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Thursday, March 24, 2011

How RV tires are developed and why some are a compromise

If 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 all trying to deliver the best overall compromise in performance characteristics. Please note than 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" to include these car and pickup manufacturers.

Compromise: Now is a good time to talk about some of the various trade-offs the 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’ 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.

The competition for a tire application might start three or more years before scheduled start of delivery with two to five tire manufacturers 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.

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 or the bare chassis for Class-C or A vehicles and in the case of trailers, may buy the tire with the lowest cost that can meet tire size requirements and expected 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.

Next time:
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Before we cover tire manufacturing -- perhaps dull to you but interesting to me -- we will do our first post on tire loading. I say first, as this topic is the single most important performance characteristic for we RV owners. But few of us truly appreciate the potential safety issues involved.

10 comments:

  1. Roger,
    I enjoy the knowledge and enthusiasm you put into your posts. You actually make tires interesting for your readers, or at least for this one. Load range and tire loading is a fine topic to bring up at this point. We just weighed our new Class C, and put the front axle, rear axle, and full weight on the scale. It wasn't fully loaded yet, but well on it's way. We're under the weights shown in the motorhome manufacturers manual, but haven't checked the load range on the tires themselves.

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  2. Roger,
    I know that there is a significant difference in tires for my big trailer applications, such as whether it is closed tandem, spread axle, and the use of axle dumps for tight turns, [ a Ranco belly dump, and a Wilson Flat w/ a spread], any ideas how the load ranges/configuration play on tires for my '79 Roadrunner 5'er, tag says 15680#, but it has a variety of 15" tires, from P195 to LT235's. I will be putting 4 new tires on, looking at LT215's, ld range E, non of the local dealer agree on tire sizes.

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  3. Heartland trailer's are shipped out with china tires and they don't balance them. I had 2 tires that got ruined before I found out the problem. Balanced tires are the most important part of a good and safe trip.

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  4. Chuck
    15,680 minus 10% on the pin leaves 14,112#. Am I correct is guessing you have three axles or are your axles rated at 8,000# or more?
    You failed to mention if you are running at the Max GVWR based on scale measurements.

    Spread Axle trailers like the big rigs you mention have special tires and axles in many cases so that situation does not apply to a standard RV.
    Rules you need to follow:
    1. All tires on an axle should be same Brand, Size and type.
    2. All tires on an axle should have the same inflation
    3. The Minimum cold inflation needs to be high enough to exceed the Load Inflation chart numbers.

    When listing a tire size it is helpful to include the complete size info such as LT215/85R16 LR-E which is rated as capable of much more load than a LT215/70R16 LR-C.

    I believe the P195/75 or P195/70 or whatever 15" size it is, is probably overloaded and probably inappropriate for your usage.

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  5. Roger,
    I'm really happy to see your blog. I'm on the staff at www.rvforum.net and we get tire questions nearly every day. Most RVers are just plain ignorant of tire loading issues and far too many RVs come with tires that are barely adequate for their GVWR. It's no wonder that RV tire failures are so common.

    Keep up the good work!

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  6. What should I do to care for or protect my tires during storage?
    My guess is
    1. cover to keep away from ultraviolet light
    2. get tires off the dirt, rocks, mud etc - use boards or what?
    3. if possible take the weight off the tires
    Thanks for your advise.

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  7. Bill
    Your list is a great start. I would offer a couple of comments.
    If you park in a garage do not park near a large electric motor or marine aquarium ozonator as Ozone is very bad for rubber. I was once involved in an investigation where the tires were destroyed in three months when the rubber became hard as rock because the person had an ozonator in a small closed garage.
    If you park on anything other than concrete slab, I would place some barrier under the tires such as plywood and be sure the entire contact area of the tire is supported. Plywood will protect the tire from the oils in asphalt, or moisture from sand or dirt from leaching into the tire and damaging the rubber or steel.
    If you can unload the tires that would prevent “Flat Spots” which can make the ride bad. If you can’t unload the tires at least inflate to the Max inflation on the sidewall of the tires to decrease the tendency to flat spot.
    Never use a tire cleaner that has “petroleum distillate” in the list of ingredients.

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  8. RE: Tire Balance
    Not sure how balancing a tire will improve its durability or being out of balance will "ruin" a tire. It may improve the ride if far enough out for the application but not all vehicles have the same sensitivity to balance.
    In my opinion improper inflation for the load is the number one item that can ruin a tire and inflation is in the complete control of the driver.
    Learn more at
    http://en.wikipedia.org/wiki/Tire_balance
    This link talks about Off-Road tires but has nice shots of resolving a ride complaint.
    http://flashoffroad.com/features/Tires/balance.html

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  9. This is a bit off the tire safety theme, but perhaps you can offer an opinion. My coach uses 295/80 R22.5 tires, which are a relatively rare size and thus pricey. According to the tire specs, the much more common 11R22.5 is nearly identical in size, Revs/mile, etc. Do you see any reason I could not substitute the 11R for the 295's? I can probably save quite a bit on that size.

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  10. Great post. Hope you could develop more tires which really fit in different sizes of cars.

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