Tires Viscoelastic Properties - Sport Compact Car Magazine

Similar to radial tires, a belt package a.k.a breakers are laid on top of the carcass and are made up of polymer fibers as well. These breakers serve the same purpose as belts in a radial tire providing restriction on the expansion of the body carcass and providing stability and strength to the tread region.

Radial tires replaced bias tires predominantly because they offer lower rolling resistance thus better fuel economy compared to bias ply tires. They also offer better uniformity and are more tuneable. Bias tires are still used in aircraft and off road (earth mover) applications although that use is dwindling as well. Some racing sanctioning bodies continue to use bias ply spec tires in order to keep costs down or for the desired tire diameter growth characteristics like in drag racing. Though the radial tire is more complex to manufacture, it can carry more lateral load for a longer distance and it has a higher breakaway point. These are the characteristics that are generated by the construction design of the radial tire.

The Wonders Of Non-linearity
Talk to any engineer and you'll soon realize the biggest hurdle in design is something called non-linearity. Most things we're familiar with are linear, like steel. If you apply a certain force, it will bend, stretch or twist by a predictable amount that we can calculate based on its material properties and geometry. Other things like rubber and turbulent flows (like a waterfall or weather system) are non-linear. Non-linear systems can't be solved with straight forward equations like we learn in physics. We can only try to approximate and try to predict its behavior using supercomputers to perform the immense number of calculations needed to get a ball park approximation of non-linear systems.

The reason why rubber is non-linear is because it acts both as an elastic and viscous material. Rubber is one of the few materials that has both of these properties, which is why it is called viscoelastic and so important for tires.

An elastic material (like steel) will stretch a known distance under a given load. As long as it isn't stretched to a point where it is permanently deformed, elastic materials will always spring back to its original shape. Materials behaving in its elastic range will obey what physics nerds refer to as Hooke's Law, which state that the force it takes to stretch a piece of material is equal to the stiffness of the material and the distance it was stretched. This is a nice linear relationship. The stretching comes from the increased spacing between each molecule.

Rubber however doesn't obey Hooke's Law in its entirety. There are two components; an elastic and a viscous component. The viscous portion is where the non-linearity comes from as viscous materials are more like liquids and don't return to their exact original shape. It also takes time to try and get back its original shape which is what all that hysteresis mumbo jumbo is about. Hysteresis is where rubber's non-linearity comes from. Initially rubber will obey Hooke's law and act like nice and linear like a spring. Stretch it more and it enters its non-linear range where the material will stretch slowly in a phenomenon known as creep. When the force is removed the rubber tries to recover to original state, but it won't fully go back.

The creep phenomenon is explained by the molecular composition of rubber. Rubber is composed of long, flexible, polymers in a 3-D network in which these polymers can alter there shape and slide past each other except at points where they are chemically bonded. The movement or rearrangement of the polymer network is known as creep.

Rubber's viscoelastic properties is what makes it so fantastic as a material for tires and such a difficult material make behave as you want it to. And that's exactly why designing a tire is sometimes considered a black art.