Is casing growth bad?

All radial tire casings, no matter the size, grow.  It’s similar to putting on a brand-new pair of denim jeans, in that they might be a bit snug in places. Wear them for a day or so and you’ll find the cross-woven fibers loosen, allowing the fabric to stretch a bit. Within a short period of time, the jeans wear comfortably, and fit like a glove, as the saying goes.

That sort of growth is not bad; it’s expected. You don’t want to buy a pair of jeans that are too loose, because they’ll never become comfortable or fit properly. You’ll look disheveled, and the hem could drag on the ground and wear out too quickly. 

Is this growth reversible?

While your jeans will get a bit smaller after being washed and dried, tire casings will never shrink back to their original size.

The fact is, all casings expand. As the tire rolls into and out of its footprint, centrifugal force causes the casing to expand. Engineers call the condition, “service growth.”

The skill in building a casing that lasts and lasts is to predict the extent of service growth and engineer a casing in such a way that it grows into a nearly perfect footprint.

Bridgestone uses many methods, including a system of belts, to control casing growth. Multiple belts with angled steel cords create a foundation for the tread so it lies flat against the road surface for slow, even wear and excellent removal mileage. This optimum footprint shape also promotes superior traction and responsive handling.

So the belts control the casing growth?

Belts, just like the ones some of us wear around our middles, can retain our tendency to become “round” in the center.

Without a belt system in a radial tire, the footprint could wear unevenly, especially at the shoulder, and fast wear could drastically reduce mileage.

As we just said, in traditional radial truck tires, the belt system controls casing growth. Wide base singles, like the Greatec, need a more robust system to control casing growth.

The patented Waved Belt™ design is such a system. It employs belts built from steel cords that are not straight, but formed into a gentle “waved” pattern to keep the casing in the right shape and hold the tread flat against the road for long, even tread life and excellent handling.

Do all the belts use the wave design?

Like radial tires, Greatec wide base singles use four steel belts for added durability. The bottom two belts are the Waved Belt design while the top two belts use angled steel cords, similar to those in other radial tires.

Why not simply add a fifth belt?

Adding another belt would require extra materials, add weight and increase rolling resistance. Many fleets are using Greatec wide base drive and trailer radials as a means of reducing weight so they can, in turn, add revenue-producing freight. If Greatec loaded up on additional belts, it might compromise its weight-saving benefits.  

Why do wide base singles need this belt package?

Remember, if casing growth gets out of hand, the footprint will not be the correct shape.

Imagine trying to break or at least bend a short, stubby piece of wood. Even if you were among the strongest in the world, you’d have a difficult time of it. Now, envision trying the same thing on a long, lanky piece of wood. The extra length makes it easier for you to flex and eventually break it.

The wide treads on wide base singles such as the Greatec are easier to flex, and therefore, especially susceptible to casing growth.  Not only does the belt package control this, but it also works to ensure Greatec’s wide tread is flat – not just in the center or at the shoulders – but across the road surface.

How does it do so?

As the name implies, the Waved Belt design uses steel cords positioned in a wave-shaped pattern. Just like sound waves have wavelengths, including high crests and low troughs, so do those in the Waved Belt System. These waves balance stiffness and resiliency to promote durability. Much of this strength comes from the steel filaments wound around the tire. Like the angled steel belt layers, they act as reinforcing elements to improve belt durability and promote long tire life.

Because of the innovative shape of the belts, it takes a lot of force – centrifugal and otherwise – to change the casing shape. 

Can casing growth be completely stopped?

Probably not. We’d have to go back to the stagecoach days and use steel wheels. Since we appreciate the ride and handling of steel-belted radial tires, we’ll learn to control – and plan for – the optimum footprint.