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RV Tire Safety: How are tires “designed”?

By Roger Marble

How tires are designed?

First off, let’s be clear on the word “design.” For this post I am not talking about the exterior “look” of the tread or sidewall but about the complete structure and how the tire performs.

Passenger, LT, and ST type tires would be considered “consumer ” tires. The 19.5 and 22.5 size tires used on Class A RVs are really “commercial” tires designed for the truck market. Some may be marketed to the RV owner but in general they are intended for commercial (truck) applications.

When a tire is being designed for a specific vehicle manufacturer such as Ford, Chevy, Toyota or BMW, there will be a number of tires submitted by competing tire companies. They will all be trying to deliver the best overall compromise in performance characteristics. Please note that all original equipment vehicle manufacturers have slightly different requirements but all make similar requests for performance improvements in many areas. In the future I will use the term “OE” (original equipment) to include these car and pickup manufacturers.

Why is there always a compromise?

Now is a good time to talk about some of the various trade-offs the tire design engineer is faced with when trying to meet conflicting goals and customer wants. I am sure we would all like an RV that has all the interior space and amenities of a 40-foot diesel pusher but gets 25 mpg and can be driven down crowded city streets without knocking off our mirrors. Oh yes, it should also cost under $30k. Well, Bunkie, that just ain’t gonna happen in real life.

The same goes for a tire that handles like an Indy tire, is as quiet as the proverbial mouse, has great off-road traction, is good for 100k miles, and costs $25. One thing few people realize is that most if not all performance characteristics are a compromise. For example: If you improve wet traction you probably hurt fuel economy unless you use a special type of rubber that costs double per pound and is more difficult to process. If you improve handling you might hurt ride and noise. When you improve noise you can significantly increase the cost of making the molds used in manufacturing.

The cost of a tire mold can be as low as $10,000 and can approach $100,000 each. Depending on the production volume needs, a tire manufacturer could need 30 or more molds. The list of trade-offs goes on and on.

Tire design engineers usually will have a couple dozen tires made for 3 to 4 specifications. Each “Spec” has a number of minor variations as we never know the exact trade-off the OE customer is willing to accept. Usually they will only say things like, I want a one step (in a 10 point scale) better steering response but will accept a two step loss in snow handling.

The test for each of a dozen different parameters are subjective and depend on the seat-of-the-pants “feel” of the evaluation engineer. So we tire engineers can’t be 100% certain how important each performance parameter really is as they all seem to start off as all being the “most” important.

How long is the tire design process?

The competition for a tire application might start three or more years before scheduled start of delivery. Two to five tire manufacturers are competing for the contract, knowing that only one or two will end up being selected to actually provide tires. The costs associated with building and testing special prototype tires can run in the hundreds of thousands of dollars and are absorbed by the tire company. The only way a tire company can afford this type of activity is by landing a contract for a few hundred thousand tires so the costs can be spread out and recovered over years of tire production.

Unlike “OE”, an RV manufacturer may only need a couple thousand tires – so a custom tire, designed for a specific RV, would be cost prohibitive. Since the RV manufacturer won’t be trying to get custom tires, it doesn’t have staff engineers working on developing specifications for such tires. The RV company will in all likelihood either take what comes already on the cut-away chassis as used on most Class C RVs or the bare truck chassis for Class A vehicles.

In the case of trailers, the RV company may simply specify the lowest cost tire in a given size and Load Range, as load capacity is one performance feature the RV company must meet along with a delivery schedule.

For RV applications the one thing that is in the control of the manufacturer is “Reserve Load”. This is the difference between the load placed on each tire with the RV normally loaded and the load capability of the tires at specified inflation.

 

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT980

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Alain Tasse
9 months ago

Thank you Roger, love your articles based on your experienced insight. Question if I may? Are OE tires, of any particular brand and model, sold to vehicle manufacturers to equip specific vehicles, the same quality as the same product sold directly to consumers. Example is bridgestone dueler hl Alenza that came on new 2018 suburban. At less than 40,000 miles they were worn to 4/32nd. Tire dealer stated that OE tires were really of lesser quality and wanted to sell me same stating would be much better? I went with Michelin finally. Is there any thruth to this as this is our 5th suburban with same OE tire issues? Thanks again.

STEPHEN P Malochleb
9 months ago

I believe the bottom line here is, OE tires are made as cheap as possible and meet DOT specs. At least in the aftermarket you can do your research for the best overall tire as far as price, quality, and durability. And yes you get what you pay for. I see so many vehicles that bought tires from large retail outlets that just barely meet the GVW of the vehicle they are on and can not safely carry a load. Owners want cheap, and company’s want the sale. Safety is overlooked. Spend the time and do your homework. Your life could depend on it. Thanks for your articles. Knowledge is priceless.

Roger Marble
9 months ago

Steve, Was that a “typo”? In my article “OE” referred to tires going to car companies i.e. GM Ford, BMW, Mazda etc not the RV Trailer ST type tire. I can assure you from 40 years of personal engineering experience that the days of lowest price wins, went away with bias tires in the 50’s. Now I might agree that many ST type, but not all, are designed with lowest cost that can pass the 1970 DOT standards in MIND. The good news is that there appear to be a FEW ST type tires now being made that exceed the DOT 109 minimums and might even pass the upgrated to 2002 DOT 139 standards but the way DOT wrote the regulations ST tires still only need to meet the 1970 standards. Might that be because the RV industry, not wanting to have the cost of ST tires go up, lobbied DOT to not upgrade ST type tires in 2002? Who knows?

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