Quick post on Dual tire air balance systems

Read a thread on an RV Forum on connecting two tires in dual position, with hoses to "balance" the inflation pressure. There were some claims made about performance improvements but IMO the conditions necessary to deliver those improvements are pretty extreme and improvements are tenuous.

The information on the function of this pressure balance equipment also ignores the potential for damage to the RV when one tire loses air but the driver is not immediately informed of the loss. There is even talk of serious air loss in excess of 25% of the pressure needed, only resulting in faster wear.

I found no mention of operating a tire that low possibly resulting in a catastrophic failure (coming apart and potentially doing thousands of dollars' worth of damage to the RV).

I can't understand why some would think that lowering the hot tire pressure (decreasing load capacity) would ever be a good thing to do.

Someone will probably point out that some of the systems that connect the air between a set of duals limit the amount of air transferred, but that still leaves a problem.

If you have two tires set to 80 psi and one gets a puncture and starts to leak, the "good" tire bleeds off air to the leaking tire until 5 or10 psi has been transferred. Now both tires are at 70 to 75 psi and are just starting to be overloaded.

But the leak continues and the tire going flat transfers its load to the "good" tire, resulting in the "good "tire seeing ever-increased the level of overload.

This can continue until one tire is flat and one tire is 100% overload. Now you are driving down the road with no knowledge you have a flat and a tire at 200% of rated load.

If you are lucky, you stop and discover the problem before the "good" tire fails, too, leaving you with two scrap tires.

This system does make airing up tires easier in that you only have one valve for the pair of tires, but I'm not sure if that "benefit" is worth the cost.

##RVT825

Leveling Blocks - Only support one dual?

I received this question the other day.

"I have some questions about leveling with ramps, specifically on vehicles with rear dualies.

The obvious temptation is to use only one ramp -- and the obvious response is that this is a 2x overload condition and should be avoided.

HOWEVER -- since tires and wheels are clearly designed with safety factors, and since the overload is a very static situation with none of the dynamic variations of a moving scenario, what do you think are the real risks involved?

In weight-sensitive designs like aircraft, I know that a typical safety factor is 2.5x. But I don't know what's in tires and wheels.

It's easy to say "just carry four ramps" to level the rear of a dually coach, but for the smaller rigs, that kind of bulk is pretty impractical.

I'm curious, but I'm cautious too. I'd appreciate your thoughts.

thx"

My reply

First "Safety Factor".
I understand the concept for aircraft and for some other items but the concept doesn't translate to tires very well. Unlike the metal used in aircraft, there is a lot more variation in the raw materials and the finished products
Tires are designed and manufactured to meet and exceed a set of requirements. The requirements are such that all tires must be capable of meeting or exceeding the standards but there is no advantage to exceeding them beyond their design goals as you can't make a claim that some tires in a line might deliver x thousand miles if all tires in the line can't and with hundreds of thousands of tires in a production run it is essentially impossible.
In your aircraft example, there are thousands of subsystems that can each be replaced or upgraded individually and while there may be a goal of a 2.5 safety factor I doubt that each and every part manages to wear out at the same point in its life.

Tires don't have replaceable parts, so while some parts may last a long time and even longer than advertised, when any one component simply meets its design goal but doesn't exceed it, the tire's life is over.


Now to your question on ramps. The answer for ramps is the same as for basic supports used to keep tires out of water when parked for the winter months.

The #1 priority is to support the entire footprint (contact area) uniformly. Never just half the tread width or half the tread length. Even only supporting 80 or 90% of the contact area is not good for the tire structure. Technically the 200% load is not a serious issue but it will definitely stretch a sidewall greater than normal. This increased stretch could, in the long term, lead to localized sidewall cracking after long-term use. RV application can also add other issues such as the tire sidewall losing some of it's "stretch" properties. RV tires are not designed to ever be loaded more than 100% of the load stated on the tire sidewall and inflated to the level associated with that stated max load.

THIS post has some pictures of what you should NOT do. It also shows an example of a tire "footprint" after parked on grass for a couple days.  A 2x4 under part the tread is particularly bad. When you use a block or ramp that is not as wide or long as the tire contact patch the edge that is not fully supported will have localized bending that can overstress that part of a tire.

Yes, tires are built to tolerate abuse and overload and overspeed and under-inflation but in each case, there will be some damage done to the structure. The damage is cumulative and tires do not repair themselves.

I carry 3 leveling blocks in my small Class-C. Here are two set up for my dual tires.  I can use just one if I need to raise a front tire. These ramps can raise the tire(s) 1-1/2" or 3". If I need more I will move the RV to a more level location.

If I need to raise one side of the rear I always use two under the duals.  If that isn't enough, I move the RV to a different spot.  

Your tires are not likely to fail simply because you only supported one of the duals overnight but I can say that putting 200% load on a tire is not a good idea if you can avoid it.

Bottom Line
Anything you use to support a tire must be at least as wide and as long as the tire contact area or "footprint". If your "blocks" aren't wider than the tire or if you do not drive upon them such that they are centered you are doing some damage that could contribute to premature failure that you probably would never associate with the use of improper sized leveling blocks.