Tire terminology: Basic tire construction, Part 2

I have previously covered basic tire construction as you would find in most passenger car tires and lower cost, lightweight RV trailer tires. I had some additional questions, so I have expanded the information.

As we move on to larger, heavier trailers, pickups and Class A, B and C motorhomes, what I would expect to see is tire constructions with 1 or 2 body ply + 2 or more steel belts + 1 or more "cap ply". The differences would also be larger and stronger reinforcement materials.

NOTE: the details of the "Cap Ply" are not listed other than the number and generic material such as "Nylon" or "Aramid".  THE KEY IS THAT AT LEAST SOME CAP PLY IS SHOWN".

IMO no cap ply shown, indicates to me an ST type tire in Load Range C through E to be avoided for multi-axle trailer use. 

For those who skipped over the "lesson" on tire construction, we are reviewing the material list as seen on the sidewall of all tires, similar to what might be seen in this picture:

OK, but what about Load Range E or F or even G and H Load Range tires? What's the difference?

I found a discussion on load range in a Forum where the question "How strong of a tire do I need?" was being discussed. Here is some of the discussion:

Below is an example of what you might see on a Class-A tire sidewall:
Load Range is rated at "H" 4,940# single, 4,675# Dual tires.
Tread is 5 ply of steel. Sidewall is 1 ply of steel.

"Strength" of steel ply in tires

Well, I feel that a better understanding of the "strength" of the steel ply might help people understand the concept of different tire constructions.

The steel in tires is not a single strand of steel, as you find in a paper clip, but a twisted cable of many smaller strands or filaments of steel. The number of "ply" or layers of cord (textile or steel) are not in itself any proof of strength. Individual cords of steel are made up of many strands. The steel or other material used to make the strands can have a wide range of strengths. Also, the number of strands and even the "twist" of the strands can affect the strength and flexibility in the end product. Some examples of just a few of the hundreds of different steel cables I might select when designing a tire can be seen in this post.

Each has a different configuration. Without more information, it would be impossible to know which is "stronger". Don't forget tires have to flex and bend millions of times, so just "max strength" may not be the best choice, as you need flexibility too.

The MINIMUM "strength" of your tires would be the load range—required to support NO LESS than the truck scale reading for your RV when it is fully loaded. (Load capacity of 110% of scale reading would be better.)

Do not try and get information on the twist or cabling of the steel in your tires. You can simply use the load range letter and load capacity numbers where higher is stronger.

You need to be careful of who you listen to when purchasing RV tires

OK, my intent is not to try and turn you all into Tire Design Engineers, but to help you appreciate that tire design is a complex topic that is seldom covered in sufficient detail in an RV Forum. Also, I want to allow you to decide on what you need to consider when purchasing tires for your RV application.

I hope that you can see that you may need to review numerous posts to learn what you need to consider when selecting different tires than what were originally provided.

I seldom see posts on tires that include the "complete" tire type and size information. When people run out and purchase tires without understanding the needs for their specific RV, there is a good chance that they may end up having to rely on a "salesperson" who has insufficient knowledge to ask the basic questions about your RV tire usage. You probably need to be better educated on your tire needs than any tire salesperson you will run into.

Be careful when trying to rely on RV Forum posters. I am only aware of two actual Tire Engineers on any of the dozen RV forms out there. I do know of at least one person who thinks they know more about tire design and performance characteristics than the tire companies do, but that person consistently provides misleading advice to the RV community.

 

 

 

Is it OK to change tire size on duallies?

Question from RVer on a forum

“I need six new tires for a 2004 38-foot Dutch Star on a Spartan chassis. Michelin tire models and sizing have me totally confused. Current Michelin size is 275/80R22.5. Dealers claim this Michelin size is interchangeable to 295/75R22.5 from brands that don’t offer the original size. They tell me there will be no ill effects on the drive line or instrumentation.” 

 My response:
I was concerned with this possibly incorrect information. In this case, the two sizes have the same load capacities, so that was not my concern.

Some folks replied to the original question on the forum by suggesting he use this tire size website or similar websites to learn about tire sizes. I did have a little problem with using such a “tire size” site, as the information is aimed at car applications and does not include special information regarding dual-tire applications. Also, the resulting numbers may or may not match published industry standards.

I pointed out my concern about the Minimum Dual Spacing (MDS), which is a published dimension in industry standards and has been covered previously in this blog.

Some folks reported that they have changed tire sizes and had no problems, implying that any change might be OK

I provided this graphic to help people understand what this dimension is:

As you can see, the wheel offset and even wheel thickness can have an effect on this spacing. I also pointed out this example: 275s have an MDS of 12.24, and 295s have an MDS of 13.19.

Let's assume that with your wheels and with the 275s you have a physical 1.00" clearance down at the bulge in the sidewall near the road. So if you don't change wheels and put wider 295s on the new, clearance will be 1.00 - ( 13.19 - 12.24) or 1.00-0.95 or a final clearance of 0.05", which is clearly too small.

I hope that those considering changing tire size consider not only the load capacity, which is very important, but also the dimensions.