Most modern tires have belts. In fact, most have 2 or 3 as part of their construction. But what do belts do? Why does it matter? And, more importantly, why do you want them?.
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Advent of Belts
Around the late 1960s, European-made radial tires started to get pretty popular, mostly due to their lasting longer than the American bias-ply tires. (To put it simply, radial tires have cords that run perpendicular to the tire’s direction of travel and bias-ply tires have cords that are arranged on an angle to the direction of travel.) American tire companies didn’t like this, for obvious reason, and so belts were introduced to tire construction. Belts had two benefits (aside from bringing business back to the American companies): they improved the tires performance and made them last longer.
By adding belts to the tire’s construction, manufacturers could reinforce the tread of their bias-ply tires without having to change out all their manufacturing equipment to make radials. While this was definitely an improvement, though, it didn’t exactly solve the problem, because they lasted longer but still not as long as radial tires.
So, by the 1970s they ended up changing their equipment anyway, and radial tires took over the market. But they kept the belts. Just like with bias ply tires, belts still improve the handling and make them last longer.
What Are Belts?
A tire belt is basically a layer of fabric or steel cords encased in rubber. The cords are made by winding the threads together to form a unit, kind of like a rope or a cable, but smaller. The cords in the belt are often wound together as well, making an even stronger, but still flexible, unit. The whole thing is encased in rubber and then cured with heat, and that’s your finished belt.
Belts are placed between the tread and the liner, which is basically the air-tight inner layer of the tire. So, when the tread hits the road, the belt helps keep it stable under the pressure while driving and cornering.
This also answers the question of why belts matter: Stability. Stability actually helps to increase your tire’s traction, so a tire with belts will perform better both in a straight line and around corners. Stability also reduces squirm, which makes the tires last longer.
Cords (and the belts they go into) are made of different materials. Steel cord in belts, or “steel belts,” are increasingly common. That’s because of their strength and ability to maintain their shape under stress. Steel is a strong substance, after all..
Polyester and nylon cords can also go into belts. These are more flexible, but still maintain their shape, and they’re cheaper. They can also provide a smoother ride than the steel belts. Polyester, because of its flexibility, is usually the type of cord you’ll find in tire sidewalls. Some other materials you might find in belts are Aramid and Kevlar, both of which are high-strength components of bulletproof vests. Just like steel, Kevlar is becoming increasingly popular.
Cap Plies
Cap plies are related to belts, although they haven’t been around quite as long. Cap plies, also called overlays, are layers of cords and rubber that are placed between the belts and the tread. They provide extra strength in high-speed tires and you’ll usually find them in tires that are H rated or above. Because high speeds generate high temperatures, these tires are under more stress, and the cap plies help discourage ply separation.
Belts and cap plies can both be what’s called “jointless.” This just means that, instead of having ends that overlap somewhere in the tire, they join the ends together with a weaving process to form one continuous unit. (You might also hear these called “woven cap plies.”)
Making Your Choice
So belts are a good thing, but how do you know what you want? Well, like most things with tires, it depends on what kind of driving you do. So the choice comes down to what’s going to work best for you:
- Polyester: This is the cheapest option and works fine for everyday situations like running around town or driving to the store. These are also good for economy cars.
- Steel: While they’re more expensive, they’re better for vehicles that are driven hard. So if you have a high performance vehicle or a heavy duty work truck, you might consider these.
- Cap plies: Extra insurance against ply separation on high- and ultra-high-performance tires. Found in premium tires for higher speeds.
- Aramid or Kevlar: These are the best choice for off-road tires. They’re tough and durable and great for muddy, rocky, or completely off road paths.
So there you have it! It’s pretty easy, and to be honest, if most of your driving is around town you don’t really need to worry about it that much. However, if you want to do some heavy-duty work or off roading, now you know what to choose!
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