Tire terminology: Basic tire construction, Part 1

 After a recent post on selecting travel trailer tires, I received a question from a reader who asked, "What is a cap ply?" I sent an email explaining that some tires include a layer of fabric on top of the belts to help stabilize the tread area of the tire. Pay attention to the different types of tire. Some have a "cap ply" and some do not.

You can learn if a tire has this reinforcement if you "read" the tire sidewall. For example, the tire in this picture has 2 body Polyester ply + 2 steel ply + 2 Nylon Ply in the tread area.  It also has 2 ply of Polyester in the sidewall.

 

 

Obviously, I am too close to the topic of tire construction and sometimes too focused on the technical terminology, so I decided to cover some of the basics. It is easiest to provide links to a few videos on tire construction. But first, I want to be sure that we are all on the same page.

1. Different tire applications, i.e., trailer, car, pickup, small motorhome, and Class A RVs all have different loading and different needs, so there are some construction differences among the tire needs for each application.

2. Having said that, tires for different applications are "different." But it is also important to understand that the general features and parts of tires are similar. So, don't jump over one video just because the tires in that video are not specific to your use. There are still enough similarities that some of the information presented will help you develop a better understanding of the features in your tires.

Basic passenger car tire

Let's start with a basic passenger car tire with a video from the U.S. Tire Manufacturers Association. This tire has 1 body ply and 2 steel belts and no "cap ply". It is just your basic, low-cost passenger car tire. The steps in this video would be what you saw if you were able to visit any tire plant in the world.

You will note that almost all radial tires, from a small 13" to a large 24.5" tire, have the basic features and components shown in the video, those being inner liner, body ply, sidewall, belts, and tread.

High-performance passenger car tire

The first video was an animation. Now, let's look at the real thing, in this case, a high-performance passenger car tire. Did you note the addition of rubber-coated fabric (time 3:15 in the video) just before the tread rubber was applied?

This is what is known as a "cap ply" or tread "overlay".  The "cap ply can be the full width of the belts, as seen in this video. Or, it may be just in the center portion of the tread or possibly two individual strips of reinforced material over the edges.

Different tires have different performance needs, and different tire companies can select different methods of meeting the performance, durability, and cost objectives for their tires.

Next week we will cover larger, all steel radials, as found on most Class A RVs.

If you have any questions on this information you can email me at Tireman9 (at) gmail.com or use the form below.

 

Why do radial tires fail when overloaded or underinflated?

 I have a number of posts here on the topic of why tires fail. There are also more than 16 posts that mention "separation" or more accurately a "Detachment", as a reason or contributor to a tire failing. But maybe, some readers want more a scientific explanation.

 

This graphic is a computer-generated color depiction of the relative heat generated internally in a tire as it is driven down the road. Light blue is the coolest area, and red is the hottest.

When tires crack

I have covered the "artificial aging" of rubber in several posts. Basically, the molecular cross-link density increases, which can be thought of as stiffening and less flexibility. When rubber loses its flexibility, the molecular bonds crack. As more bonds crack, these cracks join up until they can be seen with the naked eye, as pointed out with the yellow arrows in the picture below.

 

 When tires develop these larger cracks, they simply grow over the next hundreds or thousands of miles, as seen in the next picture.

"Free-Spin Inspection"

Tires with these larger cracks can be identified by doing a "Free-Spin Inspection," as covered in detail in this post on how I inspect my tires.

When doing a "Free-Spin Inspection", If you discover the out-of-round or side-to-side movement as seen in the video in the above post, I strongly suggest you have the tire inspected by your tire dealer for that brand tire, or simply replace the tire if it is more than 3 years old.

By the way, the steel belts do not "slip". They are no longer attached as part of the structure of a radial tire.

 

I hope this helps understand the situation.