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Apr 07

Tires : Myths, Facts, and Legends Part 2

Last time, I introduced the idea that tires should be the first mod on your car. Now we get into the construction and language of tires.

We start with a little history. Most common knowledge says that John Boyd Dunlop invented the pneumatic tire. While his invention is subject to debate, what is not, is that his company was the first major manufacturer of tires. Of course starting with bicycle tires and then moving on to automobiles, which is the subject of our topic. But one basic idea has not changed and that is the basic construction of a tire.

From the beginning, the pneumatic tire has had the basic construction. Some sort of basic woven material to form a backbone, from the beginning cord moving through nylon, fiberglass and now what we have today steel twine wrapped in kevlar. Then the rubber is applied to seal and make the air cavity. This is generally known as the tire carcass, the body of the tire.

The tire needs a rigid point to attach itself to the rim. This is known as the bead. Construction of the bead has evolved through the same lines as the carcass weave. Starting with a wide string cord, and moving up with invention of more durable materials. Some specialty tires have an actual steel wire holding the bead in place. While its not the bead but the air pressure that holds the bead in place, It is the physical link from the tread face to the wheel and sees all of the pressure and forces. In some racing instances tire failure from overheating the tire and heat from the brake system has caused the bead to fail leading to an immediate loss of tire pressure.

[img]http://i207.photobucket.com/albums/bb7/4camcobra/helpTireConstruction.gif[/img]
Picture source Discount Tire

The thing connecting the two is the sidewall. In the last 30 years, this has been one of the two parts of the tire that has seen the most advancement. Today, we see tires that have as little as a half an inch of height off the rim. And normally we see these tires on the heaviest cars and SUVs. The sidewall used to be the realm of only the cord weave and rubber. Today we see all types of silicone inserts bonded in to it in order to keep the sidewall flex down. While this makes for a more accurate handling tire, it also makes for a rougher ride. So the construction makes for a compromise. What is more important to you? Ride or road feel.

The final part of the tire is the most important, the tread face. This is the only physical link from your car to the road surface. It is what keeps a Z06 Corvette pull a full G worth of lateral side load. That is the equivalent of almost two tons of force pushing the car off the road. What’s even more incredible is the fact that with four wheels the total contact at one time is about the size of a 11×8 sheet of paper, regardless of width. The common road tire has grooves and channels cut into the tread face. The solid blocks of rubber that touches the road are known as tread blocks. The construction of the tread face has been changed radically in recent years. Not just in single or multipurpose roles, but in material. Recently the tire makers have started to increase the amount of silicone in the composition. The large improvement is both more durable tire without sacrificing grip. In fact most top tier tires have multiple layers of compound within the tread face. Giving a grippier initial surface, giving way to a more durable tread underneath.

I want to go into the carcass construction a little deeper before I move on to the next article. As stated earlier the weave was made of cotton cord. Initially, there was a double weave of cords at no more then 40-60 degrees apart within the carcass. This is known as a bias ply tire. Bias ply were much less rigid which gave a wonderful ride, but the softness gave way to bad road feel and most important durability. There is a reason such things as continental kits and full size spares were big back in the 40-60’s. Just a sharp stone could puncture a tire. Seeing a flat on the side of the road was a common occurance.

Michelin developed the first radial tire in the 40’s. The radial weave is at a side to side across the tread face pattern. It greatly reduced sidewall flex and gave the tire a much more durable construction.

Fast forward to the 70’s and the steel belting evolution has come about. In my opinion, this is the greatest advancement in cars in the last half of the 20th century. This makes possible hooking up the massive horsepower cars of today. Not only that, but it made the tire stone cold reliable and the only way to get unexpected tire failure is due to a puncture is from a perfectly placed screw or nail, or damage due to hitting a curb. The stiffer tread face makes possible the low profile tires and hard reenforced sidewalls that give us the immaculate feel and control in new cars today.

That’s about it for tire construction. Next time its how the differences come home to make a race tire a race tire.