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.
Roger Marble
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RVT1229
Always great information with even better explanations to go with it.
Thanks again, Roger!
Great info! Thanks Roger.