belt system works.
Beneath the tread area and cap ply, you’ll see the X shape created by the cords of two steel belts as they crisscross one another. Visualize a piece of paper. The cords in Belt One run from the lower left hand side of the paper to the upper right hand side. Belt Two’s cords run from the lower right to the upper left of the piece of paper, hence the X.
Immediately underneath these are the cords of the carcass plies running at 90º to the direction of travel – shoulder to shoulder. You are now looking at a triangular structure, one of the strongest geometrical shapes. Next time you’re flying, look out at the airplane’s wing. The laminates used are placed at 0º, 90º and 45º – a triangle.
By designing a triangular structure into a tire from the belt cords, engineers are able to create a strong structure that can resist flexing, twisting and shear. All three relate to comfort, steering and durability. At the same time, tire engineers must think in three directions relating to belt-system stiffness: vertical, lateral and twisting.
A belt package that’s very stiff laterally is great for handling but awful for comfort. Hence this rule of thumb: The more force a tire engineer wants to generate in a given tire, the stiffer the belt package. A go-kart tire uses a very low belt angle, while a Buick tire has a very high belt angle. Rough ride, smooth ride.
This is serious business because at high speeds, centrifugal force makes the tire want to grow. Fortunately, it can’t grow any larger than the limiting diameter of the belt (or hoop), and that’s the main message.
To better visualize the steel-belt package, picture a sandwich. The steel belts are the peanut butter in this sandwich. A rubber compound (which can be hard or soft) rests on top, the specially coated steel cords (for max rubber adhesion) come next and a second rubber compound sits on the bottom.
“We play with the marriage of stiffness to tune the tire,” said one engineer. Generally speaking, the stiffest belt packages are found in heavy-duty truck tires and ultra-high performance tires. A hard rubber compound is used in high performance tires, a softer compound on smaller broadline tires.
To this mix, add a nylon cap ply and nylon belt edge covers. One belt edge cover is placed at the edge of each steel belt, one at the top and another on the bottom. This is an area of extremely high stress. When the belts are flattened out, they want to pantograph like a Chinese finger trap.
There is lot of movement here because the belts want to roll in their calendar sandwich. In the manufacturing process, this assembly is squeezed together to form a tightly knit package.
There isn’t a lot of rubber in the area occupied by the belt edge covers and little material to absorb heat, which occurs at very high frequency. If the manufacturer isn’t careful, this is an area where air can become trapped. Air caught inside will try to permeate through the innerliner and go to that hot spot, causing a belt-edge lift or separation. Hence the use of heat-fighting nylon belt edge covers – enough of them to mitigate that possibility.
Nylon Cap Plies
As you can see, stiffness is an important benefit of the belt package. There will be a nylon cap ply, or two, in the tire used on a Mustang, but no nylon cap ply on a Crown Victoria tire. The main function of the nylon cap ply is to improve high-speed resistance to centrifugal growth. It also holds the shoulder area down and keeps it from flying around during high-speed use.
To better understand the nylon cap ply, it is necessary to realize that it is placed on top of the belt package at 0º to the direction of travel. Nylon, which doesn’t like to grow, is a good material to use in a cap ply. Usually, it is installed as a continuous 1/2-inch spiral wrap – a jointless belt.
Creating a Belt
Typically, a passenger tire belt system is made of two steel belts, each about six to seven inches wide, depending on tire width. Job One is to get the rubber to adhere to the steel, a difficult task, given that the two would rather be divorced.
To get the two to adhere, the steel is first coated with other forms of metal (say brass) and oxides. Next, three very small steel filaments are wrapped like a rope into a strand that’s roughly 7/10ths of a millimeter in diameter. There are hundreds of these wrapped filament bundles in every belt; the exact number depends on the outside diameter of the tire.
From a tire engineer’s perspective, it’s all about balance. The job is to build the right tire for the right job, and to do so, you need the right belt package for that tire. As a tire’s speed rating goes down, or up, so does the stiffness of the belt package. If a belt system isn’t very stiff, it won’t resist centrifugal force, and the speed rating will be lower.
In a passenger car tire, say a 205/55R16, the belt package and cap ply weigh about 4.4 pounds. The belt/cap package on a P235/75R16 weighs about 7.7 pounds and goes up to nine or 10 pounds on a 265/80R16. In total, belts and cap plies represent about 30% to 35% of a tire’s total weight.
Finally, let’s talk about transfer path. Energy moves from the road through the tread and to the belt and the carcass to which it is married. Next, it moves to the bead wire into the wheel and finally into the car’s suspension. So, while tire belts have a lot to do with handling and steering response (stiffness), they also have a large role in transferring power (energy) from the vehicle’s engine to the ground.
As you think about these dynamics, remember that about 85% of a tire’s stiffness is pneumatic, while 15% of its stiffness is structural. Importantly, a large portion of that 15% structural stiffness is provided by a tire’s metal hoop or belt system.
Next time, we’ll talk about compounding.