Tire Talk - Lateral Stiffness - Tire Belt Packaging
By Steve LaFerre
Adapted from Steve LaFerres article in Tire Review
In the May issue of Tomorrows Technician, we talked about the primary role of a tires bead and sidewall. Now, were going to move to the belt package.
Lets begin with some things you probably dont know. For example, did you know that there are more parts in a tires belt system and carcass than there are in a V-8 engine?
Heres 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 oclock position, the tire and the weight it carries is actually hanging from the top of the hoop, at 12 oclock. 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 dont have time to waste, so lets get back to some belt system functions we can all understand. Pretend for a moment that you have X-ray vision, and youre 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, youll 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 Twos 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 90 to the direction of travel shoulder to shoulder. You are now looking at a triangular structure, one of the strongest geometrical shapes. Next time youre flying, look out at the airplanes wing. The laminates used are placed at 0, 90 and 45 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 thats 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 and smooth ride respectively.
This is serious business because at high speeds, centrifugal force makes the tire want to grow. Fortunately, it cant grow any larger than the limiting diameter of the belt (or hoop), and thats 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 isnt 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 isnt 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 0 to the direction of travel. Nylon, which doesnt 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 thats 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 engineers perspective, its 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 tires speed rating goes down, or up, so does the stiffness of the belt package. If a belt system isnt very stiff, it wont 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 tires total weight.
Finally, lets 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 cars 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 vehicles engine to the ground.
As you think about these dynamics, remember that about 85% of a tires stiffness is pneumatic, while 15% of its stiffness is structural. Importantly, a large portion of that 15% structural stiffness is provided by a tires metal hoop or belt system.
RMA ISSUES SERVICE BULLETIN FOR NITROGEN INFLATION
The Rubber Manufacturers Association (RMA) recently issued a service bulletin on Using Nitrogen to Inflate Passenger and Light Truck Tires in Normal Service Applications.
The use of nitrogen inflation systems by tire retailers has increased in recent years. RMA, which represents tire manufacturers, issued the Tire Information Service Bulletin to provide general information about inflating tires with nitrogen.
The RMA bulletin notes that nitrogen is an inert (non-flammable) gas basically, nothing more than dry air with oxygen removed (air contains about 78% nitrogen). Because of its inert properties, nitrogen is often used in highly specialized service applications and/or demanding environments.
For normal tire service applications, nitrogen inflation is not required. However, nitrogen inflation is permissible as its properties may contribute to minor reductions in inflation pressure loss. Nevertheless, several other sources of pressure leaks, such as punctures, tire/rim interface (bead), valve, valve/rim interface, and the wheel, may negate the benefit of nitrogen.
If the tire inflation pressure is below the pressure specified on the vehicle placard, the tire must be re-inflated whether with air or nitrogen to the proper
RMA warns that depending on nitrogen alone to reduce the requirements for inflation maintenance may, in fact, lead to under inflated operation, which may result in premature tire failure.
For a printable PDF copy of the RMAs nitrogen-related service bulletin, visit www.rma.org, or e-mail T2 editor Ed Sunkin at firstname.lastname@example.org.