How are RV tires developed?

 By Roger Marble

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 that all original equipment ("OE") 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.

##RVT802

"Care and feeding" of your valve stems

I suggest that if you are looking for long "bent" metal stems you ONLY get them installed by a Truck Tire store. Preferably not just a "dealer," as anyone can sell tires, but a "company store" that is owned by a tire company or a store that is part of a large chain, as they are more likely to have a selection and actual training on proper installation of long truck valve stems. The shop should have the proper tool for bending the long brass stems without cracking them, if bending is needed.

With long stems it is also important to remember to not just press the air chuck or pressure gauge onto the end when adding air or checking pressure as you can generate a lot of force on the joint between valve and wheel. Always support the stem or hose extension with your other hand even if the stem or hose has a hard mounting, as you can loosen the mounting point too.

There are specifications for torque of the metal nut for bolt-in valve stems. (25 to 45 inch pounds) This is especially critical on your car or truck if it came from the factory with an internal Tire Pressure Monitoring System (TPMS). Many of the internal TPMS are made of aluminum for light weight and as such have very low torque specs. It is easy to break the aluminum nut or, even worse, the stem itself – which could cost you $50 to $100 to replace.

"Tire Review", a trade magazine for tire dealers, had an article on TPMS sensors and they point out the following:

"What is the torque value required for the nut?
Typical torque values for the base nuts on a TPMS valve stem range from as low as 35 in.-lbs. of torque to as much as 80 in.-lbs. of torque. That’s quite a range. This doesn’t mean that any torque value within this range is acceptable. It means that the torque specifications for the base nut on one vehicle might re­quire 44 in.-lbs., another might re­quire exactly 62 in.-lbs., yet another might specify exactly 71 in.-lbs., and so on. Don’t guess. Look up the torque specifications for the vehicle you’re servicing to make sure you use the correct torque.

Why should the nut be replaced every time a sensor is serviced?
The nut is made of a softer metal than the stem, so it will be damaged – not the sensor – if it’s over-tightened. The material of choice is typically aluminum. If the nut is over-tightened, it will develop hairline cracks."

NOTE: Those specs are INCH-Pound, not your normal Foot-Pounds. Here is a picture of my TPMS Inch-pound Torque Wrench.

Standard "metal bolt-in valves" also have specs for the nuts and those valves are usually brass or plated with nickle or chrome. I am not aware of any stainless steel valve stems for regular automotive use. There are some aluminum bolt-in stems too, but those are expensive lightweight units made for race car application which would not normally be sold without being identified as such.

Even valve core have a spec (2 to 5 in-lb) as there is a tiny gasket that can be distorted and even broken if you over-tighten the core. There are some special tools. But rather than buy some special tools I suggest you tighten core till air stops, add no more than 1/4 turn more, then confirm no air leak by testing with soapy water. When no bubble forms the core is tight enough. I then attach the metal valve cap to ensure no air is leaking. There have been a few cases of slow leak through the valve core that ended up as a tire failure as no metal cap was used. The cap is primarily intended to keep dirt out of the core area but is also a "backup" on preventing air lost past the core.

Whenever "messing" with your valve it's always a good idea to confirm there are no leaks with a quick spray of soapy water.

##RVT801