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.